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Hexagonal (6-Hole) Bolt Circle Coordinates

Understanding the Problem

When you need to create 6 holes equally spaced around a circular part, you're dealing with a hexagonal pattern. For example, a 152.4mm (6") end cap flange with 6 bolt holes:

  • Number of bolt holes: 6 bolt holes (eg: M12 bolts)
  • Angular spacing: 60° apart (360° ÷ 6 = 60°)
  • Bolt circle diameter: 127mm (5")
  • Bolt circle radius: 63.5mm (2.5")
  • Hole diameter: 13mm (0.512") for M12 clearance

Key Concept: The Hexagon Connection

When you have 6 points spaced 60° apart around a circle and connect them, you create a hexagon. This geometric relationship is the foundation for our calculations.

Hexagon Concept Figure 1: Six equally-spaced holes form a hexagon pattern on a 152.4mm flange

Setting Up Your Coordinate System

  1. Position your part so that one radius line aligns with a machine axis
  2. Set the X-axis horizontally through the center
  3. Set your dials or DRO to 0,0 at the center point

The Magic Numbers

For a 30-60-90 triangle (which appears when working with hexagons), remember these two critical values:

1. 0.5 (or 1/2)

  • In a 30-60-90 triangle, the radius of the bolt circle is always twice the length of the short side
  • To find the short side: multiply radius × 0.5

2. 0.86603

  • This is the ratio for finding the long leg of the triangle
  • To find the long side: multiply radius × 0.86603
    • PRO TIP: This value comes from sin/cos of 30°, but just memorize it!

Triangle Method Figure 2: The 30-60-90 triangle method showing how to calculate hole positions

Calculating Our Hole Positions

For the first two holes (on the X-axis)

  • Position 1: X = +63.5mm (+2.5"), Y = 0
  • Position 2: X = -63.5mm (-2.5"), Y = 0

For the remaining four holes

Using our 30-60-90 triangle relationships:

  1. Calculate the offsets:

    • Short leg = 63.5mm × 0.5 = 31.75mm (1.25")
    • Long leg = 63.5mm × 0.86603 = 54.99mm (2.165")

  2. Apply the coordinates:

    • Hole 3: X = +31.75mm (+1.25"), Y = +54.99mm (+2.165")
    • Hole 4: X = -31.75mm (-1.25"), Y = +54.99mm (+2.165")
    • Hole 5: X = -31.75mm (-1.25"), Y = -54.99mm (-2.165")
    • Hole 6: X = +31.75mm (+1.25"), Y = -54.99mm (-2.165")

The Triangle Pattern

Notice how these calculations create four identical right triangles within the circle. Each triangle has:

  • Hypotenuse = radius: 63.5mm (2.5")
  • Short leg = 31.75mm (1.25")
  • Long leg = 54.99mm (2.165")
  • Angles: 30°, 60°, and 90°

All Coordinates Figure 3: Complete layout showing all hole coordinates and four triangular sections

Working Tips

  1. Material Prep: Make sure your blank round is properly faced and centered before locating holes.

  2. Drilling Sequence: When boring the M12 clearance holes:

    • Start with a center bore or spot drill
    • Use a 12.5mm bore for initial hole
    • Ream to final 13mm size for proper clearance

Practical Tips Figure 4: Recommended drilling sequence and verification method

  1. Efficient Movements: When moving between holes, change only one coordinate at a time when possible. This minimizes setup time and reduces errors.

  2. QA / Verification: After cutting the first hole, rotate to position 4 (mirror opposite) and verify the dimension is exactly 127mm (5 inch) center-to-center.

Quick Reference Table for Common 6-Hole Flange Sizes

Size BC Diameter Radius Short Long
100mm 76.2mm 38.1mm 19.05 33.00
150mm 127mm 63.5mm 31.75 54.99
200mm 177.8mm 88.9mm 44.45 76.99
250mm 228.6mm 114.3mm 57.15 98.99

Summary

By understanding the geometric relationship between a hexagon and its inscribed circle, and remembering just two key numbers (0.5 and 0.86603), you can quickly calculate any hole position in a 6-hole circular pattern.