Charlie57
Valued Contributor
Posts: 2950
Loc: Lebanon, Tennessee
Reg: 03-30-08
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04-03-12 03:57 AM - Post#2210604
People on CT have talked about a shrinking disc, I am getting into the block sanding of my car and this seems like something I need.
How do these work, where do they work best and where do they not work? Can someone suggest one from my basic hand grinder?
Thanks,
Charlie
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 Rick_L
Honored Member
Posts: 24809
Loc: Katy, Tx, USA
Reg: 07-06-00
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04-03-12 08:06 AM - Post#2210693
In response to Charlie57
You use a shrinking disc far prior to block sanding. It is used to shrink metal (high spots). You'll need to hammer and dolly the panel as well as possible prior to using the disc. And it's possible to overshrink.
You can buy a disc off Ebay for about $30-40. Get the bigger one for a 7-9" grinder (as opposed to one for a 4-1/2" grinder).
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 Lynn63
"4th Year" Gold Supporting Member
Posts: 547
Age: 72
Loc: Vicksburg, MI
Reg: 02-07-09
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04-06-12 09:48 AM - Post#2212046
In response to Charlie57
Charlie, you place the spinning disc on the highest part of a high spot and let the friction heat only that spot rather than spreading the heat over a larger area as with a torch.
I had a local sheet metal shop cut a 5" disc out of 16ga metal and punch a perfect size center hole to match my 4" grinder.
It only takes a few seconds to heat the high spot enough to start the shrinking process. When the spot is heated, stop the grinder and immediately spray cold water or shop air on the heated spot. Do a progress check and repeat as necessary. A couple of times I went too far and had to go to the other side of the fender/door panel and shrink it back. So check it often.
By the way, after doing all the panels on a 1970 GTO the disc is still like new.
I find that the 5" disc is a little easier to get into smaller spots or hard curves. One of the pieces I did was a front lower valence for the GTO that looked like someone ran into/over one of those concrete curbs in a parking lot. After a couple hours with a 2# plastic dead blow hammer, block of wood and the disc we only needs a skim coat (1/32") of filler in any area. The reason for the plastic hammer is if you use a steel hammer on a steel fender with a steel dolly block the metal will tend to stretch. If you only need to remove bad dents try using a hard plastic or wooden hammer and a piece of hard wood as a dolly. Then start with the shrinking disc.
After thought: With the 16ga disc I was often able to apply enough pressure on the high spot with the disc spinning to physically work the high spot down as I went. If you have a spare fender laying around put a few dents in it with a ball peen hammer and get aggressive with the disc to see what you can do and how fast you can go.
Hope this helps.
Lynn63 - An old sheet metal worker. 
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Rick_L
Honored Member
Posts: 24809
Loc: Katy, Tx, USA
Reg: 07-06-00
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04-06-12 11:56 AM - Post#2212076
In response to Lynn63
I hope you turned up an edge on the disc for safety's sake. Otherwise it's lethal.
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