Tooling by Design



Hole Extrusions--Part 2

Tuesday, November 1, 2011

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.

 Fig. 1—Proper hole preparation
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.

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.

A Three-Step Process

Fig. 2—A hollow-ground shave punch
For maximum extrusion heights, the best solution is to create the hole in three singular steps: punch, shave and coin the bottom side prior to forming the extrusion (Fig. 1). For most materials, the amount of material to be removed by shaving usually equals 10 percent of material thickness. This provides good surface finish and leaves enough material in the scrap ring for slug retention. This is important, because slug retention generally is considered the most common problem associated with shaving operations.

Shaving really is more of a machining operation than a punching operation. A properly shaved hole will induce less cold working on the edges than will a tight-clearance punched hole.

To promote the cutting action, consider adding a back-rake angle on the shave punch face. This would be similar to the rake angle found on a machine cutting tool. As an example, see Fig. 2 of a hollow-ground shave punch (courtesy David Smith and Associates). This modification can improve surface-finish quality and extend punch life.

After the hole has been shaved, coin a small (0.020-in. dia.) radius on the bottom side to compress any burrs that may serve as stress risers.

Now that the hole is properly prepared, it’s ready to be extruded. Next month we’ll examine some proven extrusion-punch designs for forming extruded holes in stampings. MF


Related Enterprise Zones: Tool & Die

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