Hi Pat, Just for future reference - here's how I managed to cut glass disks for a Mendocino motor.
I needed to make a couple of 30mm diameter glass disks for the end stops. All attempts with a diamond coated hole saw resulted in disks that could be done better by a Neolithic flint knapper (Photo - bottom right).
I eventually succeeded (Photo - Top right) by first glueing scrap pieces of glass with cyanoacrylate to either side of the target piece.
The rubber ring was also glued down as a cofferdam to hold water / coolant (you can use modeling clay).
First attempt failed along with the adhesive - you need to let it cure for about an hour.
Second attempt – very much better - but my drill press chuck does not run very true - transferred to the minimill where I finally succeeded.
You do not have to apply a lot of force - being particularly prudent at the breakthroughs. Make sure the glass is spotless in order that the superglue layer is very thin (dust specs will increase the thickness).
The diamond bit went through about 10mm of glass (3+4+3) in under a minute at about 500 rpm.
After removing the three layered glass sandwich from the bit, I heated it (slowly) with a propane torch until the glue decomposed - cleaned the disk up and voila.
I suggest the sacrificial glass be thinner than the target glass – I noticed a tendency for the sacrificial pieces to crack during heating – probably due to differential stresses and the splintered edges – so if the sacrificial pieces are thicker then the target disk might crack.
When glueing – make sure the pieces of glass are cleaned spotless – any dust will hold the glass apart making the superglue layer thicker which detracts from performance – also allow the glue to cure for about an hour before drilling – if the glue fails during drilling so does the method – guess how I found that out. By the same token don’t glue large pieces of glass together – unevenness translates into areas of thicker glue, which will lead to failure of the method.
Regards, Ken
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