Page 237 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 7 Bearings, Couplers, Gears, Screws, and Springs 215

P roject 7-1: Make Your Own Gears

In this project, we’ll design and fabricate spur gears using free software and an online
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store, Ponoko, that does custom laser cutting at affordable prices. If you have access
to a laser cutter at a local school or hacker space, even better! You can also print out
the template and fix it to cardboard or wood to cut the gears by hand.
We’ll use Inkscape, a free, open source vector-based drawing program similar to
Adobe Illustrator. It plays well with most modern Windows, Mac, and Linux operating
systems (check the Inkscape FAQ at http://wiki.inkscape.org/wiki/index.php/FAQ for
details). In Inkscape, you can draw gears with a built-in tool. One glitch is that the
circular pitch is given in pixels, not inches, as in the equations in Table 7-1. You can
get different gear ratios by just choosing a circular pitch that looks good and varying
the teeth number, but if you want to make gears that interface with off-the-shelf
gears, you need to pay a little more attention.

In Inkscape, there are 90 pixels (px) in 1 in by default. So if you set circular pitch to
24px in the Gear tool, that rounds to 0.267 in (24/90 = 0.2666…). Since diametral
pitch (P) = π / circular pitch (p), the diametral pitch (P) in inches is = π / 0.267 =
11.781. You will not find any off-the-shelf gears with a diametral pitch of 11.781. As
mentioned earlier, common diametral pitches are 24, 32, and 48. So if you plan to
make gears to play nice with off-the-shelf gears, start with the diametral pitch of your
off-the-shelf gear and use the equations in Table 7-1 to work backward to what your
circular pitch should be in pixels in Inkscape.

Shopping List:

• 1/4 in wooden dowel

• Hobby knife

Recipe:

1. Download and install Inkscape from www.inkscape.org.

2. Download the Inkscape starter kit from www.ponoko.com/make-and-sell/
downloads. This will give you a making guide (a PDF file) and three templates
that relate to the sizes of materials Ponoko stocks. Unzip the file and save it to
somewhere you’ll remember.