A way to improve the laser-cut gear tooth profile accuracy (2)
In my previous post, I talked about generating gear profile with illustrator script. Now it is time to laser-cut it out of the script. The laser-cut I have access to is VLS6.60 from Universal Laser Systems. Installed with a 60W laser source, this machine is mostly used to cut plastics, wood and paper. A big problem of laser-cutting is the profile tolerance issue as the highly concentrated laser beam basically evaporates the shape profile that it moves through, although it might be only a little. Here is one example how a laser-cutter is causing some tolerance issue:
This piece of 1″ x 1″ square acrylic turned out to be 0.9895″ in width after laser-cutting. Apparently 0.005″ of the profile width on both sides was burned out by the laser beam.
This explains why my first cut of gears (shown as below) didn’t mate so well. You can see clearly the the margin between the cut gear and the design profile.
While, this is not the end of the world. The good news is that the cutting tolerance is quite consistent for the same setup ( material type, thickness, cutting power, speed, etc.). If we know the profile loss will be a certain number, and expand the profile in advance for exactly this amount. Will we be able to compensate the profile loss? The answer is YES! Ok, now let’s apply this to the gear cutting. Since I already have the profile generator code which I described in my previous post, all I need to do is to find my high school math text book, and figure out how to calculate a vector that is “d” distance parallel to another vector. Here is a diagram of the expanding algorithm. Pay special attention to joints where two different normals are rooted at the same points. For quick and easy coding, I am using angle bisector (green dash) to generate expanding profile:
Spend some efforts on the coding and make sure all the math is correct. It really pays off when you see the original gear profile and expanded profile appear in the same illustrator windows!
Time to check it out on the lase-cutter, the profile is now getting really close to its design:
Since you’re using Illustrator, there’s a much quicker way to add an offset to any shape (ideal for compensating for laser kerf) –
If your kerf is, say, 0.2mm, just add a 0.2mm stroke to the shape, centered on the outline, then use the menu command Object > Expand. On the Expand dialog, make sure Fill and Stroke are both selected, hit OK. The stroke will be turned into a solid shape in its own right – hit Union on the Pathfinder palette to merge it with the existing shape. You’ll end up with a shape that’s 0.1mm (half the stroke width) larger all round.
(If you want to make an object smaller in the same way (say, for cutting internal holes), do the same procedure, but after expanding the shape, use “Minus Front” instead of “Union” on the Pathfinder palette).
Yay lasers woo
Great, Thanks!