Probably something I’ve ticked somewhere, but I can’t get the chamfer to work on a hole edge. It either gets hidden inside, or deletes the object leaving just the chamfer 
Attackometer.3dm (10.5 MB)
It works fine for me. The only thing I noticed is that the filleted edge above the holes is quite close to them. This entails that the chamfers need to be cleaned up manually, if the radius is too large.
You can explode the object and trim the chamfer surface with the intersecting fillet surface and vis versa and rejoin the surfaces to get a closed polysurface.
