FabLayout

Flange bolt circle calculator

A flat ring or blind flange with bolt holes on a pitch circle. Enter the outer, bore and bolt circle diameters, the hole count, size and start angle; you get the hole positions and the gap between holes.

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Enter your measurements

Your dimensions are measured to the

Flange

Pinch or scroll to zoom, drag to move. Double-tap to fit.

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1:1 PDF and DXF files of this flat pattern are made in Trueflat, ready for print, plasma, laser or waterjet. It works offline on site.

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220.0 mm220.0 mm
Flat pattern: cut on the bold outline, bend on the dashed blue lines, the thin lines are marking lines

Cut length

1,481.8mm

Exact geometry

Gap between holes
50.9 mm
Sheet size
220.0 × 220.0 mm

How to lay it out

  1. Scribe the circles

    From one centre point, scribe the outer circle, the bore circle and the bolt circle.

  2. Mark the first hole

    Draw a line from the centre at the first hole angle and mark where it crosses the bolt circle.

  3. Step off the other holes

    Set your dividers to the hole spacing D3 × sin(180° ÷ k) and step around the bolt circle from the first hole.

  4. Mark and cut

    Centre punch each hole, check that the last step closes on the first hole, then cut the bore, the holes and the outside.

Good to know

  • FabLayout runs the same code as Trueflat; the result is exact geometry.
  • Source: Plane geometry of circles and a bolt circle; no published source. Checked by comparing the area and cut length against separate closed formulas

Method and formula

A flange is already flat, so the pattern is the part itself. Hole j sits on the bolt circle at the angle φ0 + 360° × j ÷ k, counted anticlockwise from the x axis.

  • Hole spacing centre to centre: D3 × sin(180° ÷ k).
  • Gap between holes: D3 × sin(180° ÷ k) − dh.
  • Outer rim (D1 − D3) ÷ 2 − dh ÷ 2; inner rim (D3 − D2) ÷ 2 − dh ÷ 2.
  • Cut length π × (D1 + D2 + k × dh).

The calculator keeps at least half a hole diameter of material between each hole and the rim, the bore and the next hole. Set the bore diameter to 0 for a blind flange. Thickness is shown for reference only and does not change the layout.

Worked example

Outer 220 mm, bore 108 mm, bolt circle 180 mm, 8 holes of 18 mm: the second hole centre is at (63.64, 63.64) mm, which is 90 × cos 45°. The gap between holes is 50.88 mm, the outer rim 11 mm, the inner rim 27 mm. The exact cut length is 1482.83 mm; drawn with 48 segments per circle it measures 1481.77 mm.

Questions

What does the first hole angle do?

It turns the whole hole pattern. At 0° the first hole sits on the x axis. Set it to half of 360° ÷ k to put two holes either side of that axis instead of one on it.

Why are my holes rejected?

It keeps at least half a hole diameter of material to the outer edge, to the bore and between neighbouring holes. Increase the bolt circle, reduce the hole size or use fewer holes.

Does the sheet thickness matter?

Not for the layout. A flat part does not bend, so the outline and hole positions are the same for any thickness; the thickness is only recorded with the part.

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