Bolt hole circle without a mill

[dlavery]

This is actually pretty easy to do, and does not require a CAD system, transferring drawings, or anything more difficult than some simple scribing skills. Remember, machinists were doing these types of operations long before CAD systems, DROs, CNC machines, or EDM systems were ever invented. So there is usually a simple way to do a lot of these operations in a straightforward manner. In this case, it actually takes longer to explain than it takes to perform.

Mount a three-jaw chuck in the lathe (if you have a six-jaw chuck it would be even better, and make this whole operation trivial). Place a sharpened tool or scriber in the tool post, and adjust its height very precisely so that the cutting edge is on the centerline of the lathe. Make a short spacer that will fit in between the ways and the jaws of the chuck, so that when the chuck is rotated by hand it will stop the rotation by blocking passage of the jaw.

Coat the face of the part with marking fluid, and mount it in the chuck. BY HAND (not under power!) rotate the chuck so that one of the jaws is up against the spacer. Move the cutting tool in so that it just touches the face of the part. Run the cross-slide across, so that the tool scribes the part. You should now have a horizontal line across the face of the part that runs precisely through the center of rotation. Back out the cross-slide.

Move the spacer out of the way and BY HAND rotate the chuck 1/3 rotation. Put the spacer back in place and run the chuck jaw up against it. As long as there is no side-play in the chuck jaws, this will index the chuck precisely 120 degrees. Move the cutting tool back in, and run the cross-slide across to scribe a second line. Back the tool out, and repeat the whole operation a third time. You should now how three lines that intersect in the exact center of the part, and extend across the face of the part. The lines define six equally spaced radial segments.

Mount a sharp scribing tool in the tool post. Align the point of the scriber with the center of rotation of the part (where the radial lines intersect). Back the cross-slide out the desired radius of the bolt circle you want. Move the scriber in until it contacts the part, and then BY HAND (not under power!) rotate the chuck. This will scribe a circle that is concentric with the center of rotation of the part, and that intersects with the six radial lines. The intersections of the lines and the scribed circle will define the locations of the bolt holes.

When you are finished, take the spacer you made and store it away somewhere safe that you will remember. You will be using it again...

-dave