Casting Study: Roof-Bolt Wedge Anchors

When the AKZO Salt Company had problems getting roof-bolt wedge anchors from its malleable-iron foundry supplier, it opened the door for zinc alloy. Mine management set out to find an alternative source for these vital parts, but they were unable to find another ferrous foundry to fill the void. They turned to a local pattern shop specializing in non-ferrous work for help. Sand-cast ZA-12 zinc alloy was recommended as the ideal solution. ZA-12 mechanical properties were more than adequate and included attractive bonuses of a 17 percent weight saving, improved corrosion resistance, and non-sparking properties.

The real bonus, however, was the prompt availability of the anchors for uninterrupted mining. Test prototypes were quickly made, using the malleable iron sections as patterns. When they were approved by the mine, economical aluminium match plate tooling was produced. The foundry delivered 5,000 sets of anchors with little difficulty, as the standard zinc sand casting tolerance of +/-0.030 in. was ample for the application. Finishing operations are limited to sawing off the gating, grinding the parting flash, and giving the parts a steel shot blasting.