These guidelines should be helpful in developing a first design approximation:
- Coated steel bolts and studs have less of a tendency to seize than uncoated steel and aluminum fasteners, making the relationship between torque and clamping force more consistent.
- Provide a margin of safety to prevent the fastener from over-torquing, which could be caused by variations in the torque applied by production tools.
- Design the joint so that the bolt fails rather than the casting. Bolt threads are usually stronger than die cast threads, so bolt failure will be by tensile failure instead of thread stripping.
- The boss diameter should be at least twice the bolt diameter. The shear strength and modulus of the elasticity of die casting alloys is much lower than that of cast iron and steel, making the die cast joint more prone to dilation than a ferrous casting. (Dilation: the expansion of the boss caused by the wedging action of the threads, weakening the joint by reducing the contact between mating threads.)
- Tapped holes should be cored to minimize porosity in the thread area. If core draft is required, particularly on deep holes, the draft will increase the minor diameter of the threads at the large end of the hole and reduce the thread height. Coarse threads have greater thread height than fine threads and are less affected. When maximum thread strength is required, the holes should be cored slightly under size and finished to the required tap drill diameter. When coring for tapped blind holes, sufficient hole depth must be provided to ensure tool clearance beyond the last full thread. If tool clearance is not enough and the tap bottoms, it can strip the threads, or worse, severely weaken the threads in a way that could go undetected.
- If operating conditions create the potential for thread relaxation, verify that the joint can maintain the required clamping force by test.
- It is possible to cast internal threads in die castings by employing spin-out cores or by unscrewing the casting from the core. But when you consider the increase in cycle time and required draft, it's usually more feasible to tap the threads in a separate operation.