Understanding Cutting Tool Angles

Cutting tools are essential in machining operations. Their geometry, defined by various angles, significantly affects the cutting performance, tool life, and surface finish of the workpiece. Two critical angles are the rake angle and the clearance (or relief) angle.

Rake Angle Explained

The rake angle is the angle between the tool face (where the chip flows) and a reference plane. It can be positive, negative, or zero:

• Positive Rake Angle: Reduces cutting forces and heat generation, suitable for machining softer materials or when a good surface finish is required.
• Negative Rake Angle: Strengthens the tool tip, suitable for machining hard or brittle materials, dealing with interrupted cuts, or using carbide tools. It increases cutting forces.
• Zero Rake Angle: Used for some specific applications, offering a compromise between tool strength and cutting force.

Therefore, a cutting tool can have a positive or a negative rake angle depending on the material being cut and the machining conditions.

Clearance Angle Explained

The clearance angle, also known as the relief angle, is the angle between the flank of the tool (the surface below the cutting edge) and the surface of the workpiece that has just been cut. Its primary purpose is to prevent the tool flank from rubbing against the machined surface of the workpiece.

Consider the implications of different clearance angles:

• Positive Clearance Angle: This is the standard and necessary condition. A positive angle ensures that only the cutting edge is in contact with the workpiece during cutting. This minimizes friction, heat, and tool wear, allowing for efficient cutting.
• Zero Clearance Angle: If the clearance angle is zero, the tool flank will be parallel to the machined surface. This would lead to significant friction and rubbing immediately behind the cutting edge. While theoretically possible under perfect alignment, it's impractical in real-world machining due to factors like vibration and tool deflection.
• Negative Clearance Angle: If the clearance angle is negative, the tool flank would dig into or rub heavily against the freshly cut surface of the workpiece behind the cutting edge. This creates immense friction, generates excessive heat, and leads to rapid tool wear or even tool failure. It would make effective cutting impossible as the tool would essentially be ploughing rather than cutting.

Why a Negative Clearance Angle is Impossible

For a cutting tool to function correctly and efficiently remove material, its flank must clear the machined surface. A negative clearance angle violates this fundamental requirement, causing severe interference between the tool and the workpiece. This interference would prevent chip formation, increase cutting forces drastically, and lead to destructive rubbing instead of cutting action.

Thus, a cutting tool can never have a negative clearance angle in a practical machining operation.

Conclusion

Based on the function and necessity of cutting tool angles, a positive rake angle is common, a negative rake angle is used for specific applications, and a positive clearance angle is always required for effective cutting. A negative clearance angle would impede cutting entirely due to excessive friction and rubbing.

Therefore, a cutting tool can never have a negative clearance angle.

Revision Table: Cutting Tool Geometry

Reviewing the key concepts discussed:

• Cutting Edge: The part of the tool that separates the chip from the workpiece.
• Tool Face: The surface over which the chip flows.
• Tool Flank: The surface below the cutting edge that is adjacent to the machined surface.
• Rake Angle: Angle between tool face and a reference. Affects chip flow and cutting forces.
• Clearance Angle: Angle between tool flank and machined surface. Prevents rubbing.

Additional Information: Other Tool Angles

While rake and clearance angles are critical, other angles also define tool geometry:

• Approach Angle (or Side Cutting Edge Angle): Angle between the cutting edge and the feed direction. Affects chip thickness and cutting forces.
• End Cutting Edge Angle: Angle between the end cutting edge and the feed direction. Prevents the tool from rubbing on the finished surface.
• Nose Radius: The rounded tip of the tool. Improves surface finish and strengthens the tool tip.

All these angles work together to ensure efficient material removal and desirable workpiece quality.