Hi Fred,
The operating guidelines must be followed for the carbide bits otherwise failure can occur. They are:-
a) the speed must be 700-900 r.pm. We have had drill bits sent back that have heavy scoring around the shanks. This indicates that the chuck of the power tool has not been tightened correctly and so speed of operation cannot be guaranteed.
b) a constant flow of water must be applied to the drill tip at all times during drilling. This is not massive volumes of water, a fine jet is sufficient from a pressure sprayer.
c) in the hardest grades of porcelain, a fairly firm pressure must be applied to keep the drill bit progressing through the tile. If the drill bit is allowed to "dwell" in the tile, not making progress, then wear can occur. If it is taking you more than a few seconds to produce a hole then it is invariably due to the fact that not enough pressure is being applied.
Best regards,
Phil.