The Virtual Machine Shop

Witness mark

A slight scoring, scribing of a line, or other evidence of a tool presence on a machined surface. This can happen when extracting or moving a tool from a completed machining operation or when pausing the tool during the cutting pass.

A witness mark happens regularly with a boring operation. One way to avoid a witness mark when boring is to stop the machine, back the tool away from the machined surface and extract the tool from the hole. Another way to avoid a witness mark when boring is to make the last spring pass from the inside (bottom of the bore) outward. The animation below shows how stopping a boring bar at the bottom of a hole and then retracting it can score the side of the bored hole. (an end mill can leave a witness mark if the feed is stopped and then started again.)