This summer I had to send my Pocket NC in for some service, when it came back, I immediately noticed that the X axis homing was very far off, something like 0.01 inches, as I was boring a hole in one side of a part, spinning it around the B axis, then boring a countersink in the other side. The two were very clearly not concentric. I suspect the homing mechanism shifted in transport or something, because the error was very consistent.

Pocket NC’s support was great as usual and I quickly received a screencast showing the location of the homing setting:

To calibrate the X axis, I just used the hole that was bored all the way through, and manually used the MDI to spin the B axes around and jog the end mill through the center of the hole. Then I used my calipers to measure the offset between the widest part of the mill and each side of the hole. Two iterations of that had the X error back to under 0.001″.

Fast forward a few months and I am running a part where the Y axis zero position matters. Sure enough, it is off too. Not as much, maybe only 0.004″ or so, but enough to make the part not work out. I tried a different technique this time, engraving an X axis line with a chamfer mill in two parts. One part with the B axis at 0, and the other half with it at 180. Any Y offset will manifest as a “jog” in the line.

I’m not sure if this was any more or less accurate than the boring method, but it was faster and seems to have also gotten me back down to under 0.001 inch of error in the Y axis.