End Mill Selection Guide: Flute Count, Helix Angle, Corner Radius and Overhang/Diameter Ratio

Selecting an end mill amounts to five decisions: flute count, helix angle, corner radius, length of cut and overhang/diameter ratio. Coating is a separate subject and is addressed in the coating guide.

1. Flute count (Z)

The flute count determines two things at the same time: feed capacity and chip evacuation.

Since vf = fz × Z × n, doubling Z doubles the feed. But as Z rises the cross-section of the chip gullet gets smaller. If the chip cannot get out, it jams in the gullet, heats up and breaks the tool.

2. Helix angle

The inclination of the cutting edge relative to the tool axis.

Axial force: A high helix increases the axial (upward) force. On thin-walled or weakly clamped parts the part can lift upward; in that case a low helix or a "down-cut" geometry is preferred.

3. Corner radius (R)

The weakest point of the cutting edge is the corner; stress accumulates there. A small radius distributes this accumulation. If the design does not make a sharp corner mandatory, a corner radius tool is preferred; it gives a markedly longer life under the same conditions and permits a higher feed.

Where sharp corners are required with a corner radius tool, roughing with the radiused tool and clearing only the corners with a square tool is a common and efficient method.

4. Length of cut (L1)

The length of cut is selected only as long as needed. A long length of cut:

• Flexes the tool (deflection increases)
• Enlarges the tendency to vibrate
• Raises the price

5. Overhang/diameter ratio and tool overhang

The ratio of the length of the tool protruding from the collet (L_overhang) to the cutting diameter is the single most important factor governing vibration. This is because deflection is proportional to the cube of the overhang:

δ ∝ L_overhang³ / (E × D⁴)

That is, doubling the overhang increases deflection eightfold.

The tool must not be drawn out of the collet further than necessary. Pushing it 2 mm further in can be more effective than increasing the cutting parameters.

Decision summary

Pre-order checklist

1. Whether the collet holds the D2 shank diameter
2. Whether the L1 length of cut covers the pocket depth
3. Whether the overhang/diameter ratio stays below 4
4. Whether the condition Z ≤ 3 is met where slotting is involved
5. Whether a radiused geometry has been selected where a sharp corner is not mandatory
6. Whether the coating suits the material to be machined

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The Tool and Coating Selector tool can be used to obtain a tool recommendation by material, and the Cutting Parameter Calculator tool to calculate cutting parameters.