Gear Cutting

OK if you have two cars or are making one for a friend as well. Or go into business on the internet?

Hedge your bets and carry a spare

+1

A dividing head isn't the best tool for the job as the work ideally needs to be horizontal. Talk of involute tooth forms and cutters is irrelevant. From the drawings/pictures of the part the tooth forms are nothing like involute. The only important parameter for the tooth form is that it matches the mating rubber coupling. The parts should be pretty simple to make; don't get bogged down thinking it's a gear, it isn't.

Andrew

No one seems to have answered the question, obviously in the OP's mind, about the difference between DP and Module.

Effectively, they are the same, other than the units used in the calculation..

DP is for Imperial dimensioned gears. Module is for Metric dimensioned gears.

A 20 DP gear with 20T will have an OD of 1.100 inches. OD = (Tooth Count +2 ) / DP (20+2 ) / 20 = 1.100

A Module 1 gear with 20T will have an OD of 22 mm, purely by using mm instead of inches as the unit of measurement in the formula.

This formula, and many others are in Ivan law's book, "Gears and Gear Cutting", No 17 in the Workshop Practice series.

Now for the bad news,

If you buy complete sets of eight gear cutters; you will need a set for each module, or DP.

Each set should allow you cut gears from 12T to a rack;. No.1 will cut from 135T to a rack, while a No 8 will cut 12T anf 13T only.

The depth of cut (Usually marked on the gear. as Df ) will be the same for all the gears of a particular DP or Module, but will differ as the DP or Module changes. (2.16 mm for a Module 1, but increasing to 8.63 mm for a Module 4 gear, or 0.0108 inch for 20 DP gear to 0.360 inch for a 6 DP gear )

to find and compare pictures of 6DP gears, likely to be used on Traction Engines, with the 20DP changegears on a Myford ML7. It will be obvious that they are different. ) Similarly compare a 0.8 Module gear with a 2.5 Module gear

You should not mesh gears with different pressure angles, such as a 20 degree PA with one with 14.5 degrees .

The tooth form is slightly different

In the absence, (Not the case apparently ) of proper Simms couplings of the correct size, I advocated using gear cutters, across the face of the couplings, rather than the OD, to produce what Meccano called "Contrate" gears, as already mentioned..

HTH

Howard

That's true for DP gears, but not for module gears. The numbering of cutters for module gears is reversed. So a #1 cutter for DP gears will cut the same range as a #8 for module gears.

The depth of cut should be marked on the cutter. This is important as it is not necessarily fixed across a given value for DP or module. For DP gears the addendum is 1/P as is the theoretical value of the dedendum. However, mating gears need clearance at the bottom of the space so the dedendum is made larger than theory. A value of 1.157/P is often used, making the total tooth depth 2.157/P. For large numbers of teeth, where less clearance is needed, a total value of 2.125/P may be used whereas for small numbers of teeth, where more clearance is needed, a total value of 2.25/P might be used.

Andrew

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Yes I did, Howard

I gave him sufficient information to work it out for himself

... which is a proven and effective way of reaching comprehension.

MichaelG.

Now there's an interesting little snippet I must scribble in the margin of my Zeus book.

I'd been warned by JS that some cutters (he said of Chinese origin) numbered "backwards". I hadn't twigged it's related to the module/DP swap-over.

Thanks for that pearl, and a Happy New Year to all.

No, DP and Module cutters being numbered in reverse fashion to each other is news to me. Although I had heard that Chinese gear cutters were marked in reverse, presumably the Module ones.

Having said that, I think that I have a couple of Module cutters that follow the original Brown and Sharpe numbering!

In reality, only one cutter should cut the theoretically correct gear, with the specified number of teeth.

But since the departures from theoretically absolutely correct tend to be small, the cutters compromise and cover a limited range of teeth. So you finish up a range of gears where the teeth are almost ideal, but not so far out that they will not work satisfactorily.

And we haven't considered the effects of tolerances on gear centres!

Howard

I double checked, just to make sure. My reference was Machinery's Handbook, so I don't think it's a case of just far eastern cutters being different.

Each cutter is correct for the lower tooth count in its range. I don't know what tolerance is applied to get each range and I'm not bored enough to do the maths.

Hmmm, RDG seem to have the same numbering for both DP and module; so as always it's buyer beware - check and double check, don't assume.

Andrew

Interesting! I think that both my Module cutters came from RDG, so that probably accounts

Thanks for that explanation.

Howard

Again, many, many thanks for all the information and the trouble so many people have gone to to explain this.

I have come to the conclusion that gear cutters are not the way to go in this instance!

To Michael G, I would say, with the help of the forum and the internet I had 'worked a lot of it out for myself' but some of us are too daft to clinch the deal!

Thus, I'm still not clear as to which set of eight I might need for a particular task - an 0.5 module, 0.6 or what.

Putting the question brutally simply - which one for a gear train on a Showman's locomotive and which for the escapement on my half-hunter pocket watch. 0.8 for the traction engine, 0.5 or less for the watch I guess.

That said....I think the way to go on the Simms coupling is as suggested by 'old mart' - use a rotary table (in my case the dividing head) to measure the divisions and cut the teeth with a small ball mill and finish the angular tips with a file, Dremel or even a very small radiusing tool.

Can't thank you enough.

Best Martyn

Martyn,

It depends on the scale of the Traction engine. as to which module or DP you use for the gears. 1 Mod would look a bit silly on a 6" scale engine, just 6 DP wouldn't fit on a 1"!

What size of "teeth" you have on the coupling will need to be calculated from the OD, and therefore the circumference of the coupling. The "teeth" will be slightly different size on one side of the coupling from the other

OD > Circumference. Circumference / 40 = cutter dia for one half. / 38 or the other halves of the coupling, if you intend to use 20T on one side and 19T on the other to get a vernier increment of 0.947 degrees.

Or is that what Michael Gilligan has already explained?

Howard

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Sorry if I presumed too much, Martyn

Are you reasonably familiar with DP, and just struggling with Module ?

If so ... the reason I mentioned Module 0.8 and 32DP is that they are comfortable sizes for most of us.

The near-equivalence is reasonably obvious when you see that 0.8 x 32 = 25.6

... it would be exact if it equalled 25.4

There’s a metric conversion and a reciprocal involved in converting between the two systems.

Thus:

0.4 is near to 64 DP

0.8 is near to 32 DP

etc.

They say a picture’s worth a thousand words, so ... **LINK**

https://sites.google.com/site/mewithmeccano/gears---general---tooth-form-charts-for-module-and-dp-diametric-pitch

MichaelG.

Hello from Rural Wales

I have made a Simms coupling not too long ago, so perhaps I can help.

Look closely at the bronze coupling in an earlier post, and I think you will see that the spaces between the teeth are (as near as a casting can be) semicircles, and the same section of semicircles from inside to outside. This is very helpful, as it means you can cut the hollows using a simple end mill or slot cutter. My guess is that this is how Simms made them in the first place, and the picture shows a later copy. So, forget about searching for fancy cutters, as they won't be correct anyway.

I don't know what kit you have apart from your dividing head, but I think I started by making a dummy spindle with the correct taper and keyway, so you can rotate the coupling precisely in the jaws of the dividing head chuck. You do need to get the taper in the new part bang on, to fit your dummy and the original spindle, as otherwise it will fidget, come loose, and wear the shaft before you notice anything wrong. It would be worth considering a lump of engineering aluminium alloy, if bronze is difficult to find locally. And I hope you have the necessary dividing plate with 19 holes (or 38) - but if not you might need to make one to be sure the angles are all the same.

If you need dimensions for the finished coupling (or half-coupling, to be precise) then come back to me and I will grope about in my freezing garage once it has stopped snowing.

Regards - Tim Stevens

Part two of my response:

Once the grooves have seen sorted, you need to round the edges of each groove. You can do this with a rounding cutter - or radius mill* - or do what I did and use a smooth file with the coupling held facing upwards. It is helpful to have a new rubber insert as a gauge for this operation. I have never seen dimensions quoted so a guess is better than nothing.

* I'm sure there is a 'proper' term for an end mill with a cut-out in each corner.

Hope this helps - Tim

Thanks again. I can take it forward now using the formulae, reading list and practical advice.

Tim, don't go out in the snow. Not yet anyway.

Martyn