Cutting a Worm Wheel

I am thinking of machining a replacement worm wheel for the autofeed on my Tom Senior M1. The teeth on the existing are so badly worn that the worm jams after about half a turn.

I have watched several videos that show a hob made from a tap, but I am not sure that technique is suitable for a 2" dia 22T wheel.

Does anyone have experience of cutting such a gear?

I have a dividing head and the mill is useable in manual feed.

I need to measure the helix angle, depth of cut and cutter profile (among other things I haven’t yet thought about). How is that done?

Any help would be appreciated

Steve

Free hobbing with a tap is simple, but it is difficult to control the number of teeth:

The mating worm needs to be of the same thread form as the tap:

Given that the worm wheel is off a machine tool I assume that the worm thread is Acme form. In the absence of a proper hobber you will need to make a hob that matches the worm and pre-gash the worm wheel blank before free hobbing:

The final worm and worm wheel set:

Andrew

What is the pitch of the worm Stephen?

Post some pic if you can.

Pete

The worm is 4 TPI o/d 1.104

That wheel looks pretty worn in places.

Have you tried cleaning it up Stephen? is there any bent bits that need filing off?

You could cut that worm with a MOD2 hob for only 2 thou per pitch error, which (IMO) is quite acceptable. I have a 32mm diameter MOD2 how which would do the job though you'd have slightly reduced contact.

Helix angle or the worm is inverse tan (pitch/circumference). You just need to decide what diameter you use to work out the circumference. It helps if you know the distance between centres and the PCD of the gear.

The numbers seem very close to the 12DP sets that HPC make. They have 0.2618" pitch worms and the 22 tooth wheel has a 2" throat (PCD at the bottom of the curve of 1.833" ).

Martin C

Edited By Martin Connelly on 04/02/2021 18:36:21