Feed-Marks

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. ...

Surface finish defects in gun drilling are the most common reason for part rejection that I encounter. The bore surface must meet the specified roughness and be free of visible defects. When the finish is wrong, the part is scrap or needs rework. I have developed a systematic approach to identifying and fixing surface finish problems by looking at the defect pattern. Problem Description The surface finish of a gun-drilled bore depends on the tool condition, cutting parameters, coolant system, and machine rigidity. A defect in any of these areas shows up on the bore surface as a characteristic pattern. The pattern tells me which area to investigate. ...