Flange / Bolt Circle DXF Generator

Lay out a bolt circle and generate a flange DXF. Enter the outside diameter, bore, PCD, hole size and count to get exact hole coordinates, spacing and a clean cut profile.

This calculator runs entirely in your browser, so it needs JavaScript enabled. The method, assumptions and worked example below are readable without it.

Dimensions

Results

Preview

Cut
Centreline / bolt circle

Hole coordinates

Measured from the centre of the plate. X to the right, Y up.

How this tool is verified

Geometry checked against hand-derived golden cases and geometric invariants — developed arc equals true circumference, metric and imperial inputs give identical millimetre geometry.
Every exported DXF is read back and audited by ezdxf, an independent DXF library that had no part in writing it (ezdxf 1.4.4).
Output opened and accepted in Autodesk Inventor and AutoCAD on a real seat (2026-08-10).

The method, the reports and the responsibility boundary are on the Approach page.

Built by Ruthen Systems

The same geometry core powers custom generators for specific shops — your rules, your standards, your output formats. If you keep redoing this by hand with your own rules attached, that is the version worth having.

How a custom generator is built

Assumptions and limits

Read these before you cut anything. They describe exactly what this tool does and does not account for.

Flat plate with plain round holes on a single bolt circle.
Hole diameters are as entered — no allowance is added for fit, clearance or machining.
Clocking angle is measured counter-clockwise from the positive X axis to the first hole.
No standards lookup: PCD, hole size and count are exactly what you enter.
No slots, counterbores, raised faces, hubs or multi-circle patterns.

Output is a planning and fabrication aid. You are responsible for verifying dimensions, tolerances and material behaviour before production — full disclaimer.

Worked example

A 300 mm plate with a 150 mm bore, eight 18 mm holes on a 240 mm bolt circle, unclocked:

Pitch angle 360 / 845°
Bolt circle radius120 mm
Chord 2 × 120 × sin(22.5°)91.84 mm
Edge margin 150 − (120 + 9)21 mm
Bore margin 120 − 9 − 7536 mm
Hole 1 at 0°120.00, 0.00
Hole 2 at 45°84.85, 84.85

The chord is the useful sanity check: it must comfortably exceed the hole diameter, or there is not enough material left between holes.

Method

Holes are placed at equal angular spacing around the bolt circle, starting at the clocking angle. With bolt circle radius Rp, n holes and a clocking angle φ, hole i (counting from 0) sits at:

angle = φ + i · 360° / n
x = Rp · cos(angle), y = Rp · sin(angle)
Chord between neighbours = 2 · Rp · sin(180° / n)

What gets rejected, and why

The tool refuses geometry that cannot be manufactured rather than drawing it anyway: holes breaking through the outside edge (PCD/2 + hole/2 ≥ OD/2), holes breaking into the bore (PCD/2 − hole/2 ≤ ID/2), and adjacent holes overlapping (chord ≤ hole diameter). Each message names the input to change.

What the DXF contains

The outside circle, the bore if present, and one circle per hole — all on layer CUT. With centrelines enabled, the bolt circle and two axis lines go to the CENTER layer so they stay separate from the cut geometry. Written as DXF R12 (AC1009) with the units recorded in the header.

Questions

What is PCD?

Pitch circle diameter — the diameter of the imaginary circle through the centre of every bolt hole. It is also called BCD (bolt circle diameter). Enter the diameter, not the radius.

What does the clocking angle do?

It rotates the whole hole pattern. Zero puts the first hole on the positive X axis (3 o’clock). For an even hole count, half a pitch angle straddles the axes instead — for example 22.5° with 8 holes.

Does it look up standard flanges?

No. There is no ASME, DIN or ISO lookup in this version. The tool draws exactly the PCD, hole size and count you enter, which also means it will happily draw non-standard patterns.

Is any clearance added to the hole size?

No. Holes are drawn at the diameter you type. If you want clearance over the bolt size, or an allowance for your cutting process, add it yourself before entering the value.

Why was my hole count rejected?

Either the holes would overlap each other on the bolt circle, or they would break through the outside edge or into the bore. The error message says which. The tool refuses rather than drawing a part that cannot be made.

Can I leave out the centrelines?

Yes — untick the centrelines option. When enabled they are written to the CENTER layer, separate from the CUT geometry, so most CAM software will ignore them or you can delete the layer.

Generating the same flange family over and over?

If you are producing a range of flanges from a table, need matching drawings or a BOM, or work to a company standard the calculator does not know about, send one example and we will look at generating the whole set automatically.

Send one example