selected surface boundary offset

[M_Hennig]

Ok PM experts. If I create a selected surface boundary, then do a 3d offset of .005, I will get a different result compared to creating a selected surface boundary with a -.005 thickness. Why is this, and which would be mathematically correct?

[Craig.Burney_DSI]

@M_Hennig  Sounds like there will be a 0.010 difference between the 2 boundaries.  You're offsetting the first one by 0.005, then the second has a -0.005 thickness.  They're going in opposite directions.  What are you trying to do exactly?  The selected surface boundary should give you a boundary that will finish the selected surfaces with the desired tool.  Maybe I'm missing something?

[M_Hennig]

I always try to make the selected surface boundary larger by toolpath stepover so no segments get snipped off. 

[Craig.Burney_DSI]

@M_Hennig  That makes sense.  There is no real right or wrong way of doing it, whatever works best for you to be honest.

[M_Hennig]

Actually that is sort of my point, there must be a right and wrong way because the results are different.  Offsetting a boundary .005 should give the same result as creating a boundary with a -.005 thickness in my head. It does not.

[Jonathan_Artiss19]

This wouldn't do the same thing, a negative offset would push the boundary into the surface keeping the boundary size  and cutting area the same where the offset would be increase the boundary size and overall cutting area. 

A transform scale would be closer to the same result as the negative thickness since it would keep the cutting area closer to the same size, collapsing or expanding the boundary depending on the selection.