Carbide-Grade

What Forging Scale Does to a Gun Drill Forging scale is the hard, abrasive oxide layer that forms on forged steel parts during heat treatment. It is one of the fastest ways to dull a gun drill. The scale is harder than the base material and contains sharp particles that act like sandpaper on the cutting edge. When a gun drill hits scale, the cutting edge micro-chips within the first few millimeters of engagement. I have seen a brand-new drill ruined in less than 15 seconds of cutting time because the operator did not realize the forging still had its as-heat-treated surface. ...

What Happens at an Interruption Interrupted cuts in deep hole drilling happen when the drill passes through a cavity, cross hole, keyway, or any gap in the material. The cutting edge hits the far wall of the cavity, loses contact, and then slams back into the material on the other side. That shock loads the edge and chips it. I have seen a sharp C2 carbide edge chip on the first interruption of a 10mm hole through a valve body with a 6mm cross hole. The chip happened in the first 0.1 seconds of re-entry. After that, the drill was cutting with a damaged edge and the bore finish was shot. ...