BIZOMAT MAKEOS · Quality
Bolt Circle Pin Check (Pitch Circle from Plugs, Readings at the Limits, Decision)
For a ring of equally spaced holes measured over plugs, what pitch circle diameter does my reading give, what readings should I expect at the limits, and does it pass?
Inspection of a bolt circle (pitch circle) of equally spaced holes with close-fitting plugs: the pitch circle diameter from a micrometer or caliper reading over two alternate or adjacent plugs and the size across opposite plugs (as the bolt circle over plugs tool), the readings that correspond to the nominal pitch circle and its tolerance limits, and the accept or reject decision.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Number of holes | holes | Number of equally spaced holes. |
| Plug (hole) diameter | mm | Diameter of the plugs fitted in the holes. |
| Measured over | choice | Whether the reading is over two plugs with one hole between (alternate) or two neighbouring plugs (adjacent). |
| Measurement over the plugs | mm | Reading over the outsides of the two plugs. |
| Nominal pitch circle diameter | mm | Pitch circle diameter on the drawing. |
| PCD tolerance (±) | mm | Plus-or-minus tolerance on the pitch circle diameter. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Number of holes
- What it is
- How many equally spaced holes lie on the pitch circle. It sets the angle between holes used to convert the plug reading into a pitch circle diameter.
- Units and format
- Enter a number in holes: holes. The form accepts 3 holes to 360 holes. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 holes as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measuring over alternate plugs needs an even number of holes greater than four.
What this means: Plug (hole) diameter
- What it is
- The diameter of the close-fitting plugs or pins put in the holes for the measurement, measured with a micrometer.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The pitch circle from the reading falls as the plug size rises, for the same reading.
- Get it right
- Use the actual plug size, not the hole size; any clearance between plug and hole adds to the error.
What this means: Measured over
- What it is
- Which pair of plugs the reading is taken over: two plugs with one hole between them (alternate), or two neighbouring plugs (adjacent). The tool uses a different angle for each.
- Units and format
- Choose one of 2 options from the list: Alternate plugs (y); Adjacent plugs (c).
- Example
- The form opens with “Alternate plugs (y)” selected as a worked example. Choose the option that matches your case.
- Get it right
- Choose the pair you actually measured over; the wrong choice gives a badly wrong pitch circle.
What this means: Measurement over the plugs
- What it is
- The micrometer or caliper reading taken over the outsides of the two plugs. It must be larger than the plug diameter.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 96.62 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The pitch circle from the reading rises with the reading.
- Get it right
- Take the largest reading by rocking the instrument slightly, so it measures straight across both plugs.
What this means: Nominal pitch circle diameter
- What it is
- The nominal pitch circle diameter of the holes from the drawing. The readings at nominal and at the limits are worked from it.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the drawing value, not a measured one; the measured pitch circle is what the tool works out.
What this means: PCD tolerance (±)
- What it is
- The plus-or-minus tolerance on the pitch circle diameter from the drawing. The measured pitch circle passes when it is within this amount of nominal.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.05 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A wider tolerance widens the band of acceptable readings.
- Get it right
- Enter the half band (the ± value), not the total tolerance width.
Method and formulas
- Alternate plugs: PCD = (m − d) ÷ sin (360° ÷ n); adjacent plugs: PCD = (m − d) ÷ sin (180° ÷ n) (as the bolt circle over plugs tool).
- Across opposite plugs = PCD + d for an even number of holes.
- Reading at a pitch circle = PCD × sin + d, at nominal and at each limit.
- Example: the reading gives a PCD of 100.02 mm (nominal reading 96.603 mm).
Limitations
- Equally spaced holes; the reading checks the circle, not the spacing of each hole.
- Plug clearance in the holes adds directly to the error.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Inspect the PCD of a flange or hub hole pattern
- Set up a go/no-go reading for a hole circle
- Check a drilled pattern before tapping
Who it is for
- Inspectors
- Machinists
- Quality engineers
What you get
- Pitch circle from the reading and size across
- Readings at nominal and the limits
- Decision
Interpreting the result
- Measure several pairs round the circle: a spread between pairs shows a spacing or roundness error, not only a size error.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 686. Topic reference; no source tables or recommended settings are reproduced.
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