Resolving Disagreement Between Optical Hole Sizes and Pin Gauges

An optical hole measurement and a pin-gauge result can disagree without either instrument being defective. The optical system may identify an entrance edge, while a pin responds to the narrowest passage along its insertion path. The first step is to define what each result represents.

Inspect the geometry behind the number

Eurocircuits describes finished-hole inspection using optical or mechanical approaches. When applying either method, document the feature boundary and access direction. A shiny finish ring, a mask opening and the actual plated bore are different edges in the same image.

Check the hole from both sides under suitable lighting. Taper, localized plating, a burr or trapped debris can reduce usable clearance away from the entrance. If the hole is slightly noncircular, an average fitted-circle diameter may not describe the largest cylindrical pin that can pass through it.

Compare methods on common samples

Mark individual hole coordinates and measure them before attempting a gauge fit. Use clean, calibrated gauges with an agreed method and light, controlled contact. Forcing a gauge can deform plating or remove an obstruction, making the subsequent optical result describe a changed specimen. Repeat the comparison across several holes spanning the reported size range; one unusually rough hole should not determine the correction applied to an entire manufacturing lot.

• Record whether the gauge enters from one or both faces.
• Keep gauge size, tolerance and condition visible in the report.
• Retain optical images and the edge-fitting settings.
• Distinguish a go/no-go fit result from a continuous diameter measurement.

Choose acceptance based on the requirement

For a soldered connector, the required clearance includes the pin’s actual cross-section and positional stack. For a tightly controlled mechanical fit, local minimum bore size may dominate. Neither application is resolved by taking the arithmetic average of two disagreeing methods.

If a section is needed, locate the narrow region using prior observations and preserve its orientation. Compare the finished-hole requirement with pin-fit budgeting and use targeted section preparation to examine the relevant wall. Close the discrepancy by identifying the physical geometry and assigning a reproducible acceptance method. Do not simply choose the method that produces the more favorable number, because that leaves the next lot’s fit problem unexplained.