Original Date: 01/26/1998
Revision Date: 01/18/2003
Information : Thread Measurement Process
Corpus Christi Army Depot (CCAD) uses a laser thread measurement machine to improve inspection methods. In 1993, the Depot was tasked to perform repairs on the UH-60 spindle assembly. One of the requirements was to perform an inspection of the critical thread characteristic on the spindle, the spindle nut, and the retaining rod. Because failure of a spindle assembly results in catastrophic failure of the aircraft, it is critical that the inspection be done accurately without adversely affecting the integrity of the assembly.
The inspection requirement prevented CCAD from using standard gages to measure the critical root radius because of the potential to scratch the surface of the assembly. The spindles are made from titanium; therefore, scratches on the surface can progress to stress cracks on the part. CCAD used pitch micrometers to measure the pitch diameters; optical comparators to measure the root radius; and lathes to measure the helical deviation. This process took approximately 30 minutes to conduct one inspection. Each spindle must be inspected twice, resulting in one hour total inspection time per spindle. The Depot repairs approximately 500 spindle assemblies per year. Using the old inspection method, the accuracy of the results depended heavily on the skill and experience of the operator, and documentation of the results was questionable.
In 1996, CCAD purchased an Aperion Laser Thread Measuring Machine for a total cost of $120,000. This price included the cost of special fixtures required to mount the spindle on the machine and all required software. The laser thread measuring machine is computer controlled, which results in a faster, more accurate inspection and provides a means of standardizing documentation methods. Complete inspection of the spindle can be accomplished in 14 minutes using the laser machine. Besides the 50% reduction in process time, the software purchased with the laser measurement machine will immediately inform the operator if any of the measurements are out of tolerance.
By using the laser measurement machine, CCAD does not incur the cost of storing, maintaining, and calibrating gages. These gages must be kept in a controlled environment and calibrated yearly in a certified laboratory, which costs approximately $80,000. The laser thread measurement machine is calibrated daily using a standard purchased from the factory. The machine also undergoes yearly calibration by a factory representative.
After using the laser thread measurement machine, CCAD detected a high failure rate in the retaining rod. The repair process has been modified to include 100% replacement of these rods. The laser thread measurement machine provides CCAD with a fast, accurate inspection system that can be used to conduct measurements to a variety of thread standards.
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