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The Artful Bodger
06-08-2012, 03:02 AM
I want to make a worm and wheel for my mill power feed. I have already made one using a steel worm and aluminium wheel but the worm wore the wheel out in no time at all.

I very much suspect the cause of failure was the rough surface of the worm and maybe the profile of the tooth.

How can I cut a smooth surface on a steel worm? The steel I am using is stainless steel shaft from a washing machine motor and I can usually get a good surface on this material but when cutting a thread at slow speed the surface is very rough. I use HSS tooling.

I suppose if I mill it I could then finish up by grinding, is that the way to go?

Thanks.

Paul Alciatore
06-08-2012, 03:27 AM
How to make a worm? Well, you need a mama worm and a papa worm to start with and a nice bucket of damp earth and ....

Oh, I see now, you are talking about making a worm in the shop. First you might try a leaded steel instead of stainless. One that has a good machinability rating, like 12L14 or possibly 41L40.

Make sure your tool is dead sharp. Use lots of a high quality cutting fluid like Tap Magic or your favorite.

If the worm still is rough, make a matching "nut" from aluminum or brass and use it to lap the worm until it is smooth. Use progressively finer grades of abrasive. You probably should use different lapping nut for each grade.

If there is room on the end for the tool to run off, you could use a higher speed for cutting it.

Oh, did I mention DEAD SHARP tool? HSS tool is good, it can be sharpened dead sharp.

The Artful Bodger
06-08-2012, 03:38 AM
Ahem, worms are hermaphrodites!;)

I have a bit of better steel here, I dont know what it is but it was given to me as being better to machine than bright mild steel so I will try that.

Thanks for the idea of running off the end so being able to use a higher speed, I can certainly do that if I cut a left hand thread (and there is no reason why I should not).

lakeside53
06-08-2012, 03:38 AM
How to make a worm? Well, you need a mama worm and a papa worm to start with and a nice bucket of damp earth and ....

.


Nope... worms are hermaphroditic. Just need two... no gender. Why do I still know that?:eek:

lakeside53
06-08-2012, 03:40 AM
lol.. Simultaneous posts. Maybe you have be be from N'Zed to know that:D

The Artful Bodger
06-08-2012, 03:54 AM
lol.. Simultaneous posts. Maybe you have be be from N'Zed to know that:D

Form 3 Biology, 1960!

Yow Ling
06-08-2012, 05:04 AM
John, maybe a quick way would be buy a right angle reduction gearbox on tardme and steal the worm and wheel from it. Might get one or 10 or 20 dollars

The Artful Bodger
06-08-2012, 06:50 AM
John, maybe a quick way would be buy a right angle reduction gearbox on tardme and steal the worm and wheel from it. Might get one or 10 or 20 dollars


Yes, thats true. But 'making stuff' is part of the.... (I cant even think of an appropriate word!)... "journey"?

sasquatch
06-08-2012, 10:53 AM
I'm thinking this subject is "Opening a can of Worms".:D

Interesting topic though for sure!!

Dr Stan
06-08-2012, 11:05 AM
I suspect most worms made in production are either finish ground or rolled like threads are rolled thus creating a much smoother finish than single point cutting.

Depending on the pitch you may be able to made a wooden or metal blade with the same included angle. Use it to push abrasive cloth into the worm to polish it. Or load it up with lapping compound and polish the worm.

Black_Moons
06-08-2012, 11:15 AM
I would also consider that aluminum is not a good material to make a gear that experiances lots of sliding out of.

Try a brass worm and a steel wheel.

Paul Alciatore
06-08-2012, 11:38 AM
Nope... worms are hermaphroditic. Just need two... no gender. Why do I still know that?:eek:


Yes, I know. It was just a joke guys; just a joke.

Paul Alciatore
06-08-2012, 11:44 AM
I would also consider that aluminum is not a good material to make a gear that experiances lots of sliding out of.

Try a brass worm and a steel wheel.


A lot of worms and worm wheels I have seen are steel for the worm and bronze for the wheel. I suspect that bronze is better than brass here for some reason. Also not sure why it is not bronze for the worm and steel for the wheel. Perhaps the wheels are easier to replace in most applications: notice I said easier to replace, not cheaper to buy. I have also seen a lot that were steel and steel. Anybody else care to jump in on the subject of materials?

jkopel
06-08-2012, 01:23 PM
LMGTFY :)

http://www.oerlikon.com/balzers/en/applications/mechanical-drives/other-drives/worm-gears/

beckley23
06-08-2012, 03:47 PM
I made a worm and gear for the vernier for my 10EE's taper attachment, it starts about post #217;
http://www.practicalmachinist.com/vb/monarch-lathes/wreck-update-146913/index6.html
In your case I would get a piece of all-thread acme stock and proceed from there, the flanks of the thread should be a lot smoother than a lathe cut one. The one I made is a very slow speed application, that so far hasn't been used. I found the formulas in "Machinist Ready Reference", and went from there in the design a fabrication. I'm quite sure you can find the same information in "Machinery's Handbook".
Harry

bytewise
06-08-2012, 03:55 PM
You did not mention what shapes you were cutting on the worm and wheel. The worm is usually a trapezoid shape. As mentioned above, manufactured acme threaded rod is the way to go for the worm. The wheel, for long wear, should be a curved pocket shape, not just a straight helical gear. The straight helical wheel will work if the load is light but the wear may still be a problem because of the small contact area between the worm and wheel.
The cup shaped teeth on a wheel are cut using a hob in a production shop. This is usually not possible in a small shop. You could do it on a universal horizontal milling machine with a dividing head and a custom ground cutter. The cutter would could be ground from a key way cutting endmill. The diameter of the cutter must be the same and the diameter of the worm.
I hope these comments help. Hugh

Forrest Addy
06-08-2012, 08:14 PM
I've cut many worm wheels early in my career. Seem to me they were made of tin bronze or prosphor bronze and they ran against a hard steel worem.

Worm gear's I've seen after long service were worn in but not worn and the worm had only a polish on the bearing area. The lesson is, keep them lubricated with the right oil and they will last generations. The oil I use is "EP lower uint oil" It's molassis colored and smells like garlic. It says on the label it meets the specs for hypoid gears, automotive transmissions, and outboard lower unit drives. One of the specs is the same as the Nacy uses for winch gearing. I LIKE them Navy specs. Time tested.

Most non-tin bearing aluminum alloys make lousy worm gears. Plain brass is better but not by much. Worm gears sets are a material sensitive pair whose parameters have been established for nearly a century. Standard practice dictates a harded steel worm against a tin bronze or phosphor bronze gear. Departures in material selection can expect reduced life.

dp
06-08-2012, 10:29 PM
Here's a really nice write up from the practical hermaphrodites:

http://www.practicalmachinist.com/vb/south-bend-lathes/making-new-cross-slide-acme-screw-150571/

Carld
06-08-2012, 11:45 PM
The thread form for the worm is a variation of the acme shape. A friend needed a worm gear for his Logan lathe. He's an engineer so he did the formulation and I did the machining.

You can't use an acme thread form for the worm, we found that out right off the bat.

dp
06-09-2012, 12:02 AM
Other than the cutter profile I can't think of any significant difference between turning a worm and turning an Acme thread. The profiles of each are well covered in the literature. The worm, in particular, is well described in Ivan Law's Gears booklet.

oldtiffie
06-09-2012, 12:25 AM
You can use pretty well any trapezoidal form for the worm - acme included - if you wish. But cutting a worm form and an acme form are very similar.

The form of the worm is at right angles to the centre of the spirals - not at right angles as a normal thread is.

The circular pitch of the wheel must equal the longitudinal pitch of the worm/screw that you cut.

The pitch circle diameter (PCD) of the wheel will be determined by the number of equi-spaced teeth in the wheel.

The outside diameter of the wheel will be in large part equal to the PCD + 2 addenda.

The helix angle (at the pitch circle) of the wheel must be the same as the helix angle of the screw/worm measured at the pitch line/s.

It now depends on just what "tooth thickness" you rerquire of the worm and wheel as well as the required worm:wheel speed reduction you require as well as the power (torque) required at the worm and wheel and what speed (feed) range you require for you mill.

Of course all of this has to be fitted into a container which has to be fitted to your mill - as well as alowing space for the elect motor.

The teeth of the worm are to be involute with a pressure angle the same as the side faces of the worm/screw.

Carld
06-09-2012, 12:27 AM
Yep, that's right, the profile (side angle and tip width I think) of the cutter is different.

oldtiffie
06-09-2012, 12:43 AM
It is quite possible and practical too to "gash" the worm teeth with a cutter similar to an involute gear cutter. The worm will only bear on the centre of the wheel teeth but unless it is a very heavy load it should not matter. The worm or the cutter is "tilted" by the helix angle.

The cutter involute form can be constructed/drawn using Ivan Laws method but doing it "by eye" and refining it as you go (ie "winging it") works OK too. Use the worm as a reference.

Use a similar sized gear as a visual reference and you will be OK.

It may need a few re-grinds of the tool and cutting passes but it usual works for light loads and low speeds.

The Artful Bodger
06-09-2012, 02:50 AM
Thanks for all the comments.

I will be having another try with aluminium wheel because thats what I have and it wont be subject to much use being only the table feed on the mill in my home shop.

Looking at some of the images on-line some worms appear to have more air than metal in their teeth. This looks like a good thing as it would mean there would be more metal in the wheel.

I have made worm and wheels before to use on my telescope tripod and they worked well using all-thread as for the worm. For this heavier job I thought it would be better to use a bigger diameter worm but I dont have a big tap to cut the wheel using instead a form of tap made from some of the steel I am using for the worm.

The aluminium plate I have is about 9mm thick and I have used 12mm all thread but the cuts in the wheel are really vary shallow using that method.

Thanks for all the comments and I will think about it for a day or two before trying again.

John