However, while a PM program provides many advantages over a purely reactive program and will decrease the number and frequency of failures, it will not necessarily prevent catastrophic failures. There also are adverse consequences of PM—over-maintenance, for example. Here, a shop performs unnecessary or ineffective maintenance. For example, consider a PM schedule that requires changing or sharpening of punches every 50,000 hits, regardless of the condition of the punch point.

Conversely, shops may under-maintain their equipment when following a strict PM regimen. Here, failure conditions may exist but remain unidentified and uncorrected in a timely manner. For example, consider stamping a material with yield strength at the high end of its acceptable limits, which requires punches to be changed more frequently than is prescribed by the PM schedule.

Predictive maintenance (PdM) improves on PM by using actual equipment performance data to determine when maintenance should occur. Shops schedule PdM tasks based on gauging deteriorating operating conditions or rate-of-decay. With this strategy, periodic or continuous monitoring detects the onset of wear or degradation. Technicians use this information to predict potential problems and schedule the required maintenance.

An example: installing force transducers behind punches to monitor punching forces. As punch points wear, the force required to produce the required hole increases. A critical force, established for each punch, can be used to signal the press technician to sharpen or replace a punch or series of punches.

PdM and PM represent very different strategies, even though the two terms often are used interchangeably. PM plans prescribe time-based activities (number of cycles, number of hours, etc.), while a PdM plan comprises event-based activities determined through the acquisition of data (force, temperature, vibration frequency, signature analysis, etc.).

Reliability-centered maintenance (RCM), or proactive maintenance, is an extension of PdM that seeks to improve process performance in addition to maintaining the equipment. RCM practices focus on determining the root causes of maintenance failures and dealing with those issues before problems occur. Shops might even perform maintenance on healthy equipment, to improve performance and save or make more money. Rigorous use of proactive maintenance can actually make maintenance profitable. Here’s an example:

The press shop’s PM schedule requires changing or sharpening punches every 50,000 hits. It undertakes process improvements to improve punch life and reduce tool-maintenance costs. It finds that by investing in higher-quality powdered-metal tool steel and by applying an engineered surface coating, punch life increases to 100,000 hits between maintenance.

Pretty good, but by practicing RCM the shop can do better. For example, design of experiments and root-cause analysis might reveal that most of the punch wear occurs due to excessive tipping moments, caused by large clearances in the press connections and slide-guiding system. Maintenance on the press restores the machine clearances to factory specifications, reducing slide tipping to within acceptable limits and, in turn, reducing punch wear. As a further cost savings, the toolroom replaces the expensive powdered-metal tool-steel punches with the original high-speed-steel punches, and eliminates the added surface coating. At the same time it maintains 200,000 hits between maintenance.

To learn more about tooling and press maintenance, attend PMA’s Die Maintenance and Press Maintenance seminars in Nashville, TN, May 3-5, 2016. To register, visit: www.pma.org/meetings. MF

Technologies: Tooling

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