I have a worm gear set with no drawings. I need to model them. The only things I know are 1) The Gear ratio is 30, and the worm gear only has one thread. It is a single start as opposed to a multi start worm. 2) The diameter of the worm is 0.7 Inches, and the length of the worm is 0.852 Inch 3) The diameter of the gear is 1.007 Inches, and it has 30 teeth.

I do not know the pressure angle, diametrical pitch or gear module.

What other piece of missing information would I need to know? How do I go about figuring out the diametrical pitch? How do I figure out the module.

  • $\begingroup$ If you have the physical parts, why not just measure them? $\endgroup$ Commented Apr 14, 2020 at 9:13
  • $\begingroup$ All I have to measure with is a micrometer. I can't measure the pressure angle or diametrical pitch with that. $\endgroup$
    – user77232
    Commented Apr 14, 2020 at 12:32
  • $\begingroup$ What about a camera? (or better, a flatbed scanner). You can measure a huge number of things optically. $\endgroup$ Commented Apr 14, 2020 at 14:08
  • $\begingroup$ If using a camera take the photo from as far way as possible (zoom in if you have a lens that allows you to), to eliminate paralax etc. Take the photo against a known grid so you can adjust for perspective. $\endgroup$ Commented Apr 14, 2020 at 14:09
  • $\begingroup$ Nope, I don't have a camera with a zoom lens or a large enough aperture to take a simulated telecentric photo of the part. $\endgroup$
    – user77232
    Commented Apr 14, 2020 at 15:00

2 Answers 2


You can use angle gages to find pitch angle. Looks like a feeler gage but for angles.

You use thread measuring wires with a micrometer and equations to find thread diameters.

  • $\begingroup$ Thread measuring wires? $\endgroup$
    – user77232
    Commented Jan 4, 2022 at 18:26
  • 1
    $\begingroup$ @user77232 Yeah. It's basically a kit with a various triplets of gage wires. You stick them in the threads, measure the overall diameter with a micrometer and run through some calculations. $\endgroup$
    – DKNguyen
    Commented Jan 4, 2022 at 19:56
  • $\begingroup$ A year and 8 months to get the right answer. sad. $\endgroup$
    – user77232
    Commented Jan 5, 2022 at 20:31

Lets say to make it simple you have the disk with 30 teeth. So the distance between each is 1.07 *pi /30= P

Now your worm circumfrance is 0.7pi. We know the slope of the thread on the worm should be such that 30P = 0.7pi.

Dividing by 30 we get P = 0.7pi/30. This is the slope of the thread on the worm.

30 turns on the worm will turn the disk 30 teeth or one turn.


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