Hobbing a worm gear Part 1 and Part 2

[kinetic]

Some pics of the steps I'm taking to attempt to make
a high accuracy worm and worm gear.
(Inspired by a few articles on the internet and also
keenly following Jeff's progress on his Project 24 thread):
http://www.iceinspace.com.au/forum/showthread.php?t=39135

This is only part 1. Hopefully, if this works, I will have
a part 2 :)

I started with the spindle.
I made it out of a pipe fitting welded to a piece of ms plate.
After turning down the two housings for the bearings, I then
made a shaft and turned a thread either end.
These parts once assembled are mounted on the tool post
cross travel on the lathe.
This gives you cross travel adjustment for the cut and transverse
travel to centre it on the thread cutter. Height is obviously critical
and some homework was done to make the spindle just the right
height. Some fine adjustment can be done with packers under the
'flange' (below).

The flange for holding the raw worm wheel blanks was turned down from
an old VCR head.
These have the most beautiful aluminium I have ever seen. Probably
what they call aircraft grade.

I cut two steps on the flange for locating centre the two types of
worm wheels I'm ever likely to make:
One for fitting on a 1.5" shaft (my R.A. axis)
One for fitting on a 1.25" shaft (my Dec axis)

To lock the flange down on the spindle I needed to temporarily lock
the spindle from rotating.
This was done with a hole drilled through the side of the spindle
housing and shaft to enable a pop rivet to be slid through for tightening
and undoing the flange. And also for pre-loading the bearings to eliminate
any play (very important).

My first test run will be the aluminium platters from a very old hard disk
drive. These are enormous, but also beautiful high grade aluminium.

The worm thread cutter is obviously also critical for accuracy.
As my lathe can cut almost any metric threads, I decided to try
an M12 thread to start with on the test case blank.
My proper cutter (not shown) is an M12 plug tap mounted on an
extension and held between the chuck and a live centre.

For the test, I will just use a piece of M12 thread also mounted in an
extension.
I have read that the best worm thread is an ACME shaped thread , the
type similar to a vice thread or car jack.
If these test runs go well, I will then try making an ACME shaped thread
in the lathe, starting with M12.

more to follow.

Steve

[GeoffW1]

My word,

I would like to send my applause and appreciation. I started my working life as a Fitting & Machining apprentice, so I can see what this involves.

Cheers

[kinetic]

Indeed, thanks Geoff, but the hard work has all been done by others.
A few really good articles on the web about this subject.
I thought I'd dive in and have a go myself.

Progress:

Tried the first test blank today. Old Hard disk platter, aluminium
mounted to the spindle flange.
M12 test cutter with splines cut with a Dremel.
(Makes it cut better just like a thread tap).

With the lathe on lowest gear I wound the platter in until it just
made contact with the cutter.
The cutter immediately began to drive it's way around the blank.
Nice cutting action. When it had driven all the way around the blank
and made a nice initial set of cuts, it didn't exactly meet up with the
first cut.
This is to be expected.
Unless I had made the blank an exact circumference to match a
theoretical thread pitch, this was never going to meet up.

All of the web articles I have read mention this.
What I then did was to just continue and watch it attempt to
'self index' by cutting deeper.
Eventually, a second set of 'mountains and valleys' appear as the
cutter made it's way around several dozen orbits.
The second set are eventually eroded away until the main set
sort of 'take control'.
The trick is to stop adding depth to the cut once the self index finds
exactly the right circumference for the start to meet the end.

It went better than I expected! :)

Pics attached.

Steve

[Gama]

Maybe look at using a friction drive method here and a worm/wheel setup on the drive shaft.
Low to no PEC this way.

Theo.

[kinetic]

Hi Theo,

not exactly sure what you mean?. Do you mean setting up
a motor driven wheel while cutting?

Or give away the machining a gear alltogether and steer towards
a friction drive/worm , a la motor driven Alt/Az Dobs?

They too have PEC, the most noticeable being non-concentric worms
and slippage/non concentric friction wheels.

All drive related PEC is measurable and manageable in my Bartels
Stepper drive system. I just rarely have it turned on! :)

Steve

[kinetic]

Last night I tried two old hard drive platters screwed together
to simulate a thicker test blank.

This will almost be the thickness of an actual blank if/ when
all of this works ok.

I also shaved about 2mm off the diameter to make a fresh
start point to attempt the self index.

Self indexing went beautiful this time.
The start met the finish. By pure luck, I suppose,
the circumference must have been just right.
I noted the diameter! :)

I gradually gave the cutter more depth until the valleys of the
thread had just the right looking amount of wall left between
them.
Once a nice looking depth was reached I just let it drive itself
around for a good half hour with some cutting compound to
smooth over the roughness.
After a bit of a polish up with fine wet and dry it looks like
quite a good serviceable worm gear! :)
Apart from the fact that this blank is made of two platters,
(you can see they don't quite sit flat together because of trapped
grit and burrs etc), I think this gives me confidence in
attempting my first real full thickness blank.

Steve

[Gama]

Friction drives use 2 rollers pressed firmly together or by a band.
They provide very low or no PEC.
The larger mount mobs all use this method.
You can use the worm setup to drive the smaller roller. A few images of one here http://www.observatoryscope.com/prototype/prototypeoperation.html

Theo

[kinetic]

thanks Theo for the advice.
I do know a thing or two about PE and different drive methods.
Are you sure you're not confusing PE for backlash?
Any gearbox drive train will have PE.
The final drive method turning the RA axis obviously contributes
the most 'error' to the final drive shaft.
I know drive belts (your example shows belts on both axes) would have
almost no backlash, but the biggest PE component would be the
accuracy of the smaller pulley wheel driving the big RA pulley.
If this is dead on concentric, then the next biggest PE would be the
accuracy of the worm/wheel driving this small wheel.
You're probably correct that this whole combination has less PE
than a rather large , well machined worm/worm wheel driving a
GEM.
But it would still have some PE.

I have measured the PE in several systems over the years (using K3
Drift Explorer) and for gear trains it is usually a sine wave.
The main sine wave is repeating/cyclic fast-then-slow PE of the very
last gear in your drive train.
This is the machining errors of that gear.
Superimposed on that sinewave is a smaller cyclic sine wave which is
the next gear back in the drive train.
These are measurable and able to be zeroed out in the Bartels system.

As there is a worm involved, albeit one gear back in the drive train of
your example, it would have a measurable error and contribute PE.

Although I fully appreciate your advice, I thought it would do no harm
to attempt to machine a worm/worm gear.

Steve

[Gama]

Well, nothing is 100%, you will always get a measurment, but in imaging youd be hard pressed to see it. Heres the site with some images, plus you may want to see the images taken by the scope. All done without guiding.
http://www.observatoryscope.com/

Theo

[kinetic]

Final part of the first test was to separate the two HD platters,
clean off all burrs and smooth them down with wet and dry.

Re-joined them and pop riveted them together.
Then I did another half an hour bedding in the
worm cutter with some polish to average out the cut.
Teeth are looking pretty clean and consistent now.

This worm gear could easily go straight on a DEC axis and do
a reasonable job.

The next step I have in mind is to turn up a worm with as close
a representation to an ACME thread as possible.

I'll use another two HD platters and see how the thread shape turns out.

Steve

[Karls48]

Hi Steve. Great effort. I have 7X12 lathe and mini mill machine. I have tried to hobble worms and it kind of worked. I also have small rotary table so I can do reasonably accurate indexing cuts. But my biggest problem is how to calculate blanks diameter. All what I can find on Web requires mechanical engineering degree to understand. Unfortunately I’m electronics guy and last time I worked on real lathe was about 45 years ago. So how do you calculate diameter? I’m casting my blanks from scrap aluminium.

[Karls48]

Steve,I think that hard disk platers are too soft to be good for telescope drive. Easy to work with yes, but I think it will wear out too quick.

[hikerbob]

For brissy based readers (and maybe elsewhere) Action Aluminium sell offcuts of aluminium and will cut to size but I don't know if there are limits on that. They often seem to have thin offcuts from quite large round stock.

I'm planning to try some hobbing so last time I was over there I picked up a number of offcuts. One is 255mm diameter and about 15 mm thick. Hardness may be a factor, I'm thinking of practicing hobbing on aluminium and once I've got the hang of it either trying to buy an offcut of brass/bronze of casting a blank. I've got a small stockpile of old brass fittings around that I've accumulated over some years. My casting skills are still at rank beginner level but that's something practice can fix.

I'm considering using the thread out of an old car jack for the worm but have not done anything to test how regular the thread pitch is. I've also looked into ACME rod, by local bolt place can get in 3/4" in 12 foot lengths for about $140 (I think, not quite what I need). It's supposedly very high quality.

I'm still thinking about what to do to hold a blank on for hobbing. I've got a 6" rotary table and I suspect that it might be a good base for the process in free running mode. Any thoughts from more experienced machinists?

Bob

[kinetic]

This one is purely just a test Karl.
I don't intend to ever use it. The alu is quite soft, you're right, but
it could always be anodised?



All guesswork this end Karl! :)
Some machinists try to aim for a 359 or 360 tooth worm.
I think that's because the motor at the start of the reduction does
1rpm or something....I vaguely recall that was why.
With microstepped steppers (my Bartel drive) this isn't an issue.
I only need a total reduction somewhere in the order of 1:10,000
for the stepper to 'sing' at a nice high pitched , smooth rate.

This test worm ended up having 230 teeth.
The diameter was purely a guess.
Some articles have the same problem calculating the theoretical
starting diameter. As I'm from the same type of background as you
it's all a bit over my head too! :)
I think indexing on a rotary table is a way better start though.
There is no way to know how accurate my cutter drove it's way
around the circumference to start with.
Some teeth might be stretched and some might be squashed etc.
Testing will show how accurate it ends up.

Steve

[kinetic]

Today I attempted to machine a nice concentric M12 worm.
I first turned a nice std M12 thread and turned a clear
shank both ends. I decided to make it standard rather than
ACME to make it match the gear. ACME can come later.
Bored it out to fit snugly on this little worm assembly (http://www.iceinspace.com.au/forum/showthread.php?t=42436) that
had some purpose built thrust/end play bearings.
You can see the original worm sitting beside it.

The idea here was to set it all up on my shed bench and
run it for an hour or so and feel the play/slop/eccentricity
in the whole setup.

I screwed the spindle and HD platter gear down to the bench first
then meshed in the worm until it was on centreline and snug.
I drove it around a bit first using a battery drill to check
concentricity of everything and see if it hadn't bound up
anywhere.

Next I bedded in the M12 worm with the worm wheel
using a 24AC motor/reduction gearbox and small chain and sprocket.
Some cutting compound every now and then was added.

The gear was SOOOO smooth and had absolutely nil slop.
After driving itself around in both directions for about an hour there
was no slack or backlash present.
With some light grease on it, it was very quiet too!

Steve

[hikerbob]

Steve beautiful little setup. That looks a lot more controlled than trying to do it all on the lathe. Food for thought in that.

Bob

[DaveGee]

Yep, making worm wheels is fun!

I turn a groove around the circumference of the wheel with a tool radiused to the root diameter of the screw. Here is a link to some photos...
http://users.tpg.com.au/users/daveg/drives.html

This allows for deeper meshing of the worm/wormwheel. it also allows a straight cut cutter to keep meshed during the cutting.

The first set of drives I made used stainless worms and aluminium Wwheels. I had the Wwheels hard-anodised by Hawker deHaviland at Bankstown. After 3 years of use there was no sign of wear. although the mount wasn't goto.

The second set were made from stainless worms again and naval brass (almost as hard as steel!) Wwheels they are still in use today. It's goto.

The latest set are using naval brass worms and Delrin Wwheels. Here is a video of them being cut...
http://www.youtube.com/watch?v=mWVqs_3IfQU&feature=channel
I've yet to finish the mount. Maybe this Easter!:shrug:

I hope this helps Steve etal

[kinetic]

Hi Dave,

what a gem of a post.....thanks for sharing!
I will have to read and digest your great site and
images.
Thanks!

I was wondering about a good clutch method for both the
RA and DEC...
And keyways too! You machined the keyways as well!? :)

Awesome.

Steve

[DaveGee]

Hi Steve,

Yes a clutch is a must to prevent an accident stripping a tooth. Not a good look!

For the hard anodised set I had the wormwheel sandwiched between two clutch faces and thin cork placemat material as the friction material. A screwed collar applied the pressure. That set of drives now lives in the Netherlands.

The current setup has a friction roller as the final drive. see picture below. it will slip if some drongo (usually me) knocks the scope.

Yes, I broached the keyways on my lathe.

[MrB]

Dave, that mount is inspiring!!
I have to get back out into the workshop one of these days.

[theodog]

Good onya Steve.
Sorry I've just caught up with this thread.
You've moved ahead and had much success. Unfortunately work committments have slowed my progress to a stop, but with my hol's here I will start on my spidle and now have a high standard to aim for.
Well done.:thumbsup:
:)

[MrB]

Not scared of a little math?

I've scanned a few things you might find useful, also found a few PDF files I'd saved from the net over the years... put them all together in a zip file that you can download here: worms.zip (http://members.westnet.com.au/foo2/worms.zip)*
Caution: 10MB!

Note that the most useful of the files in the zip are in XPS format.
XP with IE7 can open this format, as can Vista.
Otherwise download XPS Viewer from Microsoft HERE (http://www.microsoft.com/downloads/details.aspx?FamilyId=B8DCFFDD-E3A5-44CC-8021-7649FD37FFEE&displaylang=en) (~2 MB)

* File will be removed in a week or two.
If someone else would like to host it more permenantly, please feel free.

[snowyskiesau]

I found Dave's great site a couple of years ago while browsing for astronomy stuff.
I partially blame him for me now having a lathe and a mill in the spare room :)

[kinetic]

Thanks Simon, very helpful link.
Most of my previous reference URLs condensed into a single zip.
Very handy! I'll try and upload it to my site and see if it can handle 9mB! :)


Dad showed me a few Engineer's Notebook pages similar to some
of those.
I'm currently digesting a lot of that theory before I dive into
making the serious blanks.
Interesting where they say even 1 thou error on any worm face
can cause as much as 45 arc sec error. After polishing/ lapping it can be
minimised but it pretty much says your wasting your time if the
disk is less that 6 inch diameter and the hobbing method so-so.

Steve

[kinetic]

Hi,

today I started to get serious and try my first blank.
I started by turning an M12x 1.75 pitch metric thread
with a 60° tool.
My only variation was to only cut it about 3/4 of full depth
so that there were still flats at the top.

I also spun the cross travel to 29° so that the thread cutting
was only being done on one side of the tool to stop chatter.
This worked really well and cut a very clean thread.
After several dozen passes at the same final depth, it was
very smooth to the eye and dead on concentric.

(Depth of cut was only added with the top slide not the cross
travel).

I cut a long thread so that part could be the final worm and
part could be the 'hob'.
I scored the hob section again with my dremel to make it
have cutter flutes as previous.

The blank was machined concentric previously and the spindle
fitted to the toolpost.

After hobbing I drove the worm wheel blank to the 'worm' section
of the M12 and bedded it in with cutting compound for an hour.
It ended up quite a smooth job but slightly off centre as you
can see by the pics.

The beauty of this is that I can turn it down and have another go :)
This diameter, again, self indexed by pure luck, no maths was
involved.
It ended up having about 275teeth and a diameter of 148mm.

Steve

[kinetic]

Next I had a go at making a worm and housing block.

To start I turned the M12 hob/worm section down to have two
straight end shanks.
To keep everything concentric all of the next few steps had to
be done without removing it from the chuck to keep the worm
dead true. Even self centering chucks fall down at this.

I bored the final worm out to just a smidge over 8mm diameter.

This was then a snug fit over an 8mm piece of steel from an industrial
photocopier.

This 8mm would be the internal shaft the worm slides on to and
gets locked to it with a cotter pin.

I turned a shoulder to fit a ball bearing race and also an M6x1.0
thread for a nylock nut. The thrust race also is clearance on
this shaft.

Next came the housing blocks that the worm assembly would
rotate in.
I machine faced this block in the four jaw and then bored it
to suit the bearing one end and a brass bush the other.
After cutting them down to final size I then milled the rough
face where the hacksaw cut was.

Finally, all assembled except for the worm left off the sleeve
and the blocks not bolted down to a solid plate.
That's the next job.....

Steve

[kinetic]

Finally I got to test one of these serious blanks in a
real situation.

Just to finish off the worm housing it was fitted to alum
plate and a bracket made to adapt it to my DEC axis.

I also re-arranged the thrust bearing/end bearing so
tightening worked better. Had to re-turn another M6 for the
end nut. PIC 1

The finished bracket with slotted holes and set screws for
mesh adjustment shown here : PIC 2

Pics 3 and 4:
Installed on the DEC axis and in mesh.
A thrust plate locked to the DEC shaft is immediately beneath
the worm gear, which without any clutch pressure, the worm gear
freely rotates.

I drove this arrangement using a battery drill stuck on the end
straight shank of the worm.
Drove nice!!!!!!!!!!!!!!!!!!!!!!! ::D

Steve

[MrB]

Good stuff Steve.
Keep posting, love reading about all stuff DIY :D

[kinetic]

Dave, I think this is the biggest stumbling block for me.
The worm/wheel never looks completely meshed.
I have never done this pre-hobbing groove yet.
Next one I try I will give this method a go.
Cheers.

Steve

[kinetic]

I tried another blank with a radius cut in the circumference
as per Dave G's suggestion.
I attempted to make this radius the thread root depth,
again as Dave suggested.

A few pics of this week's progess.....

Pic 1 shows the index/faceplate I use to bolt blanks to
so that machining an outer diameter and inner bore are
perfectly concentric.

Pic 2 is the blank ready to start, along with radius cut.

Pic 3 is the first teeth beginning to cut and 'self index'
Note the first points of contact are both outer shoulders
of the radius.....this shows that my radius cut was too small.
There is no sign of any point contact on the centreline yet.
(this pic was dark because the flash didnt go off...so I histo
stretched it ...hence the grainy b/w :)

Pic 4. close up of the self index error.....obviously the diameter
was wrong...note the 2nd smaller set of cuts.

Pic 5. It was obvious that the self index hadn't worked out.
You can see the radius has now got teeth cut almost through
all the way so that the first points of contact on the shoulders
almost meet up.
I decided to stop here and not go any deeper.

One thing learned from this try was that the hobbing was smoother
because it was trapped between the shoulders.
I will now skim this all off and have another try to get the root thread
radius more accurate.

Steve

[hikerbob]

I've had my first go at cutting a worm based largely on idea's gather on this thread (and some previous research).

I used slice of 160mm round for the blank and the thread out of a car jack for the hob/worm.

I roughly machined a grove in the rim of the blank to try and increase the contact area. I ended up with 165 teeth (if my count was correct) and the mesh up seemed to go well. The cut's in the hob were done with a thin cutoff wheel in the angle grinder. I did not do any pre-indexing on the blank.

The support for the blank is part of an old Subaru gear box cut off and welded to another slice of the 160mm round. I've used holden trailer bearings because I had some and I figured that they are designed to cope well with a mix of thrust and side load. I machined the shaft for the support post out of an old trailer axle. I still need to set up a means to lock the shaft when I'm fitting a blank (and removing a worm gear).

I had not prepared the jack screw well enough. I should have machined a shoulder into it at the end held in the lathe chuck to stop it working it's way back into the chuck. I also should have used less side force, I broke the jack screw (thankfully at about the time I was getting ready to stop cutting). I was fortunate that the pieces included the section to be used as the worm. I'm not sure if this worm will go onto a mount but I'm planning to finish it for the learning exercise and if it works well enough I might use it.

I may have a go at cutting my own worm for a future worm gear but my screw cutting skills are not well developed yet.

[kinetic]

Bob,

thanks for sharing your results.
The Acme thread certainly looks far more suitable in shape to my
results with plain metric.

One thing though, looks like you have a big distance out from the chuck
and live centre.

You only really want to have just enough or the hobber will flex away
from cutting as you apply cross travel. I noticed this happens on mine
and it will take ages for the new cut to actually develop a deeper
thread. I use 2 thou cuts and after about 10 revolutions and cutting
compound it seems to be ready to add another 2 thou etc etc.

Was the jack thread hardened? I'm going to try turning up my first
M12 x 1.75 or M16 x 2 ACME thread soon.

Steve

[hikerbob]

Steve definately to far out from the chuck. I do need to do the hob set up differently.

Judging by the way the screw thread cut it was hardened somewhat.

I tried to buy Acme thread a while ago but the minimum length I found at the time was far to long for what I wanted to pay. I think I noticed some on the Minature Bearings site yesterday so I'll have to go back and check that later.

I do need to learn some more about thread cutting but to date I've only tried cutting one thread which worked but with a very rough finish. I've also misplaced one of my gears for the lathe and it's not readily available as a spare (but can be ordered if I ring sydney and get them to get it from the manufacturer.

It was a very satisfying experience to turn up a worm gear, I'm enjoying the learning.

Bob

[kinetic]

Thread cutting is well worth the effort Bob if your lathe is capable
of it. Mainly because, compared to a bolt or manufactured all-thread
stud, cutting your own will ensure it's concentric.
I have noticed all thread stud can be terrible quality as far as
concentricity goes.

Satisfying?.....you betcha...especially when you see it working :)

Steve

[hikerbob]

Steve my lathe is capable of the work although it's got more flex than I'd like. I've got this machine (but branded differently) https://www.machineryhouse.com.au/Products?stockCode=L159

I'm working my way through improving my skills. I recently added a coolant pump which had made a real difference. I discovered Trade Tools had them at a reasonable price. The drip tray is not a great shape for return the coolant but it's working well enough.

Bob

[kinetic]

Bob, flex in the main travels?....because you can adjust that to a minimum?
Looks nice...I wish I had the mill head! :)
With thread turning I highly recommend the 29° method (look back
earlier in this thread or just google it). Makes a FAR cleaner thread.

Hardest part of making a good thread I reckon is grinding a good tool
for it...why do ya reckon I'm scared away from ACME :)

Steve

[kinetic]

Today a better radius groove on the blank.
This self indexed better.
A nice shaped thread and depth too.

This blank ended up being a 7.5" dia with 348 teeth
at M12x 1.75 (STD Metric profile)

Steve

[MrB]

I'm going to be trying this sometime in the future.
My lathe has a precision ground 16 x 1.5mm Acme feedscrew, trying to locate a replacement to use as a worm and hob... it's about 500mm long.
With the largest blank I'll be able to turn in my lathe(180mm), I should be able to get 360 teeth at 1.5mm pitch.
If I do manage to find a replacement feedscrew, would anyone be interested in a cut length?

[kinetic]

Another attempt here.
Metric Std profile again, M12x 1.75

As close to a full depth cut from what I can judge.
Worm was lapped with polishing compound here for
about an hour.
Certainly looked and felt snug on the worm section
of the hobber.

Steve

[DaveGee]

That looks better Steve. A good wrap around the worm to ensure good meshing at all times. Well done! Now there's just another to make.:thumbsup:

[hikerbob]

Steve, nice work and thanks for the updates. I'm hoping to get back to another trial during the week.

Simon I may be interested, I do want to have another go at cutting threads however professionally made Acme should be better than what I expect to achieve. I'm also pondering building a CNC milling setup and would probably use a few worm gears in that if I go ahead. Price will of course be a consideration.

Bob

[hikerbob]

I decided to try out an approach which first occurred to me when I heard about hobbing.

Using a conventional tap held in the lathe chuck. I used a 3/4 16TPI UNF tap which I had on hand an offcut of 63mm OD alluminium round. 10 minutes or so to tidy up the round and another 10 minutes or so of hobbing and the thread is cut deeply and as a first pass looks quite passable. I'm wondering how that would work if I later bed a worm into that for the final tidy up. Possibly not up to astrophotography standards but maybe suitable for a lot of other uses.

I wanted to test how well a conventional tap would turn the gear blank and also how well it would go without having a grove already cut. Excellent on both counts. The cut as it is at the moment has a slightly raised edge on the lower side (where the cutting teeth exited the blank) but that should clean up neatly.

Bob

[kinetic]

I'm not moving on to the RA gear Dave until I'm happy with every
aspect of the DEC attempts , including how consistently I can
repeat this all.:)
I reckon I'm nearly there though.
The inspiration and reason for my success so far would all
be down to your work, and the work of others:thumbsup:



Nice looking result Bob.
On a bigger diameter wheel you would have to extend the tap from
the chuck.
I've seen this done on a few webpage examples and it seems to work
well.
I also get that same exit roughness on the teeth even with the multi
scoring on the hobber.
But as you say, it does clean up after a skim on the faceplate and/or
a bit of fine wet and dry.

Oh, and the things I could make with that 63 OD alu round!!!!:D

Steve

[kinetic]

Ok, enough stalling, finally I tried an ACME shaped thread.

As I thought, it was all down to how well I could shape the
tool.
An M12 x 1.75 ACME tool is a fairly tiny trapezoid to grind
and the errors were soon showing on the result thread that I
cut with it.

It was obvious the flat was too wide because I couldn't go
full depth. The flat at the bottom should match the flat at the
top. As I neared final depth I was sacrificing wall thickness, the
very thing that makes ACME a strong driving thread.
Anyway, I decided to do another hob with it and see how it
went.
Another successful self index.
Then the depth cuts, gradually adding 2 thou each time and
letting it drive around a few turns.

Polished on the clean part for a while then bored the worm and
parted it off.
The last pic shows the ACME worm that can now be swapped
with the other sleeve to try it out on the DEC axis

Steve

[hikerbob]

Well done Steve. I played with the results of my last effort again tonight, bedding the thread from a 3/4 UNF high tensile bolt into it and turning the bolt down so that I can have a play with it. So far all good.

Not a lot done this evening, I've had a few other things happening and also been fiddling with my first dew heater. No photo's tonight but thanks for yours.

Bob

[MrB]

Hi Bob
So far so good, may have an Aussie supply but waiting for them to get back to me.
Found a US source. US$19.95 but minimum international order is $200... grrrrr... and freight = ?
Found a UK source. £19.95, no minimum, but freight = ?
Keep in mind the feedscrew is ~500mm long, enough for many worms and hobs. Dunno how I'll go cutting it.
Here's hoping the Aussie source comes thru.

[kinetic]

Just remember guys.....unless you can get the commercial ACME thread
exactly concentric on some sort of drive shaft in your 3 jaw or 4 jaw,
you will have an error transferred to your blank.

That's why I'm machining my hobs/worms. It makes them exactly concentric.

Sorry if I'm stating the obvious.:)

Steve

[MrB]

Thanks Steve, but I'll be using a 16mm collet.

[kinetic]

I had a few test runs with turning ACME form threads
for worms this arvo and I think this is about the best
I can get without a jewelers grinder and microscope for
getting the tool perfect.

At full depth, 10.25mm minor dia the shape looks pretty
balanced. Not perfect but close.
At least the shoulder looks a bit thicker now.
I'll try this as a hob on a blank next.

Steve

[hikerbob]

Steve, nice work. I'll have to give that a go.

Simon I'll be some time before I'd be ready to buy thread if I go down that route. Still to much to learn.

Bob

[MrB]

Yeah no worries Bob, I'll still be getting a length, so if your still interested later drop me a PM.

[kinetic]

I think I'm finally there.
This one doesn't look too shabby at all.

This is a full depth M12 x 1.75mm ACME using the thread from
yesterday's post.

As in most efforts previously, self index went well.
I marked the first contact with the pre-cut groove as 0.00
on the cross-travel.
Then added cut depths of 2 thou (sorry my dials are in thou)
until I was full depth at 34 thou.

Final polishing and lapping on the section that would end up being
the worm again.
Then recovered the worm by carefully parting off the live centre end,
boring out to 8mm (so that it slips on the worm drive assembly),
and then finally parting off.

I can't wait to try this baby, it is the most snug mesh I have had
so far, and by far the cleanest teeth on the gear.

Steve

[theodog]

Oooooh that looks good!!
I found ACME thread a pain too. I found that the end of the tool kept rounding. Might be my technique. Getting the flat at the bottom and top is bloody hard.
Well done on your efforts.

[MrB]

Very nice.
Forgive me if you have already mentioned this but what diameter is your wormwheel? Looks around 100 to 125mm?

[kinetic]

It's 176mm Simon, just a tad under 7".
I counted 317 teeth this arvo.
Another shot after a cleanup with wet n dry.

Steve

[MrB]

Jeeperz, I was way off, was trying to guesstimate using the 12mm worm.
That is definitely a keeper, looks great.

[Spanrz]

What led you to do a 317 tooth Worm wheel?
Are you planning to run a servo unit or stepper motors?

I am watching this thread (yeah, ok "pun" intended) on the worm stuff.
I have got so many plans/designs, but the worm concept is on my brain.

I was thinking of going to a RC shop and buy some steel gears (worms etc) and giving it a go.
The idea of using a stepper motor, is sitting on my brain, just worried if it's geared high (say 100 : 1), that the stepper motor turns too slow (microstepping) creating jerkiness.
I figured a stepper motor turns smooth when running about 1/3 a turn a second.
Or are the steppers these days pretty good on microstepping?

What have you guys got planned to drive these wheels?

[kinetic]

Brett, yes I have a homemade Bartels Stepper drive system, dual axis.
With microstepping (steps further divided into 20 or 40 microsteps
by the software) the drive is very smooth.
Mel Bartels recommends a reduction of about 4000:1 as a guide
so there is no chance of steps showing in the final drive.
My RA reduction is about 10,000:1 and very smooth but has terrible
backlash due to spur gears. Hence the need to make a good worm.

The 317 teeth is a result of purely the choice of pitch and diameter
of blank.
An earlier test on the same blank resulted in 347 teeth and a dia of 7.5".

If this all works it is simply a matter of writing in an integer value in a
script file in Mel's software to calculate arc seconds per step.
Software does the rest.....beautiful.



In my case, most of the original geartrain that I currently use, minus
the last spur gears which will instead drive the worm/worm gear.

Steve

[DaveGee]

That's brilliant Steve! It's been fascinating to watch your efforts and to watch you learn-by-doing, surely the best way. :thumbsup:

[kinetic]

Thanks Dave ,everyone for your kind words.
It doesn't take a lot of grey matter for this dumb sparky to operate
a lathe etc. It's all down to others like yourself who paved the way
before and were decent enough to explain their journey in words and
pictures, for people like me to have a bash myself.

And yes, it has been very enjoyable:thumbsup:.

Steve

[kinetic]

Only progress this weekend has been to lock the
worm to the 8mm worm carriage with a grub screw and
test it / lap it in.
As before I screwed the whole assembly down to the bench
and ran it on a speed controlled battery drill for about an hour.

It ran very smooth but I have a slight eccentricity in the worm
sleeve now. I think I know how to fix that (thanks Dad! )

Steve

[kinetic]

I fitted the latest test blank assembly to my DEC axis
of the homemade GEM to see how smooth it drives
with the existing DEC stepper/ geartrain.

I made two pressure plate flanges with rebates
cut into their faces for two large cork washers to sit inside.
I used old cork placemats for the washers.
This is another great suggestion from Dave G! :)
The M6 pressure adjustment bolts you can see in
Pic 2 act directly on the masonite backing of the placemat
cork.

The whole setup was only temporary to get a feel for
the 'stiction' and brake effect of the pressure plates
and the ability of it to protect the worm wheel from
a bump.

The setup worked beautifully.
I am so impressed with how snug this feels and works as a
brake/clutch that I will now do a proper job of the 'temporary'
pressure plates.

Next: on to the RA Axis !!!!!!!!!!!!!!!!!!!!!:thumbsup:

Steve

[hikerbob]

Steve well done. I've been doing some more play with the remnants of by car jack thread. I used the mill to cut the slots in the hob this time, much nicer result. I've made a small gear (65 teeth I think) to drive a cheapo EQ3 lookalike which I want to put a camera on.

I picked up a gearhead motor with a nominal output of 6.7RPM at 12V (6.7RPM@12V) which is by my calculations very close to the combo I'll need to drive the RA axis of the mount (it already has a worm gear drive which seems to be need six turns of the worm per hour).

I've also just recieved four faster gearhead motors from the US which I want to experiment with to see how much range I can get with PWM speed control.

Unfortunately my normal laptop is dead at the moment so I don't have access to software to shrink photo's etc.

Bob

[saturn c]

wow, you guys are pretty crafty. awesome!

[hikerbob]

I finally got the set up for the small worm gear mounted and connected to the motor. I've used some 25mm OD (19mm ID) ally tube to hold the bearings for the worm and attached it to an EQ mount which came with a Dick Smith 4" Newt. A bit of motor noise - if the rest works out Ok I'll revisit the motor mounting and add some sound deadening. I've not tried to fit thriust bearings or spring load the worm on this one, I doubt I need either for the intended use and I'm trying to keep it simple. A good learing exercise.

I've had a closer look at the thread on the worm and it is deep with quite sheer sides, not acme but it still appears to give good contact.

I've got the motor hooked to a 12V battery and am giving it a long run to see how close the rotation speed is over a period. The mount has 24 time divisions on it so I should be able to check at any time and see how well it's gone. All being well I can hang a camera off it for wide field work.

Sorry no photo's yet, I'm still trying to work my way through a warranty fix on my main laptop.

Bob

[hikerbob]

I've edited my previous post and added a few photo's of the work gear and worm holder I've added to my EQ mount. Sorry about the quality of the photo's but along with the description it should give an idea what I've done there.

Bob

[MLyons]

You can get far smoother cutting action with a spiral flute tap.
It also has a "centre" drilled in the end for centering on a live centre in your tail stock.

One thing I noticed with the normal tap is that it shudders every time the cutting edge touches the work (3 times per rev with normal taps) and in doing so could introduce minute variations that in centering because of flexure of the workpiece.

With a spiral flute tap, not only can it be centered 100% and supported by the tailstock in a live centre (thereby staying 100% true) but there is always constant contact with the cutting edges. This makes for a much more constant force thereby ensuring a smoother, more accurate cut.

I'll take a pic of my spiral flute tap and post it.

Martin

[kinetic]

Great suggestion Martin, thanks for the tip.
Might be hard to get in an ACME form though.
Because I've started making the actual worm hob on the lathe
it is perfectly concentric. But I have only ever cut parallel facets
in the hob with a dremel so far.
I might try an angled facet with the dremel to see if it chatters less.
BTW on very low speed (the same lowest speed that I turn the hob thread with), the hobbing process does chatter but only minimally.
Once the cut is almost full depth it is surrounded by the root radius
metal and chatter is almost nil.

Steve

[kinetic]

Following the reasonable success of cutting several
gears now and fitting the best result to the DEC axis,
I was ready to start on progress on the RA gear.

Started with preparing the blank by turning it down to
the required diameter and then adding the pre-cut groove.
Also bored the centre to the size of the spindle flange
and faced two sides for clutch plates to act on.

Then turned up the worm/hobber.
Again I used M12 x 1.75 ACME profile with a 29 degree tool.
This time I chose a different approach for the worm.
Instead of the worm being a sleeve that would slide over
a 'worm carriage' I decided to make the worm and carriage
one and the same. This hopefully will eliminate the slight
eccentricity I have ended up with when I previously 'recovered'
the worm after finishing the hob processes.
I will cut the bush/ bearing shoulders and end thread on this next.

Once again I cut flutes in the hob section and put a slight
helix on these as per Martin's suggestion below.(the spiral
fluted tap).

Mounted the blank on the spindle post and wound in until contact
was made with the pre-cut groove.
This became thread depth 0.00
On the lowest speed I did the self index, again success!
I was so glad this worked because the RA I wanted to be a good
result, not just a gear with mostly a good thread and a bad part
that would be continually driven 'to the bottom' and never get used.

Then I gradually wound in 2thou cuts with cutting compound as
per previous attempts until nearing final depth of 34thou.
For the final polish I bedded it in on the section that would
be the final worm.
A few dozen rotations on full depth and it was done.

This one ended up approx 175mm dia and 317 teeth
(exactly the same as the DEC axis final working version)

Steve

[kinetic]

More progress this weekend on the RA Axis:

Pic 1: Removed the hobber section from the worm/hobber by
turning it down to a smooth 10mm shoulder both ends of
what will end up being the worm.
Also turned a shoulder for the thrust bearing hardened washer
to rest against and also turned a 6mm end thread for the
nylock nut.
Also turned a groove for the non-thrust end bush circlip.

Pic 2: Next I made the bearing/bush blocks, again the same method
as for the DEC axis. Faced a rectangular block pre-bored to
worm shaft clearance.
Then bored them to suit the bush and thrust races.

Pic 3: All the parts for the worm shown:

Pic 4: How the thrust bearings sit on the worm

Pic 5: Worm carriage assembled

Pic 6: Worm shown in mesh with the RA wheel.

regards,

Steve

[MLyons]

:thumbsup:

[kinetic]

Made one of the fixed pressure plate clutches on the weekend
for the RA axis.

This one is fixed to the RA shaft with several grub screws and has
a cork insert made from an old placemat. The same as I did with
the DEC axis. This idea, again , was from a solution suggested by
Dave G.

Pic 1 shows the clutch plate and cork insert separately.
Note the rebate the cork sits in on the pressure plate.
Pic 2 shows how it sits in this rebate.

Pic 3 shows it resting on the face of the worm gear.Cork trapped
between.

I have to make a second clutch identical to this one for
the other face now but with pressure adjustment screws.

Steve

[kinetic]

Today I fitted the RA worm and two clutch plates to
the homemade GEM.

Top clutch is fixed to the RA shaft with grub screws and
has a cork washer sandwiched between the worm gear.
The bottom clutch plate is also fixed to the RA shaft
but has 4x5mm pressure adjustment screws acting on
another cork washer.

I must say again, as with the DEC axis,the feel of this type
of clutch working on a well balanced GEM is divine....
it slips beautifully with moderate pressure, but takes up
and drives with only finger pressure twisting on the worm.
Again, at the risk of repeating myself, this clutch assembly
will stay even if the worm gear ends up with woeful PE.

As you can see, the worm carriage is only held so far with
some G-clamps to adjust mesh via shims.
All that remains is to fit the reduction gearbox and stepper
back on and I'm in business for the accuracy test......

Pic 1: side view of the sandwiched worm gear.
Pic 2: worm carriage in mesh underneath.
Pic 3 and 4: wide shots....of the whole assembly.

Steve

[Spanrz]

Steve, great stuff. Loving it. You've got my creative mind ticking.! :D

Bit of a rant here, sorry :sadeyes:, but I need to voice my opinion of a certain industry.
I've got to the point of frustration, in searching for somewhat of a Worm Setup.
I have tirelessly looked on the web and around Melbourne for someone who has knowledge of a good setup.
Tried everything from a commerical sense to a hobby sense of gears.
Tried a few local bearing joints, gave up on that one. Only 1 place had sort of what I wanted, but I knew cost was the problem.
Many many hobby shops, no good.
Tried a mob up in QLD that sells hobby stuff, they have the best worm gear setup, I've came across, but the problem is, none of their distributors here in Vic, stocks any of their parts. They have basic pictures of what it would look like, but not spending $$ until I can see what it's going to do. I have to order it first (to see it), just to hope that it will do what I want it to do.
I wanted to keep costs (as you do) low, but it's the functionality of the setup, is what counts.
I haven't tried a proper gear supplier yet (ronson), as I know cost would be into oblivion, last resort.
So, in my midst, my thoughts are Australian industries don't cater for us Astronomers. Because, we are (in their eyes) a minority. That frustrates me. I had this feeling 20 odd years ago, but thought 2009 would have produced better results. :sadeyes:

It's getting to the point, like your self Steve, to make your own.
Or the alternative, is the wife is organising a trip back to the USA (her home), and I know I could probably find 6 suppliers of this type of setup, in NY, all of which have stock and a better price than we can do in Aus.
That's sad. Probably 4 of them closed by now due to the global slowdown...figures. But I know what I want, I know someone makes it, but it's finding that person.

It's very frustrating to talk to a person in the know, who basically says "not a chance mate, you wont get that low" and they look at you stupid as if you are trying to re-invent the wheel. It's not like I'm asking for 10,000:1, just what ever I can get, which looks like 360 teeth.
I'm even up for making it myself, but the process with a lathe, is a little beyond me. (Haven't used a lathe for a very long time)

This is the website and concept I have thought about, but would this gear be suitable enough for tracking?
http://www.smallparts.com.au/store/item/wg4r64336005hs0635brz/wormsandwormgears64pitchseries3meta llic/

It's a gear with 360 teeth on it. I think it's around the 6" in Diameter, but that doesn't phase me. Everyone (retailers) think size is an issue, I said, bigger is better......Yeah I suppose what women want to hey...!
Bigger = better reduction values, but not too big.

Other than that, spare parts from a scope mount manufacturer?

Can anyone else shed some light on why it's so hard to even find a good worm setup from a local supplier?

[kinetic]

Brett,

Yep, as you mention, Byers in USA and others similar, are probably
the only source of very accurate worm /worm gear matched sets.
I'm certainly no expert to comment about how good they are, maybe
someone more experienced in that can chime in.
I'm hoping that this method can, at the very least, churn out
a gear with minimal and manageable PE.
I'm definately not an expert on a lathe, I can tell you that.
I am, however, a competent DIYer and it's not all that technical.
The pioneers like Jim Sapp and Dave G etc who take the time to
write up their engineering marvels help people like me no end! :)

You mention a dead end in Oz for manufacturers or suppliers.
I can suggest one avenue....I know some VERY talented
people dabble in miniature steam trains etc and they nearly always
have lathes and/or mills.
If you took the time to seek out one of these groups I bet you could
mention it to them...point them to the relevant websites and maybe
politely ask them to make you a gear for a fee.

PS: one alternative to worm/worm gearing is toothed belts.
Theo (Gama) kindly posted an example earlier in this thread:
http://www.observatoryscope.com/prototype/prototypeoperation.html
There are suppliers in Australia for that stuff....one source
for example of a smaller set is RC car racing.

Steve

[kinetic]

Fitted with some slight modifications to the angle iron holding
the old drive gearbox.
I have both lateral adjustment and mesh adjustment like the DEC
axis.
It feels so snug and clutches beautifully!
Now to add the drive gearbox and stepper...

Steve

PS: this hobby can be so cruel.....2 days with the scope out of
action due to motor/gear changeover....2 nights of 3 degree overnight
lows....seeing excellent....jetstream ideal!!!!!!!!!!!!!
PPS: make that 3 nights!!!!!!

[hikerbob]

Brett if you are trying to buy ready made worm and gear you could try a bearing supplier. I checked into it some time back and was somewhat dismayed by the pricing (from memory it was looking like about the $400 for a set - one worm and one gear). I did not do a lot of investigation so I don't know just how good they are for our purposes.

Hobby Parts at Mansfield (they do mail order) also supply some. http://www.hobbyparts.com.au/ Again I don't know how suitable the gears are for driving a telescope. I have purchased via the web and the delivery turned up the next day which was impressive.

Bob

[Spanrz]

Bob,
yeah that's the website I got on to. After days of frustration looking, hehe.

I have tried about 20 suppliers around Melbourne, other than actual gear manufacturers (last resort).
Every supplier says the same thing "Sorry can't help - Have no concept why you would want such a reduction - No idea mate, we don't deal in that - Sorry mate, don't know who can supply such a thing"
No one wants to help or understand what I'm asking for.

I have tried about 10 odd hobby shops, all have spur gears with a lot of backlash and bugger all reduction. Most don't even stock a worm.

However, I found 2 places that had somehwat of a chance.
The local bearing shop had a supplier called "Naismith" that made a gearbox with near zerobacklash called "Zeromax".
However, functionality was looking good until it required a minimum of 140 RPM's on the input, which is sort of pointless. It needed that speed for oil splashing. And the cost for the functionality, was basically "Astronomical" :eyepop:, excuse the pun ;)

The other, sort of success (if you call it that), is JAYCAR.
They have some gear sets there, but it's a case of, "Look and laugh".
They are plastic and very very light duty - for toys.
Some DC electric motors have a gearhead on them, however, lots of backlash, which was the last of the "last resorts".

Now, with that HobbyParts website, no one locally stocks anything.
Which is disappointing. Not so much what I was requesting for, but abolsutely nothing was stocked, yet they call themselves a "distributor".
I might as well order it online and get it myself. Pointless for the middle man.

I found a gear with 360 teeth on it, I have emailed them about it, but no repsonse.
It was about $200 for that gear and another $90 odd bucks for the actual worm gear.
I was actually after a Duplex Worm set, as both gears mesh / conform to each other and backlash (IMHO) is reduced to very low levels.
But this gear set is close to a duplex.

They key is, I want to see it in the flesh, before I purchase. I want to know it's going to work, but business doesn't work that way.
Oh well, they miss out on a sale.

I'm still searching all alternatives, even from my work.
:thumbsup:
Brett.

[hikerbob]

Brett Hoby Parts did have some worms on display last time I was in but I'm pretty certain that they don't have the gears in stock. Minature Bearings who I think supply that stuff to them are in the same building but I don't think that they do retail (and both are a long way from you).

I've seen printed catalog details on worm gears at Bsc http://www.bsc.com.au/html-bsc05/indexBSC.htm which they can get in. I think that had to come from overseas but they can get worm gears, from memory the multiple lead gears available were low ratio. I could not find anything in the online catalog.

I've taken a break from my hobbing attempts for a while as I do some other stuff but if I don't decide to buy an EQ6 and continue building myself I suspect that's the way to go. Hobbing the gear seems to be the easy bit, getting the rest of it machined properly has caused more trouble.

Bob

[Spanrz]

Bob,
I finally got a response from the HobbyStore.

This is so far the best concept of a worm gear I have found. (out of about 30-40 designs)
It's the cheapest version of a metal worm gear set that I could find, that is relatively sized to suit the scope.
The gear is around $200 inc gst and the worm is around $60 I think.
My ideal price range for the set would be around $100-$150, but the gear set went up about 25-30% after June 30th.
(and they call this a recession where prices should come down somewhat???):scared:

The other ones I have found, were cheap and plastic (...pffft)
But I suppose I can't complain, as I haven't yet hit the costs on astrophotograhy........Many thousands to go....Hahaha.
Oh the joys (and expense) of a hobby.

For what it's worth:
This gear's diameter is about 150mm and a has 360 teeth.
http://farm3.static.flickr.com/2443/3706803010_c9ea316f4c_b.jpg

http://farm3.static.flickr.com/2586/3706802730_8752492a79_b.jpg

EDIT:
I'd love to make one, I have the tools at work, but I don't think I'll have the permission to use them.
Which is the reason I have gone down the road of "purchase".

[kinetic]

Brett, Bob,

that certainly looks like a good worm but I'm not sure I'd buy
it without some details of it's accuracy.
Also, BTW, don't write off plastic or nylon gears as an option
either...

A lot of 'Bartelised' scope users report good results with
nylon, plastic and even wood machined worm gear forms.

Steve

[Spanrz]

Steve,

Haven't written off the plastic just yet, though my choices are very very limited.
A few weeks ago, I did buy a very cheap plastic worm gear from Jaycar, just on the off chance that I would have a go, seeming I was running into dead ends. It was the best of the plastics.
I have also disassembled (er... destroyed..hehe) a wiper motor, to get to the worm gear in that, as another alternative.

Also had a chat the other day to a guy who was fixing the large automatic gates at work. Mentioned where I can get any gears or anything close to the system they were using (the gate reduction box looked like a commercial worm gearbox), he didn't know, was his reply. They were heavily into mechatronics.
Hmm, can't seem to take a trick.

The hobbystore gear set, yes have thought about the accuracy a bit too.
But at the moment, it's the best candidate.

One thing that was going through my head as a real wild thing, was to try a larger stepper motor to drive it directly.
Yes a heathenous thought it is.
The torque would be well enough to drive the OTA, but the resonance of the motor at low speeds worries me. As Bojan has remarked the dropped steps is an issue, but it's playing with the finite settings that can reduce that over a analog driver.
The test on the smaller motors worked really well, just unsure how the larger motor would do, even though the driver units could accept the larger current draw.
If I was using the old analog drivers, this heathenous thought, would be driven out of my brain by the jitterness of it.
But the digital driver performs way above my expectations, to put this thought out of my mind.
I never did see a large motor drive at very slow speeds with this driver, so it's an educated guess.
No doubt a direct driven OTA with astro gear on it, will show up signs of the jitterness. It's a matter of how much.
So I am trying to think logically, but outside the square as well.

[kinetic]

Brett,

I would strongly suggest at least just having a play with a
'Bartel' type stepper or servo arrangement first before going
too far off on a tangent with ideas that may just end up
frustrating you.

The Bartel setup can be done totally on the cheap...all with
pretty much 2nd hand gear that today is worthless.
You say you have read the Scope-Drive pages tirelessly...
well dive in and have a go.:)
The best way to get a feel for it's limitations and quirks...along
with it's fantastic range of possibilities...is to do it.

You can get steppers suitable from old photocopier machines, faxes,
printers and even old 5 1/4in floppies....cheap.

You can use an old 386-486/Pent 120 era laptop/desktop....cheap.

You can make the circuit needed on a piece of veroboard in
an afternoon.....cheap.

Finally, you can implement a perfectly sound tracking platform
on a Dob (or GEM if you're adventurous) with just pressure
rollers or toothed belts....cheap.

Finally, I can highly recommend joining the Scope-drive mailing list
if you haven't already....thousands of very experienced Bartel
dabblers at hand to offer advice.

Steve

[Rod]

Hi Brett,

These worm gears I understand have a good reputation:

http://gototelescopes.com/gears.htm

Rod.

[hikerbob]

Brett I found some interesting work on Direct Drive some time ago by a group which Mel Bartels is associated with. http://www.siderealtechnology.com/DirectDriveWorkInProgress.pdf

It seems to be some way yet from being viable for amature use but was an interesting read. Mel Bartels, Dan Grey and others are part of a group working to bring large professional telescope technologies to large amature telescopes (1 to 2 meter diameter from memory). I can't find links to articles about the group at the moment but they had some interesting projects under way.

I'd not be racing into trying to use direct drive at this stage but it is an interesting idea.
Bob

[Spanrz]

Nice link. Bit on the expensive side when you factor in the Aussie dollar shipping and possible GST if directly shipped from them (if it's not marked as a gift).

I like the alum gears. 3 arc seconds seems pretty good. But I think with the cost, you could probably buy 2-3 sets from the hobbystore in QLD.
Still unsure in the accuracy of these gears from QLD.

Bob, yeah the direct drive was just a pure imagination thing, but I knew it was no good as a concept until you got the jitterness under control.
Now the digital driver, I had major smoothness. However it wasn't tested with a large motor.
So it's still a hypothesis that it could work. Because there is so much of a setting to obtain with the currents/steps etc, it change dramtically on how much vibration or jitterness you can have.

The belt attachment has been investigated (belt and pulleys).
Still looking at concepts but still an alternative.

But the worm gear is my preference.

[kinetic]

Well it's been quite a journey to get here, I have to laugh
when I look back at some of the pictures from when I started
this thread.....

Tonight, between lightning and thunder and some
pretty heavy downpours it was clear enough to point the
scope skywards and get this.

This is the first tracking session (unguided) with the new
homemade RA worm gear (317 teeth).
Stepper is belt driven through some temporary reduction using
drive belts to test the periodic error.
I didn't have a clear enough gap between showers to get several
worm revs logged but I am ecstatic at just getting my first shot.

Steve

[MrB]

Looks great, how long was the exposure?

[kinetic]

Like I said Simon, I'm just happy to get my first result.
It's only 5sec exposures Newt focus 12" F5 and Meade DSI Pro II
with IR filter.
30 exposures, seeing only average.

My serious test set will probably be something in the order
of 10-30sec shots on a VERY steady night for an hour
or so close to meridian and celestial equator.
A lot of things , including the weather have to co-operate of course :)

I'll certainly keep the thread posted.

Steve

[Spanrz]

I'd be pretty happy with that Steve. Excellent result first up.
Thunder, lighting wind and rain are all out to test us.
Pretty good shot with the elements against you.

[kinetic]

Tonight the seeing looks promising.
A front of showers has passed through and
sky was clear at sunset.
I've just finished a 10sec set at Declination -55
somewhere in ARA.
A few very faint fuzzies here.

This set is the best frames from a set in which the
worm completed several cycles.
With the Bartels Arc sec per step size almost spot on for
this new gear, the PE was minimal.
An early impression is that this 10sec set had more good frames
than a set with the old spur gear drive.
So it's looking very promising.
I still have to log a lot of cycles and put it into an Excel
spreadsheet to visualise the PE but it's looking pretty good!

Hopefully I can continue tonight, weather permitting.
A 15sec set is in progress right now.
Edit: 2nd image added

As per my usual setup: 12" F5 Newt / Meade DSI II/ Baader IR
FL 1500mm.

Steve

[MrB]

Excellent Steve, I'm itching to get started on my own wormwheel.
If only the guys at Carbatec would get back to me about cost and availability of a replacement feedscrew for my lathe. :mad2:

[kinetic]

Some PE data....

Tonight I had some success quantifying the PE of the 317 Teeth RA worm gear.
Seeing was very good, high cloud occassionally spoiling it.

Firstly, to minimise any confusion, I refined the tracking rate in my Bartels stepper drive
so that any PE would not have a sky rate error superimposed on it.
Once that was done I had a nice cyclic wobble showing with a 4.5 minute period.
This is the period of one complete worm rev. {1440mins/317 teeth = 4.5mins)

To represent this graphically, a picture tells a thousand words:

I captured a set of long exposures again but deliberately dialed in
some sky rate error so that the wobble wasn't drawn over itself
cycle after cycle.
Pic 1 shows the 4.5min wobble in RA (west is left)

Next I deliberately dialed in some Polar alignment error, again to
prevent the wobble drawing over itself (Pic 2)
Now the polar alignment error causes the main PE to show better,
still with an approximate 4.5min cycle. Note the drift now in DEC

Pic 3 is all sky rate and polar alignment errors zeroed again
and PEC tuned to the 4.5min cycle and TURNED ON.

Not bad eh? :thumbsup:

An open cluster in Norma....I think it is NGC 6067.
The clearest stars I have ever had in an image!

Steve

[Spanrz]

Love the science lesson Steve :) :thumbsup:

Is the cyclic wobble from the gear itself, or the shaft (talking about a slightly bent shaft)?
I'm assuming that's what you mean about cyclic wobble?

[kinetic]

Brett,

it is either from a non concentric worm in the carriage or the
toothed belt pulley on the end being non concentric.
Either way, I'm happy it's cyclic and the only substantial PE.
Seems the worm gear accuracy doesn't even come into it.
I have attached some shots of the jury rigged reduction which
is driving the worm at the moment.

Also attached are two old favourites from earlier tonight.
PEC was turned on for both shots but only sharpening applied to
the NGC5139 result. Seeing was only average, some high thin cloud.

Steve

[Spanrz]

Nice Steve.

Your ratio, is that 60:1 x 2:1 x 317:1? (equals in total 38040:1??)

[kinetic]

Yep, close Brett, not sure where you got the 2:1 from.
(Im assuming you guessed by the sizes of the toothed pulleys?)

Stepper 1.8 deg/step into 60:1 reduction then
Toothed pulleys 1.8:1 then finally
Edit:"sorry: it's actually 1.75:1! (24Tpulley drives a 46T pulley)
Worm driving 317 teeth worm gear 317:1

I get 33285:1 (RA axis only BTW)
That's possibly not a final value either, for the record.
Steppers as you know all have comfort zones.
This setup is close to that, but it's not ideal for fast
half step slews.

How's your options going? Has anything come together yet?

regards,
Steve

[Spanrz]

Hahaha, well yeah, it was a guess. 2 isn't far off 1.8 ;) :lol:

Had a day off last Friday, went and visited a company that distributes some gear products, that I thought were "perfecto" for scopes.
Planetary drives that were perfectly matched to scopes and it's finite movements. Best solution so far, vast devices :)
The product was made by Faulhaber. German company.
Now considering the sizes, engineering and the precision, the costs weren't as bad as I expected.
The intent, was only to see what products they had, that were good enough for functionality then price would have been an issue.
I'm looking only to see what is out there, rather than the intent to buy (just yet)

I found a size that would have suited the scope, planetary drive, which can cost about $300 and up. (500:1 and rising).
The guy that I was talking to, also had a scope and he motorised it, so atleast finally someone I can talk turkey with. (relief)
But the common problem with all these drives, is "supply".
Had to order from Germany, none kept in Aus.
I've been in a very large business for nearly 20 years, and have grown up on the words of "Can't sell from an empty wheelbarrow" (from the GM).
I find this too common of an occurance in Aus with O/S sourced parts.
Hence local products will get the nod if any.

So the concept is, either have a crack at the "smallparts.com.au" worm gear set (which is the best solution in all aspects ATM from geared POV), or direct drive.
I've also considered the epoxy/plastic threads. But that's more of an alternative.
I'm just working on the actual driveshaft mechanism now.
As it's a Bintel with the newer style brake system, I've been looking at a way to mount it all.
http://farm3.static.flickr.com/2446/3765334482_a8ab2b5410.jpg

I've figured out the internal hole is perfectly drilled for a 18mm helicoil.
Didn't want to re-drill a machined surface, if I could help it (didn't want an out of round hole)
Then I can use a 18mm bolt, and mill it down somehow to a point where I can make stuff fit to it.
It's just that I am not trying to bugger up the brake system whilst making a driveshaft that works, so I can independantly move the brake cover on the outer thread without an issue.
Once the shaft is mounted, it will be easier to see how to mount a drive.

[kinetic]

PEC was turned off for this shot this morning:
NGC 1365 in Fornax...a lovely barred spiral.

Arc sec per step size is now spot on and this frame
set which went for an hour has given me a lot of PE
data to refine the PEC curve.

Full moon was nearby so the shot suffers from lack of flats
and contrast.

Happy though :)

Steve

[Spanrz]

Looking good.

I've had a rethink about my drive (direct).....I've been in conversations with Mel Bartels.
Had a chat about direct drives and gearing. I didn't fully understand the reason for stepping down the gearing a few times now, but for "sub arc" resolutions. Like using a 360 teeth gear, then step it down to a 30:1 drive (etc).
My thoughts are now back on the local supplied worm and some stepdown system (Brisbane sourced worm).
As my efforts to get a "new age" gearing system in AUS, have failed, on the supply front.

But another alternative is, in late Jan-Early Feb, the wife has organised a trip back to the states.
I'm looking for USA companies who can supply a gearing system and a reasonable cost for it's function and most of all, have supply of them. Sheesh, do I have to physically go overseas for it as well??:screwy: Hahaha..
And the stupid thing I'll find, is that 4 companies within a 5k area will be able to supply. The USA version of bunnings "always" has everything in stock! Haha

Visiting NY (Way upstate) - yes lots of snow -yay!, Kansas, maybe New Orleans district as well , and my fav, NASA - Cape Canaveral.
I put my bidding, to the wife, for a shuttle launch visit around Feb 4th. And wanted so desperately to visit.
So the shuttle visit / launch trip is there, just organising it.

If I can get a shuttle launch on the few days we are there, that's my bucket list right there.

[kinetic]

Tonight I performed a test to see how well the RA clutch works
on either side of the meridian.
Scope was balanced, clutch engaged and a logging set done
using K3's Drift Explorer.
I used my modded SPC900 at Newt focus on the 12"
First set was done in the western sky 'going downhill'
Second was done east 'going uphill'.

A slight tracking error rate was added also.
PE shape repeats with the same sine wave structure regardless
of which side of the meridian I am on.
So I take that to mean, the clutch is reasonably reliable when balanced.
I think I'm correct on this....

I then put my DSI II mono back in and did a set on NGC 7213, a Seyfert
galaxy in Grus.
Note the diffraction spike from Alpha Grus (out of field, top right)
PEC was on and all tracking error nulled out for the set.

Steve

Steve

[kinetic]

While still continuing to refine PEC on the homemade RA gear
I have been almost completely around the circumference of the
gear following stars etc.
Arc sec per step size has not needed to be drastically altered so far.
(this is good news....this means the spacing around the gear of the
hobbed teeth is pretty accurate.
My only big PE is the non-concentric worm.
This, as in the last few posts, is manageable with Bartels PEC
turned on. Luckily the PE is cyclic and repeats almost identically.

However, to move forward from here requires many nights of
collecting accurate PE data with K3 drift Explorer and a spreadsheet.

This is the easy part. The hardest part has been getting an accurate
sync point when switching on PEC.
I have to toggle it on right at the correct position of an imaginary clock
face on the worm shaft.
The Bartels system even has a function to add this sync via software
and a pin on the parallel port. For now I just wanted some sort of
visual feedback that I was in the vicinity.

With the occasional resync needed from inside with remote desktop
this becomes impossible......enter the webcam. :)

I fitted a rough printed dial to the worm shaft and sat the dome
webcam looking at it.

All of this is now visible via remote desktop from inside and I can
call the PEC on /off via the ASCOM interface and custom LX200
commands.

It also lets me check visually there has been no drive trouble..handy.
(a grub screw came loose on the 60:1 gearbox output shaft and
the toothed belt pulley was slipping). I saw this via the webcam! :)

Pics attached. A screengrab of the dome PC with Cartes
and ASCOM connected driving the scope.
The live webcam view is top left.

Note the advertising on the webcam image. You just can't
get away from bloody advertisers can you :)

Steve

[kinetic]

This is the latest refined curve of the worm gear.
This curve is synchronised in at the index point and
activated.

This curve has been refined from many guide cycles and has
been averaged using Mel's software.
His software can extract both drift and PE from a guide session.
It implements the drift component straight away unless you disable
it.

This PE is mainly caused by the non-concentricity of the worm.
Correction cancels most of this out but of course it would be good
to have this at a minimum to start with.
I have a way to do this. Luckily I stumbled on the way because
of how my Dad suggested I recover the worm from the hob.

In the first few worm/hob/blank tests I would 'recover' the worm by
boring it out very carefully and installing it on an 8mm shaft on
the worm carriage.

The simple act of boring something and parting it off releases tensions
in the steel that otherwise was rotating perfectly concentrically.

From that point on, the worm you think was perfect and concentric
is not! It has eccentricity now but only slightly.

Dad suggested eliminating the 'recover' process of boring the worm
and doing the whole turn down/cut acme thread/cut bearing shoulders/
hob a blank/part off process in one go.
No non-concentric error should happen. Or at least it would be far
less than the 'recover' process.

Finally, the method to minimise this error on an already made worm
sitting on the 8mm sleeve is to put a set of grub screws at 90 degree
intervals around the worm.
That way I can rotate the worm until the 'high spot' is at the 'low'
spot. This should cancel out most of the worm PE.

Edit: some recent results

Steve

[Spanrz]

Steve, I see your getting some good results off the gear.... :)

I have just purchased a worm gear arrangement, from Small Parts.
The price dropped to a good number ($130 odd) including express shipping for the set.
They are apparently "on sale" as they are in surplus, so I got it, in the right time :) :)
As before when I was looking, it was over $200 not including shipping.

I was going to wait till I got over the USA (planned trip in Jan-Feb) and purchase some gears over there, but my time is very limited on looking for the parts whilst there.
I have decided to buy these gears on sale to atleast get my brain out of "hold mode" as I was going nuts not doing anything about it.
Atleast I can now look for a gearhead / reduction box now, to reduce it to arc seconds.

Your above info, is invaluable as it's the exact info I am after. :)
I would have preferred to have a crack myself at a worm gear, but the infrastructure isn't there.
Only time will tell how accurate the gearing is.

The worm gear is 360 teeth. Giving 1 tooth per degree which = approx 239 seconds per tooth.
Now to spec the reduction. See how it goes.

[hikerbob]

I've been back at it again.
I'm still not cutting threads of any merit so am sticking with recycling the worm thread from other components. The latest (and so far great) is the thread from a Trade Tools G clamp which sells for around $12.

The thread is not hard enough to do the initial cutting (it rounds over badly) but does seem fine for the final stages of cutting the worm gear. I've been roughing out the worm gear with a conventional tap of the same pitch and a slightly larger diameter then finishing it off with a slotted piece of the clamp thread.

I'm housing the worm in a piece of 25mm OD alloy tube with a 19mm ID and an 8mm shaft turned on at each end. Bearings and pullies from HE to suit and it's all pretty easy although I've not proven yet just how concentric the result is.

I've still got some cleaning up to do but this is looking promising.

Steve's mention of cork washers to allow for high load slippage solved the problem I'd had in working out how to do that. I had some old cork gasket material lining a toolbox which should be just right. I may still swap the tensioning screws over to grub screws.

Bob

[kinetic]

Brilliant Bob,

How did you tackle end thrust with the HE?
That will be a big contributor to PE and backlash if you don't have
that taken care of.
The thread looks NICE!!!!
Oh and old kitchen table placemats are a good source of cork.
Tip em upside down and they are sometimes laminated with cork.
Wait till you feel how sublime the cork /pressure plate works...
That's thanks to Dave Gee btw...:)

Steve

[hikerbob]

Steve I've not dealt with the end thrust issue yet. The bearing on the motor end of the worm is pushed up hard against an internal circlip and I've not doen the other end yet. I left some extra space in the tube at that end for that but anm still thinking about it.

Bob

[Rod]

Hi,

I hope this is the right place to ask this question. I have a bartels stepper driven scope which uses molded epoxy worm gears. These work OK but one day I would like to build something more accurate. I don't have a lathe so I wondered how good a gear I could produce with a jig like the one described here:

http://members.shaw.ca/gargwolanski/Gearcutter.html

Rather than use aluminium, I had thought about using plastic which would probably be quieter when paired with the stepper and metal worm.

What do you think?

Thanks,

Rod.

[kinetic]

For what it's worth Rod, I think it might be a viable option if you
have a good set of bearings in the drill head with no end play.
As long as you can guarantee the hobber has no slop at all and
you can slow the speed down to a nice cut speed I can't see
why it wouldn't work.

You also need fine crossfeed and transverse feed controls.
That may be a harder thing to set up. You need to add fine depth
cuts.

Having said all of that, I'm speaking from a position of ignorance,
I haven't tried it.

As far as accuracy of the lathe-hob setup for cutting medium
accuracy R.A. worms :-

A few recent images with exceptional seeing validate it as
a good, cheap alternative to the commercial option.
That's always a good thing for us ATMers :)
Latest image of Eta Carina here:
http://www.iceinspace.com.au/forum/showthread.php?t=51934

regards,
Steve

[kinetic]

Last night while capturing this (http://www.iceinspace.com.au/forum/showthread.php?t=54677) result the worm assembly made
a quite loud clunk and the live preview showed a star trail.
Lucky I was in the dome (nornally I image by remote desktop)
because I immediately stopped to investigate the noise.
I was sure the worm gear had just lost mesh and damaged the hob
surface but luckily it wasn't that bad.

First I suspected the nylock nut ( Pic 1) at the end of the thrust bearing
had , for some reason, wound itself tighter and made the worm
tighter......causing the toothed belt to jump a tooth.
When I pulled it all apart today I found the baseplate the worm
assembly sits on was to blame. Well I was to be more accurate.
I had tightened the bolt ( also shown in PIC 1) only 24 hrs earlier.
This and another 3 are what mounts the whole base plate
to the GEM and it sits on some very fine shims which mesh the
worm to the worm wheel.
I had forgotten that these were only nipped up and not excessively
tight. Of course, tightening one had meshed the worm in slightly more
and it had only driven itself to a slight high spot and got tight.
Luckily the first thing to give was the toothed belt jumping and
not a worm gear chewed out.

While apart I also noticed scrape marks from the clutch plate pressure
bolts which had chewed through the cork.(Pic 2/3)
I mounted the worm gear on the lathe index plate and re-faced this surface.(Pic 3)

All back together and meshed sweet, back in business.(Pic 4)
The advertising on the PEC index wheel is a bit greasy now and
should be replaced :) (PIC 5)

Edit: wow, I had trouble finding this old thread, I can't believe it's
got 7500 views!


Steve

[MrB]

Glad there was no damage. Things like this always help out in the long run.... eg, discovering the prob with the clutch.

7500 views!? Popular thread!

[kinetic]

The re-grease and re-seating the mesh certainly did no harm Simon.
I didn't lose the PEC index mark either so that's a bonus.
It wouldn't hurt to do some PEC curves anyway to see if anything
needs a tweak. I'm sure it should if mesh has altered.

Popular? yeah, maybe I should write an article ;)

Steve

[kinetic]

The drive train was tested last night in an all-nighter.
PEC curve is almost unchanged. The index point is exactly
at the same 'point on the clock-face' as before and the shape
of the correction curve as it was. Only a slight change in
it's magnitude.

Some results in this thread (http://www.iceinspace.com.au/forum/showthread.php?t=54677) at post #13

Steve

[kinetic]

Another clunk in the dark last night followed by another every
4.5mins....(the worm period)!:mad2:

Last night the worm gear had returned to the spot where the
problem had been where I had the toothed belt jump a tooth.
I had marked the section with red pen just in case.
Seems like there is a high spot still that has been exposed
once again by a change in mesh.:shrug: Thought it was fixed.
I will pull it all down again but this time I might try the
'paper between the mesh' method I have read in Peter's
Hypertune thread.
This will almost certainly introduce some backlash and need
a new PEC curve generated. I have had almost nil backlash with
this setup, the drive starts following stars immediately at 1524mm FL.
I'm reluctant to introduce backlash but if it removes a high spot and
protects the worm gear so be it.....

Steve

[kinetic]

One of my good tech friends has been keeping an eye out
for gearbox reductions and clutches etc after I described my
successful worm drive to him.

He gave this to me and said, "is this any good to you?" :D

It has a 30v DC motor with a drive clutch straight on the
back, driving a seriously well made, heavy duty brass gear
train. And a position feedback just prior to the output shaft.
With a few simple mods like:

Stepper in place of DC motor,
Rotary feedback pot swapped for optical encoders,
Output shaft driving a worm/worm gear,

...it could easily be the drive for a rather large GEM.

One man's junk... eh :thumbsup:

Steve

[kinetic]

After a few nights of deep sky stuff and periodic random
clunks in the dark I think I stumbled on a big reason why
the drive has been doing this.......grease!:rolleyes:
I've been driving the worm gear around well past the 'clunk'
high spot that happened weeks ago and the gear had driven
itself to an ungreased section! Doh!

I had not greased the entire 360 degrees of worm gear after the
clutch face trim on the lathe.
Obviously, once it had reached the un-lubricated section, only the
remaining grease on the worm itself was supplying the lube!
Worms and worm gears don't go well just metal on metal :)

Greased and solved last night in the wee small hours.
Another problem has been the index point is now wrong.
A few nights ago one of these clunk/ tight spots had caused the
toothed belt cog to turn about 2 hours on the index 'clockface'.
I have to re-do a PEC curve set and re-index the wheel.

Steve

[kinetic]

Ok, after a few more hiccups and a week of gusty gully breezes
messing with PE refinement, I finally got a few sets to look at
in Excel and redid the PEC array in the Bartel drive.

The final night was above average seeing which helped a lot
to both quantify and test the PE.
My index point has now changed and I have to remember this when
I engage the error correction.

First pic is a few PE sets overlaid with the same index entry point.
You can see a bit of deliberate RA rate error (the downhill slope) on
all of them.

The next set is the result of half a dozen PE sets averaged and smoothed,
then fed into the Bartels PEC array. This set has zero RA error and
represents 2 complete cycles of the RA worm with PEC ON ,
then lastly , 1 complete RA worm cycle with it off.

I haven't worked out how to adjust all Excel values up and down so they
dip above and below zero (simple Excel maths formula stuff!) so I added
a line at value 18 because this is the index start.
So for all illustrative purposes this line represents zero.
Reading this it seems I have about +/- 4 pixels error in RA with Pec
turned on and +25 / - 18 pixels with it off.
At 1.14 arc sec per pixels this represents no larger than +/- 5 arc sec
of PE with it turned on. Now that got my attention because previously
I could only get it under +/- 10 on a good night and +/- 15 on average
nights when close to the celestial equator.

Pic 3 is a close up crop of centre stars in Omega Centauri with PEC
turned on.

Steve

[kinetic]

Some more pics tonight with the refined PEC curve returning
consistent +/- 5 arc sec PE drift in RA.
Only problem was my index point has moved again! Grrr.
It was 3 o'clock on the index clockface and now is 4 o'clock.
Something that I can't fathom is shifting this and this time it's
not the worm grub screws.

Each set here was done right after a guide-save session in Bartels
scope drive and the curve smoothed.
Then dumped to the PEC.dat file...then finally used with NO GUIDING.

So it really is a custom PE for the section of the worm that has
already just been driven past...So technically it doesn't apply to
the section the capture is subsequently made on. If that makes sense.
In 3 sets , with 3 consistent PE figures under +/- 5 arc sec, this tells
me that the worm hob is fairly consistent around the circumference.

Steve

[kinetic]

I've had some recent scares with the worm gear coming completely
out of mesh twice without warning.
The scope has tracked beautifully for hours then suddenly done an out
of mesh nosedive! :(
I've luckily been in the dome both times to catch it and the second
time last week I heard the worm scrape across the mountaintops of
the worm gear hob. A truly heartbreaking moment.
I thought I had definitely chewed out the hob surface both times.
I was at a loss to explain why this had happened because the worm
carriage is completely bolted down solidly and cannot drift out of mesh.
Secondly, the RA shaft cannot drift upwards and out of mesh in it's
bearings either. There had to be something in the system that was the
cause and I reckon I have found it.

Firstly I pulled it all down and was relieved to find no obvious damage to
the hob surface. What I did find though was the worm carriage had some
slight play in the brass bush on the drive end.
Not enough to cause a complete loss of mesh though so there had to
be something else.
What I found next was the worm when bolted down firmly had a
tendency to pull sideways to one edge of the pre-cut groove
shape of the worm gear.
I've attached a rough paint pic to illustrate this.
Even though the pre-groove should seat the worm snugly, if I let
it bolt down firmly to slightly one side as seen in the right side pic
of the pic, then only a substandard mesh has been achieved.
It felt like proper mesh but when combined with the play in the worm
bush it must have been enough to come completely out of mesh at
the point on the worm gear that was slightly lower. I know I have some
ever so slight non-concentricity of the worm gear.

Anyway...PIC 2 is a shot looking straight down the axis of the worm.
Even though it' slightly oblique an not a good focus, you can see
a slight misalignment of the worm in the pre-cut groove.

All bolted back together now and tracking fine. No obvious play in
the 360 dgerees of travel. (checked with a battery drill driving the
worm direct drive).

Hopefully back in business.

Steve

[Spanrz]

My thoughts Steve, a ball roller (anti-friction bearing) to be used instead of a bush.
Or with new bushes, use Moly based grease.

Yes, I hate the sounds of metal grinding too....

[kinetic]

Brett, yes mate, I think in hindsight the bush might have been
a bad mistake. It's easy to bore the block out to suit a ball race
but I really thought a bush would be ok at the non thrust race end.
You can see from post #70 how the cradle arrangement is.
http://www.iceinspace.com.au/forum/showpost.php?p=454519&postcount=70

When first set up the entire carriage had zero end play(thrust) and
zero side play (bearing one end and bush other)
Now after probably a ballpark figure of only 10 complete revolutions
of the worm gear there is zero end play (thrust) but some obvious
play in the brass bush.
This will have to be re-made, no two ways about it, so I may as well
install a ball race.:thumbsup:

Steve

[kinetic]

An update:

Now many months on and many revolutions later of the worm gear.
Mesh is fine, tracking consistent, checking on the grease every so
often. The only thing not co operating now is the weather :)

I have the worm gear with index marks now so I can load PEC data
to suit which 'part of the pie' I'm using on a particular series of nights.
The PEC curve varies but only slightly. The PEC for '12 o'clock' isn't
the same as the PEC for the '6 o'clock' , as explained in a previous
post.

The Bartels software allows me to load up a custom PEC curve from
a list. Many nights over many months I have simply been generating
PEC curves and playing in Excel and other software to average and
smooth these curves. I have a pretty good indication of the accuracy
of the hob now and know that another gear (MkII) could have a lot
of things done slightly different to improve the errors I have in this set.

Steve

[kinetic]

A bit of prep work today making a backup RA gear for the homemade
GEM.

I faced both sides for the clutch pressure plates, cut the pre-hob radius groove,
and finally bored out the centre to interference fit the RA axis.
The blank is now ready on the spindle. Next to do is the new ACME worm and hob.

This gear hopefully will try and address the main errors which have shown up
in this entire project....which are quite minimal considering how the gears
have been hobbed with a self indexing etc.

I think I can deal with both of the main PE contributors by the sequence
that I do things this time and an idea I will try with self indexing.
I will also try some mathematics this time and see if I can target a predetermined
number of teeth on the worm gear :)

With luck this will be the working gear (with a better PE) and the current
one will be a backup should tragedy strike :)

Also attached is a beautiful set of MFM hard drive heads showing a
set of 6 roller bearings (4 visible in the shot).
Nice working parts for a Crayford focuser guts :)

Steve

[kinetic]

Made the hob / worm today.
I added a section to test thread cutting....PIC 1...the purpose was to
check the sharpness of my ACME tool shape before cutting the main
hob and final worm.
I'm glad I did this because I had assumed my thread cutting gears
were still set up to do an M12x1.75.
Turns out it wasn't and I luckily found this out on the test section.

Once sorted I cut the hob and worm and pic 2 shows a close up of the
final ACME thread profile. Good enough.
On to the hobbing....

Steve

[bojan]

Steve,
Do you plan to do some polishing as the final stage?
A friend of mine used diamond lapping paste when modifying his EQ6 (from ebay, http://cgi.ebay.com.au/12-pcs-5gram-Diamond-polishing-lapping-paste-compound-/350411795346?pt=AU_Hardware&hash=item51962bab92).

The end results are fantastic in terms of PE - it was reduced from 30arcsec pp down to 7 arcsec pp, see here:
http://tech.groups.yahoo.com/group/EQMOD/message/26509
http://groups.yahoo.com/group/EQMOD/photos/album/1587225881/pic/list

[kinetic]

Hi B,

yes I did lap the RA worm but I didn't use carborundum (from my mirror
grinding kit days) because I didn't want grit anywhere near the workings
of my lathe.
I used cutting compound and used the worm section (not the hob).
I took a little more time to lap the RA gear, the DEC didn't get any
attention I don't think.:)

Post 117 shows I measured the PE at +/- 5 arc sec but I have since
found that the PE changes 'around the clock face' of the worm gear
due to the self index creep. (That's with PEC turned on BTW)
I have different PEC files I call up depending on what 'hour' of the
RA I use.

thanks for the links mate, I'll give them a look when I'm home from
work.:thumbsup:

Steve

[kinetic]

This one ended up with 323 teeth and I calculated 323! :)
Self indexing was helped by graduations done in the 4 jaw acting as
a rotary table.
Final depth was also as calculated...0.875mm which for an ACME
thread is pitch/ 2 and a 29 degree tool.

I made a video this time of the hob/ worm polish process.
That makes 3 successful gears from this process.

Pic 1: at half depth on the hob
Pic 2: full depth and now polishing on the worm section.
Pic 3 and 4: final product

Steve

[bojan]

Steve, you are a legend ! :thumbsup::thumbsup::thumbsup:

[kinetic]

Recovered the worm/hob from the lathe today and managed
to retain it's concentricity with the recovery process.
Ready to test as a matched pair now but I will wait until
near full moon so I can keep imaging with RA worm gear Mk1 until
then :)

Steve

[kinetic]

Last night I was able to do a very fine PEC analysis and generate
a waveform for the MK II RA worm/gear set I recently hobbed.
I had cautiously hoped the 2nd worm gear would be a slight improvement
on the PE of the MK I gear as I had done a few things different in the
machining sequences. I knew which errors were the biggest contributors
after many months of watching it and measuring MK I perform on the sky.

Also,
in a testament to just how good the seeing was last night (and hopefully
tonight), I usually can only stack about 250 from 300 frame sets.
These two results are a stack of EVERY frame in a 550 frame set for the star field
and EVERY frame in a 302 frame set for Miaplacidus/NGC2822.

Not only that....the 2822 shot has NO GUIDING and PE turned OFF
The star field shot has NO GUIDING and PEC turned ON.

Do you think I was a little bit pleased with this worm gear? :D

some pics of the changeover:

Pic 3: old pair on the left, new on right
Pic 4: damage I found on the cork clutch face.
Pic 5: dismantling the worm carriage to add the new worm.
Pic 6: made a new bush for the non-thrust end.
Pic 7: in mesh and testing play.
Pic 8: up and running.

Steve

[Mighty_oz]

That's looking very good, u must be chipped :) Is this with the usual 5 sec subs or longer ?

Marcus

[kinetic]

5 sec sets Marcus....could have easily gone longer but PE measurement
and logging needed fine resolution down to 1 sec exposures so the
night was a mixture of 1sec for PE measurement and 5 sec to try
a PEC curve in operation.

[kinetic]

Ok,
here are a few Excel chart results of some initial PE measurement.

I have only done a rough PEC curve and tried it just once.

I'm pretty satisfied this gear is a winner. I hope I don't chew this one out
and can, at a later stage , make a similar one from brass or steel
and duplicate the machining sequence.
This gear has so far (gone about a 1/4 of the way around the gear)
got a PE (without PEC on) of about +/- 5 pixels.
This is slightly over +/- 5 arc seconds with a conversion of 1.14 arc sec per
pixel.:)
Worm gear mark one had +/- 25 pixel drift (see post 117)

Thanks to Andrew (Mithrandir) and Sheeny for the tips in Excel.

Steve

[kinetic]

This is a result from last night with my cooled DSI II camera.
It represents tracking for 52 minutes on the same field at 1524mm FL.
Approx 12 cycles of the worm.
No guiding or corrections done during this 52mins.
The FOV is approx 14 arc mins x 11 arc mins.
Every frame of 500 frames was used in the stack to get the result.
Seeing was very good.

The align file was loaded into Excel and correction for sky rate error
applied using Sheeny and Mithrandir's previous suggestions.
This makes the sine wave trendline flat and shows the drift of PE
better. Green is before correction, red is after.
Just over +/- 5 pixels drift over 52mins unguided.
At 1.14 arc seconds per pixel that would be around +/-6 arc sec drift.

A happy chappy :)

Steve

PS:
Note the fuzzy in the neg shot....still trying to identify what it is .....

[Astroman]

Hi Steve, excellent work, just managed to go through this whole thread. You have done a terrific job in making the gears etc.. my hat goes off to you..

Where did you get the 60:1 gear reduction from? I have been trying to find something similar for my NEMA17 steppers that I use on my little star tracker I made. Also where did you source the aluminium from to make the worm wheels from?

[kinetic]

Hi Andrew,

I just noticed your post, I've been talking to myself a bit in this thread
lately :rolleyes:
It makes a good log of my progress if nothing else :)

The 60:1 gearbox is a very well built US reduction from a heavy duty
Pan-Tilt camera housing.
I have looked around for a 2nd one so I can duplicate the drive arrangement
better on the DEC axis eventually. As it happens a good friend found
me one recently so that will be the next phase.

Worm gear blanks are aluminium 'Electrical Cables buried below' marker disks.:D

Edit: (reading forward) Bob, Andrew, i will try and post some pics of the internals of the unit....


Steve

[hikerbob]

Steve thanks for that bit of info.

I've got an old outdoor pan/tilt device that I'd been eyeing off for a while in my look at onday list. I've just pulled it to bit's for a look. The gearing is all plastic but looks well made. One beautiful bearing on the Azimuth axis. The motors are 24VAC, 7W with a nominal 3rpm output.

Bob

[bojan]

Is it something like this?
http://www.dhgate.com/outdoor-pan-tilt-device-ptz/r-ff8080812bf2ab0b012c0d92f7b153da.ht ml

Looks interesting....

[Astroman]

Thanks for the info steve, much appreciated.

[hikerbob]

Bojan was was similar, the alt axis looks identical but there were some differences in the moulding of the lower portion.

At a guess the one I had was an older model and I'd suspected it of being a lot more expensive than that. Thanks for the link.

Bob

[kinetic]

Here is a shot of the pan-tilt mechanism guys. Same as pic in post #136.

The 60:1 gearbox reduction is the heart of my GEM RA axis.
The smaller 120:1 reduction is the RA axis reduction of the Scooter GEM (http://www.iceinspace.com.au/forum/showthread.php?t=60347)
in combination with the worm set also shown.

All up, very well built and slightly over-engineered , like most good quality
US stuff. What you want for the guts of a good drive system.:thumbsup:
Steve

[kinetic]

Towards the end of an imaging set last night the
DEC axis jumped out of mesh.

The cause???

I think the cause may have been flexure of the counterweight
shaft which made the gear drive slightly out of mesh for
quite a while.
This might be due to the CW shaft being only 16mm rod.
Most of the weights are at shorter extensions most of the time.
Because I have added an ED80 recently to ride atop the OTA, the weight set
has been further down the CW shaft, as much as 4" further than
it's normal position. This may have been enough flex to drive the DEC
worm slightly out of mesh.
Solution...make the CW shaft from something more substantial maybe.

As a temp fix I spun the chewed out worm gear section to a 'retirement'
spot 180° away from the mesh point. Greased up and back in business.
As the DEC axis functions mainly in Push TO mode, the only section of
DEC that I ever seem to use is a narrow bit for slight corrections
either side of an imaging field. (In GOTO mode I would have to rely on the
entire worm gear circumference though).

Steve

[kinetic]

Dec axis all fine now. Here is a set tonight showing how well
she tracks in RA. This is Rho Oph close up at 1524mm FL and
a frame set of ALL frames stacked. Seeing was suprisingly good.
There was no guiding or corrections in half an hour of tracking
centred on Rho.
You can see the PE/ drift in the box where a hot pixel shows the worm
cycle. PEC was turned off.

Steve