On cheap mini lathes, for threading different pitches of gears, rather than having a gearbox with some gear selector, one has to place different gears on posts on the lathe to gear reduce from the lathe's spindle onto the lead screw that moves the carriage side to side.
With a number of gears included in the lathe, and five gears that can be swapped for different gears, this opens up a lot of possibilities for different gear ratios.
My Vevor mini lathe came with a table printed on the cover showing which gears to place on which post to configure the lathe for common Metric and Imperial thread pitches. However, these listed gears that the lathe did not come with. So I set out to work out a combination for the pitch I wanted that used gears that I have. I made a spreadsheet that calculated the gear ratios to try them out. To ensure my calculations were correct, I entered the provided threading table. But my values disagreed. After much investigation, I realized the included threading table was very inaccurate, to the point of being practically useless for imperial threads.\
Watch my video about this: https://youtu.be/sYw7OfeC1b8
To figure out what the best gears are for different pitches, I figured its best to just try all gear combinations with a computer program.
And this is why I wrote the python script lathe_thread_gears.py
This is a python script that is run from the command line.
Configuration for the script, such as which gears you have, metric or imperial, and target thread pitches are parameters at the top of the script. Modify these values to configure it. If your lathe has only four gears that you can swap, you will have to modify the code in the script also.
Output from the program looks like this:
Gear threading table, Lead screw gear first, gear engaging spindle last
'E=' indicates % error from target pitch value
12.9352, 20, Any,Any, 24, 66, E=-0.498%
-------- 13 tpi --------
13.0256, 72, 52, 40, Any,Any, E= 0.197%
13.9700, 33, Any,Any, 40, 72, E=-0.214%
-------- 14 tpi --------
14.1111, 80, 80, 60, Any,Any, E= 0.794%
15.6790, 20, Any,Any, 24, 80, E=-2.006%
-------- 16 tpi --------
16.1431, 66, Any,Any, 50, 52, E= 0.894%
17.9103, 50, 52, 72, 60, 66, E=-0.498%
-------- 18 tpi --------
18.0912, 80, 52, 50, Any,Any, E= 0.507%
19.9003, 50, 60, 80, 52, 66, E=-0.498%
-------- 20 tpi --------
20.0121, 60, 40, 52, 66, 72, E= 0.061%
23.8803, 66, 52, 80, Any,Any, E=-0.499%
-------- 24 tpi --------
24.2147, 72, 40, 52, 60, 66, E= 0.895%
27.9400, 33, Any,Any, 20, 72, E=-0.214%
-------- 28 tpi --------
28.2222, 80, Any,Any, 40, 60, E= 0.794%
31.5773, 72, 52, 80, 33, 40, E=-1.321%
-------- 32 tpi --------
32.2385, 72, 40, 66, 52, 60, E= 0.745%
Total gear combinations (including duplicates): 1876
Total gear ratios: 373
The first actual output line from above would be this:
12.9352, 20, Any,Any, 24, 66, E=-0.498%
This indicates:
| Actual pitch generated | 12.9352 | TPI |
| Gear on end of lead screw | 20 | teeth |
| Bottom movable post gears | Any,Any | Only one gear on this post, doesn't gear up or down so any tooth count will work. |
| Top movable post gears | 24, 66 | 24 tooth gear engages with gears below 66 tooth gear engages with the spindle gear |
| Error | E=-0.498% | The pitch is 4.98% smaller than targeted. |
The script prints the best combination it can find on either side of the target gear ratios. It omits gear combinations that:
- Would cause interference between gears
- Would not reach the spindle
- Would not fit on the tilt-able mounting bar
- Would not allow the housing to be closed
However, the checks are rudimentary so just because it passed a combination as valid, doesn't guarantee that it will fit. Your lathe may of course have a different geometry from my Vevor mini lathe, so the criteria may need adapting to your lathe.
The program is command line based and written in Python. Some familiarity with the command line and Python are necessary to use this script.
For those who insist on a "license" for this script, consider it GPL v2 licensed. Won't include the file here, its ridiculous to have a license bigger than the program!