Pilot-Holes

I use step drilling when the bore needs multiple diameters along its length. Step drilling creates a bore with different diameters at different depths, like a counterbore for a bolt head or a stepped shaft bore for hydraulic applications. It can also help manage chip evacuation and tool deflection in deep holes. When to Use Step Drilling vs Single Diameter Step drilling is not always the right choice. I use it in specific situations where a single-diameter hole will not work. Here is how I decide. ...

The Pilot Hole Is the Foundation A gun drill has a single cutting edge offset from the centerline. It does not self-center like a twist drill. The pilot hole is the only thing keeping the gun drill tip in position during the first critical millimeters of engagement. I have tested gun drilling with and without a proper pilot hole on the same material. Without a pilot hole, the gun drill wandered 0.15mm off-center in the first 10mm of drilling. With a properly sized pilot, the same drill stayed within 0.02mm of the centerline for the entire hole. ...