Why do drill bits or carbide burs sometimes break?

Question:

Why do drill bits or carbide burs sometimes break just as they penetrate the first layer of glass?

Answer:

The most common reason drill bits or carbide burs sometimes break is that the technician is drilling all the way through the first layer of glass, which is not recommended. Dull drill bits are also far more likely to break. The technician applies too much pressure trying to get the bit to drill faster, causing the bit to overheat, weaken and break. If using a tapered bur, rotating the drill clockwise in a circular motion during the drilling process helps prevent breakage and shapes the hole in such a way that air bubbles will be much easier to remove from the drill hole. This procedure also provides a perfectly shaped hole for use with the Delta Kits slide hammer and spring hammer.