Take a S.W.I.P.E. at Inspection

by Drew Koppelman

Gaging is a process, and no instrument can cover up for
failures in controlling other elements of the operation.

Every regular reader of Quality Digest who is involved in manufacturing understands that having the right kind of production equipment isn't enough to ensure that products will be of good quality. A multitude of issues, ranging from design to final delivery, influence product quality.

You can no more expect the acquisition of the proper gaging hardware to be sufficient for good inspection than you can expect production equipment alone to be capable of manufacturing good products. Like manufacturing, inspection is a process, the quality of which is influenced by innumerable physical and human elements. The role of this column will be to alert readers to the factors influencing dimensional inspection processes, present ideas for improvement and provide guidance in gage selection and use.

The innumerable influences on gaging processes can be characterized as belonging to one of five categories, represented by the acronym SWIPE, which stands for: standard, workpiece, instrument, personnel and environment. By paying careful attention to each category when analyzing an existing inspection process or when planning a new one, organizations can achieve inspection results of uniformly high quality.
Standard. In gaging, also known as comparative measurement, workpiece dimensions are established by comparing them to physical dimensional standards, including gage blocks, and master discs and rings. Physical standards are more accurate than the gages they're used to master and must be calibrated periodically in a measurement laboratory to ensure their accuracy. Even so, some calibration uncertainty exists even in standards of the highest grade. You should select a standard that is accurate enough for the job at hand without going overboard and purchasing more accuracy than you need.
Workpiece. It is dangerous to assume that just because a part feature falls within specified dimensions at one point, it will at all others. No part surface is perfectly round, flat or smooth. When a production process is under control, the amount of variation in roundness, flatness, etc. is small enough so all points on the part surface fall within the specified tolerances. If the process is out of control, some points may lie outside of tolerances even though the gage indicates that the specific point being measured is in tolerance.
It is important to understand the type of variability to which the workpiece may be subject. For example, centerless grinding often imposes a slight three-lobed condition on round parts. Most gages aren't designed to detect this condition and will register the part's average diameter rather than its effective maximum diameter.
Instrument. As noted above, the instrument or gage itself must be capable of performing the task at hand. It must have a degree of accuracy suitable to the job's requirements. It is entirely possible for a gage to be too highly accurate to perform a task properly. The gage must be in good condition, properly mastered and capable of holding calibration throughout a reasonable service period.
Personnel. No surprise here. The people responsible for operating and maintaining the gage, as well as those who summarize and analyze the results, play a central role in the success or failure of all dimensional inspection programs. The use of good practice, which includes proper gage operation and maintenance, as well as proper statistical process control methods, must be identified as a priority.
Environment. Several factors in the environment around the gage must be considered. Most gaging components are made of metal and are subject to changes in size because of thermal influences from heating/air-conditioning systems, direct sunlight, nearby machines, the workpiece and even the operator's body heat. Dirty environments can also cause problems because deposits on gages and workpieces can influence measurements, resulting in rapid wear of the instrument. Vibration from nearby machinery can also disturb instruments.

When gaging results fail to meet expectations, many users are quick to blame the gage. Remember that gaging is a process and that the instrument, no matter how good, cannot cover up for failures in controlling other elements of the operation. Before you take a strict hardware approach to establishing a new inspection program or solving a problem in an existing one, first take a SWIPE at good overall gaging practice.

About the author
As applications manager, gaging products, at Federal Products Co. in Providence, Rhode Island, Drew Koppelman provides dimensional gaging applications assistance to companies in a wide range of industries, including automotive, aerospace, packaging and electronics. He may be contacted by fax at (401) 784-3246.