Proper sized plug weld holes.

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What size plug holes should I drill on 1.5" OD x .250" Wall DOM round tubing. Im welding 3/4" in threaded bungs. I drilled four 1/4" holes. Is this enough or should I go bigger? I stayed 1/4" because I dont wanna put too much heat in the bung and warp the threads.
Reply:What process are you using?  It's going to be very difficult to weld in the bottom of a 1/4 diameter hole 1/4 deep.  With TIG or MIG, the minimum size I would use is 3/8.  I'd also chamfer the holes, leaving about a 1/16 land.  You'll have to decide if that provides enough strength for the application.Is there a reason you can't run a tap through the threads after welding?Dynasty 300DXSmith He/Ar gas mixerMM350PHobart Handler 120Smith LW7, MW5, AW1A
Reply:Take everything that follows with a grain of salt, see your PE, lawyer, doctor before taking any of my advise)I know there are some places that claim some advantage by making plug welds.  I also know that some tube inserts aren't big enough to plug weld anyway....Bottom line, I would think you risk more damage than any particular advantage and the extra time involved really makes it not worth it....The strength of a plug weld has got to be on the order of the strength of a rivet. Which, according to dusty text books is equal to the hole size times thickness of the material times material stress limits.   so .25 hole on .25 material means .0625 * whatever as the strength.   If this is a suspension component - allowable stresses have to be around 6kpsi (mild steel) so the 1/4" plug weld is good for 375lbs.   Yeah, not much.(.25 x .25 x 36Ksi / 6)  - factor of safety 6.   That BTW is 375lbs each.  If you were on a machine testing I would expect ultimate failure of a plug weld at about 3750 lbs.Assuming a good weld all the way around the head  of this tube, the weak link should be the tube itself.  The cross sectional area of the tube is roughly 1 square inch.  So the tube itself (derated) should be good for about 6000lbs.Tube cross sectional area (Pi / 4 x (1.5 x 1.5 - 1.0 x 1.0)) = .9817 sq in.I'm assuming 36ksi _yield_ strength and a factor of safety of 6x (3x if the part is isolated from axles - 6x for full shock load).  Factor of safety and actual yield strength don't matter for this discussion, they just put it in real terms.One advantage that rivets have that I'm not so sure plug welds have is that they squeeze the materials together.  If when the plug weld cools the material is squeezed so that their is a friction fit between tube and bung the advantage to plug welding goes up (potentially way up) but the note that my text book had was that its an unreliable force and for the sake of calculations and is therefor ignored.So.... considering the thermal stresses etc., I suspect you will probably do more harm than good.There might be an argument to be made that plug welds can break up the stresses.  Like a fish plate.  But, I'm assuming you are using proper rod ends on both sides and all the stresses on the tube will be along the tube (either trying to collapse the tube or stretch it out.) I'd worry more about thread engagement.  Rough rule of thumb, greater than thread diameter.  Might as well make it a full 1"
Reply:With a 1/4 inch hole 1/4 inch deep you'll be lucky to get any fusion to the insert at all. That hole is going to fill fast. Regardless of the plug weld hole size, in order to get any decent fusion you have to put heat into the insert. When you put heat into the insert you're going to get rapid expansion from the rapid-spot heat. That will mean a slight bulge or swelling of the inside at those weld spots. Run a tap through it when your done."The things that will destroy America are prosperity at any price, peace at any price, safety first instead of duty first, the love of soft living and the get rich quick theory of life." -Theodore Roosevelt