I searched the web for quite a while, but came up empty-handed when looking for information about cutting glass plates with a milling machine. I was fairly certain this would be possible since stained-glass craftspeople routinely use a diamond grinder to quickly remove material from colored glass pieces over 1/8" thick.
I started experimenting and found it quite feasible to use a Computer Numerically Controlled (CNC) milling machine to cut glass plates. I have cut many different thicknesses of glass from .075" thick first-surface mirrors and IR beamsplitters to 1/4" thick frosted glass for art projects.
Here are the critical numbers:
For thin (<.125" thick) glass, I use a .115" diameter diamond burr spinning at 2500 RPM, fed at 1 inch per minute.
For thick (>.125") glass, I use a .25" diameter diamond burr, spinning at 1500 RPM, feed at .5 to 1 inches per minute.
Both situations require flood coolant.
The real trick is to find a suitable clamping method to attach the glass to the milling table. I have settled on edge clamps that exert very little downward force on the glass, but provide a sturdy edge for the glass to butt up against, and also keep the glass from being being pulled upward from the table. Here is a picture of an edge clamp. It was made by passing a piece of thick acrylic over my table saw blade. The resulting dado cut has a peak down the middle, which is actually pretty useful, since I can use a hand file to quickly change the exact height of that peak. I like that the clamp pinches the glass ever so slightly, but exerts the vast majority of clamping force to the waste board, which is also a thick piece of scrap acrylic. Note: Do not use wood for a waste board. It will change shape when it gets wet from the cutting coolant, and will crack the glass!
I used two edge clamps like this to hold a mirror down to a 1/2" thick piece of acrylic waste board
Here is a picture of the finished mirror:
A closeup of the edge: