Natalie notices immediately that the overall mean diameter with batch 17 eliminated is 0.507527, which is higher than the specification value. Thus, the mean diameter of the rollers produced is so high that many will be downgraded in value. In fact, 55 of the 150 rollers sampled (36.67%) are above the specification value. If this percentage is extrapolated to the full year’s production, 36.67% of the 7,000 pieces manufactured, or 2,567, could not be sold as half-inch rollers, leaving only 4,433 available for sale. “No wonder we often have shortages that require costly emergency runs,” she thinks. She also notes that not one diameter is below the lower specification of 0.5072, so not one of the rollers had to be scrapped.
Natalie realizes that there has to be a reason for all this. Along with Jim Murante, she decides to show the results to Dave Martin, the head machinist. Dave says that the results don’t surprise him that much. “You know,” he says, “there is only 0.0003 inch variation in diameter that I’m allowed. If I aim for exactly halfway between 0.5072 and 0.5075, I’m afraid that I’ll make a lot of short pieces that will have to be scrapped. I know from way back when I first started here that Mr. Harnswell and everybody else will come down on my head if they start seeing too many of those scraps. I figure that if I aim at 0.5075, the worst thing that will happen will be a bunch of downgrades, but I won’t make any pieces that have to be scrapped.”
- What approach do you think the machinist should take in terms of the diameter he should aim for? Explain.
b. What do you think that Natalie should do next? Explain.
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