Magnetic Holders for Granite Surface Plate
© Frank
Ford 2006; Photos by FF
Not long ago I acquired a nice 18 x 24 Starrett granite surface plate, and since that time I've been using my old 12 x 18 Chinese number with some abrasive sheets for, er, "casual" lapping. It's sometimes hard to keep those sheets still, and when I wanted to use two sheets to cover the surface, I had even more problems.
So, this morning I rigged up a simple magnetic clamping system to hold standard 9 x 11 sheets on the top of my surface plate.
I used a regular carbide concrete drill to make a row of 9/16"
deep holes along and centered in 1/2" from each 18" edge:
After a quick bit of experimenting, I found that eight drops of epoxy would be just right in the bottoms of the holes:
You may notice that just for a bit of extra grab, I ground a little track around each of the 1/2" by 1/2" Neodymium magnets I'd be using.
After I plopped the magnets into their holes, I held them in
place with two thin steel strips standing on edge. The steel strip made
sure the magnets wouldn't sink down into the epoxy, but would stay right at
the top:
Here's a little closer view that shows the tricky part. I
folded a piece of a sticky paper label around the steel strip right where each
magnet would be attracted:
The idea was to hold the magnets just about .003" below the surrounding surface, so they would still be "attractive" yet not "stuck up."
Once the epoxy cured, it was easy to slice right off the granite
and magnets:
Now, all I have to do is lay my abrasive sheets on the granite, and drop a pair of 1/4" x 1/2" steel strips on top of the magnets. The fourteen magnets hold the two strips really well, and the paper never slips.
Here I'm flattening the bottom of my favorite jack plane. It's a job that must be done with the iron (retracted of course) and other parts in place to account for any distortion of the plane body when they are tightened:
