A video on making worm wheels

[Ajohn]

This person generally does good video's and he has recently done one on this subject

He has also  done a lot on casting at home. Especially in aluminium.

John

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[ChrisLX200]

Brings back memories, I did this (and wrote the method up) many years ago. That gear is more for an engineering project, for an astronomical mount gear there are a few other tricks that need be employed:

The worm and hobbing tool are cut at the same time so that they are identical, one half forms the tool the other the actual worm:

The 'gashing' process for the initial tooth form needs to be set at the right angle to accept the worm:

The tool receives a staggered cut (instead of straight cut) to reduce chatter and produce a finer finish:

Use a large pair of good bearings for the wheel blank jig:

Takes an hour or so to hob the gear, keeping temperatures down (no expansion wanted) and the same jig is moved down to the real worm where the two can be lapped.

After lapping the resulting gear is extremely accurate but alos extremely delicate!

Once you calculate and understand the tolerances involved, and that 0.0001" surface displacement will have a significant effect, you come to respect the gears and treat them with kid gloves!

This is that worm mounted in its carrier:

and that gear you saw being made mounted on the RA axis. Remember to make two sets...! (one for DEC)

Such delicate parts must be protected if they are to remain accurate:

Well there you go, not much new in making worm gears in your garage!

ChrisH

[Peter Drew]

I must have made hundreds of worm and wormwheel sets during my business career. I alway used a machine tap to hob the gear blank, this gave the correct pitch angle and made it possible to use standard thread forms for the wormscrews. Care was taken to ensure concentricity of the o/d of the gear blank and the bore, the worm and bearing details were turned in one setting to reduce periodic error. A 6" set used to take around 1 hour start to finish in multiples. Never had any complaints!      

[ChrisLX200]

I must have made hundreds of worm and wormwheel sets during my business career. I alway used a machine tap to hob the gear blank, this gave the correct pitch angle and made it possible to use standard thread forms for the wormscrews. Care was taken to ensure concentricity of the o/d of the gear blank and the bore, the worm and bearing details were turned in one setting to reduce periodic error. A 6" set used to take around 1 hour start to finish in multiples. Never had any complaints!      

Well that would give you a worm and worm gear all right, done it myself for a variety of projects ;-) It's not a correct thread form for this application though (20deg PA), and the final accuracy would not be as good. On the other hand I would not want to make hundreds of sets using my method, one was enough for me!

ChrisH

[lennylovit]

my new 6" rotary table just arrived , will slap a stepper on it in a week or two and run it with my CNC mill .

Lenny

[ChrisLX200]

An HV6... I have one of those ;-)

ChrisH

[lennylovit]

any good Chris ?? cant feel much backlash if any , only time will tell !!!

Lenny

[Ajohn]

I think the point ChrisH made concerning the wheel being free running is rather important for good results.  Lapping would  clearly improve things too.  That's what astrophysics used to do to a mount I own when they rebadged it.

Personally I am going to try using a metric trapezoidal tap at some point. Some have a good lead in which means that there will be less of a pcd problem. It can initially be used where the teeth are very shallow and then further along to deepen the cut. ;-) That's the theory anyway. They are also available with  various nice coarse pitches.

Personally I thought Myfordboys video was nice a clear as all of his usually are. Simple too, no over complications except a better bearing arrangement would be a good idea. It shows how trivial relieving the cutter can be. He does gash at the lead angle of the worm. I'd shim up my dividing head to do that.

The form by the way is correct as the worm will be rack form and that should generate the correct from on the wheel. Where it could get tricky is that the usual gear calculations usually assume a certain amount of clearance which could be too great for this application.

Edit - I was a little dismayed by the way he uses his hands to face off the work. That surprised me actually.

John

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[gajjer]

Thanks guys. I feel strange stirrings in my loins!

I only have a small Sieg lathe and a Sieg mill but I love to see this stuff.

I also had a go at casting aluminium with some success.

cheers

gaj

[Stub Mandrel]

Is the gear form at all critical? As the two will be perfectly matched any form within reason should maintain contact at all times, which is the main thing.

When I made the hob for my rotary table/dividing head I cut it together with the worm in a single piece of silver steel, but unlike Myfordboy I made the OD of the hob slightly larger, to make sure the worm never rubs on the top of its teeth.

[Ajohn]

His casting video's are probably the best gaj. He's helpful as well if people have problems.

John

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[Stub Mandrel]

Thanks guys. I feel strange stirrings in my loins!

I only have a small Sieg lathe and a Sieg mill but I love to see this stuff.

I also had a go at casting aluminium with some success.

cheers

gaj

I have a SEIG mini-lathe and X2 mill, you can make these things with them!

[gajjer]

I have a SEIG mini-lathe and X2 mill, you can make these things with them!

Yes I have seen some posts where people have used a small lathe. My problem is that I really need to get the lathe tuned up. I use them both for all sorts of things but nothing precise. I need someone to help me set it up properly.

It's not important at present but I might join a club now I'm nearing retirement. I fancy a petrol engine too with electronic injection. Urrrrrgh. More stirrings!

Hey it's looking a bit clear outside. I might set that tube thing up that I spent a mint on. I know the moon is up but practice makes perfect.

cheers

gaj

[Ajohn]

If I do it I intend to relieve the form of the worm at the base to avoid it bottoming there and if needed reduce it's od a bit as well so that it can only drive on the flanks.

I'd hazard a guess that the theoretical perfect profile for a telescope drive would be a square thread but that wont work out with zero clearance for obvious reasons. I reckon that all wheels I have seen have been cut with ordinary hobs which I understand will be either an acme profile or a variation on it based on the pressure angle. ACME would give 14 1/2 degrees and the metric version of this 15. The reason 20 and things like stub form are used on gears is strength which also relates to pitch. The last aspect is the most important one in that area.

I did know some one that came very near to cutting a worm wheel with whit tap - messed up because he didn't gash first. Really if things are sized correctly and they run on the flanks there isn't any real reason why these shouldn't be used either. The pressure angle will be up though 22 1/2 degrees.

I don't think a rotary table is a sensible way of doing it. If some one wants to drive the wheel they would be better off using a gear arrangement as the hobbing machines do. CNC given a steppers typical step tolerance might turn out to be a no no.

:-) One day I might get my head round how accurate a tooth spacing needs to be to achieve a certain degree of tracking error. It's not that difficult really. Say 10um error. It will give a certain amount of angular error on a 50mm dia worm wheel and  a lot less on a 200mm dia one. It's just basic trig. Tying that in with auto guiding needs is a touch trickier. Those will be at fractions of a turn.

John

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[lennylovit]

the one channel on the tube I love is doubleboost , he is good and just a bit nuts ,