BIZOMAT MAKEOS · Quality
Bolt Circle Measurement over Pins
What is the distance across opposite holes of this bolt circle, measured over plugs?
Distance over plugs in opposite (or most widely spaced) holes of a bolt circle and the bolt circle diameter, from a measurement over adjacent or alternate plugs.
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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 or in | Diameter of the plugs fitted in the holes. |
| Measured over | choice | Whether the reading is over alternate or adjacent plugs. |
| Measurement over the plugs | mm or in | Reading over the two plugs. |
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
- The number of equally spaced holes around the bolt circle.
- 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 plugs or gauge pins fitted in the holes for the measurement, which should fit the holes without play.
- Units and format
- Enter a number in mm or in: millimetres or inches, as long as every length uses the same unit. The form accepts 0.000001 mm or in to 10000 mm or in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.75 mm or in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure the plugs, not the holes; any play in the holes adds error.
What this means: Measured over
- What it is
- Whether the reading was taken over two adjacent plugs or over alternate plugs with one hole between them.
- 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
- Alternate plugs suit even hole counts above four; use adjacent plugs otherwise.
What this means: Measurement over the plugs
- What it is
- The micrometer or caliper reading across the outsides of the two plugs, adjacent or alternate as chosen.
- Units and format
- Enter a number in mm or in: millimetres or inches, as long as every length uses the same unit. The form accepts 0.000001 mm or in to 1000000 mm or in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4.5 mm or in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Read over the outsides of the plugs, not between their centres; use the same unit as the plug diameter.
Method and formulas
- θ = 360° ÷ n. Bolt circle diameter from adjacent plugs: (c − d) ÷ sin(θ/2); from alternate plugs: (y − d) ÷ sin θ.
- Even n: distance over opposite plugs x = PCD + d.
- Odd n: across the most widely spaced plugs x = (c − d) ÷ 2 sin(θ/4) + d = PCD cos(θ/4) + d.
- Example (handbook case): six 0.75 in holes 4.5 in over alternate plugs: x = 5.0801 in.
Limitations
- Equally spaced holes and plugs that fit the holes without play.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check large bolt circles with ordinary micrometers
- Find the PCD from a measurement over two plugs
- Inspect dies and flanges
Who it is for
- Quality inspectors
- Toolmakers
- Metrology technicians
What you get
- Bolt circle (pitch circle) diameter
- Distance over opposite (or most widely spaced) plugs
Interpreting the result
- Use plugs that fit the holes closely; plug clearance shows directly in the result.
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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