With so many resources exploring fastener heads, there should be nothing left for thread design. But innovation reigns there, as well. Magnesium, increasingly popular among automakers for its casting and melting properties, dimensional stability, and high strength-to-weight ratio, has one drawback if you make screws for it. 
According to Larry Pickett, who is a product manager at Textron Fastening, magnesium doesn't conform very well to thread-forming fasteners. Threads can crack and chip as they are made, and their condition worsens with the repeated removal and reinsertions of screws during repairs. Until you take out such a screw, Pickett said, it's analogous to a broken, wrapped peppermint candy. Any bits of magnesium broken during the initial forming are held intact as long as the screw is in place and unshaken by vibration. But unwrap the candy, or remove the screw, and chips can fall everywhere. To circumvent this trouble, Textron's Mag-Form screws use threads with a broad flank angle (105 degrees compared with 60 degrees for traditional screw threads) to increase compressive force and decrease shearing force during thread forming. 
The new screws for magnesium assemblies were developed very much with the needs of servicing in mind, Pickett said. 
In one instance, a magnesium die caster had to find a way to attach a cast magnesium shift tower to a sport utility vehicle's steel floor. The casting and floor were eventually mated with Mag-Form screws, and the company is currently producing 7,600 of the castings every month with no assembly or service problems reported, Pickett said. 
In another example, a maker of steering columns chose the Mag-Form screws for attaching the magnesium columns to their lower mounting brackets after first trying ordinary tapped holes. Roll forming fasteners that worked for aluminum and steel generated too many chips when working in magnesium, causing the fasteners to reach their torque points prematurely. Since switching to Mag-Form, the manufacturer has installed 1.5 million of the fasteners in steering columns, Pickett said. 
Their advancements over the last half-century notwithstanding, threaded fasteners still manage to end up broken and stuck in some very bad places. For collectors, old cars make happy hunting grounds. When he's not working as a network engineer for Philips Medical Systems Inc. of Andover, Mass., antique Volkswagen buff John Henry has been known to come upon a broken fastener crying out for extraction. He has seen so many on his way to restoring a 1950s Beetle that he's developed an odd sort of affinity for them. Of course, every one of them gives him a good excuse to fire up his MIG welder.
Henry doesn't take credit for discovering the technique, but he has used it to extract some particularly vexing screws. He found it as he tried welding an easy-out into a broken stud with the hopes of bettering the extractor's bite. With a reductionist's aplomb, he eliminated the easy-out and just built up a metal blob with the welder to which he clamped on a pair of Vise-Grips. Nowadays, he welds a dirty nut onto the shined-up end of the stud. The method takes patience. Expect to go through a lot of nuts, he said.  For Henry, coming upon a new fastener always lights a red lamp. He approaches it with the caution of a street dog sniffing a stranger's hand. Now, he always purchases the correct tool, too. 

