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Hole and thread inspection.

Assuring that threaded fasteners meet specifications can be difficult and time consuming. Recent changes to military specifications (Mil-S-8879), the Fastener Quality Act (PL101-592), and the implementation of vendor quality programs by large users of fasteners all place increasingly costly inspection requirements on manufacturers, distributors, and users.

Perhaps nowhere is inspection so important than in the area of safety-critical fasteners. Variations in the geometrical characteristics of fasteners clearly affect joint strength. For example, a nut with a 5/8" pitch diameter that is oversized by 5% will have only 60% the strength of a correctly sized nut.

A study conducted by the US General Accounting Office (GAO/NSIAD-91-309) contained a detailed analysis of two cases in which aircraft crashes were traceable to failure by fasteners that did not meet thread form specifications. Undoubtedly, part of the reason those fasteners failed can be traced to inadequate or cumbersome inspection methods, some of which are only incremental improvements over methods and devices developed decades ago.

The equipment typically used to measure a fastener that is labeled as safety-critical under Mil-S-8879C for example, requires a trained operator. Measurements typically vary significantly from operator to operator and the gages are subject to wear. In addition, a significant amount of time and space is devoted to calibration, record-keeping, and gage storage. In an evaluation of variable thread measuring systems conducted at the Charleston Naval Shipyard in 1989, officials estimated that calibration of a gage set (pitch diameter and functional diameter) from one manufacturer took eight man-hours.

Clearly, fastener producers and users must decide how to better meet these new requirements in the most cost efficient manner. Traditional methods of inspection are not likely to provide the efficiencies and reliability required. Increasing the use of current gaging methods decreases throughput and significantly increases costs.

Leading-edge technology

One of the new approaches to measuring thread form is undergoing testing at a facility that makes safety-critical parts for the aerospace industry. The Laser Thread Measurement System (LTMS) developed by Apeiron Inc of St Paul, MN, uses a laser coupled to a precision motion system to scan and digitize the form of a thread. The basic technology employed in the LTMS can be extended to accuracies of 20 to 40 microinches.

The LTMS is designed to make measurements required under the revised Mil-S-8879 and can be operated either in a quality laboratory or on the factory floor. According to Apeiron, no special expertise is required to operate the system.

The operator views the digitized form of a thread. Then, key characteristics of thread form, including flank angle, pitch, root radius, helix variation, and pitch diameter are calculated. The unit can also be used to measure other parts related to thread production including header dies, flat dies, and tangent chasers.

The LTMS solves many problems associated with traditional gaging methods. Because the system is non-contact, the problems of operator error and interpretation are eliminated. Repeatability is assured. Information gained from the LTMS is very useful for understanding and adjusting the manufacturing process. Routine, manual record-keeping and inspection documentation is also eliminated. Stored data and digitized thread form images can be readily formatted into customer reports.

For more information from Apeiron Inc, circle 161.
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:quality inspection of fasteners
Publication:Tooling & Production
Date:Mar 1, 1992
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