Question

At mechanics of metal cutting,

I cannot understand the reason why the shear angle is smaller when the cutting speed is slow.

can you explain this to me?

Answer 1

Chip thickness is inversely proportional to shear angle. Thus where the shear plane is very slight the shearing force can be up to 5 times greater. In comparision to where the angle is 45⁰. shear plane angles will vary, from 5⁰-45⁰.

many forces are encountered during the cutting process on the shear plane. The force required to form a chip depends upon the shear yield strength of the work material under cutting conditions. Shear yield strength of the work material will vary greatly. For example, the strength of nickel-chromium-vanadium steel is almost 6 times that of magnesium and almost 20 times that of lead.

In metals such as aluminium,copper,iron and nickel, cutting forces are high, because the area of contact between the rake face and the material is high. Correspondingly, when the shear plane angle is small, thick strong chips are created at slow speed. This explains why pure metals are traditionally difficult to machine. High ductility results in high contact area.

Contact area will decrease with certain types of metals. When cutting magnesium, hardened steel, titanium and hard plate, chips will be much smaller and thinner. In fact, forces will decrease as the cutting speed is raised, because there is less contact area. Temperature will also rise as the speed increases.