Prevention vs. Correction

These three points about service and maintenance can help ensure that production time is actually productive.


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CNC machine time is precious. There are only so many productive hours available in any given shift. Keeping a machine up and running requires more than just resolving problems when they occur. Understanding the following points can ensure that production time will not be lost repairing machines.

1. Preventive maintenance often eliminates corrective maintenance. Preventive maintenance involves periodic actions to ensure that any mechanical device will continue to perform within its design parameters. For CNC machine tools, these actions commonly include lubrication replenishment, filter replacement, replacement of high-failure-rate components and inspection of critical components.

Corrective maintenance, on the other hand, is required when something breaks. If a critical component that is prone to wear (possibly a bearing) is not replaced in a timely manner, it will eventually fail. When a critical component fails, the machine cannot be used until it is repaired.

Preventive maintenance can be planned for times when machines will normally be out of production, like during lunch or other breaks, off shifts and weekends. Corrective maintenance, by comparison, is unexpected and must be done whenever a machine breaks down. This is often during production hours. If you wait for a critical component to break before taking any maintenance action, the machine will eventually require corrective maintenance, which will result in lost production time.

Machine tool builders and control manufacturers provide schedules of preventive maintenance tasks that must be performed in order to keep machines in good working order. Given the importance of CNC machine tools to the manufacturing environment, it is amazing how many companies ignore these recommendations and/or scrimp on preventive maintenance efforts.

2. Operation errors are the largest causes of corrective maintenance. Given our current level of CNC technology, few machine components will fail for reasons that are not addressed by preventive maintenance schedules. If you perform preventive maintenance at recommended intervals, about the only thing that will cause the need for corrective maintenance is operation errors that cause broken components (machine crashes). 

While mistakes may be inevitable, you must strive to eliminate them. Every mishap should be investigated for the purpose of developing a way to keep it from happening again. This is especially true for repeated mistakes. 

Perhaps you find that a given task is overly complicated and operators are prone to making mistakes when performing it. In this case, you must come up with a way to simplify the task, which will lower the skill level required to perform it. If you already consider the mishap-causing task to be pretty simple, then you must provide better training or documentation to ensure that everyone knows, or can find out, how to perform it. Either way, you must take an action to eliminate future occurrences of the mistake. Without such an action, the mistake is sure to be repeated.

3. Planning and preparation minimizes corrective maintenance. It should go without saying that your company should maintain a stockpile of supplies needed to perform recommended preventive maintenance tasks. Lubrication oils, coolant and filters, among others, should be readily available when scheduled maintenance begins. And again, preventive maintenance should be scheduled for times when a machine would normally be down, or not required, so as not to interfere with production.

While certain parts, like spindle bearings, may be considered too expensive to stock specifically for preventive maintenance purposes, if you perform regular inspections as part of your preventive maintenance program, you should know long enough in advance to acquire a replacement for a faltering component before it fails. Remember, failure of a component often has a cascading effect. One component failure may cause another and another. So again, replace a faltering critical component before it fails, as part of your preventive maintenance schedule.

One often overlooked preparation step is maintaining adequate backups for CNC data. It’s likely you have backups for your CNC programs, but do you have a backup of parameters, pitch error compensation data, offset settings (if applicable) and, very importantly, programmable machine controller (PMC) ladder logic? Current CNCs make it pretty easy to create these backups. Don’t ignore them. 

Some companies make quarterly or even monthly backups of all data. This may be excessive if you rarely change the related data. The PMC ladder logic, for example, is almost never changed, meaning that, for a given machine, it is probably in the same state as it was when the machine was first installed. And a new pitch error compensation data backup should only be required when a machine's ballscrews and axis systems are recalibrated. Parameter data, on the other hand may be changed on a more regular basis.

In general, preventive maintenance can eliminate or at least lessen the need for corrective maintenance and its impact on productivity.