What is a J-Hook in Punch and How Can It Be Prevented?

October 18, 2019

A J-hook is a type of punch deformity, one that occurs due to tool wear. It’s a shape, a wear pattern that appears as an inward-curling burr of punch tip metal. Left like this, the tool will pick and snag at an opening as it performs its full stroke. If things go downhill from here, premature tool aging and blank releasing headaches become inevitable. Fortunately, a prevention strategy is available.

Signs of J-Hook Trouble Are Growing

It’s the way the punch stroke is causing tool downtime. The sheet metal is moving, the stroke is engaging, but the blanks aren’t falling cleanly away. There are burrs on the openings after the ejection phase, too. A series of strange nicks seems to be growing. Then there’s a sticking effect taking its toll on the tool armature. After an inspector checks out the troublemaking punch, the culprit is confirmed. Due to tool wear, a tiny spur of curling metal is interfering with the equipment’s performance. Just by running a finger along the once polished tool edges, the cutting snag can be felt. Not that this kind of physically conducted testing is advised, not when there’s a microscope available to view a J-Hook defect. Anyway, it’s there, in living colour, and the wear pattern is going to cause more trouble if it’s left to its own devices.

How to Polish-Erase Punch Defects

A prevention strategy must be implemented before the wear pattern ends up compromising the punch. The tool isn’t going to fix itself, not with this snagging curl of metal picking and hooking its way through what should be a series of geometrically precise punch and die-applied apertures. On taking the machinery out of service, temporarily, of course, the tip is exposed to the fine-grit abrasive action of a polishing stone. However, what if the problem reoccurs? A trained repair technician won’t keep on polishing a punch tip, not if the issue is just going to rear its ugly head all over again. There must be a problem somewhere else in the tool station, a fault that’s causing excessive J-Hook failures. As a rule, the punch guides should be checked for wear.

Wear and fatigue, that’s the system troublemaker that causes most problems in a punch and die machine. The compressive forces coursing through the gear will eventually produce a life-foreshortening defect if those forces become unbalanced. The punch guides or die driving rods loosen or wear away. Consequently, the fatigue resolves into a material-deteriorating effect, which is what inspectors eventually see as J-Hook tip wear. A potent predictive maintenance program should be used to detect such tell-tale issues before they make their way to the tip of a punch tool.

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