One of the biggest challenges when I first started building book scanners was getting the glass panes for the platen. To minimize glare, the design called for a 100 degree angle between the two panes of the platen. And to ensure a tight mesh between them, I needed each pane to be mitred to a 50 degree angle.
But there are no glass shops who offer this kid of mitre. Most commercial machines that mitre glass are only adjustable up to 45 degrees. So in order to get the panes I needed, I built a jig to let me grind them to that angle myself.
Glass Grinder Jig
The core of the jig was the hobby glass grinder. This kind of grinder is typically used to smooth and shape the edges of stained glass tiles when making windows.
I cut and glued together an acrylic stand to support the glass at a 50 degree angle. The glass was held just above the grinder table and pressed against the grinding cylinder to make a mitred edge. The stand was on a rail so it could only move left and right. So even though the grinder head is a small cylinder, I could still grind a straight miter across the edge. The rail was mounted on a vise jaw which let me move the glass slightly closer to the grinder with each pass.
In practice, this jig had a lot of room for improvement. There were too many degrees of freedom and the pane wasn't held rigidly enough. I had to keep the whole thing stabilized by hand as I moved it back and forth. While my wife and I were able to make a couple of batches with this jig, we had to check each piece against a guide afterwards to make sure it turned out.
CNC Glass Grinder
I improved this setup over the course of the next year with new iterations of the jig. The latest iteration is now a motorized CNC device which fixes all of the flaws of the original.
The glass support is much thicker acrylic redesigned to avoid useless projecting faces and sharp corners. The rail is now double gantries running on solid C beams that are mounted to the table. There are rails on both sides of the glass grinder so that the weight of the glass and acrylic holder is fully supported.
Because it is computer controlled, it can move very slowly and consistently. This eliminates the need for moving the glass closer and further from the grinder. The quality is better and more consistent. And it requires a lot less work.
The grinder is a true 1-axis CNC device. I run a simple gcode program to do the grinding. It is probably overkill for such a simple task, but it allowed me to fine tune the speed easily by just tweaking code.
More photos and a video of the device working can be seen here.
I still have a few tweaks that I want to make to the grinder. I will be adding a touch interface at some point and using a relay to control the hobby grinder. But overall, it is feeling like a project that is close to complete. Now I just need to keep grinding glass.