Spot-Facing

I use counterboring for bolt head seats and spot facing for sealing surfaces after deep hole drilling. Both operations are done after the main bore is complete. Getting them wrong means the fastener does not seat or the seal leaks — either one scrapes the part. Counterboring Deep Holes Counterboring enlarges the entry of a hole to create a flat-bottomed recess for a bolt head or cap screw. The counterbore has a pilot that fits the existing hole to keep the counterbore concentric with the bore. I use a counterbore with replaceable pilots so the same cutter body works for different hole sizes. ...

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. ...

Why Spot Facing Is Not Optional Spot facing creates a flat surface at the hole entry point that is perpendicular to the drill axis. Without it, the drill enters a curved or angled surface and the tip deflects on contact. That deflection at entry starts the hole off-center. I have seen operators skip spot facing to save time. The logic sounds reasonable — why take an extra pass if the part surface looks flat? The problem is that looks are deceiving. A surface that appears flat to the eye can be 0.1mm out of perpendicular. ...