# End milling

### Cutting edge classification

Diameter

y:   Bend / deflection (mm)

:   Overhang (mm)

E:   Young's modulus (Gpa)

I:    Second moment of area* =

π   •   D

3 • E • I

D:   Rod diameter corresponding to an end mill (rod diameter when converting to

represent the end mill diameter D1, D =

(0.7 ~ 0.8) D1)

The diameter of the end mill affects rigidity. An end mill can be seen as a round rod, so there is a relationship shown in the formula between the diameter and the amount it will bend or deflect.

The amount an end mill will bend is an inverse proportion to the fourth power of the diameter (see formula above).

That is, as the diameter increases, an end mill is less likely to bend. For example, if a diameter doubles with the same tool overhang, the deflection will be one-sixteenth with the same cutting load (see image to the right). For example, the machined surface deflection when using a 4-flute carbide end mill is 25μm, if the diameter is doubled, the machined surface deflection will be 25μm × 1/16 = 1.5μm. This shows that high wall surface accuracy can be obtained.