Gear cutting basics help needed.

I am now at the stage of cutting the gears for my clock, I have a mill and the dividing head is arriving next week, but cutters to buy?

I have never cut gears, even during my apprenticeship or college, a good bit of googling has answered some questions while leaving me bemused over others.

What's the difference between modules? .5 .75 .1

I need to cut 4 gears , 120 teeth X 34 DP

120t X 42DP, and two 68t X 34 DP

Now I roughly understand DP but not what the different module sizes mean. Will a group of say module .5 cutters cut the same as a group of 1 cutters? Does it matter what module size I get?

With the cost of cutters I don't what to make a expensive mistake.

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.

Module is the Metric equivalent of Diametric pitch.

This is the numeric ratio between tooth count and O D of the gear.

Module = (Tooth Count+2 ) / OD in mm

DP = (Tooth Count+2 ) / OD in Inches.

Buy a copy of Ivan Law's book, "Gears and Gear Cutting" No 17 in the Workshop Practice Series. You fill find it very useful in answering your questions.

Be aware that the numbering of Metric gear cutters is the reverse of that for Imperial. So a cutter of a given number of teeth for a Module gear is unlikely to be the same for an Imperial gear of the same tooth count.

Also all gears meshing in a train must be of the same Pressure Angle, as well as Module or DP..

Howard

What clock are you making?

regards Martin

Some oddities here methinks.

Firstly why is one gear 42DP the other 3 are 34DP. These won't mesh together - are they meant to?

Secondly I could imagine 32 DP is a standard size cutter, (|I see RDG advertises same) but AFAIK 34 and 42 ain't - standard cutters would offer 36 DP and 48 DP. Are cutters even available in 34 and 42 DP? And Howard's question about pressure angle is valid.

Thirdly this is for a clock. Would one expect the tooth form to be involute (i.e. use standard cutters)? If they are cycloidal that's a whole different can o' worms.

Can we please have some indication of where these numbers came from just to avoid perhaps leading you up the garden path?

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

I would refer you to the many articles by John Wilding, Dick Stevens and many others in Model Engineer about cutting clock wheels.

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

The clock is a Congreve rolling ball clock, the plans are by John Wilding.

The two 34 DP gears mesh together, the others mesh with 10, 12 and 19 pin bobbins so I guess it doesn't matter much how they are form as they only mesh with brass pins.

If I buy a set of 8 cutters is there a best type to buy for future projects?

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

With higher tooth count gears there is also the option of making your own rack style cutters which for high tooth count gears are supposed to give good results.

Details of the method are available online - see

Gear cutting with a rack form multi-tooth cutter (jeffree.co.uk)

and

Gear Cutting with a Homemade Gear Cutter (helicron.net)

Disclaimer - I have never done it, but the method looks easier than making involute cutters using Ivan Laws button method. It is on my to do list.

The two 120 tooth gears should work fine if cut by the same cutter, the other gears are meshing with what I assume are lantern pinions and should be OK, although you will need to do some math on the correct diameter of the pins and the carriers.

PS good luck with the Congreve - I own a small commercially made one (Dent). Getting it set up correctly and keeping it running is a nightmare!! They do not keep good time - but are hypnotic to watch.

Edited By Peter Cook 6 on 01/04/2022 18:16:04

Thanks for all the replies guys, they are much appreciated.

Sounded so simple to me on first thinking about cutting them, just buy a DH and a cutter and job done. Wrong!

As long as the tooth count is correct and the od of the gears is similar it doesn't matter whether I use DP or module.

So am I correct in think that changing the module/DP but keeping the same tooth count varies the od of the gear.

I think I am getting a handle on this, using the calculator posted by sillyolduffer I can use a .75 module cutter instead of a DP 34 and the od will be roughly the same.

It would also be sensible as suggested to only by the sizes kneeded.

What's the most Popular cutter bore size to buy as I haven't bought the R8 arbour yet.

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.

Hi Ian

So you need 42 DP for the Centre and Third Wheel and 34 DP for the Hour Wheels and the Great Wheel

To Convert to Module M= 25.4/DP

So 42 DP will give 0.6 Mod

and 34 DP which is near enough 0.75 Mod

The diameter of the wheel blanks are on pages 29 and 31 of John Wildings book.

I generally use PP Torntons clock wheel cutters but they are not cheap. However you only need one cutter for each module for any number of teeth for wheels. (Pinion cutters are for specific numbers of leaves but as you are making lantern pinions you don't need to worry about that) Don't get mislead by sets of 8 cutters, thats for involute gears and you are not doing that. Cycloidal for clocks as has been said above.

What have you bought for dividing?

regards Martin

Edited By Martin Kyte on 01/04/2022 22:16:48

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

I have recently made a video about making gears cutters and gears. It was not targetting clock gears but the procedure is basically the same. This is the topic: Making gear cutters an gears

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

.

The specification of the wheels is available here: **LINK**

http://www.clockmaking-brass.co.uk/congreve.htm

Also, if I recall correctly, in Wilding’s book [which is a collation of the articles in Model engineer]

MichaelG.

.

.” Cutters Module 0.75, 0.6  “

Edited By Michael Gilligan on 02/04/2022 07:38:47

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