Hardened carbide steels, all extensively heat treated, last for thousands of punching operations. Even equipped with dense material backbones, however, punch breakage is an unavoidable problem. To some extent, operator error is a known causative factor. The shut height is incorrectly set, the wrong strip-starting procedure is utilized, or some other operator-generated blunder is ongoing. Next, process clearance distances have been known to cause punch fractures.
Process-Ingrained Clearance Errors
In many machine workshops, a set stamping distance is used across-the-board in every cutting application. For mild steels, expect 10% of clearance, as matched against the thickness of the steel. For aluminium, that value drops to 8% while harder steels rely on a 12% punch-to-contact zone clearance gap. If the punch accelerates downwards and strikes an overly hardened alloy, the punch will break. As a rule of thumb, the above punch clearance distances are generally correct, at least until a harder metal makes a mockery of those fixed percentages. Avoid punch breakage here by fine-tuning equipment clearance.
Error-Driven Punch Shearing Forces
A simple centre punch drops down and focuses kinetic energy. If the punch geometry is irregular, those forces will arrive asynchronously. The propagating energies explode along intergranular stress zones, then the punch cracks. When forming the tool edge, concentric geometries are favoured over angular offsets. In other words, the punch profile should originate at the centre of the tool’s tip and move synchronously outwards so that the shear angle is minimized. Furthermore, the equipment-fitted backing plates and retainers must absorb and direct those propagating energies, especially when the punch is processing a harder alloy.
Press Deflection Problems
Counterintuitively, finely engineered punch tips, loaded with intricate geometrical details, are driven by powerful press machines. If the machinery isn’t properly aligned, perhaps because the press piston isn’t parallel to the bolster section, then the equipment stroke will generate a deflection rate that can’t be handled by the equipment’s leading edge, the punch tip. All of that poorly directed energy, it ends up tearing the punch material apart. By maintaining the press and aligning the various press sections correctly, the deflection rate is kept safely low and manageable.
Adding insult to injury, all of these issues become that much harder to manage when the punching equipment employs a substandard bridge stripping mechanism. The supplementary tool does clear scrap metal tailings from the punch, but it also introduces yet another potential punch breakage catalyst. If the stripper absorbs great quantities of impact energy, the punch will break. If the stripper isn’t lined up, then even a slight discrepancy in the shearing angle or defection rate will, yet again, result in a fractured punch.