Just to add a bit to what Wholepair wrote, I would think it could be fairly common for the DIY machine builder, to get his machine's axis directions and orientations confused from what would be considered the standard orientation.
When cutting only straight lines, you might never notice, because there is neither ClockWise or CounterClockWise to a line, but there is to an arc.
So the solution would be to carefully check through your cnc controller setup, and invert whatever axis directions are necessary, to make a G02 arc command appear to move the tool in a CW direction around the circle. In actual fact the tool does not move, the table does the circular interpolation, so it is easy to get the directions inverted
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)