- TOOLS IN PRACTICE
- HOW TO DETERMINE THE REQUIRED TAP DRILL SIZE FOR AN INTERNAL METRIC THREAD?
HOW TO DETERMINE THE REQUIRED TAP DRILL SIZE FOR AN INTERNAL METRIC THREAD?
If you know which internal thread you want to create, the first obvious question is what drill bit diameter you need to pre-drill the hole into which the thread will subsequently be cut.
Two terms are essential for selecting the correct drill bit:
Thread diameter
The basic dimension that specifies the size of the thread on a screw, measured across the thread crests.
Metric threads are also designated according to this dimension.
For example, an M8 thread has a diameter of 8 mm.
Thread pitch
It is the distance between adjacent threads, measured along the axis of the screw or nut.
It indicates how far the screw or nut moves during one complete 360-degree revolution.
For metric threads, the pitch is usually expressed in millimetres.
For example, an M8 thread with the standard pitch of 1.25 mm advances by 1.25 mm with each complete turn.
The diameter of the twist drill for thread cutting is calculated as follows:
Drill bit diameter = thread diameter - thread pitch
The drill bit diameter is rounded to one decimal place.
Example for an M8 thread with a 1.25 mm pitch: 8 mm - 1.25 mm = 6.8 mm
Our table contains all common thread sizes and the corresponding twist drill diameters.
| TAPS WITH "STANDARD" METRIC THREAD | TAPS WITH FINE METRIC THREAD | |||||
| THREAD SIZE | THREAD PITCH | DRILL BIT DIAMETER | THREAD SIZE | THREAD PITCH | DRILL BIT DIAMETER | |
| M 3 | 0.50 | 2.50 | MF 3 | 0.35 | 2.65 | |
| M 4 | 0.70 | 3.30 | MF 4 | 0.50 | 3.65 | |
| M 5 | 0.80 | 4.20 | MF 5 | 0.50 | 4.50 | |
| M 6 | 1.00 | 5.00 | MF 6 | 0.50 | 5.50 | |
| M 7 | 1.00 | 6.00 | MF 6 | 0.75 | 5.20 | |
| M 8 | 1.25 | 6.80 | MF 7 | 0.75 | 6.20 | |
| M 9 | 1.25 | 7.80 | MF 8 | 0.50 | 7.50 | |
| M 10 | 1.50 | 8.50 | MF 8 | 0.75 | 7.20 | |
| M 11 | 1.50 | 9.50 | MF 8 | 1.00 | 7.00 | |
| M 12 | 1.75 | 10.20 | MF 9 | 0.75 | 8.20 | |
| M 14 | 2.00 | 12.00 | MF 10 | 0.50 | 9.50 | |
| M 15 | 2.00 | 13.00 | MF 10 | 0.75 | 9.20 | |
| M 16 | 2.00 | 14.00 | MF 10 | 1.00 | 9.00 | |
| M 18 | 2.50 | 15.50 | MF 10 | 1.25 | 8.80 | |
| M 20 | 2.50 | 17.50 | MF 11 | 1.00 | 10.00 | |
| M 22 | 2.50 | 19.50 | MF 12 | 0.50 | 11.50 | |
| M 24 | 3.00 | 21.00 | MF 12 | 0.75 | 11.20 | |
| MF 12 | 1.00 | 11.00 | ||||
| MF 12 | 1.25 | 10.80 | ||||
| MF 12 | 1.50 | 10.50 | ||||
| MF 14 | 1.25 | 12.80 | ||||
| MF 14 | 1.50 | 12.50 | ||||
| MF 16 | 1.00 | 15.00 | ||||
| MF 16 | 1.25 | 14.80 | ||||
| MF 16 | 1.50 | 14.50 | ||||
| MF 18 | 1.00 | 17.00 | ||||
| MF 18 | 1.50 | 16.50 | ||||
| MF 20 | 1.00 | 19.00 | ||||
| MF 20 | 1.50 | 18.50 | ||||
| MF 20 | 2.00 | 18.00 | ||||
| MF 22 | 1.00 | 21.00 | ||||
| MF 22 | 2.00 | 20.50 | ||||
| MF 24 | 1.50 | 22.50 | ||||
| MF 24 | 2.00 | 22.00 | ||||