The 2-Pin Locating System
The most reliable fixture locating system uses exactly two pins: one cylindrical (round) and one diamond (relief-ground). This is not arbitrary — it is the minimum configuration that constrains workpiece position without over-constraint.
Constraint Analysis
| Pin Type | Constraints | Free Axis | Primary Role |
|---|---|---|---|
| Cylindrical (round) | X translation + Y translation | None in-plane | Primary locator — establishes the datum origin point |
| Diamond (relief-ground) | Rotation around Z only | Linear expansion along relief axis | Secondary locator — prevents rotation while allowing thermal expansion |
Why Not Two Cylindrical Pins?
Two cylindrical pins create an over-constrained system. Here is what happens in practice:
- Thermal expansion: A 500 mm aluminum workpiece expands ~0.06 mm over a 5°C temperature change. Two cylindrical pins 400 mm apart cannot accommodate this expansion — the workpiece jams or the pins bend.
- Manufacturing tolerance: The hole-to-hole distance on the workpiece has a manufacturing tolerance (typically ±0.05 mm). Two cylindrical pins at exact spacing cannot accommodate this variation.
- Insertion difficulty: Even without thermal effects, two tight-fitting cylindrical pins are extremely difficult to insert simultaneously.
Diamond Pin Design
A diamond pin has two opposing flats (relief-ground surfaces) that reduce the effective contact width to ~30% of the diameter. This creates clearance along one axis while maintaining full contact along the perpendicular axis.
- Contact axis: The narrow dimension constrains rotation (perpendicular to the pin-to-pin center line)
- Relief axis: The wide dimension allows linear expansion along the pin-to-pin center line
- Orientation rule: The diamond pin's relief flats must be oriented parallel to the line connecting the two pin centers
Pin Placement Guidelines
- Maximum separation: Place pins as far apart as the workpiece geometry allows — greater distance improves angular accuracy
- Primary pin location: Place the cylindrical pin at the most critical datum feature
- Diamond pin orientation: Relief flats parallel to the primary-to-secondary pin center line
- Avoid edges: Keep pins at least 2× pin diameter from workpiece edges to prevent deformation
When to Use Two Cylindrical Pins
There are rare cases where two cylindrical pins are acceptable:
- Very small workpieces (<50 mm) where thermal expansion is negligible
- Inspection gauges at controlled temperature (20°C ± 0.5°C)
- Temporary fixtures with manual insertion where operator can feel the fit
In all other cases, use the cylindrical + diamond combination.