Cutting Glass Circles


Picture of UBVRI Filters Glass circles.

I have recently made a "biscuit-cutter" for cutting circles out of glass. I did this for making round filters from square stock, but the technique is applicable to cutting the hole in a Cassegrain mirror blank. In fact, this is how I learned how to do it many years ago when I made a Cassegrain telescope. I was 19 when I did it and all I had to go on was a brief description in one of the ATM books. I wish I had these instructions then!

The technique is quite simple; you only need a cutter, some grinding powder, a drill-press - and lots of care and patience.


The principal is simple. You make a circular cutter out of some soft-ish metal - I used copper - that has the right inside (or outside) diameter (depending on whether you want the circle or the hole). Make an adaptor so that it may be placed in a drill-press. The cutter mustn't wobble at all, else the circle won't be the right size, and square to the glass. Slots are cut up the side of the cutter to allow the grinding powder to circulate freely during the cut.

It isn't actually the cutter that does the cutting, it is grinding powder (I used grade 80 carborundum) that does it. The cutter just provides a surface to direct the powder, just like the tool when grinding a mirror. And just like my steel tool technique, the right material (in the right shape) will make the operation progress quickly. The cutter will wear slightly but probably only noticeable if going through a lot of glass. You'll use a fair amount of grinding powder for even a small cut.

You need some way to hold the piece of glass firmly in place, and to mount it securely to the table of the drill-press. I made mine from a few scraps of aluminium screwed to a plywood base. The base is held to the table with a G-clamp.

I was cutting 1-inch diameter filters from 50mm square Schott filter glass. I first cut the glass across the diagonal just to be on the safe side (although in the few photographs below I show a 50mm square being used). I covered both sides of the glass with "magic" tape (that translucent tape for sticking paper etc.) to stop the glass from being scratched by the grinding powder. I found that the "magic" tape stayed stuck during the cutting process, while some other types of tape came off very easily. Experimentation is the key to success. If you are cutting the hole for a Cassegrain then this isn't necessary as you will be cutting from the back (probably) and not from a polished surface (although you might be finishing the cut from the front in which case you will want to use the tape).

The drill press needs to go quite slowly - a few hundred RPM or so. Mine would only slow down to 450 RPM but was still fine for this small cut. When I did the larger hole for my Cassegrain (45mm diameter) I had borrowed a drill-press that went down to about 180 RPM. I guess the rule is the larger the hole, the slower the speed.

Once the board is in place and aligned with the cutter, I set the stop on the drill-press so that it will stop a little above breaking through the glass. I then take the final ½ mm or so very slowly. (If you're making a Cassegrain hole then you'll probably not go all the way through - although I did and never had a problem.)

Make a dam around the piece so that you can hold a little water around the cutter. It also keeps the grinding powder concentrated around the hole. I stole some of my children's play-dough for this.

Take it slowly. You don't use a lot of pressure when cutting, especially if it's just thin filter material you're cutting. Pause every few seconds to let the grinding powder circulate, and try not to generate too much heat. I initially tried cementing thin glas to both sides of the main piece (to protect the glass from scratches) with Canada Balsam. The heat generated in the cutting was enough to loosen the cover-glass and allow it to come off.

It took about 10 minutes to cut through a 2mm thick piece of glass, although almost half of that time was doing the last millimetre very slowly so as not to fracture the glass when the cutter broke through. I actually tried not to break through, but leave about 0·1 mm of glass holding on. I could then snap the pieces cleanly off, rather than risk a larger chip from the more savage cutter. Of the 4 circles I made, 3 broke through and only 1 didn't. The cuts were all clean so I needn't have worried.

You'll probably find that the circles are slightly tapered. This is due to the cutting action taking place on the sides as well as the bottom of the cutter, but may also be an indication that the cutter isn't perfectly square. I squared the pieces off by rubbing against a carborundum stone for a few minutes.

And that's it. The biggest bother is making the cutter and setting up to make the cut. It's reasonably quick to make one cut, but takes as much time again to get set up, and then clean up afterwards. There is little difference between cutting circles and cutting holes, except that much more care is needed to protect the circle during the cut.


These few pictures were taken during initial tests just in case anything unexpected happened.

12Kb The cutter - a 1-inch diameter piece of copper tubing.
Note the slots up the side to allow the grinding powder
to circulate more freely.
15Kb The simple holder, made from a few pieces of aluminum
on a plywood base.
26Kb The board is aligned and clamped securely. The drill is
run as slowly as possible, a few hundred RPM at most.
16Kb The glass is covered with tape, then a dam is made to
hold water and grinding powder. Let the cutting begin!

And then I did it for real.

33Kb The remnants after 2 cuts. The 50mm square piece was
first cut diagonally. You can still see the tape holding the
pieces together. On the left are the 2 circles which have
been cemented together.


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Page last updated 1996/10/22
Steven Lee