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Drilling 2mm holes in slabs?


I am using a fordom drill press and flex shaft and number 30
handpiece for drilling 2 mm holes in agates, jaspers, woods, etc. I
am finding that in spite of the directions that have come with the
drill bits to use the very high rpms, I am drilling holes quicker
and getting more holes per drill bit when running at slower speeds.
I have a foot operated fordom so don’t really know how fast I am
drilling but it seems a lot slower than the recommended speeds for
using the diamond drills. The rocks are slab thick and submerged in
water with some water soluble oil mixture added.

Have I misread the direction for using the drill bits in rock?

Second, part of topic is a question about the best way of preventing
the “blow out” when the drill is almost all the way through the
slab. This action of breaking through seems to be what is one thing
that is cutting the life of the drill bit. I think this action tears
some of the diamonds from the bit, making its life a lot shorter. Am
I correct is this assumption?

I have another observation that I would like others to verify by
their experience. Out of any 10 drill bits, are there some (a few)
that will just be poorer in quality and produce a lot fewer holes
than the other drill bits? It seem that every few bits I will find
one that will not last near a long as some of the others.

I don’t have pictures of any of these drill stones on my web site
yet but hope to soon. If you would want to see some, I could take a
picture of some and send then to you. Email me off line.

Larry E. Whittington
Larry E. Whittington Lapidary


Hi Larry,

I’ve drilled many holes while making beads (with 3-6mm core drills)
and know what you mean about fast vs. slow speed. I use mineral oil
too because of the drill press + X-Y table set up I drill on. I think
the slower speed does 2 things. Less speed means less heat builds up
so the diamond particles aren’t dislodged as much and they aren’t
"covered up" by the metal in the diamond coating. Second, slower
speed also allows the diamond to bite into the stone better. Others
may not agree with me but this has been my observation so far.

One thing you can do to minimize or even avoid blow out is that when
you know you’re getting close to drilling all the way through, turn
your stone around to the back side and hold it up to a light. Mark
the spot where you see the light showing through then use a ball
shaped bur to get a hole started from that side. At that point, with
your 2mm bur you can just drill the remaining little bit of material
out of the channel. So how do you know when you’re getting close?
Before you even put the bur into the drill chuck, hold the bur
diamond end pointed down next to the slab and use a Sharpie marker to
mark a line on the bur just short of the top of the slab. That way
you have a good idea of how deep you are into the slab when you’re
drilling and can judge when to stop and do the above.

I know what you mean about inconsistency from one drill to another.
Guess not every bur gets dipped (is that the process?) the same way
so more diamond particles are adhered to some and not as much on
others. With sintered burs this wouldn’t be an issue. Would LOVE to
see how both of these types of burs are made on Discovery Channel’s
"How It’s Made"!

Hope this was of some help and not too “boring”. Sorry couldn’t pass
that one up! :slight_smile:



Larry…Don’t know why the instructions should tell you to use high
speed??? Are you using plated or sintered drills? In either case,
slow speeds allow the coolent to wash away the swarth and cool the
surface being cut. Don’t carry to extreme…you need around 500-1000
rpm to be effective but below 500 things will really slow down. The
drill press should be no problem in regulating the speed. But with
the Foredom it is easy to get up to 5000or more rpms quickly.

Re the blow out. I usually try to drill part way through and then
drill from the other side. Sometimes that is not convenient and I
will coat the reverse with dop wax or even dop a piece of glass to
the reverse. That way, the dumb drill doesn’t know its through!!

The quality of the drills depends a lot on what drills you are
using. If made in China, the quality may vary widely…especiall if
they are the plated variety. You might want to try 'triple-ripple’
drills. They are moderately priced but cut effeiently and the quality
seems to b e standard. Happy cutting

Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry!


Two methods are used in woodworking. First, clamp the slab against
another, sacrificial, slab so that the drill goes all the way
through and into the second slab, rather than breaking through into
air (or water :slight_smile: Both slabs have to be flat for this to help.
Second, drill a pilot hole of a smaller diameter, then drill halfway
through from both sides with the larger diameter.

Al Balmer
Sun City, AZ



I’ve been doing a lot of exactly what you describe lately, so
hopefully can answer some of your questions.

I find that the greatest success of drilling does happen at a much
lower speed than the “optimum” speed recommended for the diamond
burs. It also varies by the type and density of the rock, which I
suspect has a lot to do with the internal buildup of both sludge and
heat, even though submerged in cold water. Drilling slower also does
seem to increase the life of the bur.

I’m just using plain water without an oil mixture… provides great
clarity and better flow of sludge away from the drilling area. I keep
pointed makeup applicators (like ultra-dense Q-tips with a sharp
point) handy to wipe out the hole frequently. Keeping the sludge out
of the hole definitely extends the life of the bit… and is a bit
tough with the 2mm sizes.

Preventing blow out is a tricky matter. While I’ve successfully
drilled front to back non-stop without blowouts, that’s a matter of
some luck. The much more failsafe way (although it requires good
measurement skills) is to do a small drilling from the back of the
rock no more than about 1/4 of the way through to the front. Then
drill from the front to meet that back-drilled hole. No blow outs.

I get my diamond bits from Lasco ( and have
been very satisfied with the consistency of their bits. So I haven’t
seen a huge difference between them. Is it possible that you’re
seeing a difference based on which materials you’ve drilled with
those specific bits? For example, when I drill sea glass or a really
hard agate, I get much shorter life than when I do river stones,
lapis, malachite, or something like that.

I’m also trying right now an alternative using hollow-core diamond
drills, to see if that produces more efficient drilling and/or less
blowouts on straight-throughs (no back drilling). I’ll let you know
what I figure out.

A picture of a few of my drilled pieces using this technique are:
(this is a dense agate, set with natural padparashas)
(sea glass, with diamond and sapphire in forged setting)
(river pebbles and sea glass, with garnets and black diamond)

I hope this helps!

Karen Goeller
No Limitations Designs
Hand-made, one-of-a-kind jewelry


To prevent (reduce damage) from the drill breaking out of the slab
wax/pitch a sheet of 1/8" glass to the back of the slab.

Diamond coated drills always have a bit of variability in working
life, it’s just the way it is…



Here’s a method I’ve used to prevent breakout. For a 2 mm hole I
would use a small piece of plywood, drill a 2 mm hole, insert a 2 mm
dowel in the ply, just a nipple. I use lapidary wax to attach the
slab to a piece of ply. I drill the stone inside a small plastic
container with a hole for a drain and use water to flush and cool.
Also a lot of up and down with the drill to facilitate the flushing
action. I also use an ear syringe to aid in the flushing. I also have
an old HD press that has a cam which automatically withdraws the
drill periodically. Of course this can be done manually.

To get back to breakout: I drill part way through and then reverse
the slab and the partly drilled hole is placed on the dowel nipple
to keep the alinement. Stability is a big part of the process. I use
green lapidary wax to keep the stone on the drilling surface (ply)
so there’s no movement.

Stability, along with sufficient water flow, will increase the drill


One thing you can do to minimize or even avoid blow out is that
when you know you're getting close to drilling all the way through,
turn your stone around to the back side and hold it up to a light. 

If your material is opaque, draw a line across the stone, mark the
edges, draw another line at right angles to the first, mark the edge.
Turn the stone over and draw lines from marks on edges straight
across to match the lines on top. I have done this and it works. It
might be a tiny bit off, so use a smaller bit first if size of hole is

Richard Hart


There are actually three kinds of diamond drills available. Plated,
sintered and brazed.

The first one, plated, I’m sure most are familiar with this one. The
diamond is a mono layer that has been applied to the steel substrate
by an electronic bath process. This process is the most common type
but has the shortest life and is not very consistent. It tends to
over heat and peal the nickel adhesive coating. Very inexpensive too.

The second one, sintered, is a multilayer and has no steel substrate
except for the butt end of the drill bit which is chucked into the
drill. The diamond is throughout the thickness of the core or as in
a hollow, or core drill, the wall thickness. These last quite a long
time but will loose the actual diameter due to the wearing of the
metal that is mixed with the diamond while drilling. These also tend
to clog more and loose their cutting ability over a short period of
time. Sintered can be re dressed with a silicon carbide dressing
stick when it becomes clogged but once again you will loose the
actual diameter of the drill. Sintered drills can become very thin an
can fly apart at some time. Be very careful. Sintered drills are

The third kind, brazed, is a mono layer of diamond that has been
welded with flux and extreme heat to the steel substrate. because of
the application with extreme heat the diamond has no chance of
pealing as the bit gets hot from use. Unless, of course you are
getting the thing red hot then you have bigger problems. If any thing
the diamond itself is friable and will crumble under undue heat and
pressure or just round off due to use in the same direction of
rotation. Because of the diamond application process, these tools are
more consistent across the board. And because of the mono layer and
no metal holding the diamond in place the tools has more air space
around each diamond giving each diamond more ability to cut without
clogging as fast; making it a more aggressive drill. The price of
brazed tools falls somewhere in-between the plated and the sintered.

The quality of the drills depends a lot on what drills you are
using. If made in China, the quality may vary widely...especiall
if they are the plated variety. You might want to try
'triple-ripple' drills. They are moderately priced but cut
effeiently and the quality seems to be standard. 

As far as I know “Triple Ripple” drills are the only drills on the
market that are brazed. Not only are they brazed but they are also
fluted to allow for more efficient swarf removal. Crystalite Corp.
manufactures them. They do come in a 2.1mm size. I have no monetary
interest in them.

I agree with all of the other suggestions with regards to speed and
backing (wood works too) but I will add that a 5% solution of ethyl
glycol be used with water. Drill fully submerged. Use a light hand
and let the diamond do its business. Also release the pressure and
lift the drill out of the hole, while running, to help flush it and
the swarf on the bit regularly. Slow down and very light pressure at
the end will lessen the blow out. I recommend to make a dimple with a
diamond ball burr approx. the same size as your bit to start just
like making a center punch on metal.

Also a note about why the lasting qualities of drills with different
stones. Different stones have different abrasive qualities on the
metals that are holding the diamonds in place. So some stones will
wear away the metal thus releasing the diamond before its time.

Talk about boring… yet informative. Got to go… the lightning
and thunder is here! OOOOh Weee first of the year! Happy cutting!

Elayne K. Luer, GJG
About Lapidary
Willits, CA


I haven’t followed this thread so pardon me if I repeat someone else
or have missed the point entirely

regarding the question of why a slower speed seems to work
better… probably centrifugal force pushes the water away from the
contact point at high speed. It also occurs to me that you might
think of the speed/pressure thing as a continuum…go slower with the
same pressure and the grit actually gets a chance to dig in vs just
skimming the surface making a nasty squealing sound.


My method of drilling is the same as the consensus however I always
drill with a .75mm bit no matter what size hole I want. It’s quick to
go through, the breakout is usually very small and there is room for
slight adjustment if you are off. Making the hole bigger is trivial,
dentist reject tapered bits accomplish this rapidly.

Tony. Anthony Lloyd-Rees