I reduce feed through cross holes to protect the drill from shock loading. When a gun drill passes through a cross hole, the cutting edge loses contact with the material momentarily and then re-enters on the far side. The impact of re-entry chips the cutting edge if the feed is too high. I learned this the hard way on a batch of hydraulic manifold blocks where I chipped three drills in a row before I figured out the feed reduction. ...
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. ...