These are cheap bits. Won't stand up to hard work and tough materials (glass over 1/4 inch thick or thick glazed ceramic tile). The diamond coating will start to wear off after use. most drills are of this type, either water or cooling oil is used for drilling) The set I received only lasted one pass (each drill drilled about 4 holes, and the last one turned out to be very miserable) and then they went straight into the dumpster. Treat these bits as single use. And in case you're wondering, I have a very intense 12 years of experience drilling holes in glass, ceramic tile, marble, granite, etc. My Opinion: DO NOT BUY.