Anyone who has driven on the Jackie Robinson Parkway [Queens, NY] has already gained an appreciation of Tolerances. The posted speed limit is 40mph, but even that is too fast. Most drivers new to the parkway try to take it at 55 or 60. Big Mistake. There is a curve going to the Van Wyke or GCP where you can see all the marks made by cars as they side swipe the concrete and stone wall. The lane has a tight tolerance with respect to the width of a car and it makes it hard to navigate.
Many years ago I worked closely with a fellow who had a machine design shop. He designed machines for all sorts of industries. He was a mechanical engineer by training. He relied on my company to provide the electronics for his machines. I had an opportunity to ask him about his mechanical designs and he explained that the most important aspect (to him) was tolerancing. For instance, if you create a drawing for a one-off part used in the aerospace industry, it might be acceptable to specify a 0.1 mil tolerance on all your dimensions. Realize what is going to happen. When the machinist gets your drawing he might have to throw out the first 5 parts he makes because he had trouble holding that spec. Eventually he will learn how to make your part to that tight tolerance, possibly he might have to design his own fixtures to help him hold the part as he machines it. Eventually, with much waste, he will make your part. Your part will be expensive, but it may be less expensive than the engineering to realize a 'better' design.
A good mechanical design orients the parts so that they do not need to be manufactured with a tight tolerance. If you have ever assembled furniture from IKEA you might have noticed the large over sized screw holes to allow the part to 'float' with respect to the attached part thus alleviating the need for a tight tolerance.
One might think that a very good design then tolerates loose tolerances on all its parts. Such a design could be manufactured 'cheaper.' Well, if you remember back in the '70's how the Asian car manufacturers were advertising how well the parts of their car fit you think otherwise. Loose tolerances
could give the perception of a cheaply made product to the consumer.
You want your product to be manufactured inexpensively, but not appear cheap.
Tolerancing can take on a different aspect to the software professional. Back in 1981 John Postel coined the
"Robustness Principle". In it he advocates that a browser should “Be conservative in what you do, be liberal in what you accept from others.”
This holding of loose tolerances has resulted in a problem. See this link. For a browser the specified tolerance should be as tight as possible. Only in this manner can software manufacturers duplicate the performance of the browser over the range of web sites.
What conclusion can we make? Sometime a tight tolerance is good, sometimes its not good. But it is ALWAYS important to your design.