Making holes in soft materials like wood is easy. You just stick a standard drill bit in your drill and pull the trigger. But use this approach on harder materials like metal, masonry, glass or tile, and you'll waste time, ruin drill bits and even wreck your workpiece.
This article will help you choose the right bits and techniques for making holes in hard materials.
Getting the speed right
Running your drill too fast can cause heat buildup and ruin drill bits. Drill speed is measured in rpm (revolutions per minute). On the side of any variable-speed drill, you'll find a sticker that lists the rpm range: “0 to 2,000 rpm,” for example.
Drills don't have exact settings or speedometers, so you can't run one precisely at the speed you want. But if you know your drill's maximum speed, you can come close enough by the sound of the motor and the feel of the trigger.
Squeeze the trigger slowly and gradually increase to full speed. Then gradually decrease speed. Do this a couple of times and you'll develop a feel for where the trigger produces half-speed or quarter-speed and what those speeds sound like. This is easier with cordless drills, since most have high and low ranges.
Drilling a hole in a pane of glass or a mirror is simple. The key is to use a carbide bit made especially for glass and tile (see photos). You'll find these bits at most home centers alongside other drill bits or ceramic tile tools.
Because glass is extremely smooth and hard, the bit will want to wander as you start drilling. To give the bit a foothold, tape a small scrap of dense cardboard (like cereal box cardboard) to the glass. Begin at very low rpm to create a dimple in the glass, then remove the cardboard and continue at about 400 rpm. If you're drilling on a horizontal surface, you can pour a little oil on the area.
Make sure the glass is firmly supported on the backside and place only very light pressure on the drill; press too hard and you'll crack the glass. The bit creates a clean hole on the side it enters, but usually chips the edges of the hole on the other side.
Note: You can't drill tempered glass.
Most of the drill bits you use for wood will also bore into metal (see photos above). But a spinning drill bit tends to wander across metal's hard, smooth surface before it begins to dig in. You can give the bit an exact starting point using a center punch and a hammer. The punch creates a tiny dimple that keeps the bit in place (see photos above). With a soft metal like aluminum, you can use a nail instead of a punch.
A little oil helps you drill faster and keeps the bit cooler, so it stays sharp longer. There are special drilling oils, but you can use just about anything—motor oil, transmission fluid, kerosene, even cooking oil. On a slanted surface, keep the oil in place with a ring of plumber's putty, glazing compound or even Play-Doh.
If you're drilling holes larger than 1/4 in. through metal more than 1/16 in. thick, save time by boring smaller pilot holes first. The tips of many drill bits have a flat spot that doesn't slice into metal nearly as well as the sharp outer “lips.” By first drilling a pilot hole with a small bit—about the same diameter as the flat spot on the larger bit—you allow the larger bit to cut faster. Seal the underside of the hole with duct tape to keep oil from draining away.
Use firm, steady pressure and moderate speed (600 to 700 rpm) until you're nearly done. As the bit breaks through the other side of the metal, the lips can grab the thin remaining edges. This causes the workpiece (or drill) to spin and might leave you with a broken drill bit or workpiece or even injuries. You can avoid all of this if you ease off the pressure and go to full drill speed just before breakthrough. To be on the safe side, always clamp workpieces in place.
Tile varies greatly in hardness. You can drill some types of tile using a standard carbide masonry bit. Harder tile requires the bit you'd use for glass (see “Drilling into Glass” above). In either hard or soft tile, you can make a hole large enough for plumbing fixtures with a carbide grit hole saw (see photo above), available at home centers and tile suppliers.
To get a hole started without wandering, use the cardboard trick shown in “Drilling into Glass” above. The hole saw works best at low speeds (100 to 200 rpm). It cuts slowly and creates a lot of heat, sometimes enough to crack the tile. To prevent “heat shock,” immerse the tile in a shallow pool of water. Water keeps everything cool and actually helps the hole saw cut a little faster. Place a scrap of plywood under the tile so you don't drill through the pan.
WARNING: Water and electricity are a dangerous combination. Plug your drill into a GFCI-protected outlet or use a cordless drill.
Most forms of masonry—mortar, stucco, brick and concrete block—are fairly easy to drill. Use a carbide-tipped masonry bit (see photo above), push hard and run your drill at about 1,000 rpm. Pull the bit out of the hole occasionally to clear out the powder created by drilling. To make drilling a large hole easier, begin with a small bit and work up: 1/4 in. to 3/8 in. to 1/2 in., for example.
When you're using a standard drill, concrete presents problems that other types of masonry don't because it's full of small stones. Some of these stones are soft enough to drill through easily. But if you hit a hard one, it will stop your progress dead. When you hit a hard stone, keep drilling for a few seconds—you might break through. If not, simply move over and try another spot. If the hole has to be precisely placed, use a punch or small chisel to break up the stone.
A standard corded or cordless drill is fine for drilling a couple of holes in masonry. But if you have to make lots of holes, use a hammer drill to speed up the job (see photo above). Hammer drills not only spin the bit but also hammer it forward thousands of times per minute. Hammer drills are available for purchase at home centers and tool stores or you can rent one.