Unconstrained and fixed mode circle move

hi, can any one explain the following.

using a welding system with a torch neck (22,5deg)

when programming a full circle, on the first semicircle I have to use fixed and on the second semicircle I have to use unconstrained. sometimes it will work under unconstrained only for a full circle. why is this?

Typically when programming welding robots, we always do 4 curved moves. This is true even for our industrial robots. The torch angle is very important, and leaving it up to 2 curves can lead to rough angles or (as you’re seeing) the robot performing strange things. We leave ours fixed, so the torch remains at the desired angle. This works for everything except perfectly vertical torch positions.

thank you for replying.