Grinding down welds on sheet metal

[Little Jerry]

Hi

just wondering what the best practice / best tools would be to take down a weld on automotive sheet metal to get a seemless finish.

I have been using a 5 inch angle grinder with a 120g flap disc but I don’t feel 100% consistent and am paranoid about taking it down to far or unevenly. 5 inch is potentially too wide and taking metal off the surrounding areas unnecessarily?

Are those 2-3” roloc discs a better option?

Appreciate any guidance

adam

 

[Dcal]

I try to keep the heat input from the grinding as low as possible so I start off using the edge of a 2 or 3mm thick cutting disk.
If you hold the disk perpendicular to the weld you should be able to see when you are getting close to the parent metal.
Changing the position of the grinder (while keeping it perpendicular) helps stop you cutting tracks and will help level the grind.
When you are happy finish it off with a flap disk.
If you try to remove all signs of the weld it is very easy to go too far.

Flap discs put a lot of heat into the metal and are pretty indiscriminate in taking off material so it's very easy to over thin the, panel but are fantastic for finishing.

 

[frank horton]

it's a can of worms really.....5" u get better mileage but the over grind / polish ur worried about is slightly bigger...
the smaller disc's will help but u'll still be getting into the panel away from the weld.....
when u clean, heat means warping but it's prob there already....
I norm use the outter edge of a 4" grinder to get the knobbley bit's off then use filler.....
that thin sheet metal always was a challenge even for Rolls Royce......
they kept the welds low and used lead as a filler......
but what ever u do it will never be flat and or ripple free.....
thats why there's filler or BOG.....

 

[waddycall]

I hate grinding welds down with flap discs as they wrap around the weld and grind away at the metal either side. A sanding disc on a backing pad is much better as you can control where it cuts.

 

[RonA]

Ive had to do a fair bit of grinding back of welds on steel sheet on my ongoing resto project.
I had similar problems using a flap disc, its all to easy to remove too much material & leave the weld very thin.
The method I have finally settled on is to grind most of the weld back with a flap disc on a 4.5” grinder. I then carefully reduce it further with either an air operated die grinder sporting a roloc type 3” flap disc or a air belt sander. Final finishing to flat is done with a 2” air random sander with a semi coarse disc. Where the joint is liable to a lot of stress I tend to put weld on the non showing side. Here is a replacement transmission tunnel I made using the above technique.
RonA

 

[Parm]

I’ve used a sanding pad in a variable speed grinder once. It’s slow but gives control.

 

[awemawson]

36 grit blue Zircon sanding disk on a 4-1/2" grinder with a rubber backing pad works for me.

 

[tom2207]

edge of a 1mm cutting disc works for me , at a very steep angle , and only doing a very small part at a time , say 5mm max then off to another section takes time and practice to get good at this technique , and goggles are vital. ,, da at the end and a joint will vanish.

 

[nickk]

Flap wheels are good especially if they’re within the speed of a die grinder ,as with a narrow one ,your touching less surroundings,then finish with a narrow belt sander.

 

[RonA]

edge of a 1mm cutting disc works for me , at a very steep angle , and only doing a very small part at a time , say 5mm max then off to another section takes time and practice to get good at this technique , and goggles are vital. ,, da at the end and a joint will vanish.

Ive done similar & had to resort to a 1mm cutting disc on edge for places where the flap disc cant get in. For extremely awkward spots Ive even used 1”” dia reinforced cutting discs on my dremel like tool. Whatever it takes to get the job done.
RonA

 

[gt6s]

Grinding with cuing discs ????????. Catch yourselves on.

 

[tom2207]

Grinding with cuing discs ????????. Catch yourselves on.

try it ,,, then come back . its more accurate , generates less heat no thinning of surrounding parent metal I do it all the time , it does take practice though .

Comber.

 

[gt6s]

Bloody dangerous.

 

[tom2207]

Bloody dangerous.

nope ,,, you use the blade at about 80 degrees not 30 like a grinding disc I use 1mm laser discs for it , , very light touch ,, if your about some time and im doing light stuff ill gladly show you , failing that , give it a go but go slow and gentle and at a steep angle.
If your anle gets too low the wheel edge feathers , and with practice you will both hear and feel the change in the edge , then you know to steepen the angle ,,, if you want a finish smoother than a flap disc you can go over an area with the cutting wheel at about 10 degrees and get a real nice finish , also useful for stripping paint or filler.

 

[Pigeon_Droppings2]

I generally use my finger file...if I have a load to do then a flap disc...but I tend to clean up as I go along.

Finger file has to be the black and decker....belts have to be the blue ones torn from a belt sander belt (buying individually is too expensive).

Flap disc is good too....but got to be very careful with it.....easy to get carried away.

 

[Dcal]

Bloody dangerous.

"Cutting" with a cutting disc - Bloody dangerous, as potentially is most things in the workshop.
If you are grinding at 90 degrees on the edge of the disc, you are basically cutting.
Normal grinding with a cutting disc will thin the edge and they can break up.

I find the slitting discs too aggressive but have been known to use them.

 

[eddie49]

No-one has mentioned using double-cut tungsten carbide burrs in an air or electric die grinder...? Works for me.

 

[Dcal]

Certainly works but I reserve it for places where every other method won't.

I use 9 inch grinder, 7 inch sander, 5.25 inch grinder, 4.5 inch grinder, 3 inch cut off saw, 2 inch grinder, 2 and 3 inch sander (the 3 inch with sanding pads flap discs roloc discs etc) die grinder with carbide bits, wire brushes, stones and sanding drums, a Dremel with the full set of bits and pieces and as of a couple of hours ago, a 4 inch grinder thanks to @hotponyshoes

Every tool has its use and I might have a problem, but sometimes you can get a 9 inch grinder in where everything else won't go so it's good to have options.

I just need a high-speed air grinder now.

 

[Matchless]

I did some metal bodging today, made up wheel arch repair sections for a Rover Metro I used my 5" with plastic backing and a coarse disc, changed it regularly to keep a sharp cutting edge, less heat into the metal, will use the discarded discs when I can grind something nearer to the centre, I was welding a fag packet to a rizla paper so used the tack, tack, tack method, not pretty but it is a Rover I got for free and am giving away as a first car to a lad who helps me when I need someone young, flexible and fit, I am also teaching him how to bodge! I gave it to a mate six years ago but he kept getting very fine rust in the carburettor, despite changing the filter weekly, so he parked it up and I gave him another car, good mate! anyhow five years later I removed it from his front garden, new tank and away it goes, I hope, rattle cans arrived today,

 

[Matchless]

not pretty but better than just filler, I went over that with P40, the one with glass strands in it, then top stop, the P40 seals any pin holes and gets the rough shape better, .8 wire so I seem to deposit bigger tacks than I would like, maybe I should try .6? I should get my other mig going, no wire feed, then I could set one for thin and one for everything else!