Proven on the Assembly Line
Specific fastening solution for magnesium die-cast applications lives up to its promise.
Designers are on a continuous crusade to reduce product weight. Magnesium, at one-fourth the mass of steel and two-thirds that of aluminum, fulfills this need. Its good fatigue strength, dimensional stability, and sound and vibration dampening qualities are also very useful.
If a magnesium die-cast application requires a removable fastener to permit service, a screw or bolt is required. However, the low ductility and deformability of magnesium die-casts can lead to slivering, layer eruption, chipped threads, and fracturing when mated with standard 60º thread bolts or thread-cutting fasteners. Removal and reinsertion of these fasteners can weaken and even destroy threads, creating magnesium powder, debris and contamination.
One solution to this problem is using MagForm thread-forming screws, designed specifically for use in magnesium die-casts. These have a broad 105º thread flank angle with widely spaced threads. When driven into magnesium, a compressive action forms strong threads with minimum debris generation. The larger flank angle also allows multiple removals and reinsertion of the fasteners.
Standard machine threads and thread-cutting screws (above) can lead to slivering, layer eruption, chipped threads or fracturing.
Mag-Form fasteners, with a 105º thread angle, form strong threads and reduces debris generation.
New Design Requires New Fastening Solution
The Lunt Manufacturing plant in Hampshire, Illinois – 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 molded during casting. Mag-Form thread-forming fasteners, which form threads in 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, a global product manager for thread-forming fasteners for Acument™ Global Technologies. “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.”
Tests at Lunt Manufacturing demonstrated that Mag-Form fasteners outperformed other threaded fasteners by a factor of ten in removal and reinstallation tests.
Many manufacturers have discovered the benefits of using Mag-Form
fasteners in their die-cast magnesium applications.
Mag-Form screws also proved to be the optimal solution for DaimlerChrysler’s RS Minivan steering column assemblies at its Toledo Machining plant. By switching to Mag-Form thread-forming screws for installating the lower mounting bracket to the cast magnesium body of the steering column, they were able to virtually eliminate screw fault rejects and reduce process times by more than a third.
“Both the tilt head and the main body of the steering column are made from die cast magnesium,” said Mark Laws, product and quality engineering supervisor at Toledo Machining. They began by using machine screws, but these required costly drilling and tapping. Design engineers at the plant changed to Taptite® screws that roll-form threads as they are driven into cored holes in the diecasting. But debris and powder build-up from magnesium chips caused the screws to reach the target torque before they were fully seated, resulting in loose parts that needed rework.
“The thinking was that the 30 mm length and finer thread pitch of the Taptite screws would ensure better engagement, but the opposite proved true – their narrow thread spaces and 60-degree flank angles were too strong for the low ductile magnesium, causing debris accumulation and high torque failures,” said Laws.
When Acument™ Global Technologies recommended Mag-Form thread-forming screws, engineers at Toledo Machining were ready to listen. After a full year of testing, with special emphasis on vibration and shaking tests, Mag-Form fasteners became the sole style chosen to hold the lower mounting bracket to the magnesium body of 2001 model RS Minivan steering columns.
Mag-Form® Thread Forming Fasteners are available in a wide
variety of styles and sizes to meet your needs.
Working with Acument™ Global Technologies, Laws and his team designed a Mag-Form fastener compatible to existing assembly equipment, in which two fasteners form threads as they are torque driven into cored magnesium holes.
The solution worked. Since going on line, more than 1.5 million Mag-Form thread-forming fasteners have been installed on RS Minivan steering columns without any of the problems experienced with the machine screws or Taptite thread-forming fasteners.
“Mag-Form screws work well with magnesium because they reduce stresses in threads,” said Tom McKee, senior applications engineer for Acument™ Global Technologies. “Its broad flank angle and pitch reduce shear stresses while increasing compression, allowing magnesium to deform rather than be cut from metal. The result is a good fastener joint, virtually no magnesium thread damage, consistent torque values, and the ability to remove and reinsert the fastener multiple times in service.”
The switch to Mag-Form screws also reduced process times markedly, Laws said. Taptite screw installations took 3.7 seconds per screw, making that station the bottleneck on the line. Mag-Form screw take only 2.2 seconds per installation, a 40% cycle time reduction. The combination of shorter length and fewer threads in Mag-Form fasteners allowed the decrease in installation times, saving hundreds of hours and thousands of dollars annually.