Sun, 09 Dec 2007

Beads not Teeth

This is my first reprap post. I've established a new category for reprap posts, so expect further posts there. I plan to build a reprap, in spite of their foolish motto: "Wealth without Money". Wealth has nothing to do with money, so a motto of "Wealth without Money" is as meaningful as "A Fish Without a Bicycle". Money facilitates trade, not wealth, so if you don't like money, it's trade you don't like. And yet trade is 100% responsible for our standard of living exceeding that of a subsistance farmer.

Each of the three axes that make up the reprap 3D printer require a tooth belt, in the current reprap design. I gather that they are expensive. I grabbed 14 feet of Christmas bead garland at the dollar store. The beads are 8mm in diameter, and are spaced 2.3mm apart. That's big enough that the gear which drives them could be fabbed. In the meantime, we could use a technique like Vik's to create a gear.

The cord that holds the beads is quite inelastic. Over a 15cm section, a 5kg load only stretches it by 1mm.

Using polycapralone (capa for short), I fashioned a drive gear, to see if it could drive the beads. Glued up a circle of beads. I buried the beads about halfway (up to the string) in the capa. That turned out to be too deep. The beads need to cam in and out of their holes. When they're too deep they can't do that. I also learned how to make it into a proper pulley. Use a hole saw on some plastic. That leaves you with a small hole centered around a bigger hole. Then use the capa as the rim of the gear, burying the beads only slightly.

I'll try driving one shaft from another next.

Oops, I forgot to include the photos, so I'll let this get re-posted with the photos:

http://blog.russnelson.com/images/kif_4895.jpg(Beads forming the gear)[300,300] http://blog.russnelson.com/images/kif_4896.jpg(The empty gear)[300,300] http://blog.russnelson.com/images/kif_4897.jpg(Beads in the gear)[300,300]

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