The Specifications Approach

It was towards the latter end of the eighteenth century that people first began to think seriously about the manufacture of interchangeable components. The ideological backdrop for this step towards the techniques of mass production was The Enlightenment and the ascendancy of the natural sciences. This was, as Dr. Shewhart noted, the age of exact science,¹ when precise and rigid natural laws were established. It was held that the rigorous application of reason and scientific method would overcome the vagaries of nature. Consequently, the dreams of the Industrial Revolutionaries encompassed streams of exactly identical products pouring forth from factories and mills.

We know, now, that it is impossible to produce components which are identical in every detail. While the variation between individual pieces may be considerable, or it may be immeasurably small, we can be certain that some variation will exist. We shall look more closely at process variability, and its implications for manufacturing industry in the section entitled The Voice of the Process. For the moment it is sufficient merely to recognise its existence.

Given that it was impossible to produce identical components, manufacturers, and their customers, had to settle for similar components. This contingency led to the use of specification limits. Having accepted that there would be variation between parts, some measure was required of how much variation in the product could be tolerated. Engineers had to devise limits which were both economically viable for the manufacturer and acceptable to the customer. Given the nature of the problem, it is hardly surprising that the focus of their attention was the product itself.

Specifications do not address the nature of a process, they are simply a way of separating the output of that process. Each piece produced is either within spec or outside spec: a good part or a bad one. This has several serious implications:

• Focus on the product leads to an uneconomical cycle of testing followed by rework or scrap.
  
  
• Specifications are often viewed as goalposts.
  
  
• Conformance to specifications means relying on results measures and ignoring process measures.

Let's look at these points in a little more detail.