How to mark out a PCD (bolt circle)
Three ways to put evenly spaced holes on a pitch circle diameter. Get the numbers from the PCD calculator.
Method 1 – dividers and the chord (no DRO)
- Blue the face, then find and centre-punch the centre lightly.
- Set the dividers to half the PCD and scribe the pitch circle.
- Set the dividers to the chord from the calculator, then step round from your first mark. The last step should land exactly on the start. If not, adjust and go round again.
- Centre-punch each mark lightly, check the spacing, then deepen the punch marks. Spot drill before drilling to size.
Method 2 – X/Y co-ordinates on a DRO
- Find the centre of the part with an edge finder or a test indicator, and zero the DRO.
- Enter the PCD, number of holes and start angle in the calculator, then move to each X/Y in the table.
- Always approach from the same direction to take out backlash. Lock the axes you aren’t moving.
Method 3 – rotary table or dividing head
- Centre the work on the rotary table with a test indicator, then offset the table by half the PCD.
- Index 360° ÷ holes between holes. On a 40:1 dividing head that’s 40 ÷ holes turns of the handle.
Pro tips
- Flange bolt holes straddle the centre lines (“off-centre”) on most standards. Use a start angle of 180° ÷ holes.
- Check finished holes by measuring across opposite holes: outside-to-outside minus one hole diameter = PCD.
- For mating parts, drill both parts together or use a transfer punch.
Safety: clamp the work, never hold it by hand on the drill or mill. Remove the chuck key, and clear swarf with a brush, not your fingers.