Free Thread Milling Helical Feed Compensation Tool

1. Why CNC Thread Milling Requires Feed Compensation

In conventional straight linear milling (G01), the tool center moves at the exact same velocity as the cutting edge. However, thread milling is a 3D helical circular interpolation (G02 or G03 with simultaneous Z-axis motion).

The CNC machine's computer control regulates feed along the tool centerline path ($R_{center}$). In an internal bore, the tool centerline travels along a much smaller inner circle than the thread perimeter: $$R_{center} = rac{D_{thread} - D_{cutter}}{2}$$ If you program the tool with the linear feed rate calculated from manufacturer chip load tables ($F_{periph} = N cdot Z cdot f_z$), the tool centerline travels too fast, forcing the outer cutting edge to sweep across material at an extreme velocity, doubling or tripling the intended chip load ($f_z$) and instantly snapping the tool!

2. Mathematical Compensation Formulation

To maintain the true programmed chip load per tooth at the thread diameter:

• Internal Thread (Female Hole): $$F_{center} = F_{periph} cdot left( rac{D_{thread} - D_{cutter}}{D_{thread}} ight)$$ For example, milling an M16x2.0 thread ($D_{thread} = 16 ext{ mm}$) with a $12 ext{ mm}$ cutter requires scaling feed by $(16 - 12) / 16 = 0.25$. You must program the CNC feed rate at only $25%$ of the linear catalog feed!
• External Thread (Male Stud): $$F_{center} = F_{periph} cdot left( rac{D_{thread} + D_{cutter}}{D_{thread}} ight)$$ Because the tool centerline moves on a circle larger than the part perimeter, feed must be increased above catalog values.

3. Tool Diameter Rule of Thumb ($D_{cutter} le 70% D_{thread}$)

Best machining practice dictates that cutter diameter should not exceed $70%$ of the thread major diameter for internal threads. Exceeding $75%$ causes severe tool wrap (high arc of contact), catastrophic chip recutting, and thread profile distortion.