3/8" Rectangular Tube MIG Question

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I am working on a project with 3x5x3/8" steel tube.  Got all my pieces cut and did some test runs with my Millermatic 212 (not autoset),  and am struggling to get good penetration.  Here are the specifics.1) Butt Joint.  I have beveled each edge at ~ 45degree angle with about 1/8 inch remaining at face of joint (believe it's called root face)2) Mix Gas, 25cfh.  Tried using both 5-55 (recommended on machine) and 6-50 setting on machine.Bead looks fine but when I look in the inside of the tube I don't see good penetration. Basically can still see the original edges at the joint.  When I look inside tube right after I stop welding the metal is glowing red.  I tried gapping the joint by 1/16inch prior to welding, but that really didn't help.Any advice?Thanks
Reply:1/16" is probably too small a gap. When the students at the tech school do thier 3/8" bevel butts, fit up is usually the biggest reason they fail to get good penetration. We use a steeper bevel, 1/8" land 3/32" gap, and have them do root, fill and cap passes with mig. If they don't get a 3/32" gap, usually the gap closes up and then they fail to get good penetration. 1/8" is usually too much for most to bridge and not burn thru and make holes. The other issue a few have is simply moving too fast. They worry too much about burning thru and try to rush the weld. Even with the right prep, you can't get good penetration if you go too fast.Since your set up is very similar to ours, my guess about your issue would start with the 1/16" gap. The wider angle would mean you need to simply lay in more metal..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:My concern with running any gap at all is that when the weld cools it will shrink and close the gap which would change the relative angle of one piece to the next.  I am building an English wheel frame and on the few test pieces I did even the tack welds caused shifts in the angles I set.I did try 1/8in gap on the test pieces and things got ugly fast.
Reply:That's a completely different issue.  Usually I suggest the students set the 1st side gap at about 1/8" because it will shrink when they tack it. By doing this they usually can get it to shrink to 3/32" after a few attempts. ( It drives them nuts that I can do these by eye and get them super close without even using the gage because I've done so many for demo by that point.)  Thats' with only one plane. With 4 to deal with it gets more complicated. You will still have shrinkage issues even with zero gap.If this was me, I'd set my gap and then do heavy tacks on all 4 corners, starting with an otside corner and then going across to the opposite inside corner, followed by the other inside corner and then the final outside. Check the thing as you go. You can always cut and regrind the tacks if it's way out before you finish weld. Then I'd weld one side followed by the other, then the inside and then the outside. Again checking as you go. I'd proably do multipass welds rather than one heavy one in this case, to limit the amount of heat to help combat distortion.Getting all the welds to shrink up so you end up with the exact angle isn't easy at all. It's usually the mark of an expert that they can get things like this right the 1st time perfectly. ( That or lots of luck ) A good deal comes from experience, especially if you do the same thing over and over again.All this may still be for not. Last year I took great pains to try and keep 2 shafts lined up perfectly on my plow. I got every weld done but one and each time the shaft slid freely with no issues. The final pass on the last side however warped the top bushing enough so it was to tight to move the shaft. I probably should have just cut it out and started over, but I rushed it and tried to compensate with an extra bead, and junked the whole top bushing and had to start over.I believe many set the top of the english wheel with bolts so there is some adjustment available to compensate for weld issues and angles. I seem to remember a poster here who did something like that on theirs.Last edited by DSW; 09-04-2011 at 06:27 PM..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Thanks for the tips.  Yes, both the top wheel and bottom wheels have adjusters, so their is some forgiveness.  Any opinion on the machine setting?  Would you go with recommended 5 power or move it to 6?
Reply:to add to what dsw stated, bracing is helpful.  it won't stop warpage, but it counters it some.  as far as the gap is concerned, I can't speak to what dsw is saying as my machine (esab 250) will punch through so I typically keep a smaller gap.  I also will run a stringer root and then come back with a fill and a cap.  when you run three passes you will limit the amount of warpage.  one large weld will pull the titanic out of the deep as it shrinks.  also with too large of a weld you have the tendency to cold lap or lay filler metal on top of your weld puddle and not burn it into your parent metal.  I know these machine say they can weld 3/8 and 1/2 inch in one pass, but in reality it isn't always the smart thing to do.
Reply:I can't say for sure on the machine settings. Your machine is roughly similar to my MM185, give or take a few more top amps on your machine, and different than the XMT 304's we use at the tech school for mig. I'd need to run a few test pieces to get a feel on how it's welding. Off the top of my head I'd probably opt for the #5 tap on mine rather than the #6 simply to keep the heat down and not lay in so much metal. I might even drop down to the #4 tap possibly to reduce farther the amount of metal deposited on the root pass. I'd set up and run a 1/2 dozen tests 1st to see what worked best. I'm using .030 wire in my machine however most of the time vs the .035 wire the school uses simply because I occasionally need to do thinner steel with my mig. If I had a ton of 3/8" to do I might toss a roll of .035 in the machine and run a few tests with that as well. I won't have any chance to run any beads with either machine until the end ofthe month at the earliest, because of a trip I'm taking for two weeks..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:If you're only doing one of these splices or time isn't a big issue you could cut some 1/8th" backing plates and tack them in there. Gap wouldn't be such a big problem then and you could tack it straight and give it heck. Tack big at the corners and grind those out as you get to them. Pulling is going to be a issue no matter what. Clamp and clamp some more."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
Reply:Thanks guys.  Should have specified I was using 0.035 in my original question.  I am gonna give it a few more test runs today and then weld it up.  I will try with 1/8 in gap to start, hoping that once cool it will land at 3/32.  Sandy, if I clamp the heck out of it will the gap shrink upon cooling?  If I clamp it 'loose' then after the first tack the gap at the opposite side will have grown beyond 1/8 inch.
Reply:title says (3/8 inch tube) ? so its thin tubing correct? and advise is talking 3/8 " plate ---or am i reading it all wrong ???idealarc 250/250 ac-dc tigidealarc 250/250 ac-dc tig #2 used for sticklincoln sp100hh125dual arbor grinder polisher30 yrs of hand tools52 pitch blocks 6p-26prake gauge -pitch gaugeG&D prop repair 918-207-6938Hulbert,okla 74441
Reply:dimensions of tube are 3" x 5" x 3/8"....it is thick walled.
Reply:ok thanksidealarc 250/250 ac-dc tigidealarc 250/250 ac-dc tig #2 used for sticklincoln sp100hh125dual arbor grinder polisher30 yrs of hand tools52 pitch blocks 6p-26prake gauge -pitch gaugeG&D prop repair 918-207-6938Hulbert,okla 74441
Reply:Originally Posted by r2784cThanks guys.  Should have specified I was using 0.035 in my original question.  I am gonna give it a few more test runs today and then weld it up.  I will try with 1/8 in gap to start, hoping that once cool it will land at 3/32.  Sandy, if I clamp the heck out of it will the gap shrink upon cooling?  If I clamp it 'loose' then after the first tack the gap at the opposite side will have grown beyond 1/8 inch.
Reply:Preheat the joint before welding. Should really help with penetration.