I have been drilling deep holes in very hard rock for more than a year. We digested over 20 bits. Makita 6 cutters last twice as long as others. We also didn't use any junk bits. Bits from the American company beginning with "D" averaged about 15 holes before becoming too prone to snagging. Bits from a German company starting with "B" worked better at around 25. 6 cutters averaged about 40 holes and some bits were over 50. They also never started to stick, they cut until the carbide broke. These are the only ones I buy now.