There are a number of reasons a DIY aficionado would want to drill a hole in tempered glass.
However, you should know that truly tempered glass cannot be drilled. Not with a diamond-tipped drill, not after clamping it in place and securing it with clay, not today, not tomorrow, not ever. The simple reason for this is that tempered glass is hardened. Like diamond, it is very hard, and that means it is equally brittle.
The good news is that there is glass that has some of the hardness of tempered glass which can be drilled.
Can You Drill a Hole in Tempered Glass?
This is something that sword makers, for example, understand very well. The toughest swords are a bit soft, allowing them to bend instead of break, but the edges are known to become dull faster. Another classical solution is to use spring steel which is flexible and regains its shape after being bent. The famous Japanese sword makers of the 15th century approached a near-perfect solution by making blades with a very hard and brittle edge, and a relatively softer spine. This would make the sword salvageable even after a catastrophic failure.
Truly tempered glass panels are something like the edges of those classically made katanas. They are very hard, but when they fail they chip and shatter. If you do manage to exert enough force on the tempered structure of a pane of properly tempered glass, it will shatter, much like the windshield of a car- breaking out in spider web-shaped cracks that make the window useless for future use, except as an exhibition of error.
What Exactly is Tempered Glass?
Tempered glass, also known as toughened glass or “safety” glass, is annealed through a temperature treatment process, much like the tempering process used on swords. After it is tempered, it is compressed, making it denser and placing it into a state of elevated tension. So, what we’re talking about here is a substance that is both harder and denser than ordinary glass. This means that when a failure occurs, it will occur more violently than it would with a material that was not both dense and hard.
Let’s do a little thought experiment.
Imagine a trucking container filled with tennis balls. Suppose we also had an enormous saw that we can use to cut the trucking container as we see fit. Now, we might harden our trucking container by leaving it in the Antarctic at the right time of year. But suppose we could increase the pressure of those tennis balls by packing in more balls, or by magically shrinking the container even slightly.
Our container is now both harder, and denser. But the internal pressure from the air in the tennis balls is still present. If we were to take our giant imaginary saw to our tempered container of tennis balls, the result might well be rather explosive, with yellow fuzzy balls flying all over the icy tundra- which would probably be good material for a chewing gum commercial.
In short, the compression part of the glass tempering process packs in additional energy that would otherwise not be in there. That extra energy is what will cause the glass to shatter should you attempt to drill it.
You may have heard of people who claim to have successfully drilled a hole in tempered glass. Any successful drilling of glass is most certainly done with glass that is not fully or properly tempered- and that is where we’re going next.
How to Identify Tempered Glass
If you’re looking to drill a hole into glass and you absolutely cannot be dissuaded, hopefully, we can convince you not to try it on truly tempered glass. There are glass panel products out there that are marketed as “tempered,” but are not the real McCoy. At this point, your goal should be to get one of those types of products and drill into it, rather than an expensive piece of glass that is sure to be ruined. The way to do this is to know how to spot truly tempered glass.
Tempered glass is expensive to produce, and requires the work of a trained and experienced professional who will, in most cases, leave a watermark at one of the corners. Higher quality panes will almost always have it, but it may be faded on older tempered panes.
Just as with hardened sword steel, the high hardness factor of properly tempered glass makes it harder to eliminate imperfections. Most such pieces will show small warps, bubbles, indentations, and the like. When small and few, these can contribute to the beauty of the glass. It also is an all but sure indication that what you have is the real deal. Of course, any glass can be warped or contain other imperfections. If you look at enough truly tempered glass, you’ll learn to spot the ineffable quality of it. In fact, we strongly recommend that you make an effort to look at a lot of tempered glass to get to know its visual qualities.
Because tempered glass is quite brittle, a side effect of hardness which we have hopefully discussed sufficiently, the edges are going to be prone to chipping. This is a good illustration of why you cannot drill this stuff. Almost every good piece of tempered glass will have some chips along the edges. Such chips will often appear during transport even when all the proper precautions are taken as the glass will bend slightly under its own weight and the high internal tension is likely to kick out chips of glass. Ask any glass expert, and they’ll tell you- chips on the edges of a big piece of tempered glass are not a sign of poor workmanship.
Finally, you may have noticed that properly annealed glass has a special, misty, watery, and dreamy look to it. Light passes through it more slowly than it does through regular glass because of its density. This nearly imperceptible quality makes it both strange and pleasant to behold. This is why it’s a good idea to try to look at a lot of such glass to try and become familiar with it. Hopefully, you are able to recognize it when you see it and avoid misguidedly trying to put holes in it.
How to Drill “Tempered” (but not really) Glass
Of course, even non-tempered glass is relatively expensive, so you want to take care not to break it while drilling it. You are going to need a specialized diamond drill bit and plenty of lubricant. Know that different types of drill bits must be used at a certain speed to avoid shattering your glass.
1. Insert your diamond drill bit and tighten the bit-grip. Adjust the speed of your drill to suit the designated speed of the bit itself.
2. Clamp your glass down using c-clamps. If possible, you should support the entire pane with a bed of flattened clay (ideally) or a solid block of wood. This will absorb vibrations and reduce the chances of breakage.
3. Surround the spot you intend to drill with a loop of clay and begin drilling. The purpose of the loop of clay is to create a bowl that you can pour your lubricant into.
Keep your drilling spot fully lubricated throughout the entire drilling process. Take your time and do not, that is DO NOT exert too much pressure on the glass. You can expect the drilling procedure to take at least 10 minutes for every quarter-inch of glass.
Alright, Time to Come Clean
The truth, as you may have suspected, is that drilling truly and properly tempered glass is not entirely beyond the realm of possibility. However, to do this requires the highest quality diamond drill bit, the most fulsome and professional securement table, and a lot of time time. It could take as much as thirty minutes or more to drill a piece of truly tempered glass ¼ if an inch thick without breaking it, maybe even longer if you have a really high-quality piece.
Needless to say, the necessary equipment would be very expensive and you are running the risk of breaking your already very expensive tempered glass. It is also something that is rarely if ever done because tempered glass can be made with holes in it straight off the production line. That is the best way to get perforated tempered glass, and if you’ve got a pronounced desire to get your hands on some, your best bet is to order a fresh panel with the desired hole/s built in from the get-go.