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  #21  
Old 05-10-2017, 10:44 AM
tjratz tjratz is offline
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Well, I'm going to have to try something different now for sure. Had one more little high spot that I tried to tap down just lightly when I noticed a crack next to the MIG weld. So I took the filler off and found it was much worse than first thought. I believe I'm going to have to cut out the little patch I put in to repair rust next to the headlight ring and put in a bigger one. The metal is thin in spots and I know I've over worked it, either over stretched or over shrunk it. Ill get pic(s) when I get home. At this point I'm starting to think why bother any more..

Thanks for everything
TJ
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  #22  
Old 05-10-2017, 12:21 PM
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TJ, excuse me if I'm too forward here, but this sounds to be more of a lack of weld penetration issue or you still have too much rust in the panel issue. So I'm going to back up to square one here. First off, TIG or OA, IF DONE PROPERLY, would give you a better weld seam, easier to work than MIG. Done improperly, either is a crap shoot. Since we are discussing Mig welding, I'll share what I do in using a MIG.

Don't take any of this as picking on your work, as Lord knows I've had my share of mud daubing and other defects in trying to find a suitable method for welding sheet metal using the MIG.. But we need to discuss some things to get to the root of the problem in order to find solution.

Without seeing what your welds look like, and strictly going by your verbal descriptions, I can only surmise that your welds either have insufficient weld penetration or you still have rust surrounding your weld seam that is weakening the area. Unfortunately we can't tell the true results of the weld penetration without looking at the back side. Without any pictures of your welds to critique, lets start from the beginning.

If you have your welder set up for XX heat range to accomodate your (18? gauge) sheet metal, if we did a short stitch weld, your weld penetration is going to vary as you travel across and the panel starts to heat up. So this may show lack of penetration at the start, to better penetration toward the end of your weld pass. Next, if we were trying to continue a weld onto an existing weld poses an issue that you no longer have a requirement for XX heat range, as the previous weld could have changed the panel thickness to as much as 3 or 4 times the thickness at the weld. This in essence changes our heat requirement to a greater amount to properly fuse to the end of that previous weld bead, but the continuation of our weld pass puts us quickly into thin metal again, where we may risk blowing holes.

Through much trial and error, I have changed my MIG welding of sheet metal to a one-dot-at-a time scenario, to overcome these very issues.
Let's say we are installing a lower door skin to repair some rust. The repair panel has been trimmed for zero gap, and the panel is tacked on. We need to insure our heat setting is correct for a full penetration weld DOT, as verified by looking at the back of the panel. (I normally do a test setup on scraps the same thickness). Then, starting your weld tacking of the panel at one end, and work progressively toward the other. Don't skip around from one end to the other as you have a greater chance of misalignment in that you may have more metal on one side than the other, which will result in a buckle from the excess metal to one side of the weld. Start at one end, tack, move an inch or two, align the panels together, make another tack, repeat. This insures the panels are correctly aligned as you work progressively from one side to the other. Be sure to keep your weld tack spacing as consistent a distance as possible.



Once done with the last tack, go back to the beginning and use a hammer and dolly to planish each weld dot, working in the same start to finish pattern. Now go back to the beginning and use the 3" cutoff wheel to grind the weld dots down to just above flush, both FRONT AND BACK, as this gets the weld prouds out of the way for planishing the next sets of weld dots, and also removes all the excess weld that is acting as a heat sink/adding requirement to change heat setting. I grind to just above panel height for a weld seam like this, and final cleanup with a 3" roloc sander will be used at the end to dress the seam to the parent metal. Here's a video that shows the grinding process, but as this is a plug weld it is dressed immediately following.

https://www.youtube.com/watch?v=V2WHT_zMOE8

Now that those welds are down and out of the way, let's add the next set. Here I'll overlap the last set of welds by about 1/3 to 1/2. Whatever the distance of your overlap, keep it consistent throughout.



This method helps to eliminate any missed areas like what may occur if you kept skipping around. Again, start from the same spot you did before, overlap the first weld dot done, go the the next, overlap, repeat, until you get to the end. Then go back and repeat the planish from start to finish, then grind weld dots from start to finish, both FRONT AND BACK, and then repeat the overlap process again. Keep repeating the process until the weld seam has been finished, then use a roloc sander to dress the little bit of remaining weld to flush with parent metal, both front and back side.

If this sounds like a slow, monotonous process, that is the intent. This promotes consistency in the welding process, from fitup through to final welds dressed. At no point is it necessary to cool the welds, by the time planishing and grinding are complete, the welds on the panel are plenty cool to the touch. Any artificial cooling serves no purpose.

Planishing per Ron Covell is to flatten or straighten out a bumpy panel (if I remember that correctly). As it applies to metal shaping, one would use a blocking hammer and a shot bag to add shape to a panel, but once complete it would appear as if it had walnuts all across the surface of the metal. A planishing operation would smooth these out to a more consistent "flat" finish (where flat means the panel has some crown to it, but no more lumps). As it applies to welds, planishing is the use of a hammer and dolly (or other appropriate devices as the job dictates) to add some stretch back into the weld and HAZ for the purpose of negating any of the shrink that has occurred. I would add that seldom will you get it perfect the first time as you go through the weld process as described above. You will always have a bit of planishing to complete after the welds have been dressed out, or even some shrinking if you got too carried away in spots.
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Last edited by MP&C; 05-10-2017 at 12:34 PM.
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  #23  
Old 05-10-2017, 12:24 PM
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Just to show some actual pics of weld penetration....



First set of tacks...





Weld penetration, the back side....





Weld, planish, grind, overlap, repeat... Again, grinding after each weld dot gets us back to the original panel thickness (as close as we can) so that our welder heat setting is correct for said thickness (heat load) throughout.





Planishing as you go helps to keep the panel's shape in check...



Still needs a bit of bumping but not too bad overall..





By insuring you have full penetration welds like those shown above, you should be able to perform the weld AND planishing operations with no cracking effects as you are seeing now. AGAIN, If this seems a bit slow, monotonous, labor intensive, well it is, but it helps to maintain consistency throughout the process of MIG welding sheet metal for consistent results, including good welds with full weld penetration. The hope is that any novice can take a point and shoot of the MIG and be able to get accurate, repeatable results. The side effect, and desired result, is that you will see exactly how much work goes into welding sheet metal with consistency using the MIG, where you will opt for a TIG or OA process that should give you better control of input heat that you can revert to a full weld pass from one end to the other (after tacking in place, of course) using TIG or OA and have better results with MUCH LESS cleanup afterward
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Last edited by MP&C; 05-10-2017 at 12:31 PM.
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  #24  
Old 05-10-2017, 01:04 PM
zekeymonkey zekeymonkey is offline
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Robert,

Thanks for posting this reply. This was a great write-up and anyone welding sheet metal with mig should read it.
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  #25  
Old 05-10-2017, 01:08 PM
tjratz tjratz is offline
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starting your weld tacking of the panel at one end, and work progressively toward the other. Don't skip around from one end to the other as you have a greater chance of misalignment in that you may have more metal on one side than the other, which will result in a buckle from the excess metal to one side of the weld.

I, also do the one dot at a time method but I move around from side to side instead of starting on end and moving straight through to the other side. If I understand correctly, when I jump around tack welding in a patch and end up with extra metal, That's where my high spots and waves are coming from, correct? I was also taught to have a little gap, about a razor blade thickness for somewhere for the weld to go. I'm pretty sure you've solved my problems, not my crappy welding but the process in how I will do patch repair from now on..

Thank you Robert. Im going to try this process today when I get home. One question, how important is grinding the back side" What I mean is, what if you cant a grinder back there? Then what do you do,,file?
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Last edited by tjratz; 05-10-2017 at 01:18 PM.
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  #26  
Old 05-10-2017, 01:56 PM
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In order to planish, you'll need access from the back side for a dolly or hammer or slapper or ??. If you have multiple weld dots protruding from the back side next to each other, you MAY risk deflecting the metal depending on high/low weld dots next to each other. This is why I prefer to grind BOTH sides, it gets those last dots out of the way for unobstructed planishing of the next one, and also gets panel thickness down to close to where it started for welder settings. If you can't access it, can you make the repair panel larger where weld seams are now more accessible? Can you use more of a panel to move weld seam into higher crown areas that are less likely to deform? etc., etc.
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Last edited by MP&C; 05-10-2017 at 02:08 PM.
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  #27  
Old 05-11-2017, 09:34 AM
blue62 blue62 is offline
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Robert
I have read your write up of the mig welding process that you posted on forum some few years ago.
This current rewrite of the process goes into more detail.
Thanks for posting it. It made me think about it again and compare it to the method I use.
Also tells me why I have a few problems with mig welding.
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  #28  
Old 06-01-2017, 11:02 AM
tjratz tjratz is offline
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I must not have the ability to do this. I took an old junk fender and cut off a portion and replaced it like I would a patch or a replacement piece. I tried ( I think) and did every step and technique mentioned above but to avail. I just cant seem to keep the weld seam from becoming brittle and cracking from me trying to planish the weld. I've tried light taps, medium taps...nothing seems to work for me. I'm looking at either buying a torch set up or a tig. thank you everyone who tried to help this hapless fool out.

TJ
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  #29  
Old 06-01-2017, 11:12 AM
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How about taking some pictures of your efforts at the various steps along the way. You'll get much better feedback to help with corrective action if we can see exactly what's going on....
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  #30  
Old 06-02-2017, 04:09 AM
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Quote:
Originally Posted by tjratz View Post
I must not have the ability to do this. I took an old junk fender and cut off a portion and replaced it like I would a patch or a replacement piece. I tried ( I think) and did every step and technique mentioned above but to avail. I just cant seem to keep the weld seam from becoming brittle and cracking from me trying to planish the weld. I've tried light taps, medium taps...nothing seems to work for me. I'm looking at either buying a torch set up or a tig. thank you everyone who tried to help this hapless fool out.

TJ
Did you use 0.6 wire?
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