π§ Cutting Parameters
π SPINDLE SPEED (N)
-- RPM
N = (Vc Γ 1000) / (Ο Γ D) [metric] | N = (Vc Γ 12) / (Ο Γ D) [imperial]
π TABLE FEED (Vf)
-- mm/min or IPM
Vf = N Γ fz Γ Z
Spindle speed (RPM) & table feed rate β metric or imperial
Cutting speed (Vc) is a material and tool property β it describes how fast the cutting edge moves relative to the workpiece surface, in m/min or ft/min. It is determined by the tool material, workpiece material, and desired tool life. You find it in tool catalogs.
Spindle RPM (N) is a machine setting β it is calculated from the cutting speed and the tool diameter: N = (Vc Γ 1000) / (Ο Γ D). This is the number you actually enter in the machine control or set on the spindle speed dial.
The key insight is that cutting speed stays constant for a given material-tool combination, but spindle RPM changes with every tool diameter. A ΓΈ6mm end mill and ΓΈ25mm face mill both run at the same Vc for carbide in mild steel β but the RPM values are very different.
| Tool Diameter | Calculated RPM | Practical RPM |
|---|---|---|
| ΓΈ4 mm | 11,937 | 10,000β12,000 |
| ΓΈ8 mm | 5,968 | 5,500β6,000 |
| ΓΈ16 mm | 2,984 | 2,800β3,000 |
| ΓΈ25 mm | 1,910 | 1,800β2,000 |
| ΓΈ50 mm | 955 | 900β1,000 |
Practical note: Always round calculated RPM down to the nearest available machine speed, not up. Running faster than recommended cuts tool life significantly β most tool wear models show exponential increase in wear rate with even a 10% increase in cutting speed.