Design for fit: When designing mechanical devices with mating parts, must the designer determine acceptable size variations (tolerances) to ensure proper fit and function?

Introduction / Context:
Product reliability and ease of assembly depend on how precisely mating parts are specified. The designer’s role includes defining allowable variation so that parts consistently assemble and perform under real-world conditions and manufacturing variability.

Given Data / Assumptions:

• Mating features (bores/shafts, keys/keyways, fasteners/holes) are involved.
• Function requires specific clearances or interferences.
• Manufacturing processes introduce variability that must be controlled.

Concept / Approach:
Designers select fits (e.g., clearance, transition, interference) and specify size and geometric tolerances accordingly. These tolerances control variation so that even worst-case parts assemble and function. Without explicit limits, different suppliers may produce parts that do not assemble or that fail prematurely due to excessive play or stress concentration.

Step-by-Step Solution:

Verification / Alternative check:
Perform a tolerance stack-up calculation. If all pairs meet functional criteria within limits, the design is robust to variability, confirming that tolerances were correctly chosen.

Why Other Options Are Wrong:

• Leaving size variation to machinists creates inconsistent outcomes.
• Surface finish alone cannot guarantee assembly fit.
• Inspection merely checks conformance; it does not establish requirements.
• Stock sizes rarely match functional needs without defined tolerances.

Common Pitfalls:
Using only block tolerances for critical fits; omitting geometric tolerances (position, runout); ignoring thermal effects that change clearances.

Final Answer:
Correct: allowable variation between mating parts must be defined