RAKE ANGLE – TYPES AND THEIR SIGNIFICANCE.

Positive rake:

• Positive rake or increased rake angle reduces compression, the forces, and the friction, yielding a thinner, less deformed and cooler chip

• But increased rake angle reduces the strength of the tool section and heat conduction capacity.

• Some areas of cutting where positive rake may prove more effective are, when cutting tough, alloyed materials that tend to work harden, such as certain stainless steel, when cutting soft or gummy material or when low rigidity of work piece, tooling, machine tool, or fixtures chatter to occur.

Negative rake:

• To provide greater strength at the cutting edge, and better heat conductivity, zero or negative rake angles employed on carbide, ceramic, poly-crystalline diamond, and poly crystalline cubic boron nitrite cutting tools.

• These materials tend to be brittle but their ability to hold their superior hardness at high temperature results in their selection for high speed and continuous machining operation.

• Negative rakes increases tool forces but it is necessary to provide added support to the cutting edge. This is particularly important in making intermittent cuts and in absorbing the impact during the initial engagement of the tool and work.

• Negative rakes are recommended on tool which does not possess good toughness (low transverse rupture strength)

• Negative rake causes high compression, tool forces and friction, resulting in highly deformed, hot chip.

The rake angle for a tool depends on the following factors:

1] Type of the material being cut: A harder material like cast iron may be machined by smaller rake angle than that required by soft material like mild steel or aluminum.

2] Type of tool material: Tool material like cemented carbide permits turning at high speed. At high speeds rake angle has little influence on cutting pressure. Under such condition the rake angle can minimum or even negative rake angle is provided to increase the tool strength.

3] Depth of cut: In rough turning, high depth of cut is given to remove the maximum amount of the material. This means that the tool has to withstand severe cutting pressure. So the rake angle should be decreased to increase the lip angle that provides the strength of the cutting edge.

4] Rigidity of tool holder and machine: an importantly supported tool on old or worn out machine cannot take up high cutting pressure. So while machining under above condition, the tool used should have larger rake angle.