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Slug Detection Key to Die/Part Protection

By: George Keremedjiev

Wednesday, May 16, 2018
 

All too often in my consultancies and seminars I hear about dies that have crashed or parts that were rejected by a customer due to slugging in the die. Slugs have a nasty way of modifying the original die design. The slugs have no second thoughts about shearing punches, embedding themselves into cavities and acting in many other ways as efficient saboteurs. Slugs also can have a stealth effect in that they may not be severe enough to cause a die crash but rather, when embedded in a stripper, they can emboss patterns on the part that may not be detected until the customer sees them at its facility. Thus, after feed and part-out detection, slug detection comes next in importance for many dies.

The most efficient way to detect slugs is to monitor the stripper plate with digital inductive-proximity sensors. When the die closes, the stripper bottoms out. This should happen in a very predictable fashion. The stripper should land in a parallel manner on top of the strip. If the stripper does not arrive on the bottom with all four corners simultaneously entering the inductive fields of the four respective corner-monitoring proximity sensors, then we have a stripper condition. Perhaps a bolt has loosened, a spring fatigued or, believe it or not (as I have witnessed) an Allen wrench left inside the die by the toolroom has jammed itself within the motion area of the stripper. No one corner should arrive substantially before the other.

The die-protection system should stop the press at the top of its subsequent stroke if the stripper travels downward in a nonparallel fashion. If a slug is sitting on top of the strip, then the stripper will be lifted slightly by the thickness of the slug. Many shops use a rule of thumb where the slug detection is adjusted to be about half of the strip thickness. Thus, if a foreign object is at least one half of the strip thickness or greater, the press will stop at top dead center.

Typically, stampers mount four blocks, each with an inductive proximity sensor, on the lower die to monitor each stripper with slugging issues. These blocks can be rectangular, square or upside-down letter L configurations depending on the preferences of each shop. The respective sensor cables should enter channels or be contained within tubing to protect them. In some cases a fifth sensor is mounted underneath the stripper and as close to the center as possible, for slugs that may be thin and difficult to detect with the four-sensor setup.

Once one or more of the stripper sensors detect a slugging issue, the die-protection control will issue a stop at 0 deg. of the crank and its display will let the operator know what part of the stripper contains the foreign object. The operator then carefully inspects that area of the open die to find the location of the offending slug, broken pilot, chip from a punch, fallen screw, etc.

One could argue that stripper sensors can still permit a single bad hit. That’s true, but they will not permit any subsequent bad hits, depending on the speed of the press and the condition of its brakes. Most die damage occurs not during that single hit on top of a slug, but rather in subsequent press strokes as the slug travels with the strip and damages downstream components such as pilots, punches, lifters and pads.

The best way to detect minute slugs: Use analog (measuring) proximity sensors instead of the digital sensors discussed above. Whereas digital sensors require the toolroom to carefully adjust their positions, analog sensors allow the setter to dial-in the thickness of the slug to be detected. Cost is the only disadvantage of analog proximity sensors. They make no sense in applications where an inexpensive digital sensor will do the trick. Consider them only after digital options have been exhausted, or if in the initial analysis the use of analog proximity sensors makes perfect sense. Such an example is the detection of thin slugs (a few thousandths of an inch thick) at high speeds.

Slug detection is a necessary portion of a good sensor program’s portfolio. A metalforming shop not exploring this technology cheats itself out of a money-saving experience. Those that do maximize uptime while minimizing the rejection of parts due to slug marks. MF

 

See also: Tecknow Education Services, Inc.

Related Enterprise Zones: Sensing/Electronics


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