Bolt Circle Coordinate Calculations¶

Author: Johannes A. Nilsson

Introduction¶

Here is my practical approach when I need to machine parts with features equally spaced around a circle. This might be features such as scallops, holes, or other patterns, understanding the mathematics behind positioning while basic, is still crucial skill that I have observed many seasoned builders still don't fully grok. So I wrote this tutorial to teach the fundamental principles for calculating coordinates of holes positioned around a circle, based on radius mathematics (trigonometry).

Available Patterns¶

6-Hole Pattern (Hexagonal)¶

The most common bolt pattern using 30-60-90 triangles and simple multipliers (0.5 and 0.86603). This is the one I get asked about most often.

4 and 8-Hole Patterns (Square/Octagonal)¶

90° and 45° spacing - the easiest patterns to calculate. 8-hole is just 4-hole with a second set rotated 45°. If you can't figure these out, maybe take up knitting instead.

5-Hole Pattern (Pentagon)¶

72° spacing - requires actual sine/cosine but I'll show you shortcuts so you don't need to be embarrassed pulling out a calculator at the mill. Included some diagrams because this one's just weird.

General Principles¶

All bolt circle calculations follow the same basic approach:

Find your center point (0,0)
Determine your bolt circle radius
Calculate X and Y coordinates for each hole
Use coordinate positioning or rotary indexing to locate holes

Closing Thoughts¶

I'm thinking ahead—so the next time I get asked how to do this, I've got an easy hyperlink to send. After you've performed these operations a few times it's just like walking. You don't even have to think about it.