I'm constructing a gantry which has height-adjustable legs made from telescoping square steel tubing. The problem of how to make square steel tube slide inside another without binding or rattling excessively has been around for a long time, and this was my approach to deal with it. There are two square steel tubes in question - the outer one and the inner. The outer tube is4x4x.180" tube, and the inner is 3-1/2x3-1/2x.180" tube. If you do a little arithmetic, you see that the inner dimension of the outer tube is 3.64" which leaves .140" of slop. Also, the outer tube has a weld seam down the center of one side, which seems to be about .060" tall.
My approach was to cut sheet metal shims to fit the inside flats of the tube. On the weld seam side I used two shims which go on either side of the weld. The inner tube slides against these shims, not against the weld.
For my parts I used 13 ga. sheet metal, not because it was optimal but because it was what was available. This leaves a gap of about .05" which isn't optimal but it's a whole lot better than .140"! If I'd had 16 ga. I would have used shims on both sides.
I drilled the outer tubes 5/16" then deburred the inside of the holes. Then I clamped the shims inside and plug welded them from the outside. The picture shows the shims inside the tube. If you look carefully you can see the heat affected zone below the plug welds.
This was admittedly a little tedious, but it worked well. I used an internal countersink tool from Noga in conjunction with a file to do the deburring.
The picture isNext problem is how to paint the inside of the tubes, the longest of which are about 30".