The rake (or hook) angle is the angle between the normal to the surface being cut and the rake face, which is the face of the cutting tool against which the chip rubs. This angle is critical for making a proper, high quality cut. The optimal rake angle is largely determined by the material’s hardness. A larger rake angle results in a sharper cutting tool that will penetrate the material more easily, but also makes the edge more fragile. A smaller (or negative) rake angle reduces the penetration efficiency but increases the strength of the cutting edge and thus reduces wear. (8)
A larger rake angle is used for softer materials, while a smaller cutting angle is needed for harder materials. For example, kiln-dried wood has a lower moisture content and is therefore harder, requiring a smaller cutting angle than green wood. Likewise, hardwoods are generally denser than softwoods, so they also need smaller cutting angles. (3)
A sufficient relief (or clearance) angle is necessary to ensure than the knife heel does not rub against the cut surface. Relief angles will always be positive because of this requirement. (9)
A relief angle that is too small will generate excessive friction and the resulting heat will wear down the tool. A relief angle that is too large will make the knife angle too small, leading to a fragile cutting edge. (9)
The knife angle (also called the tool angle or wedge angle) is determined by the optimal rake angle and required relief angle.
β = 90 – (α + γ)
The knife angle affects the wear rate in two ways. First, a larger knife angle results in a stronger cutting edge and less wear. Second, a larger knife angle conducts heat better, distributing the heat over more material and carrying it away from the cutting edge, thereby reducing tool wear and increasing tool life. However, a smaller knife angle makes the cutting edge more keen and therefore better able to penetrate the material being cut. A balance must be struck, though, between strength and penetration ability. (3)
An oblique cutting angle is achieved when the cutting edge is not perpendicular to the tool’s direction of movement. Oblique angles reduce the cutting stresses on the knife edge, improve tool life, reduce edge chipping on the material, and result in an overall higher quality cut. Given these advantages, it is very common to see oblique angles incorporated into many tools used in CNC machining. (6)
Note: There are other angles in tooling. This is simply a brief overview of the more important angles you should be aware of.