Warco have been in business for a long time. Not all that they sell is faultless, but they are willing to try to right matters if there is a problem, even to replacing complete machines.
Statistical Quality Control, as per Deeming, depends up on how many standard deviations you want your product to fall within. Even if you go for 3 standard deviations, which is about the best that you can get, there can still be a small percentage falling outside the specification.
If you elect to hand build every unit, you may well get even bigger differences from item to item
Steam locomotives were hand built by craftsmen, but unless it was G W R, direct interchangeability of parts was nigh on impossible.
You can make any machine virtually perfect, if you resort to such methods, but they are slow, and therefore costly, and the parts are likely to be unique to that machine.
The skill is in design, and manufacture, such that almost 100% of machines meet the specification. You will always get a small percentage that fall at the ends of the Gaussian distribution, (Bell Curve ). Hopefully, Inspection will spot the batches that do not comply and reject them before assembly.
With our hobby facilities, we cannot hope, nor wish, to produce thousands of interchangeable parts and consistent complete assemblies..
The above may be a difficult concept to grasp if you have no experience of high volume production. But producing large numbers of complicated machines that meet specification depends upon a variety of skills and disciplne. NO it is meant to be singular (BS 5750 and it's succesor, ISO 9000, for instance )
We've all heard of Friday afternoon cars, haven't we? Sadly, some may have experienced them. They are ones where the process failed. Think of the number of components in the modern car, and wonder that so many cars work so well, for so long. Japanese cars showed, via an American's methods, and still do, the way for other manufacturers.
Mass production lives on "Fitness for Purpose", so that most do what they are intended to do.
Howard