Square tube inside of square tube?

[teamtom]

Does anyone have experience with putting a square tube say 3.5" x 3.5" inside of another square tube 4"x4"? I am thinking of doing this to make a sort of telescopic arm but I'm afraid all of the welded bits inside the outer tube would prevent the smaller tube from sliding easily inside it. I'm think of 6' of smaller tube sliding out 3' long. Any help is appreciated.
Thanks,
Tom

[Wyoming]

You can always buy receiver tube, several dimensions available, but it isn't cheap. I have taken the weld 'nipple' out of the inside of square tubing by using a section of all-thread and a piece of 1" plate to fit, but it isn't a project I'd care to ever do again. Might be easier to try that idea using the section of smaller tube for a broach shim and just pull the cutter through the difference between the two, but still not on my list of project anytime in the future. Six inches of tube might not be too bad if you care to give the idea a try. Another method would be to weld up either plate or angle for the outer tube.

[MAC702]

With that length of tubing, it will be a major PITA to get the inside of the welded seam filed down smooth to make it work well.

Look at getting DOM tubing instead of the cheaper normal tubing. This is what receiver tubes are made from as instead of being folded and welded, it is Drawn Over a Mandrel (hence DOM) and doesn't have the inside weld to deal with.

I have done short sections of tubing (up to 1 foot) and filed out the welded seam to make receivers, but it took a long time.

Another possible option is to cut the seam out. You could use a metal-cutting circular saw and just remove the seam with the kerf-width of the blade. For some projects, the groove wouldn't hurt it at all. For other projects, you could replace it structurally with a flat bar welded to the outside of the tube.

[Ferntj]

The tube they sell in hadware stores with cross holes every couple inches on all sides will slide inside another. That stuff is made to tellescope and bolt to length.

[Blacksmith]

Does it have to be square? Look at a "jack post" (or whatever it is called) that is used under the center beam in a basement. It is designed to telescope and may be just about the length you're needing.

[ronnielyons]

One easy thing you can do. We know the seam inside square tubing prevents the smaller tubing from sliding inside the larger piece with a precision fit, right? Take the larger piece and using a micrometer or something similar, measure the distance of the seam inside the one side, sometimes they're not exactly in the center. Example: 4 x 4 tubing, .250 wall. This means the inner dimension of the 4 x 4 is actually 3.5 by 3.5 inches. The seam will most likely measure 2.25 inches to one side instead of 2.0 or dead center.

Now, take your 2.25 inch measurement and transfer that to the outside of the smaller tubing. Make a 2.25 inch mark about every 12 inches all the way down the length of the small tubing. Take your 4 inch disk grinder, using a new wheel, turn it up on edge and following this 2.25 inch mark, grind a shallow groove all the way down the length of 3.5 inch tubing. This groove mates up with the seam inside the larger tubing and allows your two sizes to sleeve together.

[ozarkie]

ronnielyons,

Here is a youtube video showing the method you described for inserting square tubing. I ran across it a couple weeks ago and bookmarked it.

http://www.youtube.com/watch?v=F_IIg...2531BEA38AC107

[Rivet]

When I made a telescoping support arm for a sliding table on my table saw, I put ball bearings at the end of each tube, so the tubes slide very easily. This only works if the load on the sliding tube is always from the same side. The weld seam doesn’t matter because there is clearance between the tubes. The outer tube bearing supports the bottom of the inner tube and the inner tube bearing rides the inner top wall of the outer tube. I think some box beam cranes work this way.

See the photos. [The outer tube is 1-1/2 inch.]

If you don’t want to trouble with bearings, pick tubes that allow generous gap all around and then weld a bar on each side of the end of the inner tube to take up the gap. [Notch one of these bars for the outer tube’s weld seam.] Put 4 removable bars inside the end of the outer tube to take up the gap. Your tubes will only touch each other at their ends without binding in the middle in case there is a curve in either tube. To fine tune the fit you only need to grind the bars and not the entire tube.

[urch55]

Rivet,
The bearings is a great idea.

[Roger]

It is just like a drawer slide.
Does it have a stop to keep slide from accidentally coming out too far? Should be easy to add a stop.