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Determining the correct gear pitch for a given rack

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Adam Harris07/05/2019 19:28:31
438 forum posts
19 photos

I understand how to determine the pitch of a gear by measuring the outside diameter and number of teeth, but how do I measure a rack to get the dimensions/pitch/other of a correctly meshing gear?

JasonB07/05/2019 19:40:31
16279 forum posts
1722 photos
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For metric MOD pitches measure the pitch of the rack and divide by Pi (3.142....)

For DP divide Pi by the pitch in inches.

Best to measure the rack over say 10 teeth and then take the average rather than trying to measure just one tooth

Edited By JasonB on 07/05/2019 19:50:02

Bill Davies 207/05/2019 19:44:54
104 forum posts
10 photos

Adam, if you have some dowels or pins of a suitable and identical size to sit about halfway down in the tooth space, you can measure over the pins, over a distance of several teeth. Subtract the diameter of one pin, and divide by the number of teeth, less one (i.e., counting from zero).

By converting into appropriate units you can look up the pitch on a table of metric Module sizes, or diametral pitch (DP) for inch sizes.

Hopefully, this should get you started:


Bill Davies 207/05/2019 19:46:40
104 forum posts
10 photos


Link to gear calculations

Pete Rimmer07/05/2019 20:52:17
426 forum posts
18 photos

Set your digital caliper to 3.142 inches and offer it up to the rack teeth. If the jaws span a number of teeth exactly then count the teeth they span - that's the equivalent DP of the rack.

If the jaws don't span whole numbers of teeth then it's a module rack. Measure across ten teeth and divide the answer by 31.42.

Adam Harris07/05/2019 21:01:31
438 forum posts
19 photos

Pete, that is without doubt the most excellent response anyone could hope for! Many thanks, Adam

Michael Gilligan07/05/2019 21:23:15
14023 forum posts
609 photos
Posted by Pete Rimmer on 07/05/2019 20:52:17:

[...] If the jaws don't span whole numbers of teeth then it's a module rack.


Or perhaps, a 'circular pitch' Imperial rack ?


Pete Rimmer07/05/2019 21:40:45
426 forum posts
18 photos

Yeah it could be that Michael, good call.

Adam Harris07/05/2019 21:42:00
438 forum posts
19 photos

It is a metric machine (Schaublin 12) so indeed probably MOD

Adam Harris07/05/2019 21:48:57
438 forum posts
19 photos

I want to make a rack lever for it schaublin 12 rack lever.jpg

Michael Gilligan07/05/2019 21:59:18
14023 forum posts
609 photos


That looks like an ideal application for a circular pitch rack [be it Imperial of Metric]

... measure carefully



Edit: The first two PDFs from this page should help

Edited By Michael Gilligan on 07/05/2019 22:02:39

Adam Harris07/05/2019 22:11:34
438 forum posts
19 photos

I will revert tomorrow in the daylight with measurements

Adam Harris07/05/2019 22:12:33
438 forum posts
19 photos

Michael thanks for that link

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