Pilot-Hole

The pilot hole that guides the gun drill at entry determines the accuracy of the entire deep hole operation. I use center drilling for rotating parts and spot drilling for stationary parts. Getting this choice wrong creates problems that cannot be fixed later. Center Drilling: When and How to Use It Center drilling uses a combined drill and countersink tool to create a conical starting hole matching the gun drill tip angle. I use center drilling for rotating workpieces — shafts, axles, and cylindrical parts. ...

Why Pilot Hole Geometry Matters The pilot hole is the only thing guiding the gun drill during the first few millimeters of entry. If the pilot geometry is wrong, the drill starts off-center or at an angle, and the rest of the hole follows that bad start. I have measured the effect of pilot geometry on hole straightness across dozens of jobs. A pilot hole that is 0.1mm oversized produces a starting hole that is 0.05-0.08mm oversized at entry and the error propagates as the drill goes deeper. ...