The pitch circle diameter, PCD, is the diameter of the circle that passes through the centres of a ring of holes. It is the number behind a wheel marked 5x114.3: five holes on a 114.3 mm circle. If it is off, the part does not seat, the studs bind, and the fault shows up at assembly rather than at the machine that drilled the holes.
Checking it with a vernier is slow and depends on the operator. A hole has no centre you can touch, so the usual method is to measure across two holes and subtract a hole diameter. That is workable for one part and tiring for a shift. A PCD gauge does the job in one placement.
A fixed PCD gauge carries pins set at the nominal hole positions on the nominal circle, usually located off the centre bore. When the pins drop into the holes and the gauge seats on the face, the hole pattern is within the tolerance built into the pin diameters. It is a functional check: it asks whether the part will accept its mating component, which is the question that matters.
Because it checks position and size together, one gauge replaces several readings. On the drawing this is a position tolerance under ISO 1101, for example holes within a 0.2 mm diameter zone of their true positions, and the gauge pins are sized to represent exactly that zone. The same logic sits behind every GO / NO-GO tool, covered in what a go / no-go gauge actually proves.
For an even number of holes, two opposite holes sit exactly one PCD apart. For an odd number, no two holes are opposite, so the distance between adjacent hole centres is the chord PCD x sin(180 / n). For five holes on 114.3 mm that works out to about 67.19 mm. It is a quick way to confirm that a drawing, a gauge and a part all describe the same pattern before anything is cut.
We make PCD and other drawing-based gauges as part of our custom and special gauges. If a bolt circle is giving you trouble, send us the drawing and we will say what gauge can check it. Pins and bores wear like any other gauge surface, so our guide to wear limits applies here too.
Send us the feature and the drawing. DSN Enterprises manufactures custom gauges — form, spline, ACME, buttress and taper — for the cases where a catalogue tool cannot inspect the feature.
Or call +91 93631 22005.
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