Basic tool geometry is determined by the rake angle of the tool; which is always at the top side of the tool. With the tool tip at the center line of the workpiece, the rake angle is determined by the angle of the tool as it moves away from the workpiece center line location. Rake angles influence both tool strength and cutting pressure.
Proper selection of rake angle enhances the overall efficiency and economy of the cutting operation. There are in fact many combinations of rake angles that can be utilized as the situation demands, however for simplicities' sake, we'll narrow it down to three commonly used types of rake angles; positive, negative and neutral "zero" rake angle.
Recommended rake angles depend on tool material, workpiece material, speed and depth of cut. Negative angles result in a scraping action, while positive angles result in a cutting action. A positive rake angle is generally used in the cutting of soft material and requires less cutting force. However, the higher the positive rake angle is, it tends to weaken the cutting edge.
Generally, POSITIVE RAKE ANGLES:
- Continue sharpening of tool edge. This reduces the strength of the tool, as the small included angle at the tip may cause it to chip away.
- Reduce cutting forces and power requirements.
- Help in the formation of continuous chips in ductile materials.
- Can avoid the formation of a built-up edge.
- Increase the strength of the cutting edge and cutting forces.
- Are often used in cutting high-strength material.
- Can increase friction, resulting in higher temperatures.
- Can dull the tool.
- Can improve surface finish.
Generally, ZERO OR NEUTRAL RAKE ANGLES:
- Are easiest to use in manufacturing processes.
- Have larger crater wear when compared to positive rake angle, as chips slide over the rake face.
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