Difficulty: Easy
Correct Answer: Lower chip–tool contact area and larger shear angle
Explanation:
Introduction / Context:
Machinability encompasses ease of cutting, surface finish, tool life, and chip control. Materials that shear readily under the tool generally yield favorable cutting mechanics, lower forces, and reduced heat generation.
Given Data / Assumptions:
Concept / Approach:
A larger shear angle means the shear plane is more steeply inclined, reducing chip thickness for a given uncut chip thickness. This lowers cutting forces and power. A smaller chip–tool contact area reduces frictional heat and tool wear. Together, they indicate a material that cuts cleanly with lower energy and less rubbing.
Step-by-Step Solution:
Relate machinability to cutting mechanics: higher shear angle ⇒ easier chip formation.Lower chip–tool contact ⇒ less friction and heat.Select the option combining both favorable traits.
Verification / Alternative check:
Free-machining steels (with sulfur, lead) tend to show higher apparent shear angles and shorter contact lengths due to chip breaking and lubricity, supporting improved machinability.
Why Other Options Are Wrong:
Higher contact and smaller shear angle imply more rubbing and power. Mixed combinations with a smaller shear angle still indicate harder cutting. Built-up edge and “zero shear deformation” are unrealistic and detrimental.
Common Pitfalls:
Confusing rake angle with shear angle; although related, shear angle describes the deformation plane in the work.
Final Answer:
Lower chip–tool contact area and larger shear angle
Discussion & Comments