MIG Tack Welding on Round Tubing

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Ok, I have to ask this on here...Someone told me that professional chassis builders use a tacking method while MIG welding mild steel round tube chassis.Is this true?  I would not think you could get enough heat or good fusion by just tacking.  Although I have done some tack welds in the past that have been very strong.If this is true, any advice on the proper way to do this would be great!Thanks again for all your help!Mike
Reply:tack it to hold it in place wile you weld it chuck
Reply:You align your frame pieces, you tack, you check your frame alignment, you tack next your your first tack, you check the alignment, you tack next to the tack you tacked next to your first tack, and so on, and so on..... And don't forget: if you're not sure let a "seasoned" welder show you how!
Reply:Originally Posted by Madman StephanAnd don't forget: if you're not sure let a "seasoned" welder show you how!
Reply:Make sure you go to the Hobart school of immaculate welding too.Sorry Mods I couldn't stop myself ... Please don't ban me.
Reply:Humor should never be banned.  If you can't take a joke...right?I was looking at a neat looking go-cart/dune buggy thing being sold at a nearby sporting goods MEGA-STORE.  It looked like they used a bunch of spot welds to do the whole thing.  Not a continuous bead anywhere that I could see.  It looked like tig from a few feet, but each "dime" was recessed like a mig spot weld does, and the last one always had a pit.  It really looked like someone used the spot weld and timer on a millermatic to try to fake a tig weld look.  I don't know if it really matters on a 250 pound cart, but it might on thicker tubing if full penetration is needed, I am not really sure.  A 250 amp mig might be able to spot weld at full penetration on pretty thick tubing.Smithboy...if it ain't broke, you ain't tryin'.
Reply:I'm not sure about a "MIG tacking method" used to weld up round tubing, but I do know that some folks tack things up with MIG and then do the finish welding with TIG which could be considered a "MIG tacking method".-Heath
Reply:I guess one way to find otu if it really works is to tack some parts together and smash it in the 20 ton press.I think if you are using the tack method (if it really exists) you would want to turn up the heat a bit to get more bite for the tacks.Mike
Reply:Mike- I noticed from old posts you have the same machine i have.  I threw you a PM but am interested in how you like the machine in doing round tubing?  I am going to be doing the same thing you are doing-roll-bar for the car.  Thanks for the help beforehand!Nick
Reply:Originally Posted by smithboyI was looking at a neat looking go-cart/dune buggy thing being sold at a nearby sporting goods MEGA-STORE.
Reply:PA you got to rember that most production weldors just weld to make money and have no interest in welding after that so if it can ship there happychuck
Reply:I don't know of ANY professional chassis builders that use mig on tubular chassis. All that I know that do chassis work and design for Formula Mazda, Atlantic and other open-wheel racing all use TIG exclusively. Many of the European chassis are even brazed together.
Reply:MIG welded chassis are pretty common in motorsports applications where mild steel is used.  I know a lot of circle track cars and sand rails are MIG welded.  In applications where 4130 is used, most folks will use TIG, if not for the simple fact that racing bodies usually require it.-Heath
Reply:Something to consider with MIG welding is that the start of the weld has inferior fusion and reduced depth of penetration compared to the rest of the weld.  When the MIG arc is first establised, the base metal is cold, the wire is cold, and the arc takes a second or so to stabilize, so the first bit of weld may have lack of penetration, or even lack of fusion (cold lap).  Additionally, the sometimes violent nature of arc starts can promote porosity.  With TIG on the other hand, you can pause the arc at the weld start to form a good puddle and achieve full penetration before you begin adding wire.I think you can overcome the cold MIG start problem by starting the arc slightly away from the root, then once the arc is well established, move to the root.  Or similarly, start the arc, move back, and then forward over the cold start.Something else to think about, if you are going to use small, short duration, MIG tack welds, is that the rapid cooling rate of these welds can lead to brittleness and cracking.  As carbon and other alloy content of a steel is increased, it's hardenabilty may be increased.  A small MIG weld on a heavy section of this steel is quenched, just as if you dipped the red hot weld in ice water, so it hardens, and the weld or HAZ may crack with the weld stresses.
Reply:I have seen a chassis built in this fashion. The owner was extremly pleased with the look/outcome but I questioned it's strength. The welds looked exactly like the above go-kart referance. The welds were a series of tacks around the tubes giving a false impression of a tig weld, or a very symetrical weld. I wouldn't do it. When I do chassis work I will grind the tacks down to prep for the real weld, tig or mig.Millermatic 251Century 180 migSpectrum 625 Syncrowave 250DX
Reply:Originally Posted by malichOk, I have to ask this on here...Someone told me that professional chassis builders use a tacking method while MIG welding mild steel round tube chassis.Is this true?  I would not think you could get enough heat or good fusion by just tacking.  Although I have done some tack welds in the past that have been very strong.If this is true, any advice on the proper way to do this would be great!Thanks again for all your help!Mike
Reply:I believe the vast (greater than 90%?) of the NEXTEL cup/busch/truck chassis (the tube parts)  are mig welded. Penske is supposedly one of the few that tig weld the whole chassis.Now if you get into the higher classes of NHRA/IHRA drag racing they require chrome moly tubing and tig.Thats not saying that the IRL, FI, etc don't require TIG. I just don't know about those series. Wouldn't surprise me though if its all chrome moly and TIG.Hobbiest hack
Reply:bssically if its chromemoly must be tigged....otherwise I have no ideaIF it Catches...Let it Burn
Reply:Originally Posted by DaveDI believe the vast (greater than 90%?) of the NEXTEL cup/busch/truck chassis (the tube parts)  are mig welded. Penske is supposedly one of the few that tig weld the whole chassis.Now if you get into the higher classes of NHRA/IHRA drag racing they require chrome moly tubing and tig.Thats not saying that the IRL, FI, etc don't require TIG. I just don't know about those series. Wouldn't surprise me though if its all chrome moly and TIG.
Reply:In roundy-round racing they don't use chromoly because it fatigues, I guess it stress hardens or something like that where you weld it. In the upper drag classes their chromoly chassis are junk after one season and they sell them cheap and build another.
Reply:Chrome Molybdenum Steels can be excellently welded with plain ER70S-6 filler, whether with GTAW or GMAW, DEPENDING ON THE SKILL OF THE WELDOR.LOTS of tubular chassis are welded with both.  But it is always the purvue of the sanctioning body to specify if one MUST be used over the other.Keep in mind also, that "tacks" CAN be very deep with lots of penetration, and still allow movement due to their being short.This is not to be confused with "stitch welding."You can also MIG weld good, penetrative beads over MIGged tacks.GTAW is an awesome process, but GMAW can do an awful lot in the right hands with a powerful-enough machine.
Reply:Originally Posted by SandySo does the cage become part of the overall structure and is subject to repeated stresses? That is, it is not just setting there passive waiting for a crash.???
Reply:On a race car cage=chassis.
Reply:Pulser , you always have the right answers.  There is no way that I would sit in any type of competition vehicle if I knew it was welded that way . Like Pulser said, the beginning of every weld is the weakest point. Every time I ever seen welds start to crack it happens at the end. I built approximately 20 Brooksville Roadster chasis's for a hotrod shop when I worked there. Mig  welding the tubing together wasnt an option.  I just dont know why anyone would want to chance it (the spot welding theory anyway).Last edited by JohnnyWeld; 01-08-2006 at 11:29 PM.