Unrequested z level retracts

[Michael Moore]

I'm doing a simple, constant radius hemispherical pocket. There's a horizontal roughing with a flat end mill then horizontal finishing with a ball end mill.

I've been using RhinoCAM 4.0 for this and I haven't tried doing it in VM6 (using the Rhino model) to see if it works exactly the same but I think it would.

If I don't select optimized machining in the horizontal finishing MOP it increments in equal Z amounts downwards and won't clean out the bottom of the pocket. I haven't tried picking a Z increment that would result in an even number of Z steps ending at the lowest point of the hemisphere, but I'm going to try that and see if it cleans out.

Here is the thing that has me curious. If I select optimized machining between levels and a small scallop it will clean out the bottom. It starts out with regular increments in Z, going from 0 to -.5 in .0625" increments. When done with a level it goes down to the next. It looks like it does this following the curve of the hemisphere. But when it gets to z -.5 instead of going down to the next level it switches to doing a retract and transfer that goes to +.201875 and then comes back down to the new Z level in a vertical plunge.

What's causing the switch to all that extra retract motion?

thanks,
Michael