Special Fasteners Shift SUV Design

Redesigned threads prevent chipping and fracturing in magnesium castings

Installing fasteners in shift tower casting

Auto designers are on a continuous crusade to reduce vehicle weight, and magnesium, at one-fourth the mass of steel and twothirds that of aluminum, fulfills their needs. The Lunt Manufacturing plant in Hampshire, IL, one of the largest in the world devoted exclusively to production of magnesium castings for the automotive industry, employs cold chamber processes that transform magnesium ingots into complex and intricate assembled components in less than 90 seconds. These one-piece castings include instrument panel housings, roof structures, intake manifolds, steering wheels, steering column lock housings and jackets, and dozens of other custom-designed componentry for installation at OEM and Tier One assembly plants worldwide. When Lunt Manufacturing was selected to produce a magnesium floor-mounted shift tower for a new line of SUVs for General Motors, company engineers faced design, engineering and assembly decisions for the four fastening points where the shift tower would be installed in the steel floor of the vehicle’s body-in-white frame. These would be the only holes in the part, and their formation and the fastening system employed in them had to be flawless. Rather than drilling the holes after the casting process (a costly and time-consuming procedure conducted with coolant whose residue must be removed after work is completed), the holes in the shift tower were set during casting. Camcar brand Mag-Form thread forming fasteners, which compress threads into the magnesium, were installed in the holes at a separate assembly station.

Lunt had no previous experience with Mag-Form fasteners, and this was to be the first magnesium shift tower ever installed in a production vehicle. “We proceeded cautiously,” said Larry Pickett, global product manager for thread forming fasteners for Textron Fastening Systems. “We started by designing a Mag-Form fastener for this particular application, one that would fit the specifications for the shift tower holes, and any follow-up removals and reinstallations during warranty and service procedures over the life of the vehicle.”

Threaded fasteners have always posed challenges for magnesium component producers, Pickett noted. Magnesium’s deformability can lead to slivering, chipped threads, and fracturing when mated with threaded fasteners. Removal and reinsertion of standard fasteners often destroys formed threads, creating chips, debris and contamination.

“Mag-Form fasteners have a broad, 105-degree thread flank angle with widely spaced threads. When driven into magnesium a compressive action forms strong threads with minimum debris generation. Traditional fasteners have 60-degree flank angles and narrower thread spaces that work well with aluminum and steel but invariably leave debris and often lead to chipping and cracking when applied to magnesium.”

The larger flank angle also allows multiple removals and reinsertion of Mag-Form fasteners for service and repair procedures. Tests at Lunt Manufacturing demonstrated that Mag-Form fasteners outperformed other threaded fasteners by a factor of ten in removal and reinstallation tests.

—RM