BIZOMAT MAKEOS · Fasteners
Bolt Pattern Hole Plan (Layout, Clearance Holes, Position Tolerance)
For this bolt circle, where do the holes go, are the ligaments, edge distances and wrench clearance enough with the holes at their largest size, and what position tolerance can I allow on the holes?
Plan for a circular bolt pattern with clearance holes: hole coordinates, chord between adjacent holes, ligament, edge distances and wrench clearance with the holes at their largest (least-material) size (as the bolt circle layout tool), and the position tolerance at maximum material for floating or fixed fasteners with the bonus available at the largest hole (as the fastener position tolerance tool), checked against the least tolerance your shop can hold.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Bolt circle diameter | mm | Diameter of the bolt circle. |
| Number of holes | Number of equally spaced holes. | |
| Angle of the first hole | ° | Angle of the first hole from the X axis. |
| Fastener at MMC (largest fastener) | mm | Largest size of the fastener. |
| Clearance hole at MMC (smallest hole) | mm | Smallest size of the clearance hole. |
| Clearance hole at LMC (largest hole) | mm | Largest size of the clearance hole. |
| Fastener case | choice | Floating: clearance holes in both parts; fixed: one part is threaded or has pressed studs. |
| Share of total tolerance to the clearance-hole part (fixed) | % | For fixed fasteners, how the total tolerance is split between the clearance-hole part and the threaded part. |
| Flange and limits | ||
| Flange outside diameter | mm | Outside diameter of the flange. |
| Flange bore (0 = solid) | mm | Bore of the flange (0 for none). |
| Socket or wrench clearance diameter | mm | Diameter the socket or wrench needs around each fastener. |
| Least ligament between holes | mm | Least metal you accept between adjacent holes. |
| Least edge distance (hole edge to flange edge) | mm | Least metal from a hole to the flange edge or bore. |
| Least position tolerance the shop can hold | mm | Smallest position tolerance your drilling process holds reliably. |
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: Bolt circle diameter
- What it is
- The diameter of the circle through the centres of the bolt holes, measured centre to centre across the pattern.
- 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 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger bolt circle increases the spacing between holes and reduces the outer edge distance.
- Get it right
- Use the pitch circle through the hole centres, not the circle touching the hole edges.
What this means: Number of holes
- What it is
- How many equally spaced holes are on the bolt circle.
- Units and format
- Enter a plain number with no unit. The form accepts 2 to 360. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More holes reduce the chord and ligament between adjacent holes.
What this means: Angle of the first hole
- What it is
- The angle of the first hole, measured from the X axis of the drawing about the bolt circle centre; the other holes follow at equal steps.
- Units and format
- Enter a number in °: degrees of angle. The form accepts -360° to 360°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 45° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from the positive X axis, anticlockwise, so the coordinate table matches your drawing.
What this means: Fastener at MMC (largest fastener)
- What it is
- The largest size the fastener can be, its maximum material condition, usually the nominal major diameter of the bolt or stud.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 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
- A larger fastener leaves less room in the hole and less position tolerance.
- Get it right
- Use the fastener’s largest permitted diameter, not the thread pitch diameter.
What this means: Clearance hole at MMC (smallest hole)
- What it is
- The smallest size the clearance hole may be made, its maximum material condition, from the lower limit on the drawing.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 11 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Position tolerance at maximum material = this hole − fastener.
- Get it right
- This is the smallest hole, not the nominal or average size.
What this means: Clearance hole at LMC (largest hole)
- What it is
- The largest size the clearance hole may be made, its least material condition, from the upper limit on the drawing.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 11.3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The extra size above the smallest hole adds bonus tolerance, but the largest hole sets the ligament and edge distance.
What this means: Fastener case
- What it is
- Whether both joined parts have clearance holes (floating fasteners) or one part has threaded holes or pressed studs (fixed fasteners). It decides how the position tolerance is shared.
- Units and format
- Choose one of 2 options from the list: Floating (clearance holes in both parts); Fixed (one part threaded or pressed).
- Example
- The form opens with “Floating (clearance holes in both parts)” selected as a worked example. Choose the option that matches your case.
What this means: Share of total tolerance to the clearance-hole…
- What it is
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Ignored for floating fasteners, where each part gets the full difference between hole and fastener.
Flange and limits
What this means: Flange outside diameter
- What it is
- The outside diameter of the flange or part that carries the bolt pattern.
- 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 140 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger outside diameter increases the edge distance outside the holes.
What this means: Flange bore (0 = solid)
- What it is
- The diameter of the central bore of the flange. Zero means a solid part with no bore, and the inner edge distance check is skipped.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the bore actually machined in the part, not the shaft diameter that passes through it.
What this means: Socket or wrench clearance diameter
- What it is
- The diameter of the circle a socket or wrench needs around each fastener to fit and turn, from the tool catalogue for the fastener size.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 24 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the outside diameter of the socket you will use, not the across-flats size of the nut.
What this means: Least ligament between holes
- What it is
- The least solid metal you accept between the edges of adjacent holes, from your design rules for the material and drilling method.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Least edge distance (hole edge to flange edge)
- What it is
- The least solid metal you accept from the edge of a hole to the flange’s outside edge or to the bore, from your design rules.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 500 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.
What this means: Least position tolerance the shop can hold
- What it is
- The smallest position tolerance your drilling or machining process holds reliably, from your process capability or shop experience.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use a diametral position tolerance, the same basis as the calculated tolerance.
Method and formulas
- Hole k at angle start + k × 360 ÷ n on radius BCD ÷ 2; chord = BCD sin(π ÷ n); ligament = chord − largest hole; edge distances to the outside and bore (as the bolt circle layout tool).
- Floating fasteners: T = H − F for each part; fixed: T = H − F shared between the parts (as the fastener position tolerance tool).
- Bonus at the largest hole = H_LMC − H_MMC; total allowed = T + bonus.
- Example: the clearance holes may be 1 mm out of position at MMC (1.3 mm at LMC); the ligament is 59.411 mm.
Limitations
- Equally spaced holes on one circle.
- Position tolerance from fastener clearance only; mating parts need the same datum scheme.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Lay out and tolerance a flange bolt pattern
- Check a pattern before drilling
- Decide whether fixed or floating fasteners work
Who it is for
- Design engineers
- Drafters
- CNC programmers
What you get
- Hole coordinates, chord and ligament
- Edge distances and wrench clearance
- Position tolerance and bonus
Interpreting the result
- If the tolerance is too tight to make, use larger clearance holes or floating rather than fixed fasteners.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 686, 615–627. Topic reference; no source tables or recommended settings are reproduced.
- ASME Y14.5-2018 Dimensioning and Tolerancing
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Temperature, hole patterns and the joint register
In Bolted Joints by the Numbers
What it covers: How differential expansion changes preload in a warm joint, how to plan a bolt circle's holes and position tolerances for clearance and tapped holes, and what a joint register holds.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Bolt Pattern Hole Plan (Layout, Clearance Holes, Position Tolerance) calculator