measured the drawtube

Disassembled the focuser on my Celestron FirstScope. Removed the drawtube. Cleaned it of the tacky thick old grease goo. Measured it.

Used my callipers and thread pitch tools.

The rack pitch is weird. Is 17 TPI a thing?

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Spent a long time on the rack thread pitch. In the end, I just manually mimicked it.

So that's 33 triangle trips across the 49 mm span of the rack. Or tip-to-tip over 49mm, there are 33 peaks. To get the correct pitch I used the solid pattern repeating feature in 123D with the properties of 49mm length with 33 elements within.

Then I had to remove one of the end triangles, reducing to 32 peaks, for the cutter! 

And finally I had to offset a half-tooth.

It worked! I was able to exactly recreate the weird thread pitch for the focuser rack.

That was brain-bending.