The general guideline for holes is that there should be minimum one hole diameter's distance between glass edges, either hole to hole, or hole to side/top edge of the glass, 1.5x the diameter being better/preferable. So if you are drilling 2" diameter holes, the tops of the holes should be at least 2" from the top of the tank and the sides should be at least 2" from the end of the tank or the nearest hole.
Any time you drill a hole, you weaken the tank somewhat. having a thin glass 'bridge' between holes gives a weak point that is more prone to fracture.
Some general tips for drilling:
- Thin glass is harder to drill than thick glass and much more prone to cracking
- Have patience! It will take several minutes.
- Glass hole saws don't actually drill, rather they grind the glass. More pressure doesn't necessarily help but it does increase the risk of breaking the glass. As you get close to breaking through reduce the pressure on the drill to just the weight of the drill.
- Use a guide to start the hole and prevent the bit from skittering around and marring the glass until a groove is formed. BRS sells a drill guide, but if drilling by hand cut a hole in a ¼" piece of plywood or hardboard first.
- The biggest risk for chipping occurs on the back side as the drill breaks through. Since chips can interfere with the gasket sealing, drill from the flange side to the nut side if possible.
- Clamping a piece of plywood on the back side as well as taping the back side of the glass can help support it and prevent chipping
- Water helps keep the bit and lubricate/remove glass bits. Make a berm around the hole with plumber's putty to hold the water in place. If you have to drill on a vertical surface, have a friend with a hose spray water.
- If possible, get some pieces of glass from your local glass shop to practice with. (don't practice too much and wear out your bit, though!)
