Checking the centre distance of two holes without a CMM
On a connecting flange, the centre distance between two 12 mm holes is specified as 80 ±0.05 mm. The shop has no coordinate measuring machine available right away. The technician has to use two calibrated pins and an outside micrometer to measure the spacing to the hundredth of a millimetre.
Methods and formulas for checking the centre distance of two holes
| Dimension measured | Formula for the centre distance E | Instrument used |
|---|---|---|
| Outside dimension over pins (L1) | E = L1 - (d1 + d2) / 2 | Outside micrometer |
| Inside dimension between pins (L2) | E = L2 + (d1 + d2) / 2 | Inside caliper |
| Holes of the same diameter (d) | E = L1 - d | Outside micrometer |
All values are given for the reference environmental conditions of 20 °C.
Centre distance from the outside dimension over pins
Insert two pins that fit without play into holes d1 and d2. Measure the outside distance L1 across the outer edges of the two pins with the micrometer. The centre distance E is E = L1 - (d1 + d2) / 2. To check the related position tolerance, read our article on the ISO 1101 position tolerance at maximum material.
Checking the hole diameters before calculating
Before measuring L1, check the actual diameters d1 and d2 carefully with a plain plug gauge or a bore gauge. Using the nominal diameters instead of the actual ones introduces an error equal to half the machining tolerance.
Worked example: a flange with two holes
Target centre distance: 80 ±0.05 mm (79.95 to 80.05 mm). Holes ⌀12 H7 (12.000 to 12.018 mm). Pins used: 12.000 mm. Outside reading L1: 92.08 mm. 1. Centre distance: E = 92.08 - 12.00 = 80.08 mm. Verdict: the value (80.08 mm) exceeds the upper limit by 0.03 mm. The part does not conform.
Practical pitfalls when measuring centre distance with pins
The first pitfall is a false offset caused by a pin tilting out of square. The second is overtightening the micrometer, which bends the pins. The third comes from forgetting that the holes may be oval. To keep track of your dimensions safely, use CoteCote and its support page.