THANKS! For the many ideas and perspectives.

-- I will check with my supplier about 'telescoping' tubing.

-- The different high-density bearing materials look very
interesting. And some can be 'machined like wood' making
adjustable-clearance approaches more reasonable.

-- I will put a dial indicator inside some 4 inch tubing I
have to see the variation in size over a couple of feet.

What if: UHMW or similar was used as sliding bearing material,
grooved to fit over the 'flash' on one internal side.

What if: that side and another side adjacent to it had
fixed-to-the-outer-tube UHMW, and the other two sides had
large areas of UHMW backed up with, um, 1/8" plate,
with clearance adjustment screws thru
tapped holes on those two adjacent sides. The total raw clearance
would have to be reasonable for all that UHMW and backing plates.

What if: 'almost all' the clearance was adjusted out, pushing the inner
tube 'into the corner' with the two fixed UHMW bearings. I don't need
really low friction: probably 20 pounds would be 'pulling down' and a
strong leadscrew jack pushing up. I could 'antibacklash' like I always
do on my radial arm saw, if necessary.

This is not intended to smoothly move a cutting tool, but rather to
position and reposition the cutting tool which has it's own degree
of freedom for the cut. I need 'woodworking' accuracy here, not machine-
tool accuracy. Maybe say .016 to .032 repeatability of position would be
workable, .005 Would Be Nice.

I would appreciate your comments or suggestions on the above.

OK, this is starting to make more sense. I know some pitfalls I might
have missed. I know some new materials and ideas. I have some stuff to
research more. I hope to get closer to an approach I'll try
Real Soon Now. I'll keep in touch here.

Everyone's suggestions are appreciated.

--
Regards, Terry King ...In The Woods In Vermont

The one who Dies With The Most Parts LOSES!! What do you need?