Re: Linear bearing types?

Don Hughes wrote:

> From: Don Hughes <pencad@...>
>
> > I do have a photoplotter with 1/2" round rails, and it has some vibration
> > problems that require damping. It has an optical carriage that weighs
> > ounces, and no cutting forces at all!
> >
> John, you're plotter, is it a flatbed plotter? If so, I am desperately
> looking for one that is a "D" drafting paper size(34" x 22") or larger.
> If you happen to know of such about, or if anyone in the group here
> knows of one that some company or themselves have sitting around, I
> would be very interested in talking with them.
>
> I would only be running this with optics as well and driven with HPGL
> drivers from the LPT1 directly.

For my applications, a vector-drawing machine was no good. I did
build a light-beam writing pen for my large Calcomp pen plotter, but
it really was a horror, and somewhat of a waste of time. I did use
it while some conceptual and laser light source problems were being
worked out, but the performance was pretty poor.

My original plan, which is now fully operational and performing very
well, was a raster drawing plotter. I wrap the film around a drum,
which is conveniently machined to such a diameter that a multiple
of the shaft encoder's pulses comes to a nice unit measured on the
circumference of the drum. I used a drum such that the film's
sensitive layer is at a diameter of 6.519". 6.519" x Pi = 20.480"
If you had a shaft encoder with 20480 pulses per revolution, that
would give 1000 pulses per inch. So, I got a 1024 pulse encoder,
and attached a digital phase locked loop chip to multiply the encoder
pulses by 20. It works very well, although several 'experts' told
me it would be a cold day in hell before I ever got a PLL multiplier
to work on a mechanical system.

I have an optical carriage that slides on 2 1/2" hardened and ground
shafts, on 3 Thompson linear bearings. The carriage is moved by
a 5 TPI Kerk leadscrew with plastic anti-backlash nut. It works
quite well, too. I had a cheap ballscrew without antibacklash on
it before, and the positioning was erratic, due to the backlash.
I use a 200 steps/rev stepper motor to move the carriage in
.001" increments.

Pictures of this monstrosity are at :
http://ascc.artsci.wustl.edu/~jmelson/photoplot.html

Jon