Statistical Process Control (SPC) is often thought of as a technical and scientific
discipline that applies to sophisticated, high-volume processes. Many people also assume that
SPC is a technical approach and operators play only a supporting role. By extension, Total
Quality Management (TQM) and Six Sigma are thought of in the same way since both use SPC
as a primary tool.
The assumptions above are incorrect. SPC works equally well with simple manual operations
such as bench assembly. It also has a strong psychological component that mentally sucks workers
into the process even if they are not ordinarily inclined to participate. It can work wonders
with processes that have:
High Operator Dependence
This article illustrates the above points and tells how a manufacturer of commercial windows
reduced final assembly defects by over 90% in about seven weeks.
The company manufactured a wide variety of aluminum windows for hotels and other commercial
buildings. They had implemented workcells and each workcell built several basic models. However,
every window was made to custom dimensions and there were thousands of variations on each model.
The process consisted of manual assembly with simple hand tools by a team of operators. They
gathered parts that had been fabricated in other departments and assembled the windows on
workbenches in accordance with the engineering documents. Quality depended primarily on operator
experience and awareness.
Inspectors conducted 100% final inspections, primarily visual, with a few simple aids such as
a scale. Defects might include missing parts, crooked parts or insufficient sealant.
In a comical twist it came to light that one inspector could not read an ordinary
document because of an outdated eyeglass prescription.
The quality performance was poor. At times, over 50% of the windows built had one or more
defects. Even with 100% inspection, inspectors *missed many defects that later required
expensive field correction.