The Impact of a Material’s Inherent and Process Stress on Meeting Specification and Tolerances: A Six Sigma Case Study

Authors

  • Brittney Jimerson Department of Management & Accountancy The University of North Carolina at Asheville
  • Yusuf Fahmy North Carolina State University
  • Joe Fender Unison Engine Components
  • Linda Nelms Department of Management & Accountancy The University of North Carolina at Asheville
  • Ted Powell Unison Engine Components
  • Robert Yearout Department of Management & Accountancy The University of North Carolina at Asheville

DOI:

https://doi.org/10.23055/ijietap.2009.16.3.262

Keywords:

Six Sigma, Inherent Material Stress, Specifications, Tolerances, shot peening, and Cost Savings

Abstract

This Six-Sigma Case Study was conducted in a local aerospace company that produces high quality precision-machined jet engine components.  These complex turbine components have thin walls that must meet tight tolerances. Disks, shafts, rotating seals, plates, and cases range in size from 3" to 80" in diameter.  This case focused on a 16” (diameter) rear cooling plate whose production required 18 machining processes.  The objective was to determine if it was possible to eliminate the final manual lathing process. Manual lathing was used as the last step because the material characteristics of the plate and the stress induced by the previous processes caused the final product to expand. Stress can cause unsatisfactory changes in the plate’s dimensions. Stress is not only inherent in the material’s internal properties but is also induced during machining.    It is critical that the operator’s cut is precise and does not remove too much material.  During the two most critical steps of 18, measurements were taken.  It was theorized that relaxing the first process tolerances could allow later processes to be numerically machine controlled to conform closer to the prescribed tolerance of the final product.  Plates were tested using these revised tolerances.  After the plate was shot peened (a stress redistribution process) measurements confirmed that non-conformance had been eliminated and the final machining process could be discontinued.  Cost savings for eliminating the last machining and inspection process was $268 per plate or an annual saving of approximately 11% of total cost for the item studied.

Author Biographies

Brittney Jimerson, Department of Management & Accountancy The University of North Carolina at Asheville

Yusuf Fahmy, North Carolina State University

Yusef Fahmy, Director, Joint NC State - UNC Asheville Engineering Programs, Teaching Assoc. Professor, Dept. Materials Science & Engineering, North Carolina State University (NCSU), BS Mechanical Engineering (NCSU), MS Materials Science & Engineering (NCSU), Ph.D., Materials Science & Engineering (NCSU).  Dr. Yusef Fahmy's areas of research interest include the structure and properties of high temperature intermetallic compounds, structure and properties of nanocrystalline systems, non-equilibrium processing, mechanical behavior of structural materials and the influence of electric and magnetic fields on mechanical properties of materials.  Dr. Fahmy has published in a number of peer reviewed journals including Physical Review, Journal of Materials Research, Materials Science and Engineering, Journal of Electronic Materials, Metallurgical and Materials Transactions, International Journal of Modern Physics, Hyperfine Interactions and Scripta Materialia.  Dr. Fahmy has worked to bridge engineering education across disciplinary lines including integration of engineering and liberal arts in the delivery of a BS Mechatronics Engineering program at UNC-A.  He is a member of Alpha Sigma Mu, Sigma Iota Rho, Sigma Xi, ASEE and a Senior Member of IEEE.

Linda Nelms, Department of Management & Accountancy The University of North Carolina at Asheville

Professor of Management, University of North Carolina at Asheville (UNCA), B.A., Literature, UNCA; M.B.A., University of North Carolina at Chapel Hill. North Carolina, C.P.A., C.M.A., C.P.I.M. Mrs. Nelms awards include the Ruth and Leon Feldman Professorship for Outstanding University Service and the University Service Council Award.  She has published several articles in top tier peer reviewed international journals such as The Management Accountant and conference proceedings to include the Annual International Journal For Industrial Engineering Theory, Practice, and Application Conference.  Mrs. Nelms has served as the Director of UNCA's Undergraduate Research Program.  Her research interests are in the areas of managerial and accounting ethics, economic analysis, inventory, and other related topics that span the gap between economics, accounting and engineering.

Robert Yearout, Department of Management & Accountancy The University of North Carolina at Asheville

Robert Yearout Professor of Industrial Engineering Management, University of North Carolina at Asheville (UNCA), B.S.C.E., Virginia Military Institute, M.S.S.M., University of Southern California, M.S., Ph.D., Kansas State University. LTC (US Army Special Forces, Retired). Dr. Yearout's awards include the Ruth and Leon Feldman Professorship for Outstanding Research, Distinguished Teaching Award, UNCA Distinguished Teacher of the Year, University Service Council Award, Board of Governors of the University of North Carolina Award for Excellence in Teaching, and Annual University Research Council Award for Scholarly and Creative Achievement. Dr. Yearout has published a significant number of articles in national and international peer reviewed journals such as IEEE Transactions on RELIABILITY, International Journal of Industrial Ergonomics, and the International Journal For Industrial Engineering Theory, Application, and Practice in subject areas directly related to his teaching expertise.  He is the editor for National Conferences on Undergraduate Research (NCUR) Proceedings and a member of the Editorial Board for the International Journal For Industrial Engineering Theory, Practice, and Application.

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How to Cite

Jimerson, B., Fahmy, Y., Fender, J., Nelms, L., Powell, T., & Yearout, R. (2009). The Impact of a Material’s Inherent and Process Stress on Meeting Specification and Tolerances: A Six Sigma Case Study. International Journal of Industrial Engineering: Theory, Applications and Practice, 16(3), 162–170. https://doi.org/10.23055/ijietap.2009.16.3.262

Issue

Vol. 16 No. 3 (2009): Special Issue: Six Sigma - Its Application, Practice and Utility

Section

Production Planning and Control

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