Guys,

I was reading an article about a car restoration where the restorer said that grinding mig welds decreases their strength by 70%

I have been welding my frame as per the chevy power book but have ground the welds for a more aesthetic appearance (plus, my limited welding skills and the dirtiness of the inside of the frame mean lots of ugliness). So, what I need to know is do I need to go back and re-weld all these areas or am I paranoid?

Thanks,

Keith

 

I can't say whether the 70% number is accurate, but grinding does weaken the welds.

 

I would consult a professional welder to get their final opinion on your question if you have doubts. While your are not putting up a building you are going to drive this car around or with other folks.

My $.02 I don't grind any of my welds unless they are in the way or need to be redone. All of my grinding is done to the parts where I am about to weld them.

I have done allot of welding on my frame and several other frames in different cars. Learned allot through experience. I think you need to consider what it is you are welding (if your weld fails while your driving whats gona fall off). I've seen allot of crummy welds on these frames from the factory and they've lasted 40 or more years, depends on what they are being asked to do.

Some good simple books out there on just simple introductions to welding. Numero uno is cleanliness of the items being welded, condition of the parts being welded (not rusted away) and using the right supplies to weld the parts (prep). Then its penetration of the weld ( technique ).

If you have a pretty weld on the surface attaching two pieces and got very little penetration then grinding will certainly remove strength from an already questionable weld as its mostly on the surface. Just like welding two weak pieces of metal and not grinding is almost a futile.

Ive had a tiny tack weld kick my a$$ to the point where I had to grind it off like crazy and then Ive have a small weld break where the material itself was simply too weak to be saved.

 

I'm also not sure about the 70% number. Is it b/c your removing some material & surface coverage or is it the additional heat cycling caused by the grinding process.

If you do a good bevel between materials --\ /-- to be welded then your weld should be confined to that bevel space. So any additional weld that you can grind off would have no surface adhesion and should be safe to grind off --\^/--, unless the grinding causing heat that weakens the weld. Not sure.
Best bet, refine your welding skills so they are stronger and more attractive. A good solid weld is usually not ugly to the eye once its painted.

 

It depends on what type of welds you are grinding, and how much.
If you butt two pieces of metal together and make a sound weld that completely penetrates the joint, then the weld will be as strong as the frame is without any reinforcement (cap) required.
If your weld joint is a lap or tee joint that you are joining with a fillet weld, then it is easier to cause a problem when grinding. You want to make sure that the amount of weld left after grinding still measures as thick through its throat as the metal being joined.

 

I agree with the rest of the guys......just clean it up with the grinder and leave it alone after that.....are you using gas on your Mig? if its the inner shield wire try using a braided wire brush on a grinder to clean up the slag.

 

didn't think of that.
When I weld with my stick welder I have to hit it with the pik hammer and wire brush it to get the slag off then it looks fine, no grinding needed. Just add paint.

 

Thanks for the replies everyone,

I have been filling in the welds on the frame where the factory didn't weld, as well as strengthening some areas around the shock towers, rear kickups and the like. I had also removed the area under the front crossmember that was damaged from being jacked up and repaired it with 1/8" steel. All the areas that I ground down were to smooth out for show purposes. basically the underside that would be seen with mirrors under the car. I am thinking after reading these replies that I will go back over all these areas and re-weld them. I won't worry about aesthetics and I should get better penetration since I have since sandblasted the frame and it has been heated through on the first pass. Does this sound right?

Thanks again,

Keith

 

since factory welds stilll intact I would not worry about it, you are adding strength, not taking away. good luck