BIZOMAT MAKEOS · Quality
Bolt Circle Inspection from Three Holes (Fitted Diameter, Centre Position)
From the measured centres of three holes on a bolt circle, what diameter and centre do they actually lie on, and are the circle size and its centre position within tolerance?
Inspection of a bolt circle from three measured hole centres: the circle through the three points, its centre and diameter (as the circle through three points tool), the deviation of the diameter from nominal, and the position of the circle’s centre from the datum origin as a diametral position value (as the true position tool, regardless of feature size), against the diameter tolerance and position tolerance, with a check of how widely the holes are spread.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Hole 1 X | mm | Measured X of hole 1 from the datum origin (the datum bore centre). |
| Hole 1 Y | mm | Measured Y of hole 1. |
| Hole 2 X | mm | Measured X of hole 2. |
| Hole 2 Y | mm | Measured Y of hole 2. |
| Hole 3 X | mm | Measured X of hole 3. |
| Hole 3 Y | mm | Measured Y of hole 3. |
| Specification | ||
| Nominal bolt circle diameter | mm | Nominal bolt circle diameter. |
| Diameter tolerance (±) | mm | Plus-or-minus tolerance on the bolt circle diameter. |
| Position tolerance of the circle centre (diameter) | mm | Diametral position tolerance of the pattern centre relative to the datum. |
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: Hole 1 X
- What it is
- The measured X coordinate of the centre of the first hole, from the datum origin at the datum bore centre, as read on a CMM or coordinate-measuring set-up.
- Units and format
- Enter a number in mm: millimetres. The form accepts -1000000 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.02 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure all six coordinates in the same coordinate system with the origin at the datum centre; keep the sign for negative positions.
What this means: Hole 1 Y
- What it is
- The measured Y coordinate of the first hole’s centre, from the same datum origin and in the same coordinate system as its X value.
- Units and format
- Enter a number in mm: millimetres. The form accepts -1000000 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50.03 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Keep the sign: a hole below the origin has a negative Y.
What this means: Hole 2 X
- What it is
- The measured X coordinate of the centre of the second hole, from the datum origin.
- Units and format
- Enter a number in mm: millimetres. The form accepts -1000000 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with -43.3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Choose three holes spread well around the circle; holes close together make the fitted circle sensitive to small errors.
What this means: Hole 2 Y
- What it is
- The measured Y coordinate of the second hole’s centre, from the datum origin.
- Units and format
- Enter a number in mm: millimetres. The form accepts -1000000 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with -24.99 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Keep the sign: a hole below the origin has a negative Y.
What this means: Hole 3 X
- What it is
- The measured X coordinate of the centre of the third hole, from the datum origin.
- Units and format
- Enter a number in mm: millimetres. The form accepts -1000000 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 43.31 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The three holes must not lie on a straight line, or no circle can be fitted.
What this means: Hole 3 Y
- What it is
- The measured Y coordinate of the third hole’s centre, from the datum origin.
- Units and format
- Enter a number in mm: millimetres. The form accepts -1000000 mm to 1000000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with -25.02 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Keep the sign: a hole below the origin has a negative Y.
Specification
What this means: Nominal bolt circle diameter
- What it is
- The nominal bolt circle (pitch circle) diameter from the drawing, through the centres of the holes.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 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 diameter through the hole centres, not the radius and not an outside diameter.
What this means: Diameter tolerance (±)
- What it is
- The plus-or-minus tolerance on the bolt circle diameter from the drawing; the fitted diameter may deviate from nominal by up to this amount either way.
- 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.
- Get it right
- Enter half the total band: a total band of 0.06 mm is entered as 0.03.
What this means: Position tolerance of the circle centre (diamet…
- What it is
- The position tolerance of the pattern centre relative to the datum, given as a diameter of the tolerance zone as on the drawing’s position frame.
- 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 0.1 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- This is a diametral zone, so it is twice the radial offset allowed; the tool also reports position as a diameter.
Method and formulas
- Circle through three points: centre from the perpendicular bisectors, radius to any point (as the circle through three points tool).
- Diameter deviation = 2r − nominal.
- Centre position = 2√(c_x² + c_y²) from the datum origin (as the true position tool, regardless of feature size).
- Example: the holes lie on 100.03 mm (+0.027648 mm) with its centre 0.042354 mm (diametral) from the datum.
Limitations
- Three holes define the circle exactly, so hole position errors show up as circle errors; more holes need a best fit.
- Measurements in the drawing’s datum reference frame.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a drilled flange on a CMM or height gauge
- Find the actual pitch circle from hole coordinates
- Check the pattern centre against a datum bore
Who it is for
- Inspectors
- Quality engineers
- Machinists
What you get
- Fitted centre and diameter
- Diameter deviation
- Centre position and spread of the points
Interpreting the result
- Pick three holes spread at least 120° apart; close holes make the centre very sensitive to errors.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 40–48, 615–627. Topic reference; no source tables or recommended settings are reproduced.
- ASME Y14.5-2018 Dimensioning and Tolerancing
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Open the Bolt Circle Inspection from Three Holes (Fitted Diameter, Centre Position) calculator