welding telescoping tubing?

admin

I have two tubes that I need to weld together.The smaller diameter tube is a loose fit inside the larger one, with a gap of around 1/16" to 3/32".I should mention at this point that the item that I am welding is a model, and thus the weld is not in the least bit critical.The 'shoulder' where the two tubes meet is important to the design, so I need to avoid welding a seam around the circumference of the larger tube.I am therefore planning to drill a series of holes around the tube about 1" from the end, and plug weld the smaller tube inside.I then plan to grind the plug welds flush to blend them into the surface of the outer tube.The problem is the loose fit of the tubes.Because the tubes do not fit together tightly, I am concerned that my plug welds will not be successful.I also have the problem of ensuring that the tubes are concentric before welding them together.I can probably take care of the latter with some temporary shims, but the plug welds have me stumped.Is it possible to build up enough material that the welds will be able to 'jump the gap' between the tubes?I have been turning this over in my mind all day, and I haven't come up with a viable solution.Any ideas?Last edited by jamesbeat; 08-16-2015 at 12:23 AM.
Reply:Use some scrap pieces to experiment with.  Make a "wrap-around" shim out of some sheet metal that will make for a snug fit between the two tubes.  Drill a suitable hole in the outer tube, make your plug weld through that.  It should burn through the sheet metal wrap-around and into the inner tube to weld the two tubes together.  How thick are your tubes?
Reply:Wrap two stripes of electrical tape one at the end of the smaller tube then another about 1" below where the plug welds will be. Just enough for a slip fit. you should be far enough away from the tape not to melt it too much but it will hold your tubes centered.  Just leave the tape because you should not see it once done.  They make a fiber glass electrical tape that would take the heat if needed.Dan D.Manipulator Of Metal
Reply:Sorry, I should have included the tube wall thicknesses.The inner tube is black pipe, with a wall thickness of around 1/8", and the outer tube is EMT conduit with around a 1/16" wall.That's why I'm a little concerned about building up the weld to fill the gap. I'm worried that by the time I have put enough metal in to bridge the gap, I will have put too much heat into the outer tube.The wrap-around shim idea seems to be the best method for this situation. I'll give it a go on some scrap.Incidentally, I have stripped the zinc from the EMT with muriatic acid.I had metal fume fever once, and I don't intend to repeat the experience....
Reply:Use screws to align Concentrically.
Reply:You could even wrap a 1" strip of sheet metal around the black pipe. (clean it well first )  Then assemble the two pipes. Then just take a hacksaw or grinder and cut 3 small slots equally spaced around the outer pipe so it cuts through the strip of sheet metal and contacts the black pipe. At that point...braze or mig or even stick welding would work. The key is lo just find the right thickness of sheet metal. Those Simpson Strong ties at Lowes or Home depot may be just the right thickness.FWIW...if you get just the right amount of shim...Solder would even work for that if this is just a model. Butane torch and copper pipe solder with the flux would flow onto the shim and fill the gaps.Last edited by Tim9; 08-16-2015 at 11:07 AM.
Reply:jamesbeatThis is what I have done for near perfect concentricity.Get rid of the EMT and replace with uncoated material.Weld dimple rows [120 degrees apart] on the interior tube.Shave the dimples in a lathe for a tap-in fit - and plug weld.Opus
Reply:All suggestions seem like they would work, thanks guys.I think I'm going to go with Opus' suggestion above, as I don't have any shim material handy, but I do have a lathe.Opus, as I explained above, the EMT is uncoated bare steel.It came galvanized and with a plastic coating on the inside, but I stripped these with muriatic acid and acetone respectively.
Reply:Well, it turns out that I was over thinking the problem.When I drilled the holes for the plug welds in the outer tube, the drill bit raised large burrs in the soft steel.When I inserted the smaller tube, the burrs prevented it from sliding in and I had to use a hammer to tap it into place.The burrs held the inner tube very tight, and because the holes were equally spaced, the inner tube was held concentrically enough for my purposes.The burrs supported the inner tube exactly where I wanted it while I welded over the holes.Also, my fears about the slight gap between the tubes was unfounded, at least for this application.I did get a slight burn through at one point because I was having to put so much heat in to deposit enough filler, but I was able to fill the hole by going back over it after everything cooled down.Thanks for all the advice guys.Even though I didn't end up using any of the ideas, I still learned plenty, and I'm sure this thread will help someone else in the future.
Reply:Originally Posted by OPUS FERROjamesbeatThis is what I have done for near perfect concentricity.Get rid of the EMT and replace with uncoated material.Weld dimple rows [120 degrees apart] on the interior tube.Shave the dimples in a lathe for a tap-in fit - and plug weld.Opus
Reply:Originally Posted by OPUS FERROjamesbeatThis is what I have done for near perfect concentricity.Get rid of the EMT and replace with uncoated material.Weld dimple rows [120 degrees apart] on the interior tube.Shave the dimples in a lathe for a tap-in fit - and plug weld.Opus