Machinability criterion — key geometric indicator Which geometric parameter in metal cutting is often used as a criterion for machinability comparisons?

Introduction / Context:
Machinability refers to how easily a material can be machined to an acceptable surface finish, dimensional accuracy, and tool life. Several indicators exist, but one geometric parameter directly connected to chip formation and cutting forces is frequently used: the shear angle.

Given Data / Assumptions:

• Orthogonal cutting model for analysis.
• Chip thickness ratio and rake angle are measurable.
• Higher shear angle generally implies thinner shear plane and lower cutting force.

Concept / Approach:
From the orthogonal cutting model, shear angle phi relates to chip thickness ratio r and rake angle alpha. A larger phi reduces shear plane area, lowering energy per unit volume removed and enabling smoother chip flow. Materials exhibiting larger shear angles at comparable conditions typically show better machinability.

Step-by-Step Solution:
Link machinability to chip formation efficiency.Recognize that higher shear angle correlates with lower cutting forces and temperatures.Therefore, shear angle serves as a practical comparative criterion.

Verification / Alternative check:
Empirical tests show materials adding free-machining additives (e.g., S, Pb) exhibit increased shear angle and improved chip breakage, aligning with easier machining.

Why Other Options Are Wrong:
Cutting speed and feed rate are process settings, not intrinsic geometric indicators; tool nose radius and rake angle are tool geometry choices rather than material response metrics.

Common Pitfalls:
Assuming a single scalar defines machinability completely; in practice, machinability is multi-factor (surface finish, tool life, power, chip control).

Final Answer:
Shear angle