Conformance of Roll Formed Internal Threads to AS8879
Use of the roll forming process for the creation of internal threads has been and remains a viable method for producing internally threaded fasteners. The roll forming method uses a tool similar to a tap, but one without cutting edges. The geometry of the tool initiates material flow to fill the tool geometry, but typically leaves an incomplete minor diameter. Thread forms exhibiting this condition are typically referred to as having “open crests.”
Historically, in the aerospace fastener industry, the method commonly employed in production of structural products referencing the AS8879 thread specification has been cutting of threads from a prepared hole at the AS8879 specified minor diameter. There are various methods available to create cut threads.