Re: Stepper and driver

Wow, that is a honkin machine (18' travel!) you have in mind. Of
course I have tried my hand at building wooden boats (27' cold-molded
sailboat) so I know the need for the travel.

Personally, I would nix the oak and go with regular steel I-Beams.
You might be pleasantly surprised how cheap a 20 foot long section of
steel can be. Probably cheaper than the oak, heh heh.

I would then purchase 6 foot sections of 1" drill rod (cheap, like
$20?), drill and tap it about every 12 inches and bolt it to the I-
Beam. Then go to mcmastercarr and buy 1 inch open recirculating
bearings to ride on the drill rod (they are like $30 each or
something). I bet you will find this will give you .05" accuracy or
better over the length if you spend the time to set things up
correctly. The oak could swell in the humid months far more
than .05" over 18 feet! I am not sure of the numbers on thermal
expansion of steel, but it is reasonably low.

I have done something similar to this with plain drill rod and the
open face bearings for a 8 foot travel thingy, so I know it can be
done. I drilled and tapped my drill rod on a crappy drill press, so
if you have a machine shop available it should be a piece of cake.

Oh, one other thing you might want to try instead of chain drive is
gear rack and a pinion. A clever way to do this is put gear rack on
each 18 foot I-beam next to the 1" rod and have a shaft on your cross
beam that drives a pinion meshing with the gear track on each side.
You can get the gear track from grainger, and probably lots of other
places. It comes in 6 foot sections and is not *that* expensive
either. Of course the chain drive might be ok...36 feet of floppy
chain sounds kinda iffy though...

Cool project!

Rick