Maximum gap size

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To TIG weld chrome molly (sp?) 1.75 x .125 tube, what is the MAXIMUM gap for a "very good" weld / penetration?
Reply:Ned,I'm no expert, so please take my advice with a grain of salt. If it's a critical job where someone might die, you might want to call in a pro.Anyway, if I had to do the same job, and it was a weld that had to count I'd do this:1. Make sure ends of tube to be joined are straight and square. If possible cut them on a lathe or an accurate cold cut saw.2. Clean the area at least a 1/2 inch from the area to be welded, removing all oxides, surfaces finishes etc - so it's bright shiney steel. Do this on both sides! (ie. inside and out)3. I'd go for a 30 degree bevel with a 1.6mm (1/16 inch) land and 1.6mm gap. 4. Clean the area to be welded with acetone. I'd also wipe down the filler rods as well. You didn't state what the exact tube material or application was, so I'll leave rod selection up to you - but I'd go 1.6 (1/16 dia) - personal preferance. Remember to also clean the inside of the tube.4. Heat with a hair drier to remove any residual moisture in the metal. This is not 100% necessary, but if was me I'd do it.5. If possible back purge the tube with 100% argon. Use a separate flow meter for this. I have a separate bottle  just for back purging.6. Position the tube ends with an alignment fixture and check alginment with a 12 inch steel ruler. If alginment is more critical use a more accurate straight edge.7. I'd space it using two bits of 1.6mm filler rod and tack it in 4 places (remebering to tack apposing sides), then remove the spacers and weld using a 3/32 tungsten. Weld all the way around and over lap by about 1/2 inch once you get back to the starting point.If possible I'd welding it in a rotary welding fixture. I have an old lathe with a VFD, that I kept just for doing welds on pipe.If it's a real critical weld, get it X-rayed.Hope this helps.MikeLast edited by AAA; 03-01-2004 at 05:56 AM.
Reply:Wow...umm, err...It is a roll cage for my Jeep.  I tacked it together (a pre cut kit) and it is at a welder's now.  I added some tube to the kit and what I added was formed with an angle grinder...with, umm...kinda a poor fit.  The kit components are tight but my stuff isn't.  I was toying with the idea of asking the shop to remove my stuff and get a better fit before welding.I doubt the shop will do the things you have suggested...should I insist on having those things done for my roll cage?Thanks for all the great info!  I'll be back to ask my newbie mm175 questions!Ned
Reply:Ned, you sure that material is chromoly?  I'm not familiar with a kit for jeeps that is made of chomoly.  Generally, these things are add on's to the oem hoop.  I used a kentrol kit in my CJ7 and it was simply mild steel tubing.  We mig welded it together.  Minor gaps like 1/16-1/8 " were not a problem at all with mig.  I also added gussets at the corners.  Perhaps that's overkill, but my cage, belts and seat attachments are all stout and part of the cage, which is now attached to the frame by outriggers welded to the frame.
Reply:Ned,The specs I gave you were for pipe/tube welding - in a straight line.I didn't realise it was a roll cage. The wall thickness sounds rather thick for a chromemoly tube, so i suspect it may be a mild steel pipe. Thick wall chromemoly tube is real pricey.I've built a few roll cages and all were 1.6 mm (1/16) wall 4130 tube, which seems to be about the minimum thickness used here in Australia.For that I wouldn't use a gap at all, as I'm confident I can easily get full penetration. If I used a gap it wouldn't be more than 1mm.Fit up is real important to avoid excessive stress. You should use mild steel gussets where needed for addition joint strength.The filler rod of choice is ER80S-D2. This is actually a mild steel rod. The idea is to allow elongation in the joint and allow the weld to flex rather than crack. Back purge is used if possible.In the US much thinner tubes seemed to be used. You don't see much of that here except on aircraft frames as the stuff is damn hard to find and pricey.I've used thicker wall tube, but that was for heavy duty off road apps. What's the intended app for you Jeep?As for "should I insist on having those things done for my roll cage?" - well that depends on your intended app. For mild recreational use you should be fine, but if it's something you may have to hedge your life against I'd want it to be done as well as possible. Most of the cleaning procedures are standard for TIG.Being that the US is litigation city, I'd at least want the guy doing the welding to be certified to at least AWS D1.1, this is the American Welding Society's structural welding code for steel just to cover your ***.Mind you, certification ain't everything, I've seen welds here on CAGES that have been signed off by CAMS (Confederation of Australian Motor Sport), that i couldn't even put my name to, let alone be happy with on a roll cage, so maybe I'm just too fussy!Hope this helps.Mike
Reply:Mike,Thanks for all your information....I don't think half of it was done on my 10 hour $600 job (I'm still in shock) but I will remember your suggestions once I get the nerve to start welding more.  I'm gonna rattle can finish the cage to save some money.  I assume acetone and a scotchbright pad would be good metal prep?$600...MAN...I don't know what to say.
Reply:Wow, for 600 bucks those welds should be so bright that you have to wear shades.Common sense in an uncommon degree is what the world calls wisdom.
Reply:Did you ask what their hourly rate was beforehand? or get a quote?$60 an hour is probably a good average for hourly rates across the country. We charge more depending on what state we are working in, and a lot more on tig, mainly to cover the argon cost.Do you want to pay the plumber that hooks up the toilet more than the guy that tig's the roll cage on your heap?regards,JTMcC.
Reply:1/2 of me says, yes this is money well spent for the right welds and for the safety of me and my family1/2 of me says DAMN....that's a lot of money.  Gotta get signed up for those welding classes!
Reply:Yea, when you turn your Jeep upside down you can figure you got your moneys worth seein how even a minor trip to the emergency room will be way over $600   : )JTMcC.
Reply:At least it always is for me! : )JTMcC.
Reply:Ned,Don't suppose you've got any pics of the job that was done.10 hours seems a lot of labour. Was it a cage inside a vehicle that required removal of trim and such in order to weld inside the car - or is it a convertible?Acetone and scotchbrite should be fine for metal prep for painting. If it's exposed to elements, use some sort of etch primer first then paint with a UV stable spraypaint designed for auto use.MikeLast edited by AAA; 03-04-2004 at 04:55 AM.
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Reply:Ned,Looks nice in the pic, but it's a bit hard to see the welds from that distance. Paint came out looking great.Mike
Reply:yeah, its pretty.  I don't think you'd be overly thrilled with the welds...but I think they would be in the "good" category.  It hasn't been the best experience but it's done.I want to get better so I can do it!On a side note, I made a bracket to lift the hard top off by welding some "L" together.  As I lifted the top, the hoist kinds stopped abruptly and one of the welds gave way....now I see the importance of cleaning the metal before welding....and what penetration of weld is (or lack of penetration!)On my next weld project I'll take some pictures.Ned