4140-Steel

I’ve drilled a lot of deep holes in my career. Hundreds of them, in every material you can imagine. But there’s one that stands above the rest — a job that pushed me, the machine, and the tool to the absolute limit, and taught me more about preparation than any other job before or since. I should start by explaining what makes a hole “deep.” In gun drilling terms, deep means any hole where the depth is more than ten times the diameter. A 20mm hole drilled 200mm deep — that’s 10:1. Easy. A 20mm hole drilled 600mm deep — 30:1. Getting interesting. A 20mm hole drilled 2000mm deep — 100:1. That’s where most gun drilling applications top out. Past that, you’re in special territory. ...

A customer sent me a print for what looked like the simplest job I had seen in months. It was a 4140 steel shaft, 600mm long, with a 20mm through-hole drilled axially from end to end. The print had no tight tolerances. The bore diameter was 20mm plus 0.5mm minus zero. The position tolerance was 0.5mm. The surface finish was 3.2 micrometers Ra. I had drilled hundreds of similar holes in 4140 over the previous five years. I quoted it as a standard gun drilling job at $85 per part for a batch of thirty-six pieces. ...