When thread milling, the cutting edge follows a larger circle than the tool center. To get the right feed at the thread, the programmed center feed must be scaled down for internal threads.
Center feed = Edge feed × (Dm − Dt) / Dm
What the variables mean
| Symbol | Meaning |
|---|---|
| Center feed | Programmed feed at the tool center (IPM) |
| Edge feed | Desired feed at the cutting edge (IPM) |
| Dm | Thread major diameter (inches) |
| Dt | Thread mill cutter diameter (inches) |
Worked example
Internal thread, Dm = 0.50 in, Dt = 0.30 in, edge feed 10 IPM: center feed = 10 × (0.50 − 0.30) / 0.50 = 4 IPM. Skip this and the edge cuts far too fast.
Run it instantly with the Thread Milling Feed calculator in Cutting & Tooling.