Material Removal: Principles

There are many kinds of machining operations, each of which is capable of generating a certain part geometry and surface texture.

In turning, a cutting tool with a single cutting edge is used to remove material from a

rotating workpiece. The speed motion in turning is provided by the rotating

workpiece, and the feed motion is achieved by the cutting tool moving in a direction

parallel to the axis of rotation of the workpiece. The feed movement of the tool can be

along the axis of the workpiece, which means the diameter of the part will be turned

down to a smaller size. Alternatively, the tool can be fed towards the center (facing

off), at the end of the part. Often feeds are combinations of these two directions,

resulting in tapered or curved surfaces.

Drilling is used to create a round hole. It is accomplished by a rotating tool that is

has two or four cutting edges. The drilling operations simply consist of rotating the

drill and feeding it into the workpiece being drilled which is clamped in a vice fixed in

a machine table.

Milling is a metal cutting process performed with a rotating multi-edge cutting tool which performs high feed and speed which used to generate flat faces.

Shaping is a metal cutting operation which the primary motion is performed by the tool,

and feed by the workpiece to produce a flat surface.

Cutting tool should be provided with proper geometry (wedge form) to provide

penetration and removing of workpiece material.

He said that cutting tools are usually made of HSS. Although HSS still retains its

important, most metal cutting is now carried out with cemented carbide tools and

ceramics tools.

Experiment was performed in Institute of Technology lab in Cranfield to find out the

importance of the wedge shape.

Mild steal was machined on a planer machine:

First, a square ended cutting tool was provided (𝛾 = 0, 𝛼 = 0), the chip does not flow

naturally over the tool face and rubbing is occurred between the flank and machined

surface. This causing the tool to ride up over the workpiece until it is stop cutting.

By sloping the bottom tool away from the workpiece, rubbing is vanished.

Rough machined surface Due to rubbing action

In order to prevent the compression and the deformation over the face of the tool we

change the path of the chip. We change the angle of the tool face (The rake angle).

The workpiece material is removed by shearing along the shear plane.

For the same feed, speed and clearance angle we find that:

The larger the rake angle, the larger the shear angle, the smaller the shear plane length,

the smaller the shear area, the smaller the cutting forces, the smaller the torque, the

smaller the power consumed.

The smaller the rake angle, the smaller the shear angle, the larger the shear plane

length, the larger the shear area, the larger the cutting forces, the larger the torque, the

larger the power consumed.

Rake Angle 10° 20° 30° 40°

Total Power 2.6 kW 2.3 kW 2.1 kW 2 Kw

Net power 1.8 kW 1.5 kW 1.3 kW 1.2Kw

Excessively large positive rake angle weakens tool wedge and in danger of fracture or

thermal failure.

He said "the tool of the rake angle between 10° and 20° would be the most suitable" this

is only true if the cutting tool material was made of HSS which can withstand tensile

stresses at the tool face. For brittle tool materials, rake angle should be negative.