Hi Carol, That's a great question. Here is what our shop manager had to say: First, We use a drill bit specific for glass and a water-based lubricant to squirt as we drill into the insulators. We also use a special machine made to apply the right amount of pressure and is variable speed. If you are looking to try it yourself, you can pick up a cheap drill press to achieve the same results. The holes should only be drilled using a fixed-base drill. Never use a hand operated drill as it will not center and will walk all over your glass. Hope this helps. Thanks!
Have to ask them they had plenty there
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