Spot drill and center drill depth
How deep to go with a spot drill to get the diameter you need at the face — and what diameter a given depth gives. For pointed spot drills and center drills with a pilot.
Quick answer
Pointed spot drill with angle θ: Z = D / (2 × tan(θ/2)). At 90° the depth is half the diameter: a Ø6 mm spot → Z = 3 mm, a 1/4″ spot → 0.125″. Center drill with a pilot: Z = l + (D − d) / (2 × tan(θ/2)).
Result
- Z in the program
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- Spot diameter at the face
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- Chamfer on the hole after drilling
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Program line (zero at the face, length set to the point)
When spotting for a drill, make the spot a little larger than the drill — you get a neat chamfer and the drill doesn't wander.
Who needs it and why
Spotting is often done by eye, and then the chamfer on the hole varies from part to part, or the tailstock center rides on the end of the pilot hole instead of the cone. A Z depth worked out in advance gives the same result across the batch.
- Spotting for a drill — a spot a little larger than the drill: the drill doesn't wander and the hole gets a clean chamfer.
- Center holes for a lathe center — long shafts between centers: the 60° cone must be wide enough for the center to bear on it, not on the edge.
- Chamfering a finished hole — with a 90° spot drill to a target diameter.
- Before tapping — a chamfer a little over the thread's major diameter so the tap starts straight and leaves no burr.
The formulas
- pointed tool:
Z = D / (2 · tg(θ/2))— at 90° the depth is half the diameter - with a pilot Ø d, length l:
Z = l + (D − d) / (2 · tg(θ/2))
The hole depth itself, with the drill point allowed for, is in the drill point depth calculator.