CHUCK PRICE 2017-04-21 10:10:41
USING NONCONTACT THREAD MEASUREMENT TO REDUCE OPERATOR-INDUCED VARIATION Advancements in noncontact measurements have already demonstrated the ability to reduce variation induced by inspectors. The traditional approach to thread inspection and calibration utilizes mechanical devices. It’s time-consuming, operator dependent, not readily adaptable to automation (McCarty, 1989), and subject to human error. Many studies have shown that the tolerances for thread gages are not realistic for the variability seen in the inspection process. Yet, this is the most common and widely-accepted practice in the industry. In June of 2014 an Inter-Laboratory Comparison (ILC) for gage calibration involving 50 calibration labs was published (Solomonson, et al, 2014). In this study, major diameter and pitch diameter, pitch (lead), and half-angle measurements were compared between the labs (Solomonson, et al, 2014). After outliers were removed, the results of this evaluation showed measurement uncertainty of ±57 micro-inches for major diameter, ±98 micro-inches for pitch diameter, ±199 micro-inches for pitch (lead), and ±20 minutes for half angle (Solomonson, et al, 2014). As Hill Cox pointed out in Quality, “At 97 micro-inches, the uncertainty is about 33% of the ‘X’ tolerance” (Cox, 2015). With uncertainty values this high, it makes it difficult to settle disputes over measurements. This naturally leads to the question: Does the industry need to re-evaluate thread tolerances, or the method by which measurements are made? In the ILC study, a total of 51 reports were submitted (Solomonson, et al, 2014). Two of the reports were submitted by a pivot lab, which submitted a report on the gages at the beginning and end of the study to validate the stability of the artifacts (Solomonson, et al, 2014). While the study wasn’t intended to differentiate measurements by equipment (Solomonson, et al, 2014), the authors of the study had the foresight to involve a lab that used noncontact vision-based equipment to make their measurements. The lab using the noncontact method produced results similar to the other labs participating in the study. The advantage of using the noncontact method is that it eliminated operator-induced variation, and allowed faster measurements. Gage R&R studies for vision-based inspection have shown that a repeatability of ±26 micro-inches is achievable (Quest Metrology, 2012). With technology available that eliminates operator error, one has to wonder why this approach is not more wide-spread. The answer may lie at the heart of what the ILC study demonstrated. Coming to a consensus on what the measurement of a gage is continues to be elusive, but is the cause the inherent nature of how traditional, manual gaging works? While the noncontact gage inspection measurements in the ILC study demonstrated how technology is driving out operator-induced variation, it did not address how the gage itself is used. Consider standard go/no-go plug gages used for measuring internal threads. They are the primary method for accepting most threaded articles, but these gages are subject to wear, and are also subject to an inspector’s judgement (Folsom et al, 1999). In addition, multiple tools are usually required for each thread parameter and size to be inspected (Folsom, et al, 1999). In some industries, such as the oil industry, manual plug gaging is not sufficient for fully accepting a thread. In many cases, the shape of the thread requires that a mold be taken, and then verified with an optical comparator. These molds are time-consuming to produce, require highly-trained inspectors to confirm conformance to requirements, and must be archived for several years. Laser-optic triangulation technology provides a platform for reducing the quantity of these gages, and eliminates the molding method. In 1999, a laser-optic internal thread inspection system was able to demonstrate that using laser-optic triangulation, a measurement system could achieve a standard deviation of 16 to 66 micro-inches, and 28° to 0.37° for flank angles inspecting ring gage internal threads (Folsom, et al, 1999). Repeatability values this low demonstrate the advantage that noncontact systems have over the traditional approach to thread measurement. However, the design of the system utilized a single laser, and this approach limited the system’s capability to measuring symmetrical, positive flank angle threads. Positive flank angle and symmetrical threads may be the most common today, but in many industries the design of threads has evolved toward steeper flank angles. Thread forms with steep, and sometimes negative, flank angles present a problem to the single laser system. A limitation of many high-quality inspection systems is their lack of portability. In the oil industry, it is not feasible to move large sections of drill string, with each pipe often weighing in excess of 1,000 lbs. In order to meet the needs of these customers, it is necessary to make a portable system where the inspection equipment can be taken to the part needing to be inspected. A portable, dual laser-optic triangulation system has recently been able to demonstrate that using the dual-laser approach, even complex geometries can be profiled. Even though this new capability offers a leap from current molding methods, how can one compare a laser scan of a thread, to a rubber mold in an optical comparator? Noncontact methods offer reproducibility and repeatability that could never be replicated by a human factor dependent process, but in order to evaluate the system’s capability, one must know the dimensions of the article being inspected. Advancements in noncontact measurements have already demonstrated the ability to reduce variation induced by inspectors. This, coupled with other economic advantages, make noncontact thread measurement an attractive option to companies looking to reduce costs and improve quality. Noncontact approaches offer the potential to solve the age-old arguments regarding uncertainties and tolerances. However, without establishing standards with agreed-upon measurements, the full potential of the systems will not be realized, and disagreements about uncertainties and tolerances will continue to plague our work. Chuck Price is the engineering sales manager at Quest Integrated LLC. For more information, call (253) 480-2029, email C.Price@Qi2.com or visit www.Qi2.com. McCarty, L. H. (Ed.). (1989, January 23). Laser Inspect Internal Threads, Reduces Manufacturing Costs. Design News. Solomonson, N., Greensdale, J., & Barrows, A. (June, 2014). Thread Gage Measurement Inter-Laboratory Comparison (ILC) of Major Diameter, Pitch Diameter, Pitch, and 1/2 Angle Measurements (Tech.). Thread-View Model 3625 Five Gage Repeatability and Reproducibility Study (Tech. No. 500780). (2012). Kent, WA: Quest Metrology. Folsom, T. C., & Bondurant, P. D. (1999). Noncontact Internal Thread Inspection (Tech. No. 1999-01-3434). Warrendale, PA: SAE. 2015 Casing Reference Tables. (2015, January). World Oil, C-112, C-116.
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