Centre holes
A form B centre hole with a 2.5 mm pilot is 8 mm across its protecting chamfer and at least 5.4 mm deep; the drawing calls it up as ISO 6411-B 2,5/8. Pick the form (A, B or R) and the pilot size for the hole’s diameters and depth and the centre drill that makes it (ISO 866, ISO 2540, ISO 2541), and whether the hole must stay on the finished part, may stay, or must not stay, for the ISO 6411 symbol and designation.
The hole is drawn in section, true to scale, with a 60° centre bearing in it, and under it the part’s end as the drawing shows it.
ISO 866 type A · required
- Designation on the drawing
- Pilot Ø d
- Ø D at the face
- Cone Ø d₂
- —
- Depth t (min)
- Cone length l₁
- Pilot length, reference (min)
- Chamfer depth b
- —
- Wall radius r
- —
- Parting-off allowance a
- Centre drill
- ISO 866 type A, d 2.5, shank Ø 6.3
- Drill length (min–max)
- 43–47mm
How it’s calculated
The cone length is (D − d) / (2 tan 30°); form B adds the 120° chamfer, (D₃ − d₂) / (2 tan 60°) deep. The depth t is the cone plus the pilot’s reference length, which a reground drill must still give. The designation (ISO 6411) gives the form, d and D. Sizes in brackets are to be avoided. A dash is a value the sources do not agree on, or one the form does not have.
Form A centre holes and drills
| d | D | t min | Pilot ref. | a | Shank Ø | Drill length | Designation |
|---|---|---|---|---|---|---|---|
| (0.5) | 1.06 | 1 | 0.5 | — | 3.15 | 19–21 | ISO 6411-A 0,5/1,06 |
| (0.63) | 1.32 | 1.2 | 0.6 | — | 3.15 | 19–21 | ISO 6411-A 0,63/1,32 |
| (0.8) | 1.7 | 1.5 | 0.7 | — | 3.15 | 19–21 | ISO 6411-A 0,8/1,7 |
| 1 | 2.12 | 1.9 | 0.9 | 3 | 3.15 | 29.5–33.5 | ISO 6411-A 1/2,12 |
| (1.25) | 2.65 | 2.3 | 1.1 | — | 3.15 | 29.5–33.5 | ISO 6411-A 1,25/2,65 |
| 1.6 | 3.35 | 2.9 | 1.4 | 5 | 4 | 33.5–37.5 | ISO 6411-A 1,6/3,35 |
| 2 | 4.25 | 3.7 | 1.8 | 6 | 5 | 38–42 | ISO 6411-A 2/4,25 |
| 2.5 | 5.3 | 4.6 | 2.2 | 7 | 6.3 | 43–47 | ISO 6411-A 2,5/5,3 |
| 3.15 | 6.7 | 5.9 | 2.8 | 9 | 8 | 48–52 | ISO 6411-A 3,15/6,7 |
| 4 | 8.5 | 7.4 | 3.5 | 11 | 10 | 53–59 | ISO 6411-A 4/8,5 |
| (5) | 10.6 | 9.2 | 4.4 | — | 12.5 | 60–66 | ISO 6411-A 5/10,6 |
| 6.3 | 13.2 | 11.5 | 5.5 | 18 | 16 | 68–74 | ISO 6411-A 6,3/13,2 |
| (8) | 17 | 14.8 | 7 | — | 20 | 77–83 | ISO 6411-A 8/17 |
| 10 | 21.2 | 18.4 | 8.7 | 28 | 25 | 97–103 | ISO 6411-A 10/21,2 |
A, B or R?
Form A is the plain 60° cone: the centre bears on it from the face down to the pilot. Form B adds a 120° protecting chamfer, so the bearing cone starts below the face and a knock or a burr on the end does not spoil it: use it where the part goes back between centres for regrinding or repair. Form R has a radius wall instead of a straight cone, so the centre touches it on one circle. Whichever form, the pilot leaves room in front of the centre’s point, so the centre bears on the cone and never on the bottom.