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Old 12-18-2012, 08:03 AM   #1
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Default Seam Welding, tips and tricks

Hey guys,

Ive got all the equipment and here and the time to slowly start seam or stitch welding the chassis of my '93.

I've been warned about things like not being able to get in behind the welds and rust proof them and of course warping.

Id also like to devise a basic way to test that actual structural twisting stiffness of the car.
For that I figure If I put stands under the front sides of the car and one under the diff, and then I weigh down one side of the car I can measure weight and twist that way, if not hard data it will give me a comparison.

So, who's done this before?

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Old 12-18-2012, 08:42 AM   #2
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I have been stripping seam sealer and undercoating on the car the last few weeks so I can do a some seam welding.

I did a little test welding so far, and I am glad I took the time to clean the surfaces properly. It is a lot of work, but worth the effort to have it clean.
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Old 12-18-2012, 08:46 AM   #3
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This is pretty simple really.

Clean clean clean and clean again. Make sure all that paste they put in the cracks is all out. It should burn out, but better to be safe than sorry. Spot weld it. Make a bunch of tacks so not to warp it. Run lower amps and quicker feed rate. That "should" prevent to much spatter and from blowing holes through. Run straight over your tacks as if they weren't even there. Vertiacl down, and right to left pushing. Have fun.

Oh and edit* no swirling the gun either, if anything do a back and forth.
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Old 12-18-2012, 09:30 AM   #4
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Here is how I would test torsional stiffness if you have coil overs. Get some steel pipe with the same id as your springs and cut them the same length as your springs. set your spring perches to get even ride height side to side measured from the pinch weld. Then take off one wheel and poor a bottle Jack under the spindle and on top of a scale, note the weight. Then increase the height of the Jack by 1 inch. Note the weight and make sure the other corners are being competently held down, some friends sitting on the car helps. Then go up to 2 inches and repeat. You will need to measure from the point the Jack is touching to the center line of the car and from the jack to contact patch of the rear wheel on that side, straight back. With that you should be able to do the math to determine torsional stiffness. For the measurement to really be meaningful it your forces need to be acting on the upper shock mount area.

And for seam welding, here is a graphic that I saw on FM site once. Flyin' Miata : Tech: Seam welding
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Old 12-18-2012, 11:07 AM   #5
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I had the doors seam welded and my measurements showed a 7% increase in stiffness:
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