Keeping Things Wet
When Drilling glass it is important always to keep the drill bit and glass wet, otherwise the glass gets too hot and will break. There are a variety of things you can do to achieve this.
Drill under water in a containerDrill in a ring of clay, plasticine, etc., holding water. To do this, you need to make a ring about 50 mm in diameter and press it around the drill site. Fill the ring with water and a little diamond coolant. This will cool the drill site and glass. Diamond coolant is not necessary, but extends the life of the bits.Use a re-circulating water pump such as those made for indoor water features. Direct the small flow of water to the drilling site and catch the water in the bucket in which the pump is submerged.Use a glass drill with hollow core bits and an internal water feed. This is the most expensive but it is the best equipment with which to drill large holes.
It is best to have a drill press if you are doing a lot of drilling. It provides a stable drilling action and the pressure on the bit can be controlled. It is important to ensure the bit is running true without wobble. The drill press should have instructions to help correct any untrue running of the chuck. Make sure the drill bit is secured firmly. Core drill bits are easier to keep true, as they normally have a threaded fitting.With a drill press, you can drill continually until the hole is completed, or until a white paste or dust begins to appear. This indicates the drilling is being done dry and will in a few moments heat up the glass too much. When the white paste appears, back out of the bottom of the hole a little to allow water to flush the glass out. Then continue.
Keep a firm grip on the glass being drilled. Maintain its position, especially if you are intending to back off intermittently to allow water to the bottom of the hole. This enables you to get back into the hole without scratches.