Spot Drill Depth & Chamfer Calculator (90°, 120°, 142°)
Spot drill depth for a chamfer is Z = Dcsk / (2 × tan(θ/2)), where Dcsk = drill Ø + 2 × chamfer and θ is the spot drill point angle. With a 90° spot drill the depth is simply half the diameter: an Ø8 mm hole with a 0.5 mm chamfer needs Z = 4.500 mm. For self-centering, the spot angle should be equal to or larger than the drill point (120° for 118° drills, 142° for 140° carbide drills).
Spot Drill Depth, Chamfer Diameter & Point-Angle Calculator
Z depth for a centering spot or a finished chamfer, Z = (Dcsk − dflat) / (2·tan(θ/2)), the self-centering check against the drill point and a ready G81 / G82 line.
How Deep Should You Spot Drill?
The spot drill cuts a cone. The diameter of that cone at the surface is 2 × Z × tan(θ/2), so to reach a target diameter you invert it. A 90° spot drill is the easy case (tan 45° = 1): depth equals half the diameter. The same Ø9 mm chamfer edge needs 4.500 mm with a 90° spot, 2.598 mm with 120° and only 1.549 mm with 142°. If your spot drill has a small flat or a web at the tip, enter its diameter and the calculator subtracts it.
Drill point angles are defined in ASME B94.11M (twist drills), the US standard for twist-drill sizes, tolerances and the combined drill-and-countersink tools; the 118° general-purpose point and the flatter 135–140° points used on cobalt and carbide drills both come from that family of definitions. Knowing the exact point angle of the drill that follows matters because it decides whether the spot centers the drill or merely marks the surface. A spot that is too shallow leaves the drill to find its own center; one that is too deep leaves an oversized chamfer that has to be cleaned up later.
| Drill + Chamfer | 90° Spot Drill | 120° Spot Drill | 142° Spot Drill |
|---|---|---|---|
| Ø3 mm + 0.5 mm chamfer (Ø4) | 2.000 mm | 1.155 mm | 0.689 mm |
| Ø4 mm + 0.5 mm chamfer (Ø5) | 2.500 mm | 1.443 mm | 0.861 mm |
| Ø5 mm + 0.5 mm chamfer (Ø6) | 3.000 mm | 1.732 mm | 1.033 mm |
| Ø6 mm + 0.5 mm chamfer (Ø7) | 3.500 mm | 2.021 mm | 1.205 mm |
| Ø8 mm + 0.5 mm chamfer (Ø9) | 4.500 mm | 2.598 mm | 1.549 mm |
| Ø10 mm + 0.5 mm chamfer (Ø11) | 5.500 mm | 3.175 mm | 1.894 mm |
| Ø12 mm + 0.5 mm chamfer (Ø13) | 6.500 mm | 3.753 mm | 2.238 mm |
| Ø16 mm + 0.5 mm chamfer (Ø17) | 8.500 mm | 4.907 mm | 2.927 mm |
Why Must the Spot Angle Match the Drill Point?
When the spot is flatter (larger angle) than the drill, the drill's chisel edge touches the bottom of the spot first and the drill centers itself before the cutting corners engage. When the spot is steeper — for example a 90° spot under a 118° or 140° drill — the outer corners hit the cone wall first. HSS drills tolerate that, which is why many shops spot and chamfer in one hit with a 90° tool. Solid-carbide drills chip at the corners, so tool makers such as this NC spotting drill guide recommend a 142° spot (DIN 1897 stub length) before a 140° carbide drill and a separate 90° pass afterwards if you need a chamfer.
| Following Drill | Best Spot Angle | Result | When 90° Is Used |
|---|---|---|---|
| 118° HSS / cobalt drill | 120° spot | Chisel edge touches first; drill self-centers | 90° spot gives a chamfer in the same hit; outer corners touch first (acceptable on HSS) |
| 130–135° split-point drill | 142° spot | Self-centering | Use 90° only as a separate chamfer pass after drilling |
| 140° solid-carbide drill | 142° spot (DIN 1897 stub) | Self-centering, protects the corners | Never drill into a 90° spot: corner chipping on carbide |
| Centre / chamfer only | 90° spot | — | Standard for countersink-style chamfers (82°/90° heads need a countersink) |
The full ANSI edition of the drill standard (ANSI/ASME B94.11M-1993) and the drilling tables in Machinery's Handbook (Industrial Press) list the point geometries if you need to check a specific drill series.
G81 or G82?
For a plain centering spot use the simple drilling cycle G81. For a chamfer that has to look clean, use G82 with a short dwell (0.1–0.3 s) at the bottom so the cone is cut round instead of leaving a feed spiral. The calculator writes the line with your depth, an R plane of 2 mm and your feed; change X/Y to your hole positions or call it from a bolt circle program. For deep holes after the spot, switch to the G83 / G73 peck cycle calculator.
Spot Drill vs Countersink vs Center Drill
- Spot drill — a short, stiff 90°/120°/142° point for locating the drill on CNC; also used for chamfers.
- Countersink — 82° (inch) or 90° (metric) seats for flat-head screws; see our countersink depth calculator and countersink angle chart.
- Center drill — a 60° cone with a pilot for lathe centers, not for locating drills on a mill.
Cite this page
ToolSpecVault Engineering Data Team. (2026, October 28). Spot Drill Depth & Chamfer Calculator (90°, 120°, 142°). ToolSpecVault. https://toolspecvault.com/spot-drill-depth-diameter-chamfer-angle-calculator/