# Gear cutter module vs DP

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#### vederstein

##### Must do dumb things....
I'm in the process of building a Webster and I've reached the point of the timing gears.

I went to Stock Drive Components and the 48 tooth gear is out of stock.

So what the hell, I figured I might as well learn a new skill and cut my own gears. Therefore I've been doing some research and now have some knowledge about involute gear cutters (pressure angle, depth of cut, why cutters come in sets of eight, etc.)

Finding a set of 14.5 deg x 32DP gear cutters for a "reasonable" price is a pain. I have issues paying \$700 for a set of cutters to make two gears.

I did find a Hong Kong company (CTC tools) that sells involute gear cutters (about \$100 for the set of 8 cutters), but with a metric nomenclature I'm not familiar.

So now the questions:

1. Is CTC tools a decent company and will they ship to the USA?

My research shows where DP is teeth per inch (32dp = 32 teeth per inch) the metric gear module is teeth per millimeter. Therfore to convert DP to Metric Module, divide the DP by 25.4:
MM = DP / 25.4
So the closest MM I found to a 32DP is a 1.25MM. (32DP = MM 1.259). It's 0.8% off from exact.

2. Is my math correct?

3. Is this close enough to work?

...Ved.

CTC is a good company and yes, they do ship to the US I believe.

No, I don't think your math is correct. It should be Module = 25.4/DP, so you need 25.4/32 = 0.79375 - module 0.8 is close.

You can use the metric cutters, but you will need to calculate the centres distances of your gear selection and build to that. I haven't done the math but it's highly unlikely you'll get 'close enough' using the centres distances of imperial gears and substituting metric.

Edit to add : to make it a bit clearer when converting to metric module, you have the DP which is teeth per inch (32 in this case) so now you split the inch (25.4mm) between the number of teeth in that inch (32) and that gives your module of 0.8. Working backwards to check the math 25.4mm with a tooth every 0.8mm = 25.4 /0.8 = 31.75 (DP).

Hope this is clear.

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Travers sells 14.5 degree 32 DP gear cutters for around \$41 per cutter. I'm guessing you'd need two cutters for your timing gears.

Chuck

I don't know why I didn't suggest it earlier, but in the past I made a simple hob and I've used it to cut gears on 3 I.C. engines so far, with excellent results. Unlike involute cutter sets, you only need 1 cutter for any number of teeth.

All the info is at this link. There's a navigation bar on the right of the page with all the info you need to calculate everything and the cutter itself only took me a couple of hours at most to make. I bet you can have your cutter and gears made before any order you place would have arrived.

I've used the above method and it works very well. I should explain that you index the gear blank one tooth at a time, just with like a standard cutter. The "hobb" cuts parts of several teeth giving an involute shape approximated by a series of flats. See this for an explanation. A quick run in smooths the teeth to an involute shape. For larger (over 20) numbers of teeth the approximation is so good I can't tell the difference from the smooth involute shape.

Lohring Miller

Thank you all for your input.

I like the approximation "hobb" gear cutting. It looks inexpensive and doable. Also I can buy a carbide insert cutter (to make the hobb) that will have other uses than just to make a single gear.

...Ved.

Straight Hobb process is discussed here and here with some visual result comparisons between traditional methods shown. I Posted these recently.
Rich

Hi Ved,

No worries. Cogsy,Paul,Gus and others etc used Module 0.8 to cut 20T and 40t timing gears for the Webster and Rupnow H&M Engines with gear cutters from CTC. We also cut our own piston rings and heat treat. You need some practice and after 1/2 dozen bad gears,you will cut good gears. We also have gears with 1/2 tooth more. 19 1/2 T and 40 1/2 t paper weights. We went on to cut Mitre Gears too.
My next engine will require 12 D.I.Y. spur gears in the timing gear train. 12 gears will cost me arm and leg. In the event I move on to make the Howell V-4 Engine,that will require 24 gears.!!!!

Last edited:
I recently finished machining four 100T gears in 0.5mod (No.7 cutter) for
a Dyna Myte 2400 CNC Mill.
Bought cutter from CTC. Works like a charm.
BTW, research on the net seems to favor 20 deg PA for fine pitch gears.
Cheers, RichD (Canton, Ga)

gus
I go with the best price.
If have to place I what was used.
If trying save money I grind to fit using the old gear as a pattern

Dave

I'm in the process of building a Webster and I've reached the point of the timing gears.

I went to Stock Drive Components and the 48 tooth gear is out of stock.

So what the hell, I figured I might as well learn a new skill and cut my own gears. Therefore I've been doing some research and now have some knowledge about involute gear cutters (pressure angle, depth of cut, why cutters come in sets of eight, etc.)

Finding a set of 14.5 deg x 32DP gear cutters for a "reasonable" price is a pain. I have issues paying \$700 for a set of cutters to make two gears.

I did find a Hong Kong company (CTC tools) that sells involute gear cutters (about \$100 for the set of 8 cutters), but with a metric nomenclature I'm not familiar.

So now the questions:

1. Is CTC tools a decent company and will they ship to the USA?

My research shows where DP is teeth per inch (32dp = 32 teeth per inch) the metric gear module is teeth per millimeter. Therfore to convert DP to Metric Module, divide the DP by 25.4:
MM = DP / 25.4
So the closest MM I found to a 32DP is a 1.25MM. (32DP = MM 1.259). It's 0.8% off from exact.

2. Is my math correct?

3. Is this close enough to work?

...Ved.

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