A different approach to cutting oddball threads


Jul 28, 2017
In the last day or so I realized that the increasingly-common 3D printer offers an interesting alternative to cutting otherwise difficult-to-cut threads. This, in terms of the limited choices of change gears available for mini lathes. There's a 21-tooth gear for cutting metric threads on an imperial 7X lathe, but what if you want to make a 25TPI screw? By fooling around with the Little Machine Shop change gear calculator I found that a 22-tooth gear can produce exactly 25TPI, so that info, along with a 3D gear-printing program will do the job. Some metric threads will see an improvement as well -- a 42 tooth gear will cut .75mm threads to an accuracy of .0125%, while a commercially available 21 tooth gear gives you .023% accuracy. Either one probably isn't enough of an error to cause problems in most cases, but you get the idea.

To avoid a lot of messing with change gear calculators to find a reasonable gear, I wrote a program to help get there faster. It's attached and easy to use -- if you happen to be using a linux operating system. Sorry about that.

To use the program, open a terminal window and change directories to the location of my program, then enter "./FindGear N", where N is the desired TPI's you want to cut. BTW N can be fractional like 27.7 so you can go crazy with this. The "dot-slash" prefix just tells the OS to look in the current directory for the program file.

Due to mechanical-interference concerns I limited the search to gears with 21 to 79 teeth, but when I increased the upper limit to 127 teeth I didn't find any improvement in the accuracy of metric threads. But that might be due to some other things going on with my program.

As an example of a potentially-useful application, consider a differential screw that produces .001 inch per turn. If one of the threads is a commonly-available 32TPI screw, the second thread could be 33.05785TPI. The standard change gear set can be used to cut a thread that is accurate to .1%, but if you print yourself a 22 tooth gear you can get dead on, 0% error, It would be "nice" to have at least one standard thread so you don't have to make two oddball taps, eh?

BTW, I couldn't attach the file without adding the ".txt" file extension. To use it, rename it by removing the ".txt" part.


  • FindGear.txt
    17 KB · Views: 62
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