I have also had poor luck drilling and widening holes in things as
simple as Lapis which only has a hardness of 5.5
As you already have considerable drilling experience, I assume the
problem is in technique.
Drill - I use the regular Diamond drill bits. They are flat ended
and drill penetration is slow, you can cram it in (read that as
running a normal drilling feed speed) and it will run a little
faster, but the cost is break-outs, broken stones, bent or ruined
bits and split beads. In fact the speed of the bit and the down feed
pressure are the perfect conditions to make a normal drill bit
Drill motor - I use one of the little harbor freight drill presses,
and run it around 3500 to 5000 RPM. You can use anything that has a
stable chuck and gets the speed you need, I have drilled by hand
using a Foredom but you end up stopping a lot to let your hand and
arm rest even with a rest.
Holder - I use a regular little tool makers vise with leather over
the jaw faces. Careful on clamping pressure, while stones are hard,
they are very brittle, the same with glass. Make it just tight enough
to hold steady, but no tighter. Normal drilling we usually put
something the vice for drilling and snug it down an extra half or 3/4
turn, don't go much over an 1/8th to a 1/4 turn for stones or beads
after it 'sets'. How much you can go depends on what you are using
for a cushion, mine is thin leather and I can't go over what I
Coolant - I use Tool Kool when drilling, not because it is the only
thing, it is what I have around most often. Plain water is ok but its
better than oil, but it will work, works better if you add a drop of
fabric softener to about a cup of water. Tried oil, don't like it,
hard to clean up the pores in the hole afterward, makes a bigger
mess, and doesn't circulate in the hole as well as water.
Setup - After placing the work piece in the vise, build a little
retaining wall around your drill point to hold the coolant. I use
toilet ring wax, but anything similar will work, kids modeling clay
for example, I have even used butter once, but that was before I
bought the toilet ring.
Starting the hole - Fill the little reservoir with the coolant of
your choice. As with large bits which have a large starting area,
this little guy will want to walk all over the place. Starting the
hole is probably the most tedious part of this effort and the most
critical, like regular drilling. You can start by just bringing it
into contact and slack off a little when it starts to walk. Seems I
have my best luck and a little quicker start if I use a bouncing
motion on my quill, it could be too, that I am just doing something
to keep my mind off the amount of time that has elapsed. If the place
you want to start has any angle to it, just settle down and bear it,
it will not be quick starting the hole. Keep an eye on your bit, if
it deviates the littlest fraction from vertical, you have to back off
and slow down because now you just put a slope on your hole that the
bit is going to want to follow.
Drilling - Drill about 1/4 to 1/2 of the cutting area length and
pull up to let the coolant circulate out the debris, occasionally
bring the bit up to the surface of your coolant to clear the drill
face and it also lets coolant back into the hole. Keep the fluid
level up in the little pool.
Finishing your Hole - Way up yonder, I should have said mark your
drill bit to the depth you want to drill. Completing a hole through a
stone is as time consuming as starting one. If you break through with
normal drilling pressure, you will break out the back of the hole,
and on a bead or stone this can be disastrous. As you near the mark
on your drill, treat it like you are beginning a hole again, easy
pressure and keep the coolant moving in the hole. For a very light
break out you can use one of those little cone shaped diamond bits to
bevel the hole, don't for get to use coolant and use as fine a
abrasive as you can get. Before I found mine I used an 1/8 inch
dowel, sharpened in a pencil sharper with 600 grit carbide powder to
clean (bevel) the start and stop faces.
Widening a hole - Same as staring a hole, these little bits have
absolutely no way to use a pilot hole, if you look at the face of the
bit you will see what I mean, it is flat, square, solid and no point.
When it cuts it is like starting a new hole, it may be even worse,
your coolant will drain out the existing hole, so don't forget to
plug that and your bit will be running off course to follow an easier
path so you will have to drill slower.
General info - There is a slight benefit to drilling softer
material, but it is slim, when you are drilling you can go a little
faster, but that is it. It is compensated by the fact you have to go
a little slower and start the exit process a little sooner when you
are near the end. When drilling things with a diamond drill, all
materials (stone or glass) are created equal. Heat is bad in drilling
stones or glass, flush your bit often. I asked the guy at the rock
shop about using the diamond coated spiral bits, he said there was
not much difference, and definitely not enough to warrant the extra
expense. As he has been right so often on other things it is not
something I ever validated. In some ways drilling stone is like
drilling stainless, you get chips instead of the nice spirals, and
with rock it is a much finer chips. I have yet to discover a backing
material that will protect from break outs.
Sorry for the length of the post, but as I was drilling this
morning, and it is all fresh in my mind.
Go give it a whirl, this should give you good holes, it does for me.