Jigs and fixtures: Why is a diamond (diamond-shaped) locating pin used in a 2-pin location scheme for workholding?

Introduction / Context:
In precision location, a common strategy is to use two pins: one round (fixed) and one diamond-shaped. This controls location in two orthogonal directions without over-constraining the part. The diamond pin permits thermal expansion and manufacturing variation along one axis while still preventing rotation.

Given Data / Assumptions:

• Workpiece has two locating holes or slots.
• Fixture uses one round locating pin and one diamond locating pin.
• Goal is repeatable location without binding due to tolerances.

Concept / Approach:
The round pin fixes two degrees of freedom in the plane (X and Y at that hole) and rotation. The diamond pin, with its relieved flats, constrains the remaining needed degree perpendicular to the flat but allows slight movement along the flat. This avoids over-constraint when the hole center distance varies within tolerance, improving assembly and avoiding jamming.

Step-by-Step Solution:

Verification / Alternative check:
GD&T-based fixture design texts recommend round + diamond pin schemes to control 4 in-plane DOF while accommodating center-distance tolerance and thermal effects.

Why Other Options Are Wrong:
Hardness and wear are benefits of hardened pins but not the unique reason for diamond geometry; space saving, clamp elimination, and inspection are unrelated to the functional advantage of the diamond profile.

Common Pitfalls:
Using two round pins of tight fit causing binding; misorienting the diamond pin flats relative to the tolerance direction; failing to harden pins for durability.

Final Answer:
It allows one direction to float, accommodating center-distance tolerance during assembly.