Pitch Circle Diameter — PCD — appears on almost every engineering drawing that has a bolt-hole pattern. Understanding what it means and how to use it is essential for CNC programming.
The Definition
The Pitch Circle Diameter is the diameter of the imaginary circle on which the centres of the bolt holes lie. It is not the diameter of the holes themselves — it is the diameter of the circle that passes through all the hole centres.
For example, a flange with "6 holes on 100 PCD" has six holes whose centres lie on a circle of 100mm diameter (50mm radius).
Reading PCD from a Drawing
PCD is typically specified on engineering drawings as:
- "6 × Ø8.5 on 100 PCD" — six 8.5mm diameter holes on a 100mm pitch circle
- "4 × M8 on 80 PCD" — four M8 tapped holes on an 80mm pitch circle
- "Ø100 PCD" with hole positions shown — the pitch circle diameter with individual hole positions
Calculating Hole Positions from PCD
Given PCD, number of holes N, and starting angle θ, the position of hole i is:
- X = (PCD/2) × cos(θ + i × 360°/N)
- Y = (PCD/2) × sin(θ + i × 360°/N)
In XyzPilot, this calculation is done automatically by Points/On,C,N. Create the pitch circle with Circle/Centre,P,R where R = PCD/2, then use Points/On,C,N to create the hole centres.
Common PCD Values
| PCD (mm) | Common Use | Hole Spacing (6 holes) |
|---|---|---|
| 50 | Small flanges, instrument panels | 25mm |
| 80 | Medium flanges, mounting plates | 40mm |
| 100 | Standard flanges, pipe fittings | 50mm |
| 125 | Larger flanges | 62.5mm |
| 160 | Large flanges, heavy duty | 80mm |
Verifying PCD in XyzPilot
After creating the hole centre points, verify the PCD using Measure → Distance:
- Measure from the flange centre to any hole centre — should equal PCD/2
- Measure between adjacent hole centres — should equal 2 × (PCD/2) × sin(180°/N)
For a 6-hole pattern on 100mm PCD: adjacent spacing = 2 × 50 × sin(30°) = 50mm.