Sunday, March 6, 2016

Glass Recycling!

I hate to throw stuff away that might have another use - especially when it comes to optics and telescopes. Amateur telescope makers are known as scroungers - even going to the trouble of making mirrors from television cathode ray tubes! I won't go to that much trouble, but a couple decades ago, in the early days of the Mirror Lab, we performed some diamond generating experiments, using some 48"X48" sheet Pyrex we happened to have in storage. After the experiments, I was told to put them in the dumpster, but I set aside some of the nicer pieces, using a hammer and chisel to "score" a line, set the line on a 2X4, and jump on the piece to snap the pieces into manageable sizes. "Snapping" Pyrex up to 1.5" thick was no simple task, but I took a few pieces home where they've been sitting for the last few decades, as shown at left. While 1" to 1.25" Pyrex is a little thin for large mirrors, it is fine up to 16" to 20" diameter (I guess some might consider that large!).

With the arrival of Spring, I thought it was time to try to do something with the glass pile, so picked out a nice piece to take to a local machine shop. Shown at right (leaning against Melinda's Jeep tire) as I was leaving the house, I figured they might be able to cut out a 16.25" diameter piece of glass.  The white marks on the glass are shallow diamond generator marks on one side of the Pyrex sheet.

I've used and mentioned A-1 Fabrication They cut some 2" thick steel counterweights for my AP 1200 mount, and also did some very nice welding on the aluminum mount for my TEC 140 scope. Supposedly their water jet cutter should be able to make my irregular pieces a more useful round shape, so went to them last week to try them out when it comes to glass. I've not talked about water jet cutters before, but it uses high-pressure water with added abrasive to cut through most anything. Miguel offered to cut out my round for $80, which sounded great to me! It would have been a couple hours of tedious work to rough in the shape with a brick saw, then diamond generate the edge round. Remembering too that a 16" Pyrex substrate would likely cost in excess of $500 or so, $80 in addition to my recycling storage fee sounded good! So I dropped off a sketch and the glass about a week ago - they said it would be a few days. When I hadn't heard in the week, I called and they said they would do it that day (Friday). Sure enough, I got the call a couple hours later and picked it up.

I looked near-perfect! It was chip-free, and perfectly round to better than .030" or so. Miguel showed Roger and I the machine in use. They were cutting a piece of half-inch stainless steel, and said my 1.25" Pyrex cut at the same speed of 3" per minute. With my 16" having a circumference of 50", it would have taken about 17 minutes for the full disk. The "kerf" (width) of the cut was only .030", so there was almost no waste, and there appeared to be little or no widening of the high pressure water cutting edge over the thickness of the glass. The first thing I did was use a diamond pad to put a little bevel on the sharp edges to prevent chipping, There were some subtle marks along the edge as the water pressure, abrasive feed or linear motion surges during the process. Shown at left, they should have no effect on its performance as a mirror blank.

Now that I know they can do work like that, it is easy enough to mark out and get the other chunks cut to size. All I need is more projects to work on, but those seem to spring up as soon as the pieces appear!

1 comment:

Andrew Cooper said...

Sell a few pieces to offset the machine shop fees? Mid-sized Pyrex blanks are getting to be quite dear.