Sunday, 11 December 2011

The cost of "quality"

While I haven't had my hands on one, what I've read about them implies that, like most German designs I've worked with, the product is over-toleranced.

It's fairly easy to design a product "line-on-line." That is, you assume that every part will be made exactly to the nominal dimensions on the drawing. For instance, if the design calls for a 1/2" slot, the designer assumes that the slot will be exactly .5000 inches, every time, on every gun.

The problem is that in reality no two parts are the same. There is some range in that dimension resulting from the manufacturing process. If the process is in control, that dimension should have a normal distribution (a bell curve) centered on the nominal dimension. If the part is milled, a fairly typical capability would be +/- .005" on features less than three or four inches in size. That is about the thickness of a piece or two of xerox paper. The normal distribution means ~2/3 of the parts will be +/- .0016". ~97% of the parts will be +/-.003".

If the tolerances on the drawing demand it, the manufacturer can take their time, and reduce the manufacturing capability to +/- .001", assuming that the machine and setup can handle it. Of course, this requires more time and expense. A manufacturer will only go to this trouble if the design demands it, because it's expensive.

In general, if the tolerance is less than +/- .001", the part will have to be ground or EDM'ed. Both of these processes are fairly slow, and very expensive. If the product was designed line-on-line, with no tolerances and an inadequate amount of effort was put into exploring the design to account for variation in the components, the manufacturer will be forced into using very expensive means to manufacture the components, or extensive hand fitting will be required to make up for the inability of the manufacturing process to meet the requirements of the product.

A design that is tolerant to variation is called robust. I don't think the Korth design is robust.

That Korth uses both expensive, slow manufacturing methods, and handfitting indicates to me that the design is not robust. Now, I'm not saying they aren't good guns. They may be just as strong, durable and reliable as anything else out there. I'm saying that I suspect a less than robust design has forced Korth to charge exorbitant amounts of money for their product, because they aren't capable of making a less expensive gun.

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