Dennis:
the only real rule is that the tip fit. LazyTurn is being written as a solid's comparator. Its the only way I could figure out a path to completion,
so it creates solids from the profiles and tools, then trys to fit the tool onto the profile in scanlines. It has to compute intersections and collisions
to do the path. Its all very complex and some bugs remain in the calculations, but its getting tighter all the time, failures can be caused by the code
or the tool size, and even the pass depth, you'll notice a filure at one pass depth may be corrected by simply modiying the depth by a small amount.
even a change from .1 to .1001 can fix the trouble.
Ill be switching types of algorithsm soon to try to get the last of the intersection bugs, ( so ironically it may get more buggy while thats tracked down.

)
Until I dream up a few more checking algorithsm to fix up head on collisions with intersectional point duplicated in the tool and profile, this error
will continue, so when a profile fails, try a slightly different depth. If a profile says " Self interseting profile found" try chaning the tool to a slighly smaller
tip radius or slightly larger, typically it will fix the trouble.
Thx
Art