Cutting a worm?

Hi. I would like a few wormscrews, say 4" long by 5mm dia, in brass, but all I can find are much shorter than that. Also some spurs with concave teeth so as to mesh as well as possible.

I'm thinking that a piece of threaded M5 won't be ideal, and that a "squarish" rather than triangular tooth shape would be better, to avoid slippage of the spurs.

I've Searched several times for the worms, but found nothing but highly expensive custom-made stuff.

Can you get dies, similar to screw-cutting ones but with a different profile, for making them with?

Thanks, Graham.

PS: I only have a woodworking lathe.

Edited By Graham Rounce on 19/05/2019 22:40:57

Graham,

I have not seen dies for cutting worm profiles, only for the standard triangular/square/Acme type theads. You have said that you only have a woodwork lathe, so I assume that the worm will be made from wood. (a photo / drawing / or description of what you intend to do would help).

Threaded bar stock is available in triangular & square thread profile.

Normal bolt thtrads can be used to drive a worm wheel but the thread on it should match that of the worm. A thread tap can be used to do this.

Paul.

**LINK**

Edited By Paul Lousick on 19/05/2019 23:30:50

Never seen dies for worm threads. Acme thread form can be used for worms at a pinch but the pitch has to be a special to match a standard gear, so standard Acme threads will not work.

You don't say what application you want to use your worm and wheel for but there are small worm gear sets on Aliexpress.com for just a few quid. Search for "worm gear" reveals a myriad like this LINK   in a wide variety of sizes and materials.

At that price and with the astounding range they have available it's no longer worth messing about making your own in many cases.

Edited By Hopper on 20/05/2019 01:21:47

A normal involute gear will mesh correctly with a triangular worm profile such as a normal screw thread - after all gears are hobbed with such a profile, the thread profile is just an "involute rack". Your best approach is probably to use a standard tap as in Paul's photo. This effectively uses the tap as a rack hobbing cutter. You can make the process more controlled by turning a circular profile in the blank and "gashing" it with the appropriate number of teeth with a slitting saw to get the tap started. But you need to find just the right blank diameter, and setting up the right ratio on a dividing head may be challenging.

Not sure why you would need a worm 4" long. Just to clarify your requirements though, the two parts of the gearset are usually called 'worm' and 'pinion'. Since the worm is engaging the periphery of the pinion (or worm wheel) it is usually quite short and I suspect unlikely to be cut with a die.

I cannot imagine it would be practical to make your own worm and wheel on a woodworking lathe but ready made pairs are very easily available in vast range of sizes.

The long wormscrew you mention would more usually be called a leadscrew.

Ian P

More detail of what the actual application is would help. (Presumably this is not another MI5 job where we would have to be killed if we learned the details before the patents were taken out etc.)

An involute form will be strongest, and acme worm will demand very weak teeth on the worm wheel to allow clearance.

Neil

Not quite what you asked for, but have you considered the worm and wheel off older style windscreen wipers. The worm is a flexible cable with a spiral spring on the outside of it.

Hi, and thanks for all the advice, and the terminology info. I didn't know you could grt square threaded rod! I hope we can keep MI5 out of this.

Enc is a pic of what I have in mind. It's supposed to be a compact puller. The two gears at the left cause the leadscrews to turn in opposite directions at slightly different rates, in turn causing the pinion to move along, slowly but powerfully.

I've drawn triangular teeth because it's easier, but square would be better. Steel, probably. If necessary, there could be pinions above and below the one shown, to help stop the screws bending.

In the end, I'd like to make it even more compact - 1/4 the size shown, if that turns out to be possible.

Btw, I just mentioned the woodworking lathe. I don't think it will be of much use for this

Thanks again,

Based on the 4" length you mentioned earlier I now wonder what you mean by 'powerfully'?

If its the pinion that is doing the pulling, the force available will be restricted not only by the limited engagement of gear teeth but also by the fact that the two leadscrews will tend be spring apart.

There are many variations of differential pitch mechanisms, the one you have outlined is definitely one of the less efficient types.

A little about the what is it that you want to pull would help people offer a solution

Ian P

Oh. Well, it's just an idea. I'd like to see it in action, even so.

It started off when I saw a basque being tied (zig-zag ribbons down the back). Very decorative, but a lot of trouble! So I was idly musing about some kind of electric assist.

Now even I can think of other, more suitable, things than what I've described above, but I've become sidetracked by the interest of making one, which I haven't seen before.

That's all!

I would have thought that close quarters manual assistance was the only possible solution.

Andrew

Luddite! Lol

I was going to suggest stripping a car jack but that might now cause all sorts of unwelcome connotations.

Wouldn't you be better off with a winch (Note the spelling!) with two drums, so that both ribbons are pulled at the same rate and time?

Another question; having tensioned the ribbons, how will you tie them, whilst still attached to the winch drums, to maintain the tension?

As Andrew says, manual haulage may be the better option, for all concerned.

Howard

Obviously my mind is rather naughtier than yours.

Andrew

Howard: I haven't worked out all (most of) the details, tbh, but yes, I'm sure a "winch" will be needed.

Andrew: Yes, obviously.

Edited By Graham Rounce on 21/05/2019 01:12:30

Graham,

Without knowing exactly what you are planning to do, we are limited with making relevent suggestions and a woodwork lathe will be of limited use for making mechanical parts.

"The two gears at the left cause the leadscrews to turn in opposite directions at slightly different rates, in turn causing the pinion to move along, slowly but powerfully". Instead of this arrangement, only use 1 screw with a nut and and a set of reduction gears (or pulleys and belt) to reduce the speed from the motor. This will also increase the torque applied to the screw. Small gearmotors with a built in reduction gearbox are available which could be coupled directly to the screw. (possibly a car windscreen wiper motor ?)

Paul.

Edited By Paul Lousick on 21/05/2019 02:07:01

With the two shafts geared together 99 to 100, then coupled together with the gear via the two worms I would guess that before one revolution it will jamb up. Shouldn't the gear ratio be 1:1?

Ian S C

The gear is mounted on a moving chassis and is what moves and "pulls".

If it pulls with any force what stops the two threaded rods bending apart? It will need some bearing supports, at least at the mid points, to prevent 6mm rod from bending.