Lesson

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. ...

I was running a batch of forty-two hydraulic cylinder barrels for a mining equipment customer. Each barrel was forged 4140 steel, 2 meters long, with a wall thickness of 25mm. The print called for a 100mm bore through the full length. After drilling the first five parts, I pulled the first one off the machine and set it up on the inspection bench. The bore came out at 100.5mm. The tolerance block on the print showed plus or minus 0.2mm on all bore dimensions. By my reading, the part was 0.3mm over the upper limit. I had drilled about thirty thousand bores up to that point in my career, and I knew a 0.3mm oversize condition on a 100mm bore usually meant the drill was cutting slightly over size due to guide bushing wear or a minor misalignment. I flagged the part and told the operator to stop the machine. ...

The Fine Dust That Fooled Me I was drilling a deep hole in 304 stainless steel. The bore diameter was 12mm and the depth was 400mm, giving an L/D ratio of about 33:1. The feed rate was set to 0.03 mm/rev. The chips were coming out of the flute as fine metallic dust. I thought this was a good sign. In my mind, fine chips meant the cut was clean and the material was being removed efficiently. I had read somewhere that small chips are easier to evacuate through the drill flute, so I believed I had selected the right parameters. ...

The Setup That Felt Routine I was drilling a batch of titanium shafts for an aerospace customer. The material cost was $800 per shaft. I had 20 shafts to drill, each one 600mm long with a 16mm bore running through the center. The job specification called for a tolerance of H7, which meant the bore had to stay within 0.018mm of the nominal diameter. I’d run similar jobs before without any trouble, so I treated this one like any other production run. ...

The Job That Looked Simple on Paper I had a job drilling 3mm holes through 200mm of titanium 6Al-4V. The L/D ratio was about 66:1, which is well within the range of what a gun drilling machine can handle. I had drilled similar holes in steel without any trouble. The customer needed four holes in a part used for an actuator housing. The tolerance was ±0.05mm on diameter and the straightness requirement was 0.1mm over the full length. Nothing about this job looked unusual when I quoted it. ...