Stop Doing Bolt-Circle Trig by Hand: A Faster Way to Drill Hole Patterns

If you run a CNC mill or router, you have hit this job: a part needs six, eight, or twelve holes evenly spaced around a circle. A flange. A motor mount. A faceplate. A rotary table fixture. The geometry is simple in principle and annoying in practice — and getting one hole position wrong means a scrapped part.

The math behind a bolt circle

A bolt circle (or bolt hole circle) is a set of holes whose centers all lie on the same imaginary circle. Three numbers define it: the bolt circle diameter (BCD), the number of holes, and a start angle. From there, each hole sits at an equal angular step around the center.

The position of each hole is straightforward trigonometry:

```

angle_i = start_angle + (i * 360 / hole_count)

x_i = center_x + (BCD / 2) * cos(angle_i)

y_i = center_y + (BCD / 2) * sin(angle_i)

```

That looks tidy until you are doing it for 12 holes, in inches, with a non-zero center, a 37-degree start angle, and your calculator in radians. One transposed digit and you are re-running the part.

Where it goes wrong

The common failure points are predictable:

• Degrees vs radians. Most calculators default to radians; most prints are in degrees.
• Start angle reference. Is hole one at 12 o'clock or 3 o'clock? Mixing conventions rotates the whole pattern.
• Diameter vs radius. Prints call out BCD (diameter), but the math uses radius. Halve it or every hole lands twice as far out.
• Rounding. Truncating coordinates early stacks small errors that show up as a visibly lopsided pattern.

A CAM package handles all of this, but firing up a full seat and building a setup for a single drilled circle is overkill. Hand-coding it invites the errors above.

A small tool for a small job

This is exactly the kind of repetitive, error-prone task worth automating once. The Bolt Circle G-code Generator takes your inputs — diameter, hole count, center, start angle, depth, feed, and units — and writes the complete drilling program. Every hole is spaced exactly, the trig is done in full precision, and you choose mm or inch up front so there is no unit guessing at the machine.

You paste the output into your controller and cut. No subscription, no account, no learning curve.

If you find yourself drilling circular hole patterns more than once in a blue moon, it is worth keeping a dedicated tool in your back pocket. Try the Bolt Circle G-code Generator and skip the trig.