Distortion

Stress relieving before deep hole drilling removes residual stresses from the material. If these stresses are not removed before drilling, the part can distort when the material is removed from the bore area. I have seen this happen enough times that I now flag every new job for a stress relief review before I touch the machine. The underlying mechanism is straightforward. A part that has internal residual stresses is in a state of equilibrium. When I remove material from the center – which is what deep hole drilling does – that equilibrium shifts. The remaining material redistributes the stress and the part moves. In extreme cases, the bore comes out straight but the part bends around it. I have had parts that looked fine on the bore gauge but were visibly curved when placed on a surface plate. ...

The Core Tradeoff in Process Planning The choice between drilling before or after heat treatment depends on the tolerance requirements and the material. Both approaches have tradeoffs. I have made this decision hundreds of times over my career, and I have developed a framework for deciding which sequence works best for a given part. The wrong choice can add 30% to the production cost or cause a 5% scrap rate from distortion. ...