Off-Center

Every shop drills a crooked hole eventually. The question is not whether it will happen, but what you do when it does. I have a simple framework for deciding whether to salvage an off-center hole. It comes down to three things: how far off the hole is, how much material is left around it, and what the part is worth. Step 1: Measure How Bad It Is Before I decide what to do, I need accurate numbers. I measure the hole position at both ends and, if possible, at the midpoint. ...

I spot face the entry before drilling eccentric holes. Eccentric holes are holes that are not centered on the part axis. The drill enters the material at an angle relative to the surface, which causes the tip to deflect if the entry is not prepared properly. A bad setup here almost always produces a scrapped part. Preparing the Entry Surface The first thing I do is create a flat entry surface with a spot face cutter. The spot face must be perpendicular to the drill axis, not to the part surface. Without the spot face, the drill hits the curved surface at an angle and slides down it, shifting the hole position by as much as 0.5mm on a 50mm diameter shaft. ...