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Quality Scan: Develop a Process Control Plan

David Bengston Process validation and documentation is a critical component of success in today's manufacturing world. From Production Part Approval Process (PPAP) to development of comprehensive production and quality assurance methodologies, measurement of the production process defines the manufacturing world.   

The development and implementation of process-control plans for the manufacture of threaded components is complex. Used in a wide variety of components from aerospace to automotive applications, the screw thread is one of the more difficult characteristics produced in general manufacturing operations. In many instances, the breakdown of process validation has its origin in the design phase. Incomplete product details and unrealistic tolerances are all too common.The net result increases production costs, extends delivery, and complicates the entire manufacturing process. As a manufacturer and designer of both standard and special inspection systems, our company is often required to manufacture special out-ofthe- box gaging solutions for unique requirements, when a simple design modification will permit use of standard off-theshelf gaging. Occasionally, complex designs coupled with excessively stringent dimensioning produce a part incapable of quality verification at any cost. The old adage "keep it simple" should be a design standard.

As products evolve from design to manufacture, the challenges increase. A first step in the development of a processcontrol plan is a complete understanding of the potential sources of variation. Unlike most manufacturing characteristics, the geometry defining the thread form defies easy solutions. Variation is not limited to change in the pitch diameter, but also to the interdependence of size and form variation as indicated by the functional size. Defined as the cumulative effect of profile variation, functional size includes the entire effect of thread-angle and thread-lead error on the pitch diameter size, and is related to the ability of a thread to assemble. In any control plan, it's critical to control and minimize the difference between the functional and pitch-diameter size as a means of variability reduction.

Hand-in-hand with an understanding of variation is the selection of an inspection system.The ability to measure variation and target the manufacturing process is essential to efficient production of threaded components. The inspection system should have the ability to differentiate variation in both size and form as it occurs. For threaded components, this includes errors in thread angle and thread lead in addition to pitch-diameter size variation. Further, the basic inspection system should also include the capability to provide real-time measureable data to both verify ongoing capability and record process degradation. Variables inspection systems are a basic requirement of any process-verification plan. Traditional GO and NOT GO attributes data are inadequate to record and document change in manufacturing operations.

Efficient manufacturing depends upon the selection of a key process variable to indicate control or change in threading processes. While both pitch diameter and functional size are necessary to assure acceptability in accordance with military, government, and many private-industry acceptance standards, control of pitch diameter by itself is inadequate to verify process. Pitch diameter does not reflect change or degradation. Functional size is a better indication of actual variability, as it includes the effects of both size and form.

Even with optimal methods and systems for the control of threaded products, it's critical to target production within the manufacturing control or acceptance limits. In any mature control plan, it is important to move from identification of random process error to overall control of the process. Unlike many manufacturing processes, degradation in thread manufacture is unilateral. Variation occurs in one direction only. Degradation in setup, tooling, and process increases the functional size of the external screw thread, and decreases the functional size of the internal screw thread. A properly targeted threading process recognizes this essential and unique characteristic, and plans for initialization of the thread manufacturing process at the low end of the external thread tolerance, and at the high end of the internal thread tolerance.

While it's common practice to verify both pitch diameter and functional size as part of the inspection and acceptance process, very often the basic inspection protocol can be expanded to incorporate a specific and sequential practice that will detect errors in thread lead and angle.


This article was first published in the September 2008 edition of Manufacturing Engineering magazine. 

Published Date : 9/1/2008

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