Stress-Relief

I drilled a bore that came out 2mm off position. The part was a hydraulic cylinder barrel forged from 4140 steel, about 700mm long with an outside diameter of 180mm. The bore was supposed to be 80mm diameter through the full length, positioned within 0.2mm of the centerline. When I put the part on the CMM after the first drilling pass, the CMM reported the bore centerline was 2.1mm offset from the print position at the mid-length point and 2.4mm offset at the far end. ...

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. ...