# Bolt Hole Circles

*Many of us need to bore holes in a circle but don’t have a rotary table, don’t want to remove the vise or need to create a circle too big for the table.*There is a solution in Machinery’s Handbook. Find the section on JIG BORING in your copy of the Handbook. In my 23rd edition, it starts on page 950. Look for the four tables labeled “Hole Coordinate Dimension Factors for Jig Boring.” Definitions: There are two types of bolt hole circles:

- Those that start with hole number 1 centered at the top of the circle are a Type A circle
- Those that start with the holes evenly centered around the top of the circle are a Type B circle.

*as shown in this first drawing.*and 2. Bolt hole circles where the reference is found at the origin or center of the circle as shown in the second drawing below. The two types of hole layouts and the two reference locations create the need for the four tables.

*As with any system based on coordinates, we have X and Y directions for movement, just as we do on our milling tables.*Now for the tables themselves: As you look at Table 1 in your copy of Machinery’s Handbook, you notice the table is broken into groups by the number of holes per circle. You will also notice that there are no numbers larger than 1. That is because these numbers are based on a bolt hole circle with a diameter of 1. So, if you want to make a bolt hole circle with a diameter of 2 (

*inches, millimeters or furlongs, it doesn’t matter*) you just multiply the numbers by 2. The same method works for a circle of any diameter including those with a diameter smaller than 1. Suppose we want to drill three holes on a circle where the first hole is at the top center of the circle and the circle diameter is 4 inches. Furthermore, we are going to use as a reference point for the circle, the center of the circle. We have four important pieces of information to help us select the proper table and create the X-Y coordinate pairs for the three holes:

- It is a three hole circle.
- The first hole is at the top center.
- The reference point is the center of the bolt circle.
- The diameter of the bolt circle is 4 inches.

Coordinate Name |
Table 3 Value |
Bolt Hole Circle Diameter |
Calculated Coordinate Value |

X1 |
+0.00000 |
x 4 |
= 0.0 |

Y1 |
-0.50000 |
x 4 |
= -2.0 |

X2 |
-0.43301 |
x 4 |
= -1.73 |

Y2 |
+0.25000 |
x 4 |
= 1.0 |

X3 |
+0.43301 |
x 4 |
= 1.73 |

Y3 |
+0.25000 |
x 4 |
= 1.0 |