LatheCalculator

PCD / bolt circle calculator

Gives the X/Y position of every hole on a pitch circle diameter (PCD) for co-ordinate drilling on a mill with a DRO, the chord distance between holes for marking out with dividers, and a quick drawing to check it.

Use any unit (mm or inches); the results come out in the same unit.

How it works

The holes are spaced evenly round a circle. Either set dividers to the chord and step round, or move to each X/Y position on a DRO. There’s a step-by-step PCD marking-out guide.

Hole i sits at angle θ = start + i × 360°/N. Its position is X = cx + (PCD/2)·cos θ and Y = cy + (PCD/2)·sin θ. The straight-line distance between neighbouring holes is the chord, PCD × sin(180°/N). Set your dividers to that and step round the scribed circle.

Worked example: 6 holes on a 100 mm PCD. The chord is 100 × sin 30° = 50.000 mm, so the dividers are set to the radius, the classic hexagon. With the first hole at 3 o’clock, hole 2 is at X 25.000, Y 43.301.
Safety: clamp the work, not just the vice, when drilling larger holes, and lock the axes you aren’t moving. Centre-drill each position before drilling.

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