How to Fix Feed Marks and Scratches in Gun Drilled Holes
Problem Description Feed marks are visible helical lines that follow the tool advance pattern on the bore surface. They look like thread crests running along the hole wall. I have measured feed marks that were 0.02 mm deep on a 15 mm bore — enough to fail a 1.6 Ra surface finish callout by a wide margin. Scratches are different. They run axially along the bore and look like the surface was dragged across something rough. Scratches come from chip drag or recirculating debris. I have seen both problems on the same hole, and each needs a different approach. Understanding the distinction is the first step to fixing surface defects in gun drilled holes. ...