The relationship between end mill diameter and length

• Cutting Diameter (ØD): The diameter of the cutting edge.

• Overall Length (OAL): The total length from the tip to the end of the shank.

• Cutting Length / Flute Length (CL): The effective cutting length of the tool.

• Shank Diameter (Ød): The diameter of the shank, usually equal to or larger than the cutting diameter.

Standard Proportion

Cutting length (CL) ≈ 1.5 × D

Overall length (OAL) ≈ 3 × D

Example:

-Ø10mm end mill → CL ≈ 15mm, OAL ≈ 60–75mm

-Ø20mm end mill → CL ≈ 30mm, OAL ≈ 90–120mm

Long Flute Type

Designed for deep slots or cavities.

Cutting length can reach 3D – 5D.

Lower rigidity, more prone to vibration, requires reduced cutting parameters.

Extra Long / Long Reach Type

Overall length can reach 5D – 10D.

Used for deep cavity or mold machining.

Shank is often reinforced (thicker) to increase rigidity.

• Rigidity: Larger diameter = higher stiffness, stronger against bending. Smaller diameter + long length = easy to break.

• Stability: Small diameter with long reach is prone to chatter.

• Application: Deep cavities and narrow slots require longer flute length.

• Small diameter (Ø1–3mm) → OAL typically 38–50mm, flute length ≤ 2D.

• Medium diameter (Ø4–12mm) → OAL 50–75mm, flute length ≤ 3D.

• Large diameter (Ø16mm+) → OAL 100–150mm, can be designed as long or extra long.

• High-precision machining → Always choose shorter flute and shorter OAL for better rigidity.

Summary

• Larger diameter → can have longer length

• Smaller diameter → should be kept shorter

• Standard ratio: CL ≈ 1.5D, OAL ≈ 3D

• Long or extra-long tools are only for special applications and require reduced cutting parameters