Geometry of Single Point Tools

1. Rake angle:

It is the angle formed between face of the tool and plane parallel to its base. If this inclination is towards shank, it is known as back rake or top rake. When it is measured towards side of the tool, it is called side rake.

These rake angles guide the chips away from the cutting edge, thereby reducing the chip pressure on the face and increasing the keenness of the tool, so that less power is required for cutting.

An increased rake angle will reduce the strength of cutting edge. Therefore tools used for cutting hard materials are given small rake angles, whereas those used for soft metals contain large rake angles.

2. Negative rake angle:

The above rake angles are called positive rake angles. When no rake is provided on the tool, it is said to have zero rake angle.

When the face of the tool is so ground that it slopes upwards from the point, it is said to contain a negative rake. It reduces keenness of the tool and increases the strength of cutting edge.

Such rake is usually provided on carbide tipped tools when they are used for machining extra- hard surfaces, hardened steel parts and for taking intermittent cuts.

The values of negative rake on these tools normally vary from 5 to 10.

3. Lip angle:

The angle between the face and flank of the tool is known as Lip angle. It is also called angle of keenness of the tool. Strength of the cutting edge or point of the tool is directly affected by this angle. Larger the lip angle, stronger will be cutting edge and vice-versa.

This angle varies inversely as the rake angle. It is only for this reason that when harder metals are to be machined a stronger tool is required, the rake angle is reduced and consequently the lip angle is increased.

This calls for reduced cutting speeds, which is dis-advantage. The lip angle is therefore kept as low as possible without making the cutting edge so weak that it becomes unsuitable for cutting.

4. Clearance angle:

It is the angle formed by the front or side surface of the tool which are adjacent and below the cutting edge when the tool is held in a horizontal position. It is the angle between one of these surfaces and a plane normal to the base of the tool.

When the front surface is considered it is called front clearance and when the surface below cutting edge is considered, the angle formed is known as side clearance angle.

The purpose of providing front clearance is to allow the tool to cut freely without rubbing against the surface of the job. The side clearance is to direct the cutting thrust to the metal area adjacent to the cutting edge.

5. Relief angle:

It is the angle formed between flank of the tool and a perpendicular drawn from the cutting point to the base of the tool.

6. Cutting angle:

The total cutting angle of the tool is the angle formed between the tool face and a line drawn through the point, which is a tangent to the machined surface of the work at that point. Its correct value depends upon the position of the tool in which it is held in relation to the axis of the job.

7. Nose radius:

If the cutting tip of a single point tool carries a sharp cutting point, the cutting tip is weak. It is therefore highly stressed during the operation, may fail or loose its cutting ability soon and produces marks on the machined surface. In order to prevent these harmful effects the nose is provided with a radius, called Nose radius.

It enables greater strength to cutting tip, a prolonged tool life and superior surface finish on the work piece.

As the value of this radius increases, a higher cutting speed can be used.

If it is too large, it may lead to chatter. So a balance has to be maintained. Its value normally varies from 0.4mm to 1.6mm depending upon several factors like depth of cut, amount of feed, type of cutting and type of tool.