Page 32 - Design for Six Sigma a Roadmap for Product Development

10 Chapter One

Before the industrial revolution, quality was assured by the work of
individual crafters. The production is rather like an art, and crafters
were trained and evinced similar behavior to that of artists.A crafter was
often the sole person responsible for the entire product. Quality was con-
trolled by the skill of the crafter, who usually had a long training period.
The assembly line and specialization of labor were introduced during
the industrial revolution. As a result, the production process became
more productive, more routine, and also more complicated. Compared
with artistic production, where a single worker makes the whole prod-
uct and the worker’s skill is very important, the new production process
employs many workers, each making only a portion of the product with
very simple operations, and the worker’s skill level became less impor-
tant. Thus the quality can no longer be assured by an individual worker’s
skill. In the modern production system, the volume and number of parts
in the production increased greatly; therefore, the variation in assembly
and variation in part quality became a major impediment in production
because it destroyed the consistency of product and part interchange-
ability. Also, modern production assembles parts from many suppliers;
even a small number of defective parts can ruin a big batch of produc-
tion, and the rework is usually very costly. Therefore, there is an urgent
need to control the variation and sort out defective parts from suppliers.
This need is the impetus for the creation of modern quality system and
quality methods.
The historic development of the modern quality method actually
started at the last stage of the product development cycle: production.

1.3.1 Statistical process control (1924)
Statistical process control (SPC) is the application of statistical tech-
niques to control a process. In 1924, Walter. A. Shewhart of Bell
Telephone Laboratories developed a statistical control chart to control
important production variables in the production process. This chart is
considered as the beginning of SPC and one of the first quality assur-
ance methods introduced in modern industry. Shewhart is often con-
sidered as the father of statistical quality control because he brought
together the disciplines of statistics, engineering, and economics. He
described the basic principles of this new discipline in his book
Economic Control of Quality of Manufactured Product.

1.3.2 Acceptance sampling (1940)
In the production stage, quality assurance of incoming parts from other
suppliers is also important, because defective parts could certainly
make a defective final product. Obviously, 100% inspection of all
incoming parts may identify defective parts, but this is very expensive.