Core drill bits are very useful, as only a thin wall of glass is removed, leaving the main part of the hole as a single piece. This means that it is quicker than a solid drill bit through not having to remove so much glass. However with small diameter core drills they often jam up with waste glass. This is evident when the drill ceases to drill through the glass, leaving a broad circular mark as it moves around on the glass.
Inevitably, at one time or another, the glass core will get stuck inside the drill bit. This needs to be cleared before any further drilling can be successfully done. It seems to be more common with the smaller diameter drills – from 5mm down.
The broken off core needs to be cleared from the back of the bit, not the diamond coated end. So you need to take the bit out of the drill and use a thin nail or piece of wire into the hole to push the glass out.
|My collection of core drills. At the front you can see copper wire and a thin punch for clearing blocked drills|
Core drills need a more accurate means of steadying the bit than your hands. Imagine that you are trying to get that core out of the glass in tact. Any wobbling as you drill will break that core. When the core breaks, it is inclined to jam the cylinder. So a drill press is almost essential when using a core drill.
They also need water like any other drilling of glass. Normally this is supplied through the hollow core of the bit, which is beyond hand held drill motors.
For something as small as 3mm, a solid drill that removes all the glass rather than a core is more likely to be most successful. You can get diamonds bonded to piano wire that will give you holes from a fraction of a mm up to 2 or 3 mm and these can be used in dremmel-like tools that take small diameter bits.
It is absolutely NOT recommended to use a core drill at an acute angle to start the hole. The wall of a 3mm core drill is very thin and easily damaged. It is ok to start a solid drill that way, but not a core drill.
There are a number of related posts beginning with this one which give advice on the many ways to drill holes in glass.