Optimal Dremel Speed for Drilling Gemstones with Diamond Bits

Question
Hi Thomas
I read your answer to Mary about drilling into gem stones and understand the concept.  I have a variable speed Dremel and need to know if the speed of the Dremel needs to run at the highest or lowest or somewhere in the middle speed when using the diamond bit.
Thank you very much.
Paul

Answer
Dear Paul,

Start the drill tip with tight control and a speed just fast enough to allow the diamond to take a bite of the material without walking around. Too slow and the bit wants to wander and not get started drilling into the stone, leaving surface scars.  Once you have a starting point, keep the bit wet and the hole cleaned out as you drill, using a medium speed.  Mostly you can tell from feel if the bit is drilling and is staying cool enough.  Too much speed and heat and the bit will be ruined or break off and seize inside, the last being a frustrating bit removal generally by dissolving the bit with acids over a period of time or trying to rust it out.  Overall this is it: A medium speed which is drilling down effectively as observed by slowly increasing depth and removed material around the hole, kept moist during work, bit removed and hole rinsed ever so often.

Try first on some scrap material, even broken glass.  The practice will provide a feel of the operation.

Best wishes with the work.  God Bless and Peace.  Thomas.