Have you tried different size abrasives? I don't have much
experience with ultrasonic drills, but with resin bonded diamond
blades, 12 micron diamond can cut silicon crystal and sapphire
crystal, but not glass. I have to change to 53 micron diamond to cut
There should be a frequency adjustment knob for tuning an ultrasonic
drill. Without it, you could not compensate for small amounts of
frequency drift, or more importantly, different mass loads from
changing the cutting tool on the horn.
To avoid blow-out, you have to have something underneath the stone,
clamped or bonded so that it does not separate during drilling. Ease
up on the pressure too.