techniques for capping steel tube?

Hello all-
I'm building some fixtures out of rectangular mild steel and would like to find a better way to cap the open ends of the tube.
I've tried cutting down some flat bar and welding a fillet around the edge w/ some 7014, but the results were none too pretty, and needed some effort w/ a grinder to dress them up. I was wondering if trigger welding w/ some S3 wire wouldn't be easier? (no TIG on premises) This joint is strictly cosmetic and doesnt have to be a work of art, but I'm guessing there is an easier way.
Any suggestions are appreciated mucho!
Thanks!
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utahparx wrote:

Cut a short piece of the tube to use for a template and use it to mark your sheet you're cutting the plug from. (inside the tube, with a sharp soapstone). When you cut the plug, hold the torch so that you're cutting at an angle- you're cutting a tapered plug that just fits. Hammer the plug into the tube and weld/grind.
John
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utahparx wrote:

Unless the tube plug needs to be welded steel, look at the thousands of plastic/rubber tubing plugs available from McMaster-Carr. Pop it in with a hammer, paint to match, done.
Stuart
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utahparx wrote:

The nicest looking that I have found is to miter the joints at 45 degrees and there is no hole to cap. It is also more work to get dimensions right.
Bob
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utahparx wrote:

Cut the plug from sheet metal or flat stock so it fits *inside* the tube, leaving quite a liberal gap (1mm or more). Big gap, flat weld, less grinding.
Nick
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"Nick Mueller" wrote: Cut the plug from sheet metal or flat stock so it fits *inside* the tube, leaving quite a liberal gap (1mm or more). Big gap, flat weld, less grinding. ^^^^^^^^^^^^^^^^^ I understand John M's technique (tapered, tight-fitting plug), but could you fill me in? How do you keep your loose-fitting plug from dropping into the tube?
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I have been cutting my plugs out of 10 gauge with my CNC plasma cutter. I then tack a piece of 1/4" rod to the plug, insert it into the tube and weld away. Steve
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I lay the undersize plug on the steel table and place a short piece of flat bar across the plug standing up. I put one small tack joining the plug to the edge of the flat bar. Now it is a simple matter to set the flat bar across the end of the tub and tack the plug to the inside edge of the tube. Once the plug is tacked in two places snap the flat bar off the plug. you can now weld the plug. This works for one inch tubing or even six inch tubing.
Randy

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Now that sounds easy enough even I should be able to swing it!
Thanks to all for the great ideas!
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Leo Lichtman wrote:

Grab one end of the cap with pliers and insert the other end so it sits flush with the tube's end. Tack-weld that end. You just made a one-time hinge. Get a hammer (or the pliers) and hammer the end you've just held with the pliers flush. Tack that end and finish-weld. If you have made a generous gap, the cap gets in easily. With a tight end, you'll have to hammer harder. :-)
Nick
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could
dropping
sits
one-time
held
tight end,

Small magnet across the plug / tube gap works as well, but don't try welding too close to the manget as ferrite or rare earth ones will shatter with the heat (don't ask!!!) Saves putting a slight bend in the cap plug as you hammer it.
AWEM
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Andrew Mawson wrote:

... but then it's bending outwards when it cools. And you need a hammer again.

No comment! ;-)

You don't get a bend. I'm using a water-pump pliers (if you know what that is) and hold it with them. Then a short tack and with the back of the pliers a little smack while the tack is still hot. And flush it is. Also, I'm using something much thicker as cap then the wall's thickness. Something like double or triple the wall thickness of the tube.
Nick
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Leo Lichtman wrote:

Use a rectangular magnet with a straight edge long enough to span the tube. Use that to hold your workpiece until you can get one small tack on the cap, then use screwdrivers or whatever you need to get the cap aligned, then make the second tap. A small gap will be easy to weld up, and you will get much better penetration and the bead will be much flatter, i.e. easier to grind.
GWE
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utahparx wrote:

This seems to come up for me fairly regularly. I cut a piece of flat stock to fit inside the end of the tube (not too tight) and hold it in place using rectangular magnets until I get the first tack on it. Once tacked solidly, and pretty square to the tube axis, then I run an angle grinder's grinding wheel down the seam if it's tight, and just weld it up using hardwire MIG, and knock it off flat with a flap wheel on an angle grinder. As it's all flat, it's very easy to get it smooth and reasonably straight and square. If you needed it more square, at that point you could mill it square if you had to.
They come out looking great.
GWE
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So, it sounds like your fit-up has a pretty generous gap all around before you start welding? Do you make any adjustments like lower voltage or longer stickout? The magnet idea mentioned on a couple of the posts made me do a Homer Simpson style head-slap. Brilliant!
UTP
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It depends, you did not give us the wall thickness of the tube? do you want a flat end with sharp square edges? or do you want the edges softened and with a rounded edge?
IMHE, the object in fab fitting is to design the joint so there is room for the weld puddle to fill and not be forced to sit on top above the surface level. IMHO, the better the weldor and the joint design, the less grinding will be required.
If you want to have sharp square edges then any of the other suggestions for fitting the cap material inside the tube are good but I would suggest you use some combination of bevel and/or gap to give the weld someplace to fill so the surface will be reasonably flat, but without undercut. Using a thicker capping material will make the job easier as this will tend to carry the excess heat away and reduce any tendency to burn through and leave holes. IMHO, this will be the easiest joint to make and if the tubing wall is very thin will be the best.
If the tubing is heavy walled and you want to have a rounded edge, then I would suggest the cap plate be a thickness just slightly smaller than the tube wall and be fitted to sit on top of the tube and be only slightly larger than the tube inside. This is the classic outside corner weld, this joint will require more control of weld shape and the choice of rod size is more critical, smaller rods (3/32-1/8") and lower heat will make it easier to control the puddle shape and prevent the puddle from bulging excessively. The edge of the nicely convex bead should just reach the edges of the tube and the cap plate while still fully filling the corner. If done properly and depending on how smooth you desire the finish to be, this joint should require a minimum of grinding to clean up. IMHO, This joint is more demanding of operator skill, and the best weldors will do it with only minimal cleanup as the weld itself is a demonstration of the weldor's skill.
If the tube wall is thin and you desire rounded corners, then a combination of these joint designs may be best. Here I would suggest using a thick cap plate cut to fit tightly inside the tube and driven inside so that the top of the plate is the same distance above the tube end as is the tube wall which will also result in a classic outside corner weld.
IMHO the worst joint design would be to cut the cap the same size as the outside of the tube and to bevel the tube end or the cap or both and then weld around the outside of the tube. This joint design will be difficult for all but the best weldors to make visually appealing.
Good luck, YMMV
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your point about cap thickness is well made- my first efforts involved a cap that was much thinner than the tube wall and as I made the corner weld, i kept burning the cap away. more material (and possibly a flatter drag angle) would have made it way easier.

thanks for the great post.
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A good source of 'caps' is any shop that has a punching machine.... the machine spits out the bit of steel when it punches the hole, usually into a bucket provided for the purpose. If they happen to do a lot of punching in a steel size you commonly use then these are great, easy and fast. you can usually get them for free.
Shaun

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