Bolt-hole patterns are one of the most common features in precision engineering components. Flanges, mounting plates, gear blanks, bearing housings — all require accurately positioned holes on a pitch circle. Getting the positions right is critical.
The Pitch Circle Diameter
A bolt-hole pattern is defined by three parameters:
- Pitch Circle Diameter (PCD) — the diameter of the circle on which the hole centres lie
- Number of holes — how many holes in the pattern
- Starting angle — the angular position of the first hole, measured from the positive X axis
The holes are equally spaced around the pitch circle. For N holes, the angular spacing is 360°/N.
Calculating Hole Positions
The position of each hole centre is:
- X = PCD/2 × cos(start_angle + i × 360°/N)
- Y = PCD/2 × sin(start_angle + i × 360°/N)
For a 6-hole pattern on 100mm PCD starting at 0°:
- Hole 1: (50, 0) at 0°
- Hole 2: (25, 43.301) at 60°
- Hole 3: (−25, 43.301) at 120°
- Hole 4: (−50, 0) at 180°
- Hole 5: (−25, −43.301) at 240°
- Hole 6: (25, −43.301) at 300°
In XyzPilot, Points/On,C,N computes all these positions analytically in a single operation. No manual calculation required.
From Points to Drill G-Code
In XyzPilot, bolt holes are defined by points — not circles. The point positions are the drill centre positions. The CAM module uses these points to generate drill cycle G-code (G81 or G83).
You do not need to draw circles for drilled holes. The drill diameter and depth are specified in the CAM setup, not in the geometry.
Drill Cycle G-Code
A typical drill cycle for a 6-hole pattern:
G81 R2.0 Z-15.0 F80
X50.000 Y0.000
X25.000 Y43.301
X-25.000 Y43.301
X-50.000 Y0.000
X-25.000 Y-43.301
X25.000 Y-43.301
G80
G81 sets up the drill cycle (rapid to R plane, drill to Z depth, retract). Each subsequent X/Y line drills at that position. G80 cancels the cycle.
Verifying Hole Spacing
Always verify hole spacing before machining. For a 6-hole pattern on 100mm PCD, adjacent hole spacing = 2 × 50 × sin(30°) = 50mm. Use Measure → Distance to verify.