The Importance of Die Clearance in Punching Operation

May 1, 2019

Without die clearance tolerances, punching strokes would become woefully inaccurate. The equipment could still send the punch operation downwards accurately, with the tool moving as if it were on an invisible rail, but a poorly configured die-to-punch gap would then deflect the stroke before it made contact. To forestall such undesirable process results, equipment techs pay a great deal of attention to that finely positioned die clearance notch.

Talking About Clearance Buffeting Factors

There’s a number of impact-heavy forces in play between a die and punching tool. As a stroke operation commences, huge quantities of kinetic energy propagate. Heat is produced by the operation, metals expand, and the gap between the tool parts diminishes. Bits of knock-through workpiece debris further shrink tool clearance. Cooling lubricants absorb the heat and detritus, but the fluid film also carries its own set of clearance-shrinking repercussions. If repeatable punch strokes are to stay accurate for hundreds, perhaps thousands of operations, then equipment configuration procedures must factor in die clearance allowances.

What’s Happening In The Strike Zone?

Because of the above issues, an out-of-sorts die clearance configuration causes punch deflection. Supposed to be an array of perfectly defined indentations, half of those marks are ill-defined. Elsewhere, multiple operations should be striking down on the same spot. Perhaps there a cut-out to make, and that operation is being followed up by a complex countersunk strike that’ll produce a beautifully profiled sheet metal aperture. Unfortunately, because of poor tolerance die clearance problems, the different tool strikes hit half off centre and half at some weirdly incongruent angle. Again, without a properly configured die clearance, there can be no high-quality, highly repeatable equipment strikes.

Checking Out the Ejected Blanks

Left like this, both the die and the punch tool accumulate damage. Already, the holes and indentations are off-centre. Rejection rates are skyrocketing, so expensive equipment downtime is incurred until the problem is found and corrected. There’s too much heat, too many abrasive marks on the tools. Worse still, the expensive tool metals, which should last for ages, are fracturing prematurely. Calling in the services of a lead technician, this trained expert assesses the blanks. Ideally, those ejected waste slugs should look like a mirror-image of a properly configured tooling stroke.

When the slugs are covered in rough fracture planes and/or thin burring lines, the case for a die clearance problem strengthens. Burnished areas and heat discoloured regions on the slug ejection planes further demonstrate a tooling issue. Taking no chances, the repair technician decides to reconfigure the gap. While he’s at it, an adjustment to the flow of the cooling lubricant might be in order, too.

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