Sunday, August 30, 2009


Giving Mendel Teeth

I am particularly fond of Vik's bath-plug drive, but it's too lumpy for the X and Y axes. His model-helicopter belts will work well for these, but we can't print the drive gears for them, as the teeth are a bit too fine.

Toothed timing belts are a perennial problem for RepRap: we can reprap drive gears for the 5-mm (0.2") pitch ones with no difficulty, but not for a finer pitch. But the 5-mm pitch belts generally have a minimum width of 10mm, whereas we really need 5mm-wide belts. Otherwise the whole thing gets too chunky.

I think I finally have a solution: split a 10mm-wide belt to give two 5mm-wide ones. I made a jig...

...that you can put the belt in, stab it with a box-cutter/Stanley blade, then pull the belt through. The AoI file for this is in the Mendel section of the subversion repository here. Because timing belts have cords (usually kevlar or steel) running along them lengthwise to stop them stretching, the split works particularly cleanly as it cuts between two cords. And 10mm-wide belts with 5mm (0.2") pitch seem to be universally available and cheap. You can see the result at the top of this post.

I've designed a belt drive gear (here) that you can also see on the NEMA 14 motor at the top. This should work for all three axes of Mendel.

Incidentally, when reprapping something like this with a low cross-sectional area there can be problems with each layer of the part getting too much material deposited on it if you use the same reprapping settings as for larger parts. To avoid this:

  1. Print the part along with other things of the same height or higher (this effectively increases the cross-sectional area), and
  2. Increase the layer thickness slightly. I use 0.3mm for normal builds, and 0.5mm for tall thin things.

The drive gear does need the motor shaft to be filed to have two flats:

The way to do this is to:

  1. Put Blu-tack in the bearing hole to prevent iron filings getting in the bearing,
  2. Clamp the shaft, not the motor; carefully get it parallel to the vice by eye,
  3. When filing, check frequently: if the flat shape is a rectangle, it's parallel to the axis; if it's a trapezium, it's not.
With a little care you can get the size right within 0.1mm.

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Have you considerd the red 2mm pitch ball chain this is avalible in B&Q?
No - I haven't. Can you post a link? (URL, not a bit of chain :-) )
B&Q sell this chain but getting their seaech engine to find it is nigh on impossible. here is a Pic from the forum.
Philwad beat me to using the chain I bought 4m its a nice colour too. nb it is a metal chain not plastic chin.,17846,page=1,file=1689,filename=photo_2_.jpg
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