A Study on Mitigating Head Gap Defects due to Radius of Externally Threaded Fasteners in Aerostructures Assembly

Externally threaded fasteners such as HI-LOK™, HI-TIGUE™, and HI-LITE™ fasteners are used in Aerospace Industry for their light weight, high strength, and controlled preloaded designs. These pins are offered in a variety of head styles, materials, finishes, and coatings. The pins are easily installed from one side, by one person, and quietly without the conventional riveting noise. In the design specifications of these pins, there is a radius R provided under the head which is to be accounted during manufacturing. The head radius, R comes with a manufacturing tolerance of 0.015 to 0.025 inch for standard sizes of 5/32 inch and 0.020 to 0.030 inch for higher sizes for protruding heads and similarly vary from 0.020 to 0.030 inch for 5/32-inch flush head style. As a general assembly practice, it is essential to deburr the edge of the holes to accommodate the head radius of the pins. The improper seating of the head can possibly result in the head gap defect of the fastener after installation and results in rework due to quality nonconformances. Inspectors use standard feeler gauges of 0.003 inch or less for checking the head gap and determine its acceptance criteria. Also, these fasteners are generally to be installed in a Transition fit, which could lead to popping of fastener heads resulting in head gaps after installation after some time. This rework of fasteners has a significant effect on the cost of assembly as the fasteners once installed cannot be reinstalled due to their loss of coating during interference fit installation and the loss of the broken collar which cannot be reused. This article analyzes the externally threaded fastener head designs and their effect on head gaps, and the suitable methods and means to avoid them.