Maintaining the balance

There’s little doubt that preventive maintenance is a far better option than waiting for something to break down, but what technologies are available and how should they be used? Martin Oakham investigates.

Today’s CNC machine tools have extremely fast spindle speeds, feeds and rapid traverse rates, yet must also achieve high positioning accuracy and maintain tolerances throughout a production run. Such demands are of course the driving force behind the current trend to implement preventive maintenance (PM) programmes on the shop floor.

Preventive maintenance, by definition, is a schedule of planned actions designed to preserve and enhance equipment reliability in order to prevent breakdowns. Therefore, in a production engineering environment, the chief objective of PM and any similar programmes, such as Reliability and Maintainability (R&M), Machine Tool Variability Management System (MTVMS), and Total Productive Maintenance (TPM), is to predict when a machine will fail or go out of tolerance to reduce unplanned downtime, particularly at critical production periods.

However, the key misconception about PM is that it’s unduly costly, and regularly scheduled downtime and maintenance is more expensive than waiting until repair is absolutely necessary. While this may be true for some equipment, it doesn’t take account of the long-term benefits and savings associated with PM.

for example, without preventive maintenance, costs for lost production from unscheduled equipment breakdown will be incurred. Also, PM will result in savings due to an increase of effective service life. Bearing these facts in mind, it’s reasonably safe to say that the PM route is a viable one.

Although programmes are designed to reduce downtime, extensive diagnostic procedures can take machines out of service for longer periods than some companies can tolerate. Therefore, new ‘quick-check’ techniques are being integrated with PM programmes to cut downtime to the minimum. Using laser calibration equipment, volumetric accuracy can be checked without having to remove covers or other components of a machine tool.

In short, the ‘quick-check’ technique allows volumetric accuracy of a machine to be evaluated with diagonal displacement measurements. The laser beam is directed from corner to corner of the work zone. As the machine executes a test movement pattern, the instrument reveals errors in squareness, as well as in linear position, reversal, pitch and yaw.

There are two different types of laser-based measurement systems – the laser interferometer and the Doppler system. the major difference between the two is that a Doppler requires only two optics – a laser head and a retro reflector. A laser interferometer requires three optics, one of which (the laser head) must be mounted on a tripod outside the machine tool. So the Doppler system is much faster to set up, and has the advantage of automatic data collection, further reducing measurement time.

In a typical PM programme, each CNC machine tool is checked bi-annually for accuracy and vibration characteristics. An analysis is made by comparing the baseline data, past data and new data. A prediction is then made for when the machine tool should be calibrated and serviced.

In this way corrective actions can be scheduled at a time not critical to production. For example, a machine tool that has to maintain a tolerance of say 0.005mm (5 microns) is checked every six months. At each of the last three inspections, the tolerance has been off by 0.0005mm (half a micron). Based on the historical information, a prediction can be made that, although the tool has appeared to remain stable, it will still need to be calibrated within a year. Similarly, vibration analysis can reveal much about the condition of the spindle and other critical components.

In an actual working example, aluminium manifold production at Nissan’s Sunderland site, operating 24 hours a day, five days a week, required frequent component checks and a full quality check three times a day. Any detected problems required the services of an experienced technician to determine suitable tool offsets. As a result downtime was considered unacceptable. The solution came from implementing a PM programme to continually determine current production capability of individual machines. Renishaw’s QC10 ballbar system was used to perform regular machine ‘health checks’, and its ML10 laser system for more extensive accuracy assessment.

The results of these tests are used by Nissan to set all production machines to a ‘nominal’ level. Using this method, if the control programme remains at the correct setting, all components produced will be within tolerance.

As part of the programme, operators have a series of documented guidelines to follow when they detect a problem with their machine. Implementation of the guidelines will return the machine to its ‘nominal’ position. All machines are now routinely checked and re-calibrated every three months using both QC10 and ML10, allowing preventive maintenance to be scheduled while historical records are analysed to determine a machine’s remaining operational life to plan for refurbishment and replacement.

It is likely that in future engineers will become more concerned with total machinery monitoring, combining process engineering and maintenance concerns. This is perhaps a return to days when a ‘Mike the mechanic’ character would take care of the machines. He was an expert in all aspects of maintenance and the machine’s processes.

So despite recent technical specialisation and the divisions of plant management that have separated these functions, tomorrow’s process engineers and maintenance managers will be one and the same. They will possess the tools, knowledge, and experience to deal with total machinery health and process efficiency.

Emphasis will be placed on engineers/managers who understand the overall impact of maintenance decisions on production, and the role that output plays in profit. The trend toward plant-wide management will place even greater emphasis on the integration of process engineering and maintenance management.

In addition, future machine tools will feature on-line calibration systems. These will automatically take measurements and correct error, while the tool is in production. The data will be automatically collected and charted for later analysis to determine the tool’s condition and to predict future maintenance requirements. On-line calibration will assure consistent and error-free machine tool performance.