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Revitalizing a Used Mechanical Power Press, Part 4: Die Protection

By: George Keremedjiev

Sunday, May 01, 2011
 

This fourth installment of the series on revitalizing a used mechanical power press focuses on the inseparable relationship between press and die—an often-misunderstood relationship that can lead to inadequate use of die protection. To quote a plant manager with whom I recently chatted: “I come from the plastic injection-molding world where perfect setups are the norm. Therefore, I expect my pressroom to be the same.”

One could not be further from the truth when comparing injection molding with metal stamping. The variables in metalforming are much more complex and unpredictable than those in a typical injection-molding operation. For one, a progressive die typically running unattended in a mechanical power press must endure the following and seemingly random occurrences within even a seemingly perfect die setup:

• The sheetmetal strip entering the die will randomly, throughout its coil length, vary in thickness, hardness, camber and edge burring. These variances may result in the strip being improperly fed or binding within the stock guides on the die.

• Pilot-release timing is not properly set nor calculated, causing the pilots to struggle to precisely locate the strip before the die closes, resulting in part-quality issues, premature pilot wear, punch shearing, etc.

• Air pressure in the shop fluctuates as several air-driven machines and processes increase or decrease their air consumption, leading to an occasional part-ejection problem, double hits or partially trimmed parts.

• During die assembly, toolmakers may over- or undertighten critical bolts that, over time, leads to a section in the upper die dropping down onto the strip and causing a major crash.

• The purchasing department, in its desire to save money and without notifying anyone else in the shop, decides to replace the bolts the company has been buying for years with a lower-priced and inferior brand. Results are the same as those noted above.

• Lubrication mechanisms used for spraying, layering, misting, rolling or dripping can, over time, become clogged or may deposit blobs of lubricant onto the strip or into the die. Clogged lines also can lead to inconsistent lube composition and viscosity.

All of the aforementioned process inconsistencies (among others) can lead to a die crash and, in some cases, severe press damage. No amount of nervous hovering over the process by the press operator can catch, much less react to, these types of surprises. Therefore, metalformers must enact a series of internal safeguards within their dies to detect these problems before they lead to crashes.

Even with perfect die assemblies and press setups, there exists within all press shops the possibility of human errors. I therefore suggest that when purchasing a used mechanical power press, stampers look for the following die-protection capabilities:

• A minimum of 16 (ideally 32) sensor inputs on the die-protection control. Otherwise, look for upgrade capability.

• Resolver and control logic should be from a reputable vendor available to troubleshoot, audit, rebuild and service the system.

• Tonnage monitoring, to patrol the forces being exerted and protect the press.

At the same time:

• Shy away from home-brewed die-protection mystery boxes and systems that may have survived beyond their inventors.

• Never fully trust the die-protection system until it’s been tested under actual working conditions. Ask a qualified electrician to check the wiring, with a keen eye tuned any bypassing or tampering of the sensor-monitoring and press-stopping circuitry. He may, for example, discover a bypass switch cleverly installed to cause a sensor channel to be ignored.

The intertwining of the monitoring of internal die activities with the down-and-up motion of the press ram and its rotating crank must be monitored carefully to avoid the costly issues of die repair, premature die maintenance, part quality and rework, and press repairs. When purchasing a used mechanical power press, be sure to check for the above-noted die-protection requirements, to ensure that your dies will run within a relatively modern press. MF

 


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