Thread: How do properly grind down a weld?

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Well ,I am welding up the truck frame,filling in holes ,and cutting out small sections and filling in.I guess part off the problem is that I am useing a grinder with a 3"disc.If I use a big grinder with a 5-6" disc maybe I won't gouge the metal so much.What type of sand paper do I use to SAND metal?The stuff I am buying at Home depot does good for paint and rust,but with bare metal all it does is polish it.Any suggestions?

My favorite tool is a long shaft 110 volt die grinder with a standard 3 inch cutoff wheel on it, You can get the die grinder from harbor freight for about $60 bucks, you'll need a cut off wheel arbor about $10 bucks and the cutoff wheels usually come in packs of 5 or 6....that thing makes short work of dressing down welds.

In addition I would add..........carefully! Especially with a fillet weld. If you don't have good penetration you'll severely weaken the weld. You probably know that, but it's always worth emphsizing.

I use HW's method for fillet welds. For welds on the side of a frame rail such as filling holes I take it down with a 36 grit 3" Roloc disc on a 90* die grinder. If I want a smoother finish I'll use a worn 50 grit disc.

yes 36 to 60 and and soft pad 80 grit . i like the zirconia on steel and i use the norton backer and sanding dics with the four holes punch in them you can see how much you are taking down while you are grinding they work great

I like to use a flapper disk in my 4" Makita. The courser the sandpaper on them the better it works. I like to knock the top down with a grinding wheel and change over to the flapper disk. They work great! Your local welding supply house should have them. You can buy them mail from Northern Tool too. Google on Norther tool.



http://weldersource.com/6342503532.html

gator discs. They really cut, and leave a nice finish.

On body metal, I use a 3" diameter x 1/16" thick abrasive wheel on an air-powered rotary die grinder---this is a bit tricky because I grind with the edge of the abrasive wheel at 90 degrees to the weld, and it takes both hands to contol the grinder so you don't gouge the parent metal. When the weld is close to flush, I use a 36 grit disc with semi-flexible backing pad to finish the weld down flush.
On something like a frame weld, I use a heavy abrasive disc 7" diameter, on a heavy duty right angle grinder and knock most of the weld down, then switch to a 36 grit sanding disc on a semi-flexible backing pad to bring the weld down flush.------if you are grinding frame welds, be sure that the metal you are grinding has been V'd out to almost its full thickness before you start the welding, or grinding it flush will remove all the strength from the weld.
You can do it with a small grinder, but its very time consuming.
One of the best sanding disc thingies I have ever seen has a circular shape, but is made up of many peices of what appears to be about 36 grit abrasive coated heavy fabric, and all the peices overlap each other, kind of like a fan of cards.----I don't know exactly what they are called, but they cut very fast, and blend excellently.

they are call flap discs a lot of guys like them. i do not. to much money and will not get thing flat but if you want to blend and not get it file flat they work the 3m psa soft 8 inch pad works .with 80 grit works better i think better for this

Originally posted by pat mccarthy
they are call flap discs a lot of guys like them. i do not. to much money and will not get thing flat but if you want to blend and not get it file flat they work the 3m psa soft 8 inch pad works .with 80 grit works better i think better for this

I have been using flap disc with a elect. grinder,and they don't work for me.I have to angle the grinder too much too get it to even cut.I think thats why I am gouging the metal so much. I think the 90 degree air die grinder well work better ,I am going to try that setup. Thanks

Originally posted by BigTruckDriver
Well ,I am welding up the truck frame,filling in holes ,and cutting out small sections and filling in.I guess part off the problem is that I am useing a grinder with a 3"disc.If I use a big grinder with a 5-6" disc maybe I won't gouge the metal so much.What type of sand paper do I use to SAND metal?The stuff I am buying at Home depot does good for paint and rust,but with bare metal all it does is polish it.Any suggestions?

...as someone already mentioned; don't grind the welds any more than you have to. In fact, if no ones going to see it, don't grind them at all. Also, if you make verticle welds on your frame they will have a tendency to crack. I'm not sure what type of vehicle, the type of repair or what. But in some cases you should weld a plate over over your repair that is cut in such a way where your welds are not verticle or horizontal. Like at a 45 degree angle. I see that your user name is 'big truck driver'. Are these repairs on an 18 wheeler or ??? Bill

I cut the old front shock mounts out ,and the area where the emergency brake passes through the frame.I want a smooth frame rail.I will use a aftermarket E.B. I am going to get the frame sand blasted and then weld from the back.I will then box the frame with 3/16 inch metal.After boxing the frame do you think it will be strong ,and not crack, and handle 600 horses? Should I weld a plate behind the sections I cut out then box,they are vertical and horizontal welds.I am redoing my 65 chevy short bed to handle racing abuse.After I weld the boxing plates I will weld in all the crossmembers.I will also install a simple roll cage for safety ,this should help keep the frame from twisting.Thanks for the input.

Originally posted by BigTruckDriver
I cut the old front shock mounts out ,and the area where the emergency brake passes through the frame.I want a smooth frame rail.I will use a aftermarket E.B. I am going to get the frame sand blasted and then weld from the back.I will then box the frame with 3/16 inch metal.After boxing the frame do you think it will be strong ,and not crack, and handle 600 horses? Should I weld a plate behind the sections I cut out then box,they are vertical and horizontal welds.I am redoing my 65 chevy short bed to handle racing abuse.After I weld the boxing plates I will weld in all the crossmembers.I will also install a simple roll cage for safety ,this should help keep the frame from twisting.Thanks for the input.

..... well it's hard to say without seeing it, but if you stand back and look at the entire project and just try to imagine what twisting, pushing and pulling forces will be where, you should be able to get a good idea. Just keep in mind that Vert. & Horz. welds have a tendency to crack when repairing a frame. The boxing just might do it, but if I were you, just to be on the safe side, I would weld what they call a 'fish plate' on the inside of the frame over your welds. You weld this metal patch on at a 45 degree angle over your repair. Since its on at a 45 degree, your welds will also be at a 45 degree. hope this helps, Bill

Ya I am going to put a fish plate behind it, then box the frame!I will also drill some 1/2" holes in the fish plate and weld.Thanks

I would not buy the paper at home depot or a hardware store go to a parts store that sells paint, I use the 4" Makita to knock down the weld and then use a Mac angle grinder with 36 grit roloc disc and finish with the roloc polishing pad? I would talk with others about getting the frame to stiff as this could break stuff.