Dull punches and other troublemakers in metal stamping

Modern Machine Shop, Oct, 1990 by R.J. Rizzo

Dull Punches And Other Troublemakers In Metal Stamping

When the cutting members of a steel die are sharpened, the tool usually produces clean, burr-free stampings which are also dimensionally acceptable. But later, the die is often allowed to run longer than the optimum number of strokes prescribed for it, and as a consequence, certain tell-tale symptoms begin to appear which may indicate trouble. Some of these and other problems will be discussed.

Since the decision to shut a job down or keep it running depends primarily upon economic considerations, the operation is usually permitted to continue. Because of this, the quality of the product suffers, and the die will require abnormal refurbishing to restore it. Sometimes a dull die causes the sound, or "tone", of the press to change. This signal is sufficient to prompt an alert operator to stop the press. If undecided about what action to take, he then calls for help. At this point, the stampings are inspected visually for burrs and blemishes which indicate dull or chipped punches, and so on. If this is not conclusive, the strip is then pulled out of the die, a segment cut off and inspected for mis-feeds, or, if the die is being run wet, for slugs which may have backed up onto the die face, stuck under the strip and carried along by it. Should any one of these conditions exist, the cause must be identified because, allowed to continue, a jam-up could result which could break a punch, a pilot or even a die section.

If a strip hesitates to move smoothly through the die, a different problem may be present. Since the raw material being stamped is purchased from a mill, it should be inspected along 12-foot lengths for possible edge, surface and lateral wave defects, and also for sporadic oversize variations in thickness and width. Anyone of these could cause the strip to mis-feed, jam at a cutting station and damage the die.

When the stock being cut is of a "chalky" nature, a cutting member may show signs of galling or metal pickup--or slugs could be packing in a die section. Any of these should be corrected before proceeding. If the width of the web separating successive blanks in the strip skeleton fluctuates, the feed should be checked accurately and adjusted to precise pitch. Cumulative errors in strip movement could cause punches to make partial cuts which, in turn, could deflect and shear them, shortening their life.

A frequent check should also be made for excessive burring in the stamping itself and in the strip skeleton, especially at the sharper corners of the contour. Since punch wear is exponential, these points wear fastest, causing the clearance between punch and die to increase and generate troublesome "spikes", which could impede strip movement.

Dull cutting members could also affect the desired "break" in the edges of a stamping. In extreme cases, edges could even roll-over a bit, prevent the strip from advancing freely over the face of the die, and jam. Even a casual inspection of today's products will invariably show signs of dull dies and related defects. These are apparent as burrs and sharp edges, especially in many automotive stampings and, in some of our household items such as can openers, toasters, and ranges. An adage of mine warns, "Repeat an aberration enough times, and soon it will begin to appear normal and proper." This means that today, instead of correcting offensive tooling, more and more manufacturers continue business as usual--or they redesign their products to eliminate stampings altogether in favor of molded plastics--a change which does not always cure the problem.

It is obvious that many of the problems discussed are the result of dull cutting members, production over-runs, oversights or economic considerations. These, in turn, cause costly tool damage, downtime and unnecessary repairs which could be prevented by adherence to conservative operating schedules and wise maintenance procedures.

R. J. RIZZO, M.E., P.E., Consulting Engineer 400 Kingston Drive, Ridge, NY 11961