Surface-Defects

Types of BTA Surface Defects BTA surface finish defects have their own language. After years of looking at bores under a loupe, I have learned to read the patterns. Unlike gun drilling — where the single cutting edge and the bushing dominate the finish — BTA surface defects come from three main sources: the guide pads, the chip mouth, and the coolant hydraulics. Here is how I break them down. ...

How Surface Defects Affect the Bore Surface defects on the workpiece do not stay on the surface. They transfer into the bore during deep hole drilling. The drill encounters the defect at the entry point and carries the effect of that encounter throughout the hole. I have seen a part with a 0.2mm-deep rust pit on the entry surface. The drill hit the edge of the pit and deflected. The resulting hole was 0.15mm off center at 200mm depth. The defect was on the surface, but the problem was in the bore. ...