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Saturday, May 26, 2012

Python Involute Spur Gear Script

It's pretty difficult to find reasonably priced gears around and even if you can find them, they're often not exactly what you want. Finding a gear with the right number of teeth, width, pressure-angle and bore is often not possible.

To this end I have written a python script for generating involute gears. Other scripts are available, an OpenSCAD script thingiverse for example, however i) I like to do things myself, from scratch and ii) I'd prefer my setup work with a more mainstream programming language. The source-code is available at the bottom of this post.

The script generates involute spur gears, with pressure angles up to about 30 degrees. I hope to extend it to include racks and internal gears eventually, but it is quite useful now. Output is in SVG for easy editing in graphic-design/laser-cutting or DXF for use with OpenSCAD. I am currently working on a CGAL-based constructive solid geometry module for python that will allow CSG operations to be performed by python scripts. This would allow fully parameteric CSG design to be done in an open, mainstream language.

I've generated some examples using the following script:
# import the gears script
from gears import *

pa = 14.5   # pressure angle, in degrees
P  = 24     # pitch, teeth per unit distance

# generate three gears
ax, ay = gears_make_gear( pa, 12, P )
bx, by = gears_make_gear( pa, 24, P )
cx, cy = gears_make_gear( pa, 48, P )
dx, dy = gears_make_gear( 30.0, 8, P )

# write them out as svg files, scaled uniformly by 150 and 300
gears_svg_out( ax, ay, 'section_a.svg', 150 )
gears_svg_out( bx, by, 'section_b.svg', 150 )
gears_svg_out( cx, cy, 'section_c.svg', 150 )
gears_svg_out( dx, dy, 'section_d.svg', 300 )


The output is below:



14.5 degree pressure angle, 12 teeth




14.5 degree pressure angle, 24 teeth




14.5 degree pressure angle, 48 teeth




30 degree pressure angle, 8 teeth


Since I haven't finished the CGAL CSG python CSG library, I've been calling OpenSCAD from the command-line via python to generate actual gears. I 3D printed some of these on the Vancouver Hackspace (VHS) Makerbot, with reasonably good results:



These parts will eventually be used as part of a 1:4 drive for the leadscrews for my CNC. Currently I have ample torque, but cannot spin the motors fast enough to get fast rapid traversals. With a 1:4 drive, I will hopefully be able to drive the machine quite quickly.



Sourcecode for the script can be downloaded from: http://sites.google.com/site/jamesgregson/tmp/gears.py. Use it for whatever you'd like, but please don't redistribute it.

2 comments:

Klintbørre said...

Excellent! Just spent 12 hours compiling pythonocc for pygears, only to find there was a bug concerning my graphics driver.

This was exactly what I was looking for. Simple, to the point, and q fine addition to CAD/CAM that runs on linux.

James Gregson said...

Glad to hear that it works for you! If you have any photos of projects where you've used the gears, I'd love to see them.

James