Centre drills (combined drills and countersinks) are sized by number: #00 is the smallest, with a 0.025 in pilot on a 1/8 in body, and #8 the largest, with a 5/16 in pilot on a 3/4 in body. Every size cuts a 60° countersink that matches a standard lathe centre.
Use the inch table below for ASME B94.11M number sizes and the metric table for DIN 333 form A sizes, which are named by pilot diameter × body diameter (for example A 2.5 × 6.3). The pilot drill makes a small clearance hole so the point of the lathe centre never bottoms out; the 60° cone is the bearing surface the centre runs on. Pick the size from the work diameter table, then feed in until the countersink is the size you want. All sizes listed are the plain (60°) type. Switch between inch and mm with the toggle; values in mm are exact conversions (× 25.4) rounded to 0.01 mm.
Inch centre drill sizes #00 to #8 (ASME B94.11M)
| Size | Body diameter | Pilot drill diameter | Pilot drill length | Overall length | Countersink angle |
|---|---|---|---|---|---|
| #00 | 1/8 (.1250) | 1/40 (.0250) | .030 | 1-1/8 (1.1250) | 60° |
| #0 | 1/8 (.1250) | 1/32 (.0312) | .038 | 1-1/8 (1.1250) | 60° |
| #1 | 1/8 (.1250) | 3/64 (.0469) | 3/64 (.0469) | 1-1/4 (1.2500) | 60° |
| #2 | 3/16 (.1875) | 5/64 (.0781) | 5/64 (.0781) | 1-7/8 (1.8750) | 60° |
| #3 | 1/4 (.2500) | 7/64 (.1094) | 7/64 (.1094) | 2 (2.0000) | 60° |
| #4 | 5/16 (.3125) | 1/8 (.1250) | 1/8 (.1250) | 2-1/8 (2.1250) | 60° |
| #5 | 7/16 (.4375) | 3/16 (.1875) | 3/16 (.1875) | 2-3/4 (2.7500) | 60° |
| #6 | 1/2 (.5000) | 7/32 (.2188) | 7/32 (.2188) | 3 (3.0000) | 60° |
| #7 | 5/8 (.6250) | 1/4 (.2500) | 1/4 (.2500) | 3-1/4 (3.2500) | 60° |
| #8 | 3/4 (.7500) | 5/16 (.3125) | 5/16 (.3125) | 3-1/2 (3.5000) | 60° |
Inch values show the fraction with its decimal in brackets. Pilot drill length is the length of the small drill tip before the 60° countersink starts.
Metric centre drill sizes: DIN 333 form A
DIN 333 form A is the plain 60° centre drill, the metric equivalent of the inch number sizes. The size is written pilot × body, e.g. A 3.15 × 8. Form B adds a 120° protecting chamfer around the centre hole (so a knock on the end of the shaft does not damage the 60° seat) and form R has a curved (radius) profile; those forms are not tabulated here.
| Size (d1 × d2) | Pilot d1 (mm) | Body d2 (mm) | Overall length l1 (mm) | Pilot length l2 (mm) | Closest inch size (pilot × body, mm) |
|---|---|---|---|---|---|
| A 1 × 3.15 | 1.00 | 3.15 | 31.5 | 1.3 | #1 (1.19 × 3.18) |
| A 1.25 × 3.15 | 1.25 | 3.15 | 31.5 | 1.6 | #1 (1.19 × 3.18) |
| A 1.6 × 4 | 1.60 | 4.00 | 35.5 | 2.0 | no close inch equivalent |
| A 2 × 5 | 2.00 | 5.00 | 40.0 | 2.5 | #2 (1.98 × 4.76) |
| A 2.5 × 6.3 | 2.50 | 6.30 | 45.0 | 3.1 | #3 (2.78 × 6.35) |
| A 3.15 × 8 | 3.15 | 8.00 | 50.0 | 3.9 | #4 (3.18 × 7.94) |
| A 4 × 10 | 4.00 | 10.00 | 56.0 | 5.0 | #5 (4.76 × 11.11) |
| A 5 × 12.5 | 5.00 | 12.50 | 63.0 | 6.3 | #6 (5.56 × 12.70) |
| A 6.3 × 16 | 6.30 | 16.00 | 71.0 | 8.0 | #7 (6.35 × 15.88) |
| A 8 × 20 | 8.00 | 20.00 | 80.0 | 10.1 | #8 (7.94 × 19.05) |
| A 10 × 25 | 10.00 | 25.00 | 100.0 | 12.8 | no close inch equivalent |
The "closest inch size" column is only a rough match for choosing a substitute; inch and metric centre drills are not identical. Pilot length l2 is the nominal value listed by DIN 333; manufacturers' catalogue values can differ slightly.
Which centre drill for which work diameter?
Bigger, heavier work needs a bigger centre hole so the 60° cone has enough bearing area to carry the load and resist wear on a dead centre. The table below is the traditional lathe guide for choosing a centre drill by the diameter of the work. It is a rule of thumb, not part of the standard.
| Centre drill | Typical work (shaft) diameter | Body diameter |
|---|---|---|
| #1 | 3/16 to 5/16 in | 1/8 (.1250) |
| #2 | 3/8 to 1/2 in | 3/16 (.1875) |
| #3 | 5/8 to 3/4 in | 1/4 (.2500) |
| #4 | 1 to 1-1/2 in | 5/16 (.3125) |
| #5 | 2 to 3 in | 7/16 (.4375) |
| #6 | 3 to 4 in | 1/2 (.5000) |
| #7 | 4 to 5 in | 5/8 (.6250) |
| #8 | 6 in and over | 3/4 (.7500) |
- Between two ranges? Use the smaller size for light finishing cuts and the larger one for heavy cuts, long work or a dead (non-rotating) centre.
- #00 and #0 are for very small work (instrument and model parts) and for starting tiny drills.
- Depth: stop before the countersink reaches the full body diameter. A centre hole cut to the full body diameter leaves a sharp edge that is easily damaged, and the centre has no clean seat left if the hole is re-cut.
- Metric work: A 2 × 5 or A 2.5 × 6.3 covers most small lathe shafts of about 10 to 20 mm, A 3.15 × 8 suits roughly 25 to 40 mm, and A 4 × 10 and larger suit heavier shafts. Check the work drawing: many shaft drawings call up the centre hole per ISO 6411 / DIN 332, which may also say whether the centre may stay on the finished part.
Spot drill vs centre drill
A centre drill and a spot drill look similar but do different jobs. A centre drill makes a 60° seat for a lathe centre. A spot drill (a short, stiff drill with a 90°, 118°, 120° or 142° point and no pilot) makes a small locating cone so a twist drill starts exactly on position.
| Feature | Centre drill | Spot drill |
|---|---|---|
| Main job | 60° seat for lathe centres (between-centres turning, tailstock support) | Start a twist drill on position; can also chamfer the hole |
| Point | Small pilot drill + 60° countersink | Single point: 90°, 118°, 120° or 142° |
| Weak spot | Thin pilot snaps easily if fed hard or run out of square | Very stiff; nothing thin to break |
| For spotting before drilling | Not ideal: a 118° drill meets the 60° cone at its outer corners first, which can chip the corners and let the drill wander | Use a spot angle equal to or larger than the drill point (e.g. 120° spot for a 118° drill, 142° for a 135°/140° drill) so the drill tip touches first |
Worked example
You are turning a 1-1/4 in (31.75 mm) steel shaft between centres. The guide table puts 1-1/4 in in the 1 to 1-1/2 in range, so use a #4 centre drill (1/8 in pilot on a 5/16 in body). In metric, the nearest equivalent is A 3.15 × 8. Centre-drill both ends, stopping when the countersink is around two thirds of the body diameter, and support the work on a 60° live centre.
Related calculators and charts
- Countersink Calculator: depth for a given countersink diameter and angle.
- Drill Point Depth Calculator: allow for the drill point when drilling to depth.
- Feed and Speed Calculator: spindle speed for the pilot diameter.
- Taper Calculator: Morse and other tapers for centres and tailstocks.
- Tap & Drill Size Reference, Countersink Bolt Size Chart, Carbide Insert Designation Chart and Keyway and Key Size Chart.
Frequently asked questions
Sources: ASME B94.11M (combined drills and countersinks, plain type) for the inch sizes and DIN 333 form A for the metric sizes. The work-diameter guide is traditional lathe practice, not part of either standard. Always check critical dimensions against the current standard or the tool manufacturer's catalogue.