Lecture-5 Theory of Metal Cutting - Orthogonal and oblique cutting,

The process of Metal Cutting is divided into the following two main classes: 

A caparison between these two methods is clearly illustrated in Figs. 5.1 and 5.2. In Fig. 5.1 is shown as to how these two Cutting Methods differ while turning a job on a lathe. 

Fig. 5.1 Orthogonal and Oblique cutting processes in Turning. 

Similarly, Fig. 5.2 shows the difference between these two methods as applied to the Planing work, in which the tool remains stationary and the workpiece reciprocates past it. 

Fig. 5.2 Orthogonal and Oblique cutting processes in Planing. 

Basically, in Orthogonal cutting, the cutting edge of the tool remains at right angles to the direction of cutting velocity [Fig. 5.1 (a)] or work feed [Fig. 5.2 (a)]. This type of cutting is also known as Two-dimensional Cutting. In Oblique cutting, the cutting edge of the tool is inclined at an acute angle with the direction of tool feed or work feed, the chip being disposed of at a certain angle. This type of cutting is also called Three-dimensional Cutting. The main features of the two types of cutting are summarised below : 

Orthogonal Cutting 

1. The cutting edge of the tool remains normal to the direction of tool feed or work feed. 

2. The direction of the chip flow velocity is normal to the cutting edge of the tool. 

3. The angle of inclination 'i' of the cutting edge of the tool with the normal to the velocity Vc is 'Zero'. 

4. The chip flow angle ' β’, i.e., the angle between the direction of chip flow and the normal to the cutting edge of the tool, measured in the plane of the tool face, is 'Zero'. 

5. The cutting edge is longer than the width of the cut. 

The last condition may not be fulfilled in some cases. It is then called Semi-Orthogonal or Restricted Orthogonal cutting. 

Oblique Cutting 

1. The cutting edge of the tool always remains inclined at an acute angle to the direction of tool feed or work feed. 

2. The direction of the chip flow velocity is at an angle ' β’ with the normal to the cutting edge of the tool. The angle is known as Chip flow Angle. 

3. The cutting edge of the tool is inclined at an angle ‘i’ with the normal to the direction of work feed tool feed i.e. velocity ‘Vc’. 

4. Three mutually perpendicular components of cutting forces act at the cutting edge of the tool 

5. The cutting edge may or may not be longer than the width of the cut. 

An interesting feature to note here will be that most of the metal cutting carried out in workshops is through Oblique Cutting method, but all our further discussions on metal catting will be in the context of Orthogonal Cutting because of its simplicity. However, it won't matter much since most of the general principles of Orthogonal Cutting are equally applicable to Oblique Cutting.