Developing Operating Procedures to Avoid Future Complications
A long- and well-established customer in the aerospace supply chain came to us about a problem their customer had with a few parts that are critical components in jet engine fuel systems. The particular part had a long and slender 5-16-24 UNF thread for about 1-1/4” on this part that measure nearly 1-1/2” overall length. It is manufactured from 17-4 condition A.
For a gas-turbine engine, particularly for a jet engine, the automatic fuel control is one of the most important aspects. As an aircraft’s main part, an appropriate amount of operational safety and highest reliability must be built into it. Jet engines for aircraft are built in a large range of performances and types depending on their specific tasks, civil or for combat.
Whatever the engine’s use might be, it can get pretty complicated when several output parameters are affected by only a few input parameters. In order to make the engine a safe-operating aircraft part, it is a must that specific automatic control systems be controlled to keep the output parameters in the desired range, whatever the flight conditions are.
The customer’s client complained of a slight back taper over the length of the thread of about .0001-.0003”. Of course, this deviation was not noticed using traditional GO/NOGO ring gages; they would only pick up back taper. Due to the small amount of taper in the threads, a comparator would not pick this up with any repeatability and reliability either. The customer had measured the threads over wires and verified that there was taper in the thread.
Not knowing that Specialized Turning used a full-profile thread, the customer could have easily determined that there was back taper in the thread by measuring the major diameter or outside of the thread over the length of the part to determine in there was back taper. Likewise, the customer was not aware that Specialized Turning had a vision inspection system that could detect back taper at first piece, in-process, and final inspection.
Specialized Turning was not using the vision system, all parts were inspected 100% by the ring gages, which required frequent calibration due to the duty cycle of checking each part at the machine and at final inspection. Prior problems with matching thread measurement between the customer and Specialized Turing had led this to be an agreed-upon standard practice.
The customer was not aware of any form, fit, or function issues with the part in its intended use; being a component in a jet engine, no one dared ask. The important lesson for all involved—and more than anyone, Specialized Turning—is that the problem could easily be corrected at the machine with a taper offset, and inspected quite handily along the axis of the thread with the vision system. At the machine, and in process, the machinist could verify any taper concerns by monitoring parts coming off the machine with a micrometer.
When the part was loaded on the vision inspection equipment, an adjustment was made to measure the pitch diameter, thread angles, major diameter, and root radius (a larger root radius is a critical component of the “J” thread) at five separate intervals over the length of the thread. If any of the thread dimensions were out of tolerance, the machine would report a red signal in the display, indicating a non-conforming part.
Going forward, this part and all similar parts will go through a standard operating procedure that defines first piece, in-process, and final inspection with the combination of the use of micrometer, vision inspection, and GO/NOGO ring gages.
Contact Specialized Turning to make sure your threads are spot on.