Almost seventy years after its introduction, Statistical Process Control is an integral part of modern manufacturing. Unfortunately, despite well-documented success, the full potential of SPC as a adjunct to efficient production of threaded components has not been realized on a widespread scale. In many cases, the manufacturing and inspection technologies associated with threaded products are consistent with practices introduced in the early part of the 1900s.
However, there are companies and industries which embrace the challenges associated with state-of-the-art Statistical Process Control techniques and the manufacture of threaded components. In some instances, programs were developed as a direct response to a real or perceived quality assurance issue or concern. Still others evolved as the end-result of excessive cost burdens directly attributable to inconsistent assembly or in-service failure. Finally, many of the programs integrating SPC and thread manufacture are simply an outgrowth of the desire to unify all manufacturing processes under the umbrella of process control. Regardless of the reason for initiation, all successful programs share an identical philosophical basis: an intense desire to reduce variation, a never-ending focus on continuous improvement, and an unshakable belief in customer satisfaction.
Prior to the introduction of Statistical Process Control to the fabrication of threaded components, a examination of the overall Quality Assurance System as well as manufacturing operations and capability is a prerequisite. While this seems elementary, the uncertainties in the actual manufacture of threaded components mandate its inclusion in program planning. Too many SPC initiatives have stumbled as a result of poor initial preparation.
Among the prerequisites for the establishment of a Statistical Process Control Program for thread components is an in-depth understanding of the technical issues affecting the screw thread manufacture. The complex nature of the thread form is perhaps the most difficult characteristic to produce in generalized manufacturing operations. A fundamental understanding of this complex relationship is critical to the on-going nature of the program. Unfortunately, this level of expertise is deficient in all too many instances.
Another topic demanding consideration is the selection of the Thread Inspection System. The ability to target the manufacturing process is critical for efficient production of threaded components. Further, the inherent difficulties in threading operations virtually demand an inspection system with the ability to differentiate between variation directly related to size or variation related to the interdependence of size and form. Without this capability, any understanding of threading operations is limited. Given these basic requirements, the incorporation of variables thread inspection into Process Control System is a virtual necessity. Attribute data is simply inadequate for Statistical Control of threading operations.
The relationship between Statistical Process Control and specifications or standards for product acceptance is complex. SPC is only valid when a viable index for the assessment of process variation is firmly in place. If the standards and specifications defining these tolerances and allowances are erroneous or misleading, the entire concept of SPC loses all meaning. Unfortunately, many of the acceptance documents used for threaded components suffer are inadequate. Some even eliminate the necessity for the inspection of Thread Pitch Diameter, a critical element in any process control system. Careful evaluation of the governing standards and specifications is a necessity when developing a System for the orderly control of threading operations. An illogical structure with standards and specifications inevitably creates a unresolvable conflict between acceptance and Statistical Process Control.
Upon completion of the initial evaluation phase, it is critical to identify the key process variables directly related to thread manufacture. Unfortunately, the complex geometry defining the screw thread defies easy solutions. Unlike many characteristics in manufacturing operations, variation in the screw thread is not limited to simple variations in size. While variation in the Pitch Diameter Size is significant, the Functional Size is also a key process variable for the efficient control of threading operations. Defined in FED-STD H-28 as "the cumulative effect of all profile variation," Functional Size shares the tolerance limits defined for the Pitch Diameter Size and includes the entire effect of both Lead and Flank Angle Error on the Pitch Diameter Size. Differences between these different interdependent manifestations of thread geometry reflect the sum of inherent Thread Form Error in the process.
Compounding the difficulty in selecting the key process variable for the statistical control of thread manufacturing is the relative importance of these two different but inter-related expressions of thread geometry. From an engineering perspective, Pitch Diameter is a critical consideration due to its central role in ultimate performance. Virtually all mathematical calculations for the strength of threaded connections are dependant upon the Pitch Diameter Size in some respect. Conversely, Functional Diameter Size is equated with the ability to assemble. While these considerations are not central to the ability to statistically control the threading process, they are cornerstones in the overall Total Quality tenet of customer satisfaction, and should receive careful review and consideration.
The central dilemma is apparent: control Pitch Diameter and assure optimal performance yet risk selective assembly, or control Functional Diameter Size and assure assembly while sacrificing long-term reliability. In many instances, this paradoxical impasse is resolved by eliminating neither Pitch Diameter Size nor Functional Size from consideration. Both are considered key process variables, and are used simultaneously in the control of the process. Although burdensome, this approach has unquestionable appeal, especially at the early stages of the program.
As the program for the Statistical Control of the threading process evolves, the desire for simplicity without sacrifice of critical information is paramount. At this point, standard SPC techniques have been utilized to identify and isolate random sources of variation. As the process reaches the point of statistical control, there is sufficient information about the threading process to refine not only the process itself, but also in the means and methods of control. An examination of the data may suggest that there is a small differential between the Pitch Diameter Size and the Functional Diameter Size. In other words, the effects of Lead Error or Angle Error on the process have been minimized. Given this process knowledge, control of a single variable or characteristic may assure control of the interdependent characteristic. Simply stated, it may be possible to monitor Functional Diameter Size to the exclusion of Pitch Diameter Size without sacrificing process integrity. While this assumption may be statistically valid and will certainly simplify the control mechanisms, it is critical that any change in mechanisms utilized to monitor and control the process are based on empirical data.
In advanced systems, control of an independent variable critical to the success of the threading process has shown great promise and benefit. Through the control of blank diameter prior to the actual thread fabrication, the thread forming operation has shown significantly reduced process variation. While this method of control is extremely useful as a part of the overall statistical control of threading operations, it is not a substitute for control of the actual process.
Selecting and locating the process on a target is another prime consideration when developing a statistical process control system for thread manufacture. Generally constructed in parallel to selecting key process control variables, this step is critical in the ability of the process to meet specifications or customer requirements on a consistent basis. Once again, the presence of the interdependent variables of Pitch Diameter Size and Functional Diameter Size presents a challenge. Despite the refinement of a statistically controlled process, no threading operation can successfully eliminate all sources of process error. Inevitably, Lead Error or Angle Error will cause a separation between the Pitch Diameter and Functional Size. In a stable environment, process deviation in thread form has a overwhelming tendency to Maximum Material Limit. As a result, it is entirely normal and even adavantagous to target the Functional Diameter Size at a different location than the Pitch Diameter Size. Given normal circumstances in the thread manufacturing environment, the optimal target value for control of the Pitch Diameter Size is generally at a point between the process mean and the Minimum Material Limit, while the optimal target for the Functional Diameter Size is between the process mean and the Maximum Material Limit. To assure process stability as well compliance to expectations of performance and assembly, the entire focus of process improvement should concentrate on the elimination of the effects of the differential between these different but inter-related expressions of thread size.
Unfortunately, the complex behavior of the inter-related variables in the manufacture of threads pose several unique dilemmas regarding SPC protocol which define process variation. As the Functional Diameter Size reflects the consequences of process variation and degradation, it is a much more dynamic variable than Pitch Diameter Size, and may indicate a higher degree of process instability. Such circumstances are entirely normal, and indicate normal process behavior. A well-designed SPC System should acknowledge this occurrence in its basic premise, and develop control systems accordingly.
The specific indices defining the process capability of the fabrication operation have also created controversy and confusion. CP, a statistical index defining the process capability without regard to process centering, and CPK, a statistical index describing the process capability with respect to the mean of the tolerance, are both widely used in a wide variety of SPC programs. However, the inherent bias of CPK with regard to the necessity to target the specification mean negates much of its value for the control of threading operations. Unless specific compromises and clarifications regarding use of CPK are built into the system, CP may be a more valuable index than CPK when expressing capability of the thread manufacturing process.
In circumstances where use of CPK is encouraged or requested, the index does retain validity if its inherent bias to the process mean is tempered through the introduction of other indices of process capability. As stated earlier, it may be desirable to target the process at some other location within the specification limits. This method of process targeting will yield a CPK which indicates that there is a probability that a significant portion of manufactured output will exceed that specification limits. Yet depending on the application, this condition may indicate optimal process targeting. In these instances, it may be advantageous to compare CPK to CP. If the CP indicates a high degree of process capability, the necessity to "target the mean" is minimized. The process can be centered at a point ideally suited to efficient production, and the degree of centrality around the optimal target expressed by an intentional offset of the CPK. In these instances, while CPK does not represent optimal process distribution in the traditional sense, it does maximize the capability of an individual manufacturing operation. An in-depth knowledge of the manufacturing process including the capability for the control of all possible process variation is integral in the development and continuation of this type of program. Typical applications include situations requiring additional processing of the threaded product subsequent to actual manufacture, such as heat treating or the application of an additive finish. In these cases compensations for the potential and actual changes to the material conditions of the thread geometry are in integral part of process planning and control. Failure to modify basic statistical indices in light of subsequent operations is simple ignorance of manufacturing reality and a prime example of excessive rigidity without the temperance of process knowledge.
The Ford Motor Company is most successful advocate and proponent for the development and refinement of statistically-driven programs for the control of process variation in the manufacture of threaded components. The basic program requirements are defined in the Ford Worldwide Quality System Standard Q-101. Specific program parameters for Metallic Threaded Fasteners are included in an Appendix to the document. Applicable to both Ford manufacturing operations as well as outside suppliers of finished product, the program requirements retain sufficient detail to assure an operable system while retaining sufficient flexibility to accommodate real-world experience. According to sources at Ford, the program has radically reduced warrantee and repair costs, virtually eliminated the expense associated with receipt inspection, and has provided a higher quality threaded product at a reduced cost.
In a more recent development, the Defense Industrial Supply Center announced it intention to redefine its procurement policies for Class 3 Threaded Fasteners. In an historic meeting with its supply base on September 28, 1994, DISC provided advanced notice that future contracts would include mandatory utilization of Statistical Process Control technology for the manufacture and verification of all incoming product. According to DISC, all current specification requirements will be strictly enforced. Individual manufacturers and contractors will create and implement specific SPC methodology for the control of individual characteristics and processes. In all instances, DISC will retain the responsibility for final control plan approval.
