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

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.

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.)

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.

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

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

Andrew

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

Obviously my mind is rather naughtier than yours.

Andrew

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

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.

Have you tried searching for ACME threaded rod Graham? Now that 3d printing is popular it's quite cheap in 8mm, 10mm and 12mm diameters. Here's an 8mm on ebay, £4.19 for 300mm long. (ACME is easier to make than true square and much more common.)

For a small worm wheel you'll have significant undercut of the teeth to mate with a square thread so the teeth are likely to break in bending instead.

Square threads are almost obsolete for good reason. They have three interference possibilities all of which are independent.

Andrew