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Cutting a Worm Wheel

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Stephen Osborne04/02/2021 11:53:24
30 forum posts
4 photos

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


Andrew Johnston04/02/2021 12:16:16
6129 forum posts
674 photos

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

cutting governor worm.jpg

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

governor pulley pinned.jpg

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:

worm wheel hobbing.jpg

The final worm and worm wheel set:

worms and worm gears.jpg


Pete Rimmer04/02/2021 12:55:52
1004 forum posts
57 photos

What is the pitch of the worm Stephen?

Post some pic if you can.

Stephen Osborne04/02/2021 17:48:19
30 forum posts
4 photos


The worm is 4 TPI o/d 1.104img_0869.jpg

Roger Best04/02/2021 18:05:34
263 forum posts
31 photos

cool That wheel looks pretty worn in places.

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

Pete Rimmer04/02/2021 18:24:42
1004 forum posts
57 photos

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.

Martin Connelly04/02/2021 18:32:31
1789 forum posts
190 photos

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

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