Spot Drill Depth Calculator by Chamfer Diameter
Spot-drill depth depends on the included point angle and the target spot or chamfer diameter. Use drill-point geometry for a spot in solid material, or countersink geometry when an existing hole is being chamfered.
Spot diameter in solid material
For a centered conical spot in solid material, the target spot diameter is the cone diameter at depth.
z = D_spot / (2 * tan(A / 2))
Chamfer on an existing hole
When the spot drill is used to chamfer an existing hole, subtract the existing hole diameter before solving depth.
z = (D_chamfer - D_hole) / (2 * tan(A / 2))
Blind-hole point allowance
If the job is a blind drilled hole rather than a spot chamfer, the drill point calculator adds the point length to the required full-diameter depth.
L_p = (D / 2) / tan(A / 2)
Tooling caution
Spot drills, center drills, split points, tip flats and regrinds can depart from a perfect cone. Use the actual tool point angle and verify tight chamfer diameters with a measured test spot.
Frequently asked questions
Which calculator should I use for spot drill chamfer diameter?
Use the drill point calculator when spotting solid material. Use the countersink depth calculator when chamfering an existing hole because it subtracts the hole diameter.
Does spot drill depth equal drill point length?
It is the same cone relationship when the target diameter is the spot drill diameter at the surface. For blind-hole full-depth allowance, add point length to the cylindrical depth requirement.
Related pages
- Drill point length calculator - point allowance from drill diameter and included angle.
- Countersink depth calculator - chamfer depth from target diameter and existing hole.
- Peck drilling calculator - programmed depth and peck count after point allowance.
- Drilling feed-rate calculator - feed rate and drilling time from RPM and feed per rev.