A spade bit is a flat, paddle-shaped drill bit used to bore relatively large holes in wood quickly. It matters in workshops because it is inexpensive, easy to recognize, and useful for rough carpentry tasks such as running wires, making clearance holes, or starting larger openings. Unlike a twist drill, a spade bit removes wood with a broad cutting edge rather than spiral flutes.
Its simple shape makes it fast, but it requires careful control to make clean and safe holes.
Key Facts
- Hole diameter is set by the width of the spade blade, such as 13 mm, 19 mm, or 25 mm.
- Cutting speed at the outer edge is v = πDN, where D is bit diameter and N is rotation rate in revolutions per second.
- Torque demand increases as bit diameter increases because the cutting edge acts farther from the rotation axis.
- A center spur guides the bit and helps keep the hole centered before the cutting lips engage.
- Clamping a scrap backer board under the workpiece reduces tear-out as the bit exits.
- For cleaner holes, drill until the spur just breaks through, then finish drilling from the opposite side.
Vocabulary
- Spade bit
- A flat wood-boring drill bit with a central point and two cutting lips used to make medium to large holes quickly.
- Center spur
- The pointed tip at the center of a spade bit that starts the hole and helps guide the bit straight.
- Cutting lip
- A sharpened edge on the flat blade that slices wood fibers as the bit rotates.
- Shank
- The straight or hex-shaped end of the bit that fits into the drill chuck.
- Tear-out
- Ragged splintering on the exit side of a drilled hole caused when wood fibers are pushed out instead of cleanly cut.
Common Mistakes to Avoid
- Drilling too fast with too much pressure, which overheats the bit and tears wood fibers instead of cutting them cleanly.
- Skipping a backer board, which lets the exit side splinter badly when the spur and cutting lips break through.
- Holding the drill at an angle, which makes an oval or crooked hole and can cause the bit to bind in the wood.
- Using a dull or damaged spade bit, which increases force, reduces control, and leaves rough edges that require extra cleanup.
Practice Questions
- 1 A 25 mm spade bit rotates at 600 rpm. What is the cutting speed at the outer edge in meters per second? Use v = πDN with D in meters and N in revolutions per second.
- 2 You need to drill a 19 mm hole through a 38 mm thick board. If your drill advances at 4 mm per second, about how many seconds will it take to pass through the board, ignoring setup time?
- 3 Explain why drilling halfway from one side and then finishing from the other side can produce a cleaner hole than drilling all the way through from one side.