Slug Pulling in a Punching Operation: Causes and SolutionsJuly 2, 2019
Slug pulling occurs when a “slug” gets stuck to a punch as it’s pulling back from a strike. The tool pierced a panel of sheet metal, the slug (punched scrap) separated from the sheet, but it hasn’t dropped free. In lieu of the expected scrap removal action, the ejected waste clings to the tool. This is slug pulling, a process glitch that can, if left untreated, cripple a punch machine.
What Causes Slug Pulling?
A number of waste trapping factors exist. They’re all potential sources of ejecta ensnaring concern. One source of trouble is found in tools that use complex geometries. The slugs eject with tiny voids, which create a vacuum suction effect. Next, cutting clearance inconsistencies cause slugs to compress and release in ways that can cause slug pulling problems. The piercing/withdrawal process generates “spring-release” energy, which manifests as a matrix obstructing effect. And these are just the familiar punch-effected causative factors. Overly lubricated cutting zones cause true stickiness problems. If that oily stuff is overly viscous or full of metal cuttings, equipment damaging slug pulls become that much more likely.
Don’t Give Slugs a Chance
Keep high-tolerance punch clearances in place by adding this issue to a preventative maintenance plan. Make sure operators know that cutting clearance inconsistencies can cause slug pulling problems. For the lubricant sticking headache, the oily fluid should run thin and without contaminants. A small vent aperture in the centre of the punch will clear up that vacuum sticking effect, by the way. At last, things are running smoothly, but there’s a few other waste attracting headaches to deal with before putting this matter to rest. Magnetic fields, which appear as actual metal attracting forces, can get into the process. In this case, the tool edges have been sharpened, perhaps by a surface grinder. Supported by a magnetized mount, the fields transfer to the ferrous components of the punch, where they take up residence.
Magnetic fields draw ferrous materials together. If the feed metal is ferrous and slightly magnetic, a demagnetizing service will need to be carried out after the tool sharpening work concludes. Oily stickiness or vacuum suction, magnetic clinginess or cut clearance inconsistency, these issues cannot be ignored. If they are, well, larger problems are imminent. Fast-approaching, a single slug pulling incident falls free of the punch phase after the tool withdraws. An expensive punch fractures and breaks because the waste metal has doubled-up the thickness of the cutting zone. Alternatively, the slug gets carried through to another station, where it destroys a key component. Please, if any of these red flags are waving, bring the issue to the notice of the floor foreman.
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