Cutting Plug Welds?

More of an art than a science - discuss metalworking and welding here.



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PostPosted: Thu Mar 19, 2015 12:42 am
I had a radiator core support & headlight buckets swapped on my Mitsubishi 3000GT in order to use newer style headlights. I hired a "mobile autobody guy" and he came and drilled out the original one and welded in a the newer style one I had on hand. He did not use a spot weld drill bit and instead just drilled right through the spot welds, I didn't know at the time that this was the wrong way to do it. I had this done 2 years ago when I was a lot dumber and trusted him to do a good job. Anyways, I was trying to align my front bumper, headlights, fenders and hood and there was no way it was going to line up because the core support was not aligned correctly. He also painted the bay which was also a hack job. $400 down the drain but I did learn to just do everything myself. This guy had good references and pictures of beautiful paint on his website.

I am now attempting to cut out that core support to weld in a new one myself using an actual spot welder. However, his welds are much larger than a standard spot weld, most are probably 1 square inch. I am attempting to drill them out with a spot weld cutter and in about 6 hours, I only managed to free maybe 3 welds. I am positioning the spot weld corner in 4 the 4 corners of the plug weld in order to cut out the entire surface area of the weld and it is not going great. I am using the blaire style spot weld cutter. There has got to be an easier way to do this. Any advice?

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PostPosted: Thu Mar 19, 2015 12:45 am
I was going to save this for when I actually got to it, but I will go ahead and post it now. The core support pictured will be junk after its cut out, I have another good replacement ready to go, however where it welds to the frame is obviously drilled through. Am I correct in thinking that I can weld the holes shut on the frame, smooth it all out until its back to its original shape, place the new core support onto it, and then use a spot welder to weld it to the frame in the correct position?

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PostPosted: Thu Mar 19, 2015 6:31 am
If I had a 1" diameter spot weld to remove, I might try a 1-1/4" bimetal hole saw, center it on the spot weld, drill through the top layer of unwelded sheet metal, fill the 1/4" hole left by the pilot bit, then grind it flat after the radiator support is removed.

Just an idea.
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PostPosted: Fri Mar 20, 2015 3:18 am
natemoore wrote:If I had a 1" diameter spot weld to remove, I might try a 1-1/4" bimetal hole saw, center it on the spot weld, drill through the top layer of unwelded sheet metal, fill the 1/4" hole left by the pilot bit, then grind it flat after the radiator support is removed.

Just an idea.


That actually seems really obvious considering the spot weld driller I am using is basically a mini hole saw. Thanks, I will give that a shot.



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PostPosted: Thu Mar 26, 2015 3:39 pm
Drilling spot welds out with just a regular drill bit is fine so is using a specialized cutter I have done both. I actually like using regular drill bits and step drilling the original spot welds bigger and bigger then cutting the panels with a panel knife.

Just me but once the part has been plug welded with a Mig its permanent. Not a mess that I would want to drill apart to replace a second time.
I'M NOT SAYING IT CANT BE DONE but Its just not something I would wanna do.
Mig welds are HARD a lot harder than the original metal.
Using a hole saw is gonna leave huge holes that will need to be repaired before installing the new piece, again more work.
You can try sanding down all his welds with a 2 inch sanding pad on a die grinder then drilling the plug weld but its gonna get ugly no matter what you do at this point.

Kinda hard not to be at least be close when installing and fitting a core support??
Got to remember your modifying from one style to another not every thing is gonna line up perfectly as original parts would.
If it was mine I would make it work as best I can beat to fit and paint to match as they say.
Or Sell it off as a parts car.
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PostPosted: Thu Mar 26, 2015 4:23 pm
The easiest way to remove those plug welds is with a Dynabrade Dynafile belt sander. It's a versatile tool and I find it invaluable in my shop. I rarely use a spotweld bit anymore. The Dynabrade quickly and easily removes factory spotwelds, and it is great for many other tasks. Less than $300. 3M makes them at a higher cost.



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PostPosted: Fri Mar 27, 2015 9:24 am
I can see the benefit of sanding a spot weld instead of drilling it Scott.
I have used one of those small belt sanders before as well, But I never thought of using one to sand off spot welds or Plug welds though. I can see the benefit of using one for that purpose depending on belt cost and longevity of the belt.
If It cut's down on the use of the 2 inch sanding disks I'm all for it!
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PostPosted: Fri Mar 27, 2015 3:40 pm
Scott, that sounds a lot better than a 2 or 3 inch disc.

99, For welding that back in, I doubt that your spot welder is up to the task, keep in mind that it is a structural part of the car and must be solidly welded in.

I have a miller spot welder and it does a good job on 20ga metal, but I wouldn't trust it on a structural part, or on different gauges of metal. It sounds like you may have a hole in one or both of the parts anyway, or might be trying to use a spot welder on an area that was made with built up weld material.



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PostPosted: Fri Mar 27, 2015 5:29 pm
Thanks everyone for the info, but I found a way to do it with the tools I had on hand.

I managed to get one side done today. While searching, I came across a method of using a die grinder with a cut off wheel and going back and forth over the weld. This worked really well to accomplish this task. It completely destroyed the panel that I am replacing, but it left the panel underneath untouched. I got it close with the die grinder and then when the metal was paper-thin, I used a tool similar to a panel splitter to cut the remaining metal. It is not the fastest thing in the world, each weld took 5-10 minutes but I got the result I wanted.

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Now I just have to do the other side, then grind off the remaining welds on the bottom panel and them I'm ready to weld the drilled holes shut.

Chevman:

I was thinking I would weld the replacement panel back on with the 220v spot welder from Harbor Freight (I know, I know... but it has good reviews) This seems to be a very common method for doing this on these cars. The metal is not very thick, unfortunately I couldn't tell you what gauge it is. If is is not advised to use a spot welder and a mig is used instead, would it be appropriate to grind the welds down flush or would that ruin the integrity of the weld. Even though it doesn't look it yet, this is hopefully going to be a top-notch build so I really want it to look as factory as possible.



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PostPosted: Fri Mar 27, 2015 6:09 pm
You can buy a 3/16"(or 1/4")x3" wheel for grinding welds, or stack 2-3 cut off wheels instead of using just one. If that low budget spot welder will not achieve very good penetration with adequate heat, plug weld the new panel on. You can dress the plug welds and leave them proud. They are only as strong as your welding skills provide.

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