This discussion went a bit further in another thread where I outlined the short-comings of the current mill facing tool:
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The part that got really hacky was facing the fixation lugs on each side. If I set material allowance to get it to clean up the corners, it digs into the body of the piece. Here, I could not find better than to edit the sketch and pretend the central block was larger to protect it, then edit the sketch back down and profile the central block. That's two ops where one should be enough, and messing with the model to trick FreeCAD to do what is required.
Now, maybe I'm missing a trick but it seems that there should be a way for FreeCAD to know the difference between air and the part described in the model. The basic milling op does not clean up the face since it dumbly stops at the boundary, even when there is nothing but air around it. If I use material allowance it ignores the fact that it is destroying the workpiece.
If doing this manually, the obvious solution is to run off each side into unused stock or the surrounding air. The leaves no residual bits at the corners and a nice clean finish with straight, regular paths.
How can I achieve that with FreeCAD?
From replies there, it seems there is no simple way to get a clean facing operation unless there is air or unused stock all around , ie. it is the top surface of the work. dubstar-04 suggested putting fake "helper geometry" in the model, which was better than my workaround of two cuts with model edits in between using fake dimensions. All this is really hacky and there probably should be more flexibility in the face milling path tool.
I see two principal issue here:
1. Material allowance
does not seem to differentiate between air or unused stock and remaining parts of the model, meaning it will happily remove part of the work piece. I would consider that a serious bug.
2. A facing operation by default does not clear the selected face and thus does not fulfil its primary task in any situation. It requires a specific option parameter to be set by the user to clear the corners even if there is nothing but air/stock outside the boundary. Once the above issue is resolved , this could probably be handled in a more helpful way, overstepping by the tool radius where it does not damage the surrounding part. Material allowance
could then be used to set more over step if desired to get a cleaner finish ( no turns on the work ).
If there is a step in the model, this requires some rather imaginative and ugly workarounds to clear a face, while there are obvious tools paths to achieve the result.
The model referred to is here: