View Full Version : 21 Tooth Worm Gear

10-17-2005, 11:36 AM
Need a replacement for a 21 tooth bronze worm gear. About 2.75" dia pitch diameter, 3/4" thick, 1" keyed bore. Boston gear only shows a 20 tooth. Any ideas? Thanx.

10-17-2005, 11:49 AM
Do you need the worm or the worm wheel?

10-17-2005, 01:03 PM
Worm wheel not the worm.

10-17-2005, 01:29 PM
I use my rotary table to cut odd tooth gears. 360/21 = 17.142857 degrees per tooth.
Most rotary tables are accurate to +/- 45 seconds or so. On a 2.75" diameter gear,
a one minute variation would be:
3.14159 * 2.75/360 * 60 = .0004.

Probably having a variance of +/- 1/2 mil
on a gear will work pretty well.

Of course the RT is marked in minutes, no decimal fractions of a degree, so you need to convert. I always make up a chart of the reading in degrees and minutes for each tooth, and check it off as I cut.


Jeff E.

10-17-2005, 02:26 PM
You can hob one.

Make a hob using an Acme type profile with the correct pressure angle. Grind a few slots in the threads to create the teeth and case harden or make it from drill rod.

You need to make a fixture to hold the gear blank so it can rotate and offer it to the hob in the lathe at the right height. It will generate the correct number of teeth if the diameter of the blank is correct. It helps to gash the blank first to the pitch circle depth.


10-17-2005, 02:45 PM
Evan, obviously the results speak for themselves, but how did you get away with not backing off behind the teeth?

Al Flipo
10-17-2005, 02:55 PM
You so called amateurs never cease to amaze me. After 40 years in the business, I would really like to run as an apprentice with one of you.

10-17-2005, 02:56 PM
It cuts slowly without relief but it cuts. It wouldn't be hard to relieve the teeth but it isn't really necessary for aluminum or brass.

10-17-2005, 04:21 PM
Here's a nice site with lots of pics on hobbing some gears on a small lathe-

Paul Alciatore
10-18-2005, 02:22 AM
<font face="Verdana, Arial" size="2">Originally posted by Evan:
It cuts slowly without relief but it cuts. It wouldn't be hard to relieve the teeth but it isn't really necessary for aluminum or brass.</font>

So Evan, did you feed the cutter radially or axially into the blank? That's a detail that is often not clear in the descriptions of hobbing.

Paul A.

Paul A.

10-18-2005, 02:37 AM
I don't follow you Paul. The blank is mounted as in the pics at the link above and is fed in at a right angle to the hob as if the hob is the worm driving the worm wheel you are making. It just cuts as it drives...

John Stevenson
10-18-2005, 05:07 AM
Evan, what Paul is asking is valid as some hobbing machines do feed axially.

In your case you fed in radially if you had wanted to feed in axially you need a longer portion on the hob in front of the teeth at clearance diameter and the first few teeth are tapered.

Using this method you fedd in to full depth in the clearance groove then move the hob into the work untill it's reached full depth.

It's obvious from your picture that your hob is for Radial feed and not Axial by it's design.

There is nothing wrong with Evans setup as regards his hob, granted it has no relief on the back but in practice these cut just fine, especially for the purpose that the home shop guy needs.

I have cut many worms with a cutter made out of a piece of mild steel, case hardened, no tool steels or other special steel.


This started life as the unworn end of a Myford leadscrew, fluted, part relieved and then hardened.


This tap, as that's what it is, has cut many brass, bronze and a few cast iron wheels.
Not sure on how many but it's over a 100 and it's been licked a couple of times with a dremel wheel.

Dont underestimate the edge you can get on mild steel with modern commercial cyanide hardening processes.

Sir John.

[This message has been edited by John Stevenson (edited 10-18-2005).]