178 Introduction 

Boeing was contracted to deliver flight qualified PBNs for use on the ISS. Prior to the current contract,  PBNs had not been manufactured since the original ISS contract with NASA in the mid 1990s. Recently,  PBNs were fabricated in 2007 and delivered to Boei ng in Huntsville Alabama for acceptance testing.  Acceptance testing resulted in a PBN functional failure rate of 34%. Due to the complexity of the issue, a tiger team consisting of different engineering facets  was established to determine the failure mode and to  introduce corrective actions. The Assembly and A cceptance test-bed was investigated for possible  causes of failure. These investigations proved the Assembly and Acceptance test-bed were not the cause  of the failures. PBN build records and destructive testing of failed PBNs were also investigated for  possible causes of failure. Through destructive testing and evaluation, and review of previous build records, the failure mode was identified to be a combi nation of out of tolerance thread dimensions, out of  tolerance thread surface conditions, and improper application of dry film lubricant. Following the root cause investigation, improved process controls were implemented and a new lot of 113 PBNs were fabricated in 2009. These PBNs were acceptance tested and displayed a 0% functional acceptance test  failure rate due to thread dimensions and lubrication issues; validating the implemented process controls to be appropriate. The 96 PBNs manufactured in 2007 were scrapped.  

Thread Fabrication 

Internal thread geometry of an encapsulated nut is di fficult to inspect following machining, leaving the  thread geometry acceptability to rely primarily on th e fabrication processes used during manufacturing.  The PBNs are kept to a tolerance of 23 µm (0.0009 in) on the internal thread pitch diameter dimensions to  ensure proper bolt to nut clearances after the application of  lubricant. It is difficult to maintain these thread  dimensions because the nut must be machined and then lapped to ensure surface finish requirements. The PBNs are made of cold worked Nitronic 60, a di fficult austenitic alloy to machine because of its  susceptibility to internal stresses  caused by cold working. If the machinist does not take the proper  precautions during machining, these internal stresses will cause the metal to distort during the machining  process. These distortions, which may cause mating issues later dur ing acceptance, w ill be grounds for  rejection of the hardware. 

The machinist must ensure adequate and uniform lapping over all the internal threads with a hand tool specially made for the PBN thread dimensions to en sure burrs are removed and the required 0.41 µm (16  µin) finish is obtained. The lapping process for the 2009 PBNs consists of machining a bolt to proper dimensions and covering it with lapping compound. When choosing a hand tool to lap the nuts and remove burrs left from machining, the hand tool material must be softer than the nut base material. This is to ensure the burrs will be removed and the base metal will not be dam aged by galling with the hand tool.  The best way to ensure an adequate finish is to fully engage the softer material tool with lapping compound multiple times to wear down any burrs le ft from machining. This process was implemented for  the 2009 PBNs to smooth out burrs and meet required  finishing while opening the nut pitch diameter  before the application of lubricant. Figure 2 is a surfac e finish error (burr) that occurred due to inadequate  machining and lapping of the 2007 PBNs. This error may seem minuscule, but when requiring the Powered Bolt to autonomously atta ch two pressurized elements of the ISS together, a small burr will  quickly break through the thin dry film lubricant and cause galling of the Powered Bolt Assembly. The PBNs fabricated in 2007 were polished with a lathe and polishing sponge. The lathe and sponge method is unable to polish in the grooves of the PBN threads; therefore this method will not meet the thread call  out and finish of the specified PBN drawing. This proc ess not only leaves room fo r potential burrs but also  does not evenly open the thread pitch. Using a la the and sponge technique could also damage the  threads of the nut if the machinist calculates dime nsions wrong or a sizable previously made burr gets  caught on the lathe. The PBNs fabricated in 2009 were correctly dimensioned and lapped to open the pitch diameter in order to meet the PBN drawing specifications.   NASA/CP-2010-216272