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.
The method behind it: How to lay out a bolt circle by hand — step by step, with sources.
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 is deterministic: the same inputs produce the same file, byte for byte, with a version tag in the header.
The method, the reports and the responsibility boundary are on the Verification page.
Built by Ruthen Systems
Free, with no account, and the dimensions you type never leave your browser. If a result looks wrong, or the option your shop needs is missing, say so — reports of a wrong result are dealt with first.
Report a problem or request a toolAssumptions 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° / nx = 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 a hole count that is a multiple of 4, half a pitch angle straddles both centre lines instead — for example 22.5° with 8 holes. With 6, 10, 14 … holes, half a pitch straddles one centre line and puts holes on the other.
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?
Because the holes would overlap each other on the bolt circle. Holes that would break through the outside edge or into the bore are refused too, but on the bolt-circle or hole-diameter field, whatever the count. The tool only checks that each hole clears its neighbours, the edge and the bore — not by how much, so check the web left between them for your process.
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.
Missing an option?
A different datum, an offset, a shop allowance, another shape altogether — say what this tool does not cover yet. Requests decide what gets built next, and a report of a wrong result is dealt with before anything new.
Rather have the whole part drawn — a shop drawing or a cut file from your sketch? That is paid drafting, quoted per job: drafting for small shops.
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