Concentricity Control in Stepped Core Pins: Measurement & Specification
Understanding Concentricity in Stepped Pins
Concentricity measures how well the centerlines of all step diameters align. In a perfect pin, the small-diameter section, mid-diameter section, and large-diameter section all share exactly the same centerline. In practice, manufacturing tolerances create slight offsets.
Concentricity is measured as TIR (Total Indicator Reading) — the total range of movement on a dial indicator when the pin is rotated 360° between centers. A TIR of 0.01 mm means the center of one step is offset by up to 0.005 mm from the center of the reference step.
Concentricity Tolerance by Application
| Application | TIR Requirement | Pin Type |
|---|---|---|
| General structural features | ≤ 0.015 mm | One-step or two-step |
| Standard connectors | ≤ 0.010 mm | One-step or two-step |
| Precision connectors (high-speed data) | ≤ 0.005 mm | One-step preferred (single transition) |
| Optical components | ≤ 0.003 mm | Custom specification required |
Impact on Part Quality
The relationship between concentricity error and wall thickness is direct and linear:
- 0.01 mm TIR → ±0.005 mm wall thickness variation → acceptable for wall ≥ 1.0 mm (0.5% variation)
- 0.02 mm TIR → ±0.01 mm wall variation → borderline for wall ≥ 1.0 mm (1% variation)
- 0.05 mm TIR → ±0.025 mm wall variation → unacceptable for wall < 1.5 mm (causes warping)
For multi-cavity molds, concentricity variation between pins across cavities creates cavity-to-cavity part variation. This is why incoming inspection of stepped pins is critical for high-cavity production.
Measurement Method
- Equipment: V-blocks or precision centers + dial indicator (0.001 mm resolution)
- Setup: Mount pin on the largest diameter section (reference). Rotate 360°.
- Reading: Record max and min indicator values at each step. TIR = max − min.
- Multiple locations: Measure at the midpoint and near the tip of each step section.
According to ISO 1101 (Geometrical Product Specifications), concentricity is defined as the deviation of the center of a cross-section from the datum axis, and should be specified as a diameter zone on the engineering drawing.
Manufacturing Best Practices
- Single-setup CNC — All step diameters turned in one setup without rechucking. This eliminates rechuck error and achieves TIR ≤ 0.005 mm.
- Post-grind verification — After final grinding, measure concentricity and reject pins exceeding specification.
- Pin head concentricity — The head mounting surface must also be concentric to ensure the pin sits true in the plate bore.
Frequently Asked Questions
What concentricity should I specify for stepped core pins?+
How does concentricity error affect the molded part?+
Can concentricity be improved on existing stepped pins?+
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