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A trained operator in basic electronics can save the day

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When you stand back and think about it, a CNC machine of whatever variety is nothing more than another power tool that depends on a single or multiple electrical motors to operate, and electrical motors have parts that fail, usually at most inopportune time.

A CNC machine typically has two or more types of electric motors. The first is a spindle motor that drives the tooling, and there may be more than one. The others are AC servomotors, which move the gantry or bed to precise positions during the machining process.

An AC servomotor houses a rotary encoder, a electromechanical device, that generates light or low-voltage DC electrical signals to indicate how fast the motor shaft is turning, accelerating or decelerating, for example. This feedback automatically generates very small adjustments via the servo position controller that can be made while the AC servomotor is running at any speed and in any direction.

Another major component of the servomotor system is the programmable logic controller (PLC). The servo position controller takes instructions from the PLC and converts them into step-and-direction commands, which are sent to an amplifier and then the rotary controller.

The last part of the system is the operator interface (usually a touch screen monitor) that gives specific commands to the servomotor system that in turn controls the movements of the CNC machine.

If one of these components fails, the CNC machine is out of commission. One of the best ways to limit such a failure is to keep the equipment clean, especially in hard to reach areas where motors and cabling tend to be placed. Though there are some that find regular and systematic cleaning a waste of time, and hard to manage, loss of production is a real time waster and is impossible to manage.

Another good practice is to have one or two complete servomotor systems in reserve, including a servomotor, servo position controller and amplifier. An extra touch screen monitor is also a good idea, since it is a prime candidate to fail because it is used so much.

But it’s a bad idea to start replacing AC servomotor components without investigating the cause of the failure. Therefore it is a wise investment to have at least one operator trained in basic electronics. This training is often available from a CNC manufacturer or a servomotor repair service.

Unfortunately, there is an amazing array of ways the system can fail. A multimeter is probably your best diagnostic tool. But proceed with caution.

The first step is to shut off and lock out the machine. High-voltage, 3-phase AC power is nothing to mess around with, and so is DC power in a servomotor system that can range from 5 to 90-volts.

It’s best to wait 15 minutes after locking out the machine. This provides plenty of time for the DC bus and any capacitors to discharge.

A good starting place for static checking is a visual inspection of all the components. Do a smell test for burned connectors or wires, a sure sign that something has shorted. Check that connections are tight and secure, too.

Besides a multimeter, have a MegOhm-Meter in your toolbox. Also known as a “megger”, it can generate up to 15,000 DC volts necessary to test wire insulation and motor windings for levels of resistance.

But if you don’t know what you’re doing, call a professional. Though if there is a breakdown, a properly trained operator with basic electrical problem-solving skills can have the CNC machine up and running much faster than having to wait for outside servicers. 

This article originally appeared in the February 2018 issue.

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