I have a strange issue where if I try do a arc in the XZ plane with G43 tool height compensation enabled the machine goes on the wrong arc path then does a sharp jump with a loud bang!

The following program should do an arc from X50, Z100 to X100, Z50 of radius 50 (center at relative I0, K-50). If I run the program with no tool height offset it works exactly as expected. However, when I enable tool height offset (G43), the 50mm tool offset is applied before the arc is started, then the arc begins, however radius is wrong and it heads off on a wrong path (radius seems to be 100 now). Then part way through the arc the machine jumps with a loud bang.

G90 G94 G17 G49 G40
G21
G54
G0 X50. Y0 Z110.
G43 Z105. H1 <---- This is the offending line! H1 is set to 50 in tool offset table
G1 Z100. F175
G18 G3 X100. Z50. I0. K-50.

Below is a link to a video showing the problem happening. The video shows the program working as expected with the G43 line removed. Then it shows what happens when the G43 line is enabled. After that it shows the machine screen while stepping through the program (I stopped before it crashed again as this is exactly repeatable!).



Any ideas what might be going on?

It scared the ****e out of me when it went bang! Note that the rapid rate was on 10% and the feed rate slow so I don't know why it moved so fast! The machine when into a fault state with a Z axis alarm and had to be power cycled after the bang. There are no mechanical problems with the machine motion.