My rotary part came out the wrong diameter (too thin or too fat). What did I zero wrong?

Why it happens

On a rotary job every Z height is a radius. That has two consequences:

• Any Z zero error shows up twice in the diameter. In one public owner report, a center offset that was 0.045 inch off produced a part 0.09 inch under size (1.16 inch instead of 1.25 inch).
• The job and the machine must agree on what Z zero is. The Z Zero Position in the job is either the cylinder axis or the cylinder surface. If the file expects the axis and you touched off on the surface (or the reverse), the whole part is off by the radius.

The official documentation recommends the cylinder axis, because the cylinder center stays constant whatever the irregularities in the blank's diameter or errors in centering the blank in the chuck.

The fix

1. Open the Material Setup summary on the Toolpaths tab and read the Z zero setting. Zero the machine the same way.
2. For an axis zero you cannot touch the center with the blank loaded. Touch a fixed reference (top of the chuck or the mounting bracket) and enter the measured distance from that point down to the chuck center.
3. Verify that distance yourself. Cut a short test cylinder, measure it with calipers, and correct the offset by half the diameter error.
4. Zero the unused linear axis so the bit is directly over the rotation center. One published method: before loading stock, bring the bit to the point of the tailstock center and zero that axis there.
5. Re-zero Z after every bit change using the same reference.

Pro tips

• One machine-maker guide says to use the surface option only when you are setting all work offsets by hand. A probe routine that measures to the chuck center needs the axis option.
• A touch-off pad fixed at the same height as the tailstock point lets you re-zero without removing the blank.
• If the part is the right size at one end and wrong at the other, check whether the blank sits on center at the tailstock end. A change to the Z zero will not fix that.