On a twist drill bit, these chisel tips are called “lips,” and the grooves that spiral around the shaft of the bit are called “flutes.” Flutes don’t really cut; their primary job is to evacuate “swarth,” the cut material (usually chips or windings). If the swarth isn’t removed, it will jam up and bind the bit, causing it to snap.
Most drill bits have a shank that’s the same diameter as the rest of the bit, so an adjustable handpiece (#30 style) is required to accommodate the different sizes. For those who love their quick-release style handpiece, there are choked drill bits. The various shaft sizes of these little beauties all emerge from a 3⁄32-in. (2.38 mm) shank, the most common size used for flex-shaft accessories and for the collets of most quick-release handpieces. The downside to choked bits is that they cost about twice as much as their variable-shank cousins. An economical solution is the adaptor chuck, which holds small bits in an adjustable collet, much like a pin vise, and has a 3⁄32-in. (2.38 mm) shank.
The tip of a twist drill bit can vary in steepness (the “point angle”) according to the material the bit is designed to cut. An all-purpose point angle is 118°; I use these to drill through metal, plastic, wood, pearl, and even slate. (Bits don’t cut metal well after attacking slate, but they still work on that stone, so I designate them for that task.) Although all-purpose bits will cut plastics, steeper point angles are used to cut Plexiglas and other materials.
Whether you’re buying plain or choked bits, you’ll usually have to order them in multiples of five, six, or 12, although smaller suppliers may offer them individually. Drill bits are consumable, and I buy the sizes I use most often in bulk.
NOTE: Speaking of size, I select my drill bits by their millimeter measurement. For ordering, though, I remember the drill-bit number and wire gauge equivalents of the bits I use most often:
Diamond drill bits: Twist and core
For drilling stone, glass, ceramics, and hard materials, diamond drill bits are the ticket. Unlike the chisel action of a steel bit, the diamond grit cuts abrasively — it grinds away material rather than chipping it away. The diamond coating is usually plated onto two basic shapes: twist and core. You may also be able to find solid-cylinder diamond bits, but they’re less common.
The diamond twist drill bit is essentially a standard twist drill bit plated with diamond from its flutes to its lips, with the shank left bare. Diamond core drill bits consist of a diamond-coated hollow cylinder.
The cylinder’s rim does all the cutting, and waste material is evacuated through the hollow center and a hole in the side. Diamond drill bits come in a variety of sizes and with choked or straight shanks.
The one weakness of a diamond drill bit is the bond between diamond and steel. This is where these bits eventually fail due to heat build-up; that’s why they’re meant to be used wet. I periodically dip the end of the bit in water as I drill or hold my work in a shallow, water-filled container.
A note about safety. Specifically, protecting your eyes: safety goggles, safety glasses, face shield. Pick one. Use it.