I have not used a diamond bit in a mill but when I grind glass I use a diamond wheel and cool and lubricate it with water. The secret is 'slow'. Go to any stained glass shop and for $20 you can get the grinding wheel. (they have 1/8" shaft and 3/8" diameter with a 1/2 to 3/4 inch length.) They also sell the lubricant you put a couple drops in a quart or water. Works great. I have been using the same grinding wheel for 15 years in my glass shop. Granted it is not used heavily.