Terry King wrote in message ...
Ok, I'm designing another machine. This is for making
accurate, repeatable cuts on / into large wood beams,
therefore posted to both groups...

I need to build a telescoping vertical elevation section,
much like some Radial Arm Saws have. I want to use standard
steel-supplier square tubing, like 4 to 5 inches, probably
.125 wall or, so. There will be a threaded rod actuator,
possibly based on a trailer jack.

This is a one-off at this point, although I'd like to be
able to document how to make one. IF it works well. So I need
easy-to-get parts. The question:

** What is a good approach to providing bearings for a
sliding, telescoping mechanism with square steel tubing??**

I don't THINK I need antifriction (ball or roller) bearings.
There won't be high-speed or continuous motion.

So, I see something like this:

Two sizes of square steel tubing about 5 inches or so
that telescope with a clearance of, um, .25 inches to .5 inches.
The wall of the outer tube has cutouts of some size and
shape which hold 'things' that reach through and 'bear'
against the inner tube. There probably need to be 'lots'
of these, say 2 per side x 4 = 8 at each 'level' of
contact. Maybe there are only two levels of contact.

What are your thoughts? What is used in existing commercial
designs?

My only thought so far is nylon (other plastic?) bolts in
threaded holes in the outer tube, hex heads inside, jamb
nut on the outside. Adjust for just a little clearance.

Any suggestions or pointers appreciated.

Tell your metal supplier that you want "Telescopic Tubing". It may
cast a little more but it will save you a load of work. -Steve