How to Grind Metal
This information is provided here as a re-post from this article on the 67-72 Chevy Truck Forums.
there seems to be allot of confusion on how to grind metal, I am gonna try my best at helping everyone out some, I still have more to add, but this will be a start.
FIRST: THE TOOLS NEEDED.
6″ Dynabrade for my finish work, this does finer swirl marks in the metal, creating a more uniform appearance.
6″ Dual Action sander, AKA,”THE KNUCKLE BUSTER” if you know the meaning of this, you have used one. used for medium sanding
8″ Mac sander/grinder, used for initial knockdown and leveling.
“Don’t be afraid to dive in and tackle these tough jobs, the right tools and techniques will see you through!.”
:Grinders, there are several, this is what I use
Bosch 4.5″ electic grinder with either a flap disc or grinding stone, or cut off wheel. This is my favorite.
El Cheapo air grinder for all of my Carbide bits, these are used for corners and any place I can not grind with a stone
Air tool for my cut-off discs and 3″ grinding stones
Mac 90 degree grinder for my ROLOC discs, ranging from 36 to 80 grit, also comes with abrasive pads and surface pads.
:Safety Equipment, A MUST HAVE!!!!!!
proper gloves, I use tig welding gloves
3M particle mask
So you have a spot weld you want to grind down, what now? Should I just hit it with a hard stone until it’s beyond flat and now I’ve
heated the metal along the way, NOPE. stop right there.
you never want to heat the metal when grinding, especially when it starts turning blue, bad, very bad, you just warped your metal and
now have more work to deal with.
The first thing to do is grab a 3″ hard grinding stone, or one on the electric grinder and slowly, I mean slowly, grid down that weld, get
it close to the surface of the original metal, without touching the original metal. It takes practice, I know.
Once you have it close to the bottom, switch over to your ROLOC disc and start smoothing it out, never put too much pressure on the
metal itself, just let the grinder do the work for you, nice and easy does it. This will take some getting used to but once it is mastered,
you will have won half the battle.
Now you have some marks either in 36, 50, or 80 grit leftover from the grinder, what now? Leave them? nope, depending on how bad
you have cleaned up that weld, you can now use your sanders and smooth them out.
I always use 80 grit on my pads to do this. Sand it for a bit and you are done, 8 out of 10 ten times, no “mudwork” is needed.
You have just mastered spot weld grinding.
This is where it gets tricky. Lets say you have a weld 10″ long, well that is a major heat source waiting to warp. Time must be taken to
do these types of welds, Patience will prevail.
Well which tool should i start with? For a weld this large I either start with my 3″ air grinder or my Bosch 4.5″ hard stone grinder.
Depends on what you have and how you feel. Now remember to go slow and let the stone do the work for you, nice and easy,
Do not to rush into things! otherwise it just gets messy.
Grind a small section at a time or go back and forth over it slowly. Never sit in one spot too long or you just heated the metal and
caused more work for yourself.
Since you have now ground down the majority with the stone, I switch over to my Bosch grinder with a flap disc using 36 grit, this
allows me to remove material quickly without pressure and does a nice smooth job. I never go to hard with this one either.
Start at one end and go back and forth over the metal, these flap discs do an amazing job on leveling the weld and creating a nice
uniform surface. If you still feel wearily about getting too close with an electric grinder, then just finish it up with you ROLOC disc and
smooth it out.
Now that I have gone this far, I will normally take my 8″ MAC sander and smooth over the whole surface with 120 grit. After this is
done, I use my D/A sanders and finish the job, nicely ground down and sanded, nothing looks better than this.