For a hush hush project, I need to use gears and I plan on designing and laser printing them.

Theory

I found this old article which explains a lot on how to make gears with (older) inkscape. It relies on the following technical reference. With new version 0.91, things have been a bit simpler.

Unit can now be express in human readable values (i.e. mm). To make gears compatible, you need to use the same circular pitch – let’s use 5mm.

I want to have a 2x transmission ratio : let’s build a 12 teeth gear and a 24 one. I’ll use a turnion as axis, 8mm central hole is then fine.

Gear	Teeth	Pitch Diameter
Gear 1	12	teeth*p/pi=12*5/3.1415=19.09
Gear 2	24	teeth*p/pi=24*5/3.1415=38.19
Gear 3	18	teeth*p/pi=18*5/3.1415=28.65

Distance between gears is (Di+Dj)/2

	Gear 1	Gear 2	Gear 3
Gear 1	19.09	28.65	23.87
Gear 2	28.65	38.19	33.42
Gear 3	23.87	33.42	28.65

 

Inkscape

Render the 3 gears using the extension:

Prepare guides to hold support. Motor gear will be Gear 2, offset 100mm. Keeping some space for security (0.3 mm), secondary gears will be Gear 1, offset 100-28.65-0.3=71 and Gear 3, offset 100+33.42+0.3=133.8. Using an horizontal line and snap on center on bounding box, you should end with something like that.

Move the gear parts to a dedicated layer, hide it and start with a fresh one. Use my plugin to create the holding box. You’ll want a holding box whose internals are roughly: 95*46*16 (we’ll be using 3mm thick plywood, 16= 3*5 + 1mm margin)

Duplicate the top and bottom lid, prepare holes (9mm) fot the secondary gears

Control will be done using a sg-90 servo (or a chinese equivalent). Mine require a rectangular hole of 23×12, axis is 6mm from top. Prepare hole and drill it.

Set drawing to 0.1mm red (cutting), 0.1mm blue (engraving, used for center of drill) and coloring to yellow pale, copy everything to a new layer and setup on page:

Test

I cut the prototypes and have a first round of feedback:

• It’s working !
• My drilling indicators are wrongly place (for servo), need to measure better, easy to fix
• The offset I used to allow cranking (0.3 mm) is too much, as the laser itself as a few 1/10th of mm thickness. 0.1 would have been enough (or even 0)
• Assembly is non trivial, I need to order additional hands
• I didn’t fully assemble it so there is no picture (just snapped the pieces together to check)
• The real version based on this proof of concept worked 🙂