Holemaking and tapping
Drill point length calculator
Tip length = (D / 2) / tan(point angle / 2); a 118° point is 0.3004 x D long.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Tip length
in
L = (D / 2) / tan(A / 2) = (0.5 / 2) / tan(118° / 2) = 0.1502 in
3.82 mm. Tip length is 0.3004 x D at 118°.
Programmed Z depth
in
Z = H + L = 1.5 + 0.1502 = 1.6502 in
Program Z-1.6502 from the top of the hole to get full diameter at 1.5000 in.
Source: Machinery's Handbook (twist drill point geometry); ANSI/ASME B94.11M.
| Point angle | Half angle | tan(A / 2) | L / D | Tip on a 1 in drill (in) | Tip on a 10 mm drill (mm) |
|---|---|---|---|---|---|
| 90° | 45.0° | 1.00000 | 0.5000 | 0.5000 | 5.00 |
| 118° | 59.0° | 1.66428 | 0.3004 | 0.3004 | 3.00 |
| 120° | 60.0° | 1.73205 | 0.2887 | 0.2887 | 2.89 |
| 135° | 67.5° | 2.41421 | 0.2071 | 0.2071 | 2.07 |
| 140° | 70.0° | 2.74748 | 0.1820 | 0.1820 | 1.82 |
Tip length = L / D x drill diameter. Factors are computed from the formula and are unitless.
How it works
L = (D / 2) / tan(A / 2)
| Symbol | Meaning | Unit |
|---|---|---|
| L | Tip (point) length, from the corners of the cutting lips to the point | in or mm |
| D | Drill diameter | in or mm |
| A | Included point angle | degrees |
Programmed depth
Z = H + L
| Symbol | Meaning | Unit |
|---|---|---|
| Z | Programmed depth to the drill point | in or mm |
| H | Full-diameter depth from the drawing | in or mm |
| L | Tip length | in or mm |
Worked example: 1/2 in drill, 118°
A 0.500 in drill with a 118° point.
- Half the diameter: 0.500 / 2 = 0.250 in.
- Half the angle: 118° / 2 = 59°. tan(59°) = 1.66428.
- Divide: 0.250 / 1.66428 = 0.1502 in.
Result: 0.1502 in (3.82 mm) of tip.
Worked example: 10 mm drill, 140°
A 10 mm drill with a 140° point.
- Half the diameter: 10 / 2 = 5 mm.
- Half the angle: 70°. tan(70°) = 2.74748.
- Divide: 5 / 2.74748 = 1.82 mm.
Result: 1.82 mm of tip.
Shop notes
- Full-diameter depth: a 0.250 in, 118° drill to 1.500 in at full diameter has a 0.0751 in tip, so Z = 1.500 + 0.0751 = 1.5751 in.
- Spot drill and center drill angles (90° or 120°) matter when you chamfer with the spot. Match the spot angle to the chamfer the drawing calls.
- Web thinning and split points do not change the point length.
- Blind tapped holes also need room for the tap chamfer below the full thread depth. Drill deeper than the full-thread depth the drawing asks for.
FAQ
How long is the point on a 1/2 in 118° drill?
0.1502 in: 0.250 / tan(59°) = 0.250 / 1.66428.
How long is a 135° point?
0.2071 x D; 0.2071 in on a 1 in drill.
Do you add tip length to the programmed depth?
Yes, when the drawing calls a full-diameter depth: 1.500 + 0.0751 = 1.5751 in for a 1/4 in, 118° drill.
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