Drill Bits

Twist drill bits are the most common type of drill bit. They are used for everyday drilling in all types of material: wood, metal, plastic, and more. They are the least expensive of the drill bits and are available in a huge variety of size, tip, and material specifications. They are well suited for drilling very small diameter holes (1/16” or less), but are impractical (outside of specialty applications) for anything larger than about one inch. The length of a twist bit is usually proportional to its diameter to balance strength and reach, but extra long and extra short variations are readily available. Most general purpose twist bits have a 118° tip angle, which is great for a range of materials including wood, non-ferrous metals, and mild steel. More obtuse angles (135°) are better for harder materials like stainless steel, and sharper angles (60°) are good for soft materials like plastic. 

Materials: wood, metal, plastic

Typical size range: 1/16” to 1”

Typical length: variable

Special properties: inexpensive, easy to re-sharpen, available in very small diameters

Brad point drill bits have specialized geometry for drilling clean holes in wood. The long center spur helps position the drill accurately and keep it from skating across the surface. It also allows you to tip the drill bit to an angle instead of straight up and down. The spurs on the outer edges shear the grain and leave a clean entry and exit hole. On the downside, they cost a bit more than regular twist bits and are harder to sharpen. They are very helpful for precision woodworking operations like making dowel joints. 

Materials: primarily wood and some plastics

Typical size range: 1/8” to 1/2”

Typical length: 2-6”, proportional to diameter

Special properties: accurate placement, angled drilling

Forstner drill bits are also specialized for drilling holes in wood. The cutting geometry includes a short center spur, chipper blades, and a cutting rim that can be smooth or serrated. They cut clean, precise holes with a flat surface at the bottom. Unlike other bits, you can drill overlapping holes or holes extending over the edge of a workpiece. Forstners are good for large-diameter holes ranging from ¼” up to 3” or bigger. They are not well suited for drilling at an angle without additional fixturing (the short center spur means that the outer edge makes contact first and may skid around), and they are not very long. They are substantially more expensive than twist bits and difficult, though not impossible, to sharpen. 

Material: wood only

Typical size range: ¼” to 3”

Typical length: 3.5”

Special properties: flat-bottomed hole, overlapping holes

Spade bits, also known as paddle bits, have a flat shape with a long center spur and (sometimes) a pair of nickers on the outer edges. They tend to cut fast and somewhat roughly. The long center spur allows drilling at fairly sharp angles. They are available in larger diameters and longer lengths than typical twist bits, and tend to be inexpensive. They can be resharpened with a file, or even ground down to a custom diameter. Due to their length and narrow shanks, they are best used in a hand-held drill rather than a drill press.

Material: wood

Typical size range: 1/4” to 1-½”

Typical length: 6”

Special properties: angled drilling, fast cutting

Auger bits have tapered lead screws for centering and drawing the cutter forward, as well as long deep helical flutes for effective chip removal. This allows the auger bit to cut quickly, cleanly, and drill deep holes without overheating or clogging. They are available in a range of sizes, including specialty longer lengths. Older styles have a tapered square tang to interface with a hand brace, but modern ones have a standard hexagonal or round shank suitable for a hand-held drill. Do not use on a drill press: the threaded spur can cause it to aggressively self-feed and get out of control.

Materials: wood (although there are specialized auger bits for many different materials)

Typical size range: ¼” - 1”

Typical length: 8”-18” or longer 

Special properties: chip clearing, long length

Hole saws are cup-shaped cutters with saw teeth around the edge and a pilot drill in the middle to keep it centered. This arrangement creates a disc-shaped core plug, which must be pried out of the hole saw. These large cutters require a low RPM and high torque, so a drill press or hand-held drill with side handles is preferred. They are not very good at clearing chips, so they tend to get overheated or clog, which can cause powerful kickback. Also, they tend to have limited depth and are therefore best suited to thinner material. 

Materials: wood, plastic, metal

Typical size range: ¾” - 5”

Typical length: 2”

Special properties: large diameters available, remove disc of material

Self-centering bits are used to accurately place screws for attaching hardware like hinges. They consist of a twist bit sleeved inside of a spring-loaded shell with a chamfered tip. The shell registers against the countersunk hole in the hinge plate and keeps the drill bit centered as it advances. They are typically available in a range of small sizes matched to screw sizes. 

Materials: wood

Typical size range: 1/16” - ¼”

Step bits allow you to drill a range of different sized holes with a single bit. It is cone-shaped, so the further you drill into the material, the wider the hole gets. Each step is fairly shallow, so it it best used with thin materials like plastic and sheet metal. The cutting angle is less aggressive than a twist bit, so it scrapes material away rather than grabbing a large chip. Step bits are substantially more expensive than twist bits. 

Materials: thin sheet metal, plastic, wood

Typical size range: variable (¼” - 1-⅜” is one example)

Masonry bits drill into tough material like concrete, brick, and stone. They typically have a carbide cutter brazed onto a fluted steel body. Some work with a standard hand-held drill, and some are designed for use with a hammer drill for more impact. 

Materials: concrete, brick, stone

Typical size range: ⅛” - 1-½”