Re: [ATM] drilling glass

[Dominic-Luc Webb <dlwebb@canit.se>]

On Sun, 13 Nov 2005, James P Crombie wrote:

> to do so core drilling of a mirror blank I ran across some diamond core
> drills.  They are in Hong Kong but the prices are pretty cheap.  No
> affiliation, just thought they were interesting :-)
> http://cgi.ebay.ca/Assorted-28-40mm-Diamond-coated-drill-bit-hole-saw-6pcs_W0QQitemZ6576919585QQcategoryZ4843QQrdZ1QQcmdZViewItem
> _______________________________________________


Hi James,

These really look like they should make your life easier, but
having used such drill bits, albeit not these particular ones,
I should say that they ended up expensive and not very efficient
in my hands. I have great success with common metal drill bits
for smaller holes and iron plumbing pipes for bigger holes.

I use a drill press and make a well with plastacine and then
put a little water and grit into the well. I tie a string with
a block of wood to the lever to get only slight pressure against
the glass. The wood is cut to whatever size is needed to get
the small amount of pressure I want. This is cheap and works
great. It takes a bit of time so you need to be patient. I have
bored holes as small as 3 mm and cored full thickness 12" and
larger mirrors with holes as large as 100 mm diameter and more than
60 mm thick. When I get really close to the front surface, I stop
the job in order to score the front surface with a glass cutting
tool to ensure the break actually occurs right where the drill will
come through. It is at the end on the front surface where horrible
breaks can happen. My bits are generally re-usable, but with me,
stupidity and impatience sometimes take over and I break a smaller
drill bit. But then again, those bits are discount store cheapies,
so I do not care and impatience speeds the job along with the drill
bit being acceptable collateral damage.

The diamond ones on the other hand, tend to have comparatively
softer shafts and the diamond usually wears out very quickly.
Once worn out, I do not try to use these with grit. I just feed
them to my eagerly awaiting garbage can. Even at the low price you
mention, I am so content with my homemade bits that I am not planning
to ever use diamond again. Remember, we ATMs already have plenty of
SiC grit, which clearly excels at such jobs, which is why we use it
for grinding. I suspect the shape of SiC particles gives this compound
a slight advantage even though its hardness is less than diamond
(but not much, 9.5 for SiC vs 10 for diamond), and grit comes in vastly
greater quantities per unit money.

Bottom line, the myth of the greater usefullness of a diamond bit is
busted (but that is only my opinion regarding this application).


Dominic-Luc Webb

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