Question

A positive rake angle is generally preferred for:

• A. Cutting tool materials that are hard and brittle
• B. Cutting tool materials that have poor thermal conductivity
• C. Brittle work piece materials to reduce cutting forces
• D. Ductile work piece materials to reduce cutting forces

Hint:

Think about how a positive rake angle "lifts" the chip away from the workpiece, reducing resistance and force, which is particularly beneficial for the continuous chip formation in ductile materials.

Answer 1

The Correct Option is D

Step 1: Understand the concept of rake angle.
The rake angle is the angle between the face of the cutting tool and a line perpendicular to the cutting velocity. A positive rake angle means the tool face slopes away from the cutting edge. Step 2: Analyze the effects of a positive rake angle on cutting forces.
A positive rake angle:
Reduces the shear angle, leading to a longer shear plane and thinner chips.
Decreases the cutting forces required because the material is sheared more efficiently.
Lowers the power consumption during machining.
Can lead to better surface finish.
Step 3: Consider the implications for different workpiece materials.
Ductile materials: These materials tend to form continuous chips. A positive rake angle helps in the smooth flow of these chips over the tool face and reduces the cutting forces significantly.
Brittle materials: These materials tend to fracture and form discontinuous chips. While a positive rake angle can still reduce cutting forces to some extent, the primary concern is often to avoid excessive impact and tool chipping, which might necessitate a smaller or even negative rake angle in some cases for increased tool strength. Step 4: Consider the implications for cutting tool materials.
Hard and brittle tool materials: These materials have high wear resistance but low toughness (resistance to fracture). A large positive rake angle might weaken the cutting edge, making it susceptible to chipping, especially under interrupted cuts or with hard materials. Tool materials with poor thermal conductivity: A positive rake angle can reduce the contact area between the chip and the tool face, potentially leading to higher temperatures at the cutting edge due to less heat dissipation into the chip. Step 5: Evaluate the given options.
Option 1: A positive rake angle is generally not preferred for hard and brittle tool materials due to the potential for reduced tool strength.
Option 2: While a positive rake angle can affect heat generation and dissipation, it's not the primary reason for preference based on tool thermal conductivity alone.
Option 3: For brittle work piece materials, the preference for a positive rake angle to reduce cutting forces is not as strong as for ductile materials, and sometimes smaller or negative rake angles are used for better tool life.
Option 4: A positive rake angle is generally preferred for ductile work piece materials because it significantly reduces cutting forces and power consumption, promotes better chip flow, and can improve surface finish. Step 6: Select the correct answer.
A positive rake angle is generally preferred for ductile work piece materials to reduce cutting forces.