Making square tubing telescope

[Dusty]

For a project I'm working on I need to make a few telescoping legs. The leg would be made of two pieces (1 inner, 1 outer tube). A piece of threaded rod will go through the center to move the tubes. To keep the leg from spinning I decided to go with square tubing. I found some 3/4 in Al tubing for the outer tube, and 1/2 in. steel. Does anyone have any suggestions for truing up the inside of the Al tube? The longest this piece would be is 1 ft. The first thing that came to mind was a broach. This would be an expensive option. A machined bar with sandpaper adhered to it may also work (use it similar to a lap). What are your opinions/ideas? I'm sorry if someone has already asked a similar question.

Dusty

[metal mite]

I would trim the inside piece to fit the outside part.
mite

[SGW]

The inner surface is that rough, you need to smooth it out?

Not knowing exactly what the state of the inner surface is, here's one idea. I might be inclined to make a square block of a suitably snug-push fit in the tubing and push it through to see if that would iron out the imperfections.

[darryl]

This is sort of a poor man's broach. Make a short length of square steel bar for a close fit to the tubing, probably with about 3 thou of play. Drill and tap a hole for a threaded rod in the end. Now make a 'chip' out of hacksaw blade, either with a hole in the center, or a slot to center, wide enough so it will slip over the threaded rod. A nut on the rod will let you tighten this blade piece up against the end of the length of square steel bar. Careful positioning will give you an edge that will skim metal as you pull the threaded rod through the tubing. Rotate this tool, and repeat, to trim all inner sides of the tubing. Readjust and sharpen the blade piece as needed. This piece should be made the width on the inner tubing, minus a few thou, and cuts from only one edge. A trick to positioning this piece on the holder while tightening it down, is to stick some scotch tape to one side of the square length, then place this side downwards on a flat surface. Then have the blade piece rest against the surface as well. Do the tightening. Strip the scotch tape, now the cutting edge sticks out from the side of the holder uniformly by about 3 thou. Use cigarette paper as a shim instead of the tape, if a lesser bite is wanted, or some shim stock. Ideally, the blade piece should be somewhere along the length of the holder, with that side of the holder relieved somewhat for clearance for chips, or shavings in this case. It's harder to position and hold the blade there, or maybe I just can't think of how to do it. Cut a crosswise slot to insert the blade into, then adjust the blade with two setscrews from the other side, perhaps? Just another idea for you to consider. Whatever you do, make sure the shavings won't be jamming the tool holder.
Actually, in hindsight, I realise that sq al tubing is pretty good inside already. It might be better to leave the inner leg just enough smaller so that it can have some freezer tape wrapped around it, and then be a close fit. Put one wrap of tape at the top of the inner leg, and one wrap, or four individual pieces, inside the outer tube at the bottom. Telescope the inner leg in from the top. That will let the inner leg run up and down without having metal to metal contact, and the freezer tape has a fairly slick surface. If you can find some stick-on teflon tape, so much the better. Hmm, replaceable wear surfaces.

[This message has been edited by darryl (edited 02-23-2004).]

[Weston Bye]

The machine tool shop where I worked years ago used to do this: The square tube had a weld seam running the length of the inside of one wall. Rather than trying to get rid of the seam, they would mill a channel along the outside of the inside telescoping tube.