Difference in MOD is simply how coarse or fine, the teeth are.

For cutting DP gears you need to be working in imperial units, not metric.

What clock are you making?

regards Martin

Diametral Pitch and Module are two different ways of expressing the same idea. The size of the cutter needed depends on the diameter of the gear blank and the number of teeth needed. Imperial gears decide this using DP; metric gears use Module.

DP is the number of teeth per inch diameter.

Module is the inverse: diameter / number of teeth

Try playing with an online calculator like this one to get a feel for it. With pressure angle = 20°,

120 teeth at 34DP, gives a gear about 3½ inches in diameter:

OD Reference: 91.1412 mm 3.5882 in
Pitch Diameter: 89.6471 mm 3.5294 in
Root Diameter: 87.7794 mm 3.4559 in

Addendum: 0.7471 mm 0.0294 in
Dedendum: 0.9338 mm 0.0368 in
Working Depth: 1.4941 mm 0.0588 in
Whole Depth: 1.6809 mm 0.0662 in
Circular Pitch: 2.3470 mm 0.0924 in

Same website has a DP to module converter. It says 34DP is:

Module: 0.7471 mm
Diametral Pitch: 34.0000 1/in
Circular Pitch: 0.0924 in

So, 34DP could be done close enough with a 0.75 Module cutter

If a 124 teeth were cut with a 1.0 Module cutter, the blank diameter would need to be 122mm, about 4.8 inches.

When gears are made to run together the teeth should all be cut to the same DP or the same Module.  The number of different cutters rises quickly when gears of many different sizes and tooth numbers have to be made.

Getting a copy Ivan Law's 'Gears and Gear Cutting' is recommended. Gears aren't simple!

Dave

Edited By SillyOldDuffer on 01/04/2022 16:23:40

What is important is the ratio between meshing gears, possibly more so than DP or Module (Within reason )

You can get a 5:1 ratio by using 20:100, as well as 15;75 or 12:60 etc.

Gear sizes will, be determined by the centre distances.. If the centre distance is quoted in Imperial units, look for DP gears, rather than Module.

As already said, being a clock, the tooth form may not be involute, but cycloidal, so finding cutters may be less easy.

The clock making fraternity will ,be able to give guidance as to sources of supply, or how to make them.

Again, the book may provide some guidance.

Howard

If you are thinking of branching out into projects other than clocks, you need to decide whether to opt for DP or Module cutters, ie Imperial or metric gears.

Will Involute gears mesh happily with lantern pinions?

It could be argued that to cover most possibilities, Module would be better, since for most purposes 1, 1.25 and 1.5 Module would cover a lot of uses (Such as gears on a lathe. Mini lathe changewheels are 1 Mod, larger Chines lathes use 1.25 and 1.5 Module gears. Imperial gears could be 14, 16, 18 ,20, or maybe even finer DP, and 14.5 or 20 degrees Pressure Angle )

If you choose Imperial, you would need to cater for a wider range of DPs and at least two pressure Angles to cover many possibilities

With 8 cutters to a set, at upwards of £20 each cutter, it could get costly.

AND which ever route you choose, you might never use some of the cutters. (Admittedly, not making gear boxes, I only cut gears when I have to, so the few cutters, 20 DP or Module, that I have see very little use )

FWIW, only buy a cutter when you need it

(I have boxed sets of taps and Dies, some of which have, and probably, never see use! )

Howard

You need a copy of Ivan Law's book Gears and Gearcutting. It's one of the Workshop Practice Series, so cheap and available. Covers all the basics and more.

Buying a set of cutters isn't the only way of making gears , which is just as well because they're expensive.

Peter Cook mentions rack cutters (Sanderson Method), which are easy enough to make. A straight cut rack generates involute gears. Picture of my version several posts down this forum thread about making gears with a mill and Neil Wyatt's tidier version appears on the next page.

Clockmakers originally filed teeth by hand with the aid of some sort of divider or index plate – hacksaw blades rolled round a diameter can be used. More patience and skill than I have!

Another way is to hand-file a single tooth shape into a silver steel rod, either by eye, or scientifically with buttons (see Ivan Law), or with a cone drill. The latter method relieves the cutter it creates the tooth profile. After hardening, the form tool is spun sideways by the mill and moved gradually into the gear blank edge to cut one tooth at a time. After each valley is cut the blank is stepped round by a rotary table, dividing head, or home-made indexer.

Dave

According to this, the theory that cycloidal gears are better than involute is an old wife's tale. I've made clocks using involute, but they were very simple and driven by stepper motors (plenty of grunt). Involute are more tolerant of centre distance errors, but you're not likely to find 34 dp or 42 dp. I'd use nearest metric equivalent. I'm not sure how well they work with pin wheels, but the Congreve isn't a high accuracy clock anyway. Fascinating to watch, there used to be one in a shop window in Richmond (North Riding of Yorkshire) when I were a lad, stood watching it for hours. I wouldn't want one in my bedroom, very noisy. As others have said, buy Ivan Law's book, he even tells you how to make your own cutters

Presumably, clock wheels will be thin, and since the normal, and recommended method is to cut to full depth in one pass, the blank will, need to be supported by some form of sacrificial backplate?

Howard

Ian, if the clock is to a Wilding design and as Michael's post indicates uses module cutters, then those are the ones to use, don't faff around with DP cutters. Also if the wheels work on lantern pinions then you need "cycloidal" teeth to work properly. Get the right cutters, or make them.

Another site on the lines of the one that Andrew posted designs cycloidal gears and is useful to show what they look like. You can also download dxf files of the designs, and I have used these to produce g-code to cut them with CNC.