Hole Extrusions—Part 2
November 1, 2011Comments
Last month we proposed a simple approach for estimating extrusion height using the constancy-of-volume rule: The volume of material available to form an extrusion must be greater than or equal to the volume of material contained within the extrusion. The amount of material available to produce an extrusion of a given diameter and height is proportional to the punched hole diameter, the diameter of the extrusion and the amount of permissible wall thinning.
However, material volume alone does not ensure success when forming extrusions. All hole extrusions start as hole expansions, with a punch forced into a blanked hole to cause circumferential elongation or stretching of the cut edge. Expansion limits depend on the material type, tool design, lubrication, edge quality of the punched hole and extruding direction relative to punching direction. When the amount of stretching required to form the hole extrusion exceeds the residual stretchability of the punched hole edge, fracturing of the expanded edge occurs.
Fig. 1—Proper hole preparation
Metalformers have options for restoring the edge stretchability of a punched hole:
• Improve the quality of the original cutting operation;
• Make an additional cut of higher quality; or
• Use a shaved cut and deburred edge.
Reducing punch-to-die cutting clearance can improve the edge quality of a punched hole. This results in a larger shear (cut) band, a correspondingly smaller fracture zone and small burrs forming on the backside of the hole. A larger shear band also produces a larger cold-worked zone that, combined with the small burrs, still limits maximum edge stretchability.
Another option for improving hole quality is to use a step punch. The first point diameter cuts an initial hole while the second point diameter re-cuts the hole to provide a higher-quality cut and a more precise diameter.
Many of the problems associated with step punches result from the elastic behavior of the material being punched. Remember, when punching holes using conventional punch-to-die clearance (approx. 5 to 8 percent per side), punching stress forces the hole-edge periphery outward into compression. When the slug breaks free, the compressive stresses relax and the punched hole relaxes inward toward the punch point.
The opposite occurs when applying engineered cutting clearances (approx. 9 to 20 percent per side). In this case, the punching stress pulls the material around the punch hole-edge periphery inward in tension. The hole relaxes outward (a) from the punch point when the slug breaks free.
Because the hole often changes shape during the second cutting step, problems such as chipping, wear, galling or adhesion arise in as little as a few hundred or a few thousand hits. These conditions ultimately degrade the edge quality of the hole and its ability to deform into deep extrusions.
