Re: Digest Number 5

Jonty50@... wrote:
Subject: Re: CNC Router How-To's

I've been mulling several different translation options. There are linear
driver steppers that have a built in lead nut and ride on a lead screw. I had
thought to place a lead screw on either side of the table for the 96" axis
and just allow a linear stepper on either end of the gantry to carry it. This
plan avoids the whipping problem associated with turning long lead screws at
high RPM.

Another option would be a belt drive using a toothed belt and gearwheels. I
have all the parts for this option and am currently designing the gearboxes
required to allow a 0.0005"/step cog belt system. Belt drive is nice because
it has inherent dampening and cushioning properties and also doesn't mind a
some dirt and grit much. Use the larger size belts (I'm using 1" wide "L"
belting) and they are remarkably strong.

Another idea which I'm getting from my Astronomical interests is a form of
friction drive. Before you think I'm completely whacked out, let me tell you
that large telescopes around the world are gradually moving from a worm and
gear style drive to friction drive to track the stars (rotation of the earth
actually) in order to perform long duration astrophotography. Friction drive
is both more accurate and more easily machined than a worm and gear. Yeah, it
surprised me too...

Anyway, I envision an aircraft type cable stretched the length of the axis
with a friction type drive working its way back and forth down the length of
the cable. The cable will describe a path in much the shape of the greek
letter omega as it wraps around two passive pulleys at the left and right
bottom of the omega and a somewhat larger driven pulley that will form the
upper loop of the omega. I've done a couple of quick and dirty experiments
that have made me fairly confident that I could produce such a system that
would work to my requirements.

[I believe the "shopbot" system started with a cable system like this, but
abandoned it in favor of rack and pinion gearing, which is less subject to
stretch and more positive.]


[One of the other nice attributes about this type of guide is that it
[ doesn't require surface friction lube'ing like shafting or ways, eh?
[ That can be a problem when routing things like wood and especially foam
[ or plaster. This fact was brought to my attention somewhat "late" in my
[ own project plans, by a fellow who sent me some in-progress plans for a
[ big router table he had built outta "rollers on tubing". I'll get his
[ reference out if anyone's interested (actually be surprised if he wasn't
[ in the group already, but I've forgotten his name, and his plans are in
[ storage, sigh). He mentioned his design goals were to avoid having to
[ continually worry about cleaning linear bearings and keep the
[shafts/ways wet with lube. I thot that was a pretty important point I
[ had missed when planning me own, so I mention it to "the wise". B)

[This is something I've been wondering about as well. If these things are
always hanging out covered with lube, it seems like they would be magnets
for dust, which is abundant in machines like this.]

I've tried the "rollers on tubing". It works fine until you get dust under
the rollers and then the dust cakes under the intense pressure. The nice
thing about the BW wheels riding on hardened rod is that the contact area is
so small and hard that it just cuts right through most crap that gets on the
rods.. BW touts the "wiping action" of their wheels on their V track and it
does seem to work reasonably well that way too...

[Perhaps brushes could be mounted in advance of the wheels, to push chips
out of the way?]

Andrew Werby

Andrew Werby - United Artworks
Sculpture, Jewelry, and Other Art Stuff
http://unitedartworks.com