We have a good idea of what "management" means, but what does the word "variance" mean?
A variance is a deviation from a desired standard. For instance, the desired standard for a washer may be that it be made of a certain alloy of stainless steel an eighth of an inch thick, an inch in diameter, and with a hole one half inch in diameter with the center exactly centered on the center of the washer, with the main faces perfectly flat. A deviation from that standard could be many things. For instance, a deviation is that it would bemade of copper, tin, or an alloy of stainless steel with more or less carbon in the mix that would change its durability and resistance to being deformed. It could be thinner or thicker than an eighth of an inch across the main face, not making it perfectly flat. It may be thinner than an eighth of an inch in one part and thicker in another part. It could be bigger or smaller in diameter than an inch, or not perfectly circular. If measured across one side, it could be less than an inch, while it could be bigger than an inch across if the other side was measured. The hole could be bigger or smaller than a half inch, or oval shaped and not circular. Also, the hole may not be exactly centered or the inner face of the hole may not be perfectly flat. The face of the outer edge may need to be flat as well. All of these differences from the desired dimensions or make-up of the washer are variations or deviations.
And I bet you didn't think that defining a washer wasn't complicated, hmm?
Now, unless you are able to enter Plato's Realm of Ideals and bring back into this world The Perfect Washer, there is, in fact, no Perfect Washer. There are many causes of variance, and each cause creating one or more variances. The key to the Industrial Revolution was the realization that, for parts to be mass produced and interchangeable, they had to be precisely fabricated within tight specifications. The key to higher quality was the realization that one had to progressively manufacture those parts to tighter and tighter specifications. Eli Whitney is credited with the first attempt in the United States to introduce a manufacturing process that was precise enough to create interchangeable parts for a rifle. His first contract went beyond schedule and 400% over budget, and the parts were not entirely interchangeable, but subsequent contracts were awarded and fully met. His problem was that, in order to manufacture precise and interchangeable parts for a rifle, he had to manufacture precise machinery to create those parts, and those machines required different machines equally precise to manufacture their parts. Precision is not free: one has to bootstrap oneself up "the precision branch of the tree of technology". All the high-precision machinery of that day was in Europe: France had already succeeded in mass producing rifles, and it was Thomas Jefferson who saw the potential and recommended that the manufacturing process be replicated in America. Whitney may have copied the idea of precise mass production from France, but he had to invent every machine his business eventually used to produce every part that went into the rifles.
What does higher precision machining of parts have to do with quality? A good illustration of this is the toilet paper roll holder in your bathroom (or water closet if you're British). The roll of toilet paper goes on the shaft that is held by the two mounts at either end of the shaft. Everything is built with very generous margins of space when it comes to the diameter of the shaft and its length because the design requirement is that the toilet paper roll should roll freely. However, note that one can take the toilet paper roll and slide it sideways so that it alternately hits one mount, then the other. This can represent a part in an engine. It should be easy to see that the large margins allow the roll to "gain speed" while flying sideways between the mounts. The damage comes when the roll hits the mount, since the damage is caused by the energy transferred from the roll to the mount. The energy is kinetic energy, whose magnitude is proportional to the square of the velocity. That is, doubling the velocity quadruples the kinetic energy in the object that is moving. The large margins necessary to allow the toilet paper to roll freely work against a reliable engine since the part would be "rattling" between its mounts. But, if there was only a quarter of an inch of margin, so that the space between the sides of a centered roll and each of the mounts is an eighth of an inch, then there's less space to allow the roll to "gain speed", meaning that when it hits the mount, less energy is transferred to it, resulting in a reduced magnitude of "rattling". When that happens the relationship between velocity and energy works for you, not against you, since reducing the velocity by half reduces the energy, and resulting damage, by three quarters. Reducing the velocity by two thirds reduces the energy by eight ninths. Thus, an engine manufactured to tighter specifications is more reliable since it suffers less internal damage from the internals striking each other while working together.
The process of controlling the amount of variance in the parts being manufactured so that they can fit together with the desired margin so as to attain the desired level of quality is what I call variance management. The methods for designing and manufacturing such parts, acquiring or creating all the machines required to do that, and the training and direction of the workers tasked with the job, requires a lot of planning and direction that is properly a function of the management of the manufacturing firm.
Why talk about the problems and benefits of manufacturing high precision parts? If one likens parts to thoughts, and the human heart as a generator of those thoughts (machine), then the simliarity in the problems becomes evident, so maybe the solutions for one would work for the other.
My research, and experience, indicates that this is true.
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