Enhancing Safety Through Preventive Maintenance

The primary reason companies perform preventive maintenance on their production equipment is to minimize, if not eliminate, unpredicted downtime. Potential problems should be corrected before they cause machine components to fail.


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Most manufacturers would agree that if you’re doing your preventive maintenance properly, your machines should never go down for corrective maintenance—at least, not due to a component failure. Again, if preventive maintenance is done properly, faltering components will be found before they fail.

There is a second benefit, this one safety-related, to properly maintaining your machines. Well-maintained machines are safe machines to run. Keeping machines properly maintained provides a safe environment for your setup people and operators.

An obvious comparison can be made to operating an automobile. Think of all the components in a car that impact your safety. You surely wouldn’t drive a car if you suspected problems with the brakes, if it had bald tires or if warning lights were illuminated on the dashboard. Yet, I’ve seen companies that allow CNC machines to continue running, even if they have known safety-related maintenance issues. Here are a few examples:

• Failed indicator lights—Most CNC machines have all kinds of indicator lights. Many are related to control panel buttons and switches. When a button is pressed, a light comes on (or goes off) to indicate a function’s condition. If this kind of indicator light is burnt out, of course, the operator won’t be able to tell whether or not the function is activated. Other lights may be related to maintenance functions. This kind of indicator light will come on, for example, when the way-lube level is low. If burnt out, the operator will not replenish way lube, which will cause the need for corrective maintenance.

• Failed (or disabled) safety interfaces—Almost all CNC machines have built-in safety interfaces to ensure that the machine will not run when something is wrong. The most common is the door interlock. The machine will not run when the door is open. I’ve seen companies that disengage this interlock—sometimes temporarily—so the setup person can see inside the machine’s work area more clearly. This places operators in a dangerous situation if the door interlock isn’t engaged once the setup is completed.

• Running with known problems—Many companies continue using machines even when they know something is wrong. The intention is to fix the problem eventually, but the current job is so hot that they feel the machine cannot be stopped.

Another possible reason for running with known problems is that the needed component might not be available yet, so the company hopes the machine can limp along until it arrives. This may be acceptable if there are no safety-related issues, and if there is no possibility of further damaging the machine. However, I’ve seen very questionable decisions in this regard. In one company, the toolchanger mechanism was intermittently dropping tools from its magazine. A person was actually assigned to stand under the magazine to try to catch falling tools in a padded box—a very dangerous assignment.

• Failure to perform preventive maintenance tasks—If any needed preventive maintenance task isn’t done, something on the machine will eventually fail. Consider the safety-related implications of random failing components. People will be in a constant state of danger. Again, compare this to driving a car. If you never perform maintenance on the car (replacing fluids, filters, tires, brakes and others), something is eventually going to fail. Would you want to be the one driving the car when it does? While you’d never consider treating a car in this manner, I’m amazed at how many companies have a “run it ‘till it breaks” attitude about their CNC machines.

Remember that preventive maintenance requires more than simply replacing filters and replenishing oils. All machine tool builders provide—usually in their operation manuals—a complete list of preventive maintenance tasks as well as their recommended frequency. These tasks include inspections of components (such as bearings, ways and ballscrews) that are most prone to wear and failure.