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Cutting Blanks

Cutting Blanks. Trepanning. Trepan. Tre-pan.

A 38" square tabletop of .75" plate glass was purchased at a garage sale for $15. A couple weeks later I have three 14" blanks, one near-blank, and enough large pieces to get a 10" and a 6" sometime down the road. Initial calculations showed I could get three 14" and four 12" but that was too optimistic.

The machine is based on a cheap drill press, using the pulley stacks plus one 8" wooden pulley to drive the "cookie cutter". The pulley was made from a scrap disk but a 10" or 12" size would be better. The torque was just enough to add a 2.5lb weight but not enough for 10lb. The pulley drives a 1/2" copper pipe which runs through two greased, wooden blocks and drives the cutter. A small 2x4 block is screwed to the pulley and a brass rod skewers the block onto the pipe. The cutter is made of one scrap disk of .75" plywood and one matching ring of the same screwed together with a strip of galvanized roofing flashing screwed to the outside edge. I could not get the cutter accurately lined up with the pipe, after four redrilling attempts it was still wobbly. So I put two bolts in the cutter and let the pipe drive them with another brass rod. That allows the cutter to rest flat on the glass and wobble just a bit, the pipe runs into the cutter and keeps it on center. This was all made with scrap, if I had done it from scratch it could be a little cleaner.

You can see the pink Play-Doh wall around the cut. Play-Doh is adequate, but do NOT let the cutter touch it while spinning, it will peel it off the glass faster than you can say "Doh"! The plastic jar holds 80# silicon carbide grit and the blue bottle holds water which I dribbled into the pipe. For my three-and-a-half 14" disks I used about three jars full of grit, or about 4-5 pounds. One of those was remarkably coarser than the other two even though they were all marked 80, it was all from a box of lapidary supplies I got at another garage sale. And there was also a huge difference in volume among the three packages all marked "1.5 lbs".

The cutter turned at about 100-120 rpm. First successful cut took about two hours. Second one was probably about one hour with the coarser grit, but I did not realize it was through and spent about 30 minutes slowly grinding into the supporting board. Third cut was at 90 minutes when I heard a faint "thunk" and sure enough it was through. The rate of cutting seemed to be determined by how frequently grit was spooned into the cut, with my attention wandering I managed about one-fourth spoonful every 30-60 seconds.

As you can see I just set up everything on the ground in the back yard. A big piece of carpet and a .75" board with melanine surface supported the glass and the machine. It was hardly level but that was not a problem. If I could have I would have turned the glass 180 degrees halfway through, but that was inconvenient. As a result, the machine cut one side through about 1/8" before the other and spent time cutting the board. On the second disk I put a wedge under the machine to level it and that one cut through evenly.

Now for the hard part. I started the first cut on Saturday before Memorial Day, knowing I would be out of town Sunday and Monday. With the cut about half-way through it got dark and I had to shut down so I wrapped up the machine and left everything out. Returning Monday night I had a quick look and discovered a crack running across the tabletop directly at the cut. Again it was dark and I could not do anything with it. Tuesday morning the crack had proceeded to the cut, followed it around a little ways, then ran right across, clipping about .25" off the disk. I decided to set that piece aside and start another cut. There was another small crack started across a large area so I determined to cut a disk in its path. If it worked, ok, if the crack went through, it was going to do it anyway. Success! The disk cut nicely and when I lifted the tabletop the crack completed to the cut but too late to injure the disk. That was the last of the cracks.

The moral of this story is: when the weather forecast is for 90-degrees, partly sunny with passing clouds and chance of brief showers, DON'T LEAVE GLASS LAYING OUT IN THE YARD. I covered the glass with newspapers while cutting the other two disks and had no more problems. When I went back to the first piece I had to reset the sheet metal to allow enough cutting depth, unfortunately it seemed to be a slightly different size than before and would not stay down in the old cut. After running for 30 minutes or so it had started a new cut, slightly offset from the original. So I set it aside again, some day I will cut a 12" or 13" out of it.

You can see that exposure to water is not a good thing for the drill press, but only the table showed any rust and that cleaned up with a little steel wool. The motor ran hot, and at one point was dripping parafin. But it continues to run.

We had recently pruned the apple tree and I laid some of the sticks down on the newspaper. Now the dog likes to chew on them and at one point when I was "zoning" he ran up and grabbed one. A loud "TINK" woke me up and I had to go over the glass very carefully several times before I conluded that he had just bounced his tags on it.

You will ABSOLUTELY NEED EAR PROTECTION for this project. The sound was high-pitched and painful after a few minutes. After a few hours you will have ear damage which may be unrecoverable. I used an old stereo headset that was a huge relief. Also, use a thick rag or newspapers when lifting and carrying glass remnants or have bandages standing by.

Some decent links from which I learned much, especially on using the drill press:

And a great machine from Norm:
Front Side
Front Side
Back Side
Back Side
Ack! a Crack!
Ack! a Crack!