When a ball end mill cuts at a given depth, the actual diameter doing the cutting is smaller than the tool's nominal diameter. This calculator finds the effective diameter and adjusts your RPM accordingly.
Calculate ↓The formula: De = 2 × √(R² − (R − ap)²) where R = tool radius, ap = depth of cut.
A 10 mm ball end mill cutting at 0.3 mm depth has an effective cutting diameter of approximately 3.4 mm — only 34% of the nominal diameter. If you program RPM based on 10 mm at 120 m/min, you get 3,820 RPM. But the actual cutting happens at 3.4 mm diameter, which gives a surface speed of only 41 m/min — far below the recommended range for carbide in steel. The tool rubs instead of cuts, generating heat and accelerating wear.
The correct RPM based on effective diameter is 11,200 RPM. This is a 3× difference. Running at the nominal-based RPM means the tool is moving at one-third the required cutting speed. The result is poor surface finish, rapid edge wear, and scrapped parts — especially in 3D finishing operations on molds and dies where ball end mills are the standard.
In mold finishing on a Haas VM-3 or Okuma MB-4000, ball end mills are used at light depths of cut — typically 0.1-0.5 mm. At these depths, the effective diameter is a fraction of the nominal diameter. A 6 mm ball end mill at 0.15 mm depth has an effective diameter of only 1.9 mm. The RPM should be calculated from 1.9 mm, not 6 mm. The feed rate must also be adjusted because the smaller effective diameter means fewer RPM per mm of tool path.
CAM systems like Mastercam and Fusion 360 include effective diameter compensation in their HSM toolpaths. But when programming manually or editing existing code, this calculator provides the corrected values. For critical surface finishing passes, some programmers increase the programmed RPM by an additional 10-15% as a safety margin against effective diameter error.
What is effective diameter in ball nose milling? The actual diameter of the cutting zone at the depth of cut. Since a ball end mill is spherical, the contact point moves up the radius as depth decreases, reducing the effective cutting diameter.
How do you calculate effective diameter? De = 2 × √(R² − (R − ap)²). The calculator above does this instantly for any tool size and depth.
What RPM should I run a 6 mm ball at 0.15 mm depth? At 0.15 mm depth, the effective diameter is 1.9 mm. At 120 m/min Vc, RPM should be approximately 20,000 — near the limit of a standard 10k spindle. This is why mold finishing often requires high-speed spindles or smaller stepovers.
Does effective diameter affect feed rate? Yes. Feed rate = RPM × flutes × fz. Since RPM increases at small effective diameters, feed rate also increases. But the chip thinning effect at light depths means the programmed fz may need to be reduced to maintain the actual chip load.