I still use the technique I described last March on my blog (Rock drilling 101) to drill beach rocks, and once in a while a few pieces of sea glass. Depending on the material I’m drilling, the drill bit will make a clean exit on the other side of the rock, but most of the time it does not. Especially with drilling glass, the exit end chips off instead of making a clean hole.
(Did anyone else just start thinking about jelly beans?)
The same thing happens with granite pebbles.
My solution is to drill into the rock (glass) 3/4 of the way through, (using the hot glue and class cup with water technique), pop the rock out of the glue, and determine where to start drilling from the opposite side to have the holes match up in the middle of the rock. There may be an easier way to do this, and if anyone knows a better way please leave me a comment!
But here is what I do, and I have about an 85 to 95% success rate (…depending on how recently I have been drilling. Practice makes perfect.) With glass it is easy to line up the 2nd drill hole by just holding the glass up to the light. You can usually see where to mark the opposite side, because the existing drill hole shows up easily from the other side.
With rocks it’s a little more difficult to determine. I start by placing the 3/4 drilled rock on the opposite end of a discarded drill bit.
Then I draw a line up and just over the top of the rock, trying to keep the pencil in line with the drill bit. I turn the rock 90º and draw another line. I do this until I have four lines going up and over the top of the rock. With any luck, they will all intersect at one point, and that will be the point where I start drilling the second hole. I usually make a good sized mark on that point. I check it by spinning the rock on the drill bit axis and looking down at the mark. If the mark stays at the center of the spin, I know there’s a good chance that I have the mark in the right place.
Even with the mark in the right place, I still have to be careful to place the rock at the correct angle in the hot glue to drill my way neatly through to the original hole. The advantage to drilling the first hole at least 3/4 of the way through is that you will know if your second drill hole goes deeper than 1/4 of the way through, you probably are not drilling in the right place.
The best advice I got when starting to drill rocks still holds. Go slowly and be patient!