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Most CNC retrofit service provides say, up-front, that a new control will not turn into a magic bullet that solves the mechanical problems of an aging machine tool. A new control won't push spindles any faster or solve myriad mechanical problems that crop up on older equipment.

True, AC digital technology, far superior to the old DC systems, has done a lot for machines within the past decade, adding unforeseen encoder-feedback technology and processing speed. Just four years ago, many controls had encoder count rates of approximately 64,000 per revolution. "Now, we can get to 16 million counts per revolution," says Pat Harrington, vice president of CNC Engineering, Enfield, Conn. "It increases the resolution of the system and also acts as a tachometer replacement, so you actually get better motion control."

Nevertheless, a new control won't solve every problem, Harrington says—and neither will new drives and motors. "If you have a motor-drive situation that allows you to go 200 inches a minute, and I put a system on that allows 800 inches a minute, it doesn't mean the mechanical system is capable of achieving it."

However, there are some mechanical problems a new control can indeed help fix. Whether to buy a new machine or upgrade existing equipment depends on many factors, and on the specific application. But, says Harrington, knowing the complete capabilities of newer controls can aid the decision—including how that control affects a machine's mechanical capabilities.

Compensation Tables

What have become rote tools in industries like aerospace, compensation tables in newer controls account for backlash and other errors. This represents one case where sophisticated electronics can improve a machine's mechanical accuracy.

With ball-screw error, for instance, "you can take some of the error out, utilizing pitch-error compensation, and error-plot it into the control," Harrington says.

Using a laser, a technician commands a position; then it is read and compared with true machine positions as measured by the laser. For example, a technician commands a laser to take readings down a machine-tool's 20-inch slide, 0.001 inch at a time. The laser measures each commanded position in one or both directions. An error plot comparing what the control expects with the actual measurement is stored within the control's memory.

The technician can then perform "another complete run, too," Harrington says, "essentially correcting the correction table," so control-compensation accuracy can be made as accurate as possible.

With more sophisticated equipment and high-end controls, such error compensation tables can be made in three dimensions. Harrington says, measuring on two different planes enables correction using control options such as straightness compensation for minor correction of pitch and yaw.

Linear Scales

Linear scales, Harrington says, represent another electronic enhancement that can have a direct effect on a machine's mechanical accuracy, taking advantage of the soaring feedback rates of new controllers. Adding a scale can remove/improve backlash and, in critical-tolerance applications like grinding and other operations, may help control/compensate thermal growth of the ball screw and other machine elements.

The linear scale takes what the encoder on the motor does one step further, ensuring that the control receives measurements of "actual movements happening on the slide" and not what the motor's encoder sees.

The motor encoder could be called near-sighted. It receives position feedback, but filtered, or "blurred," through mechanical drive-train elements between it and what's happening on the slide. First, the motor sees the turning of the motor, and the motor directly coupled (at least in the ideal situation) to the ball screw.

In a worst-case situation, the encoder could receive readings that have gone through belt systems with multiple ratios. This translates to slippage in a belt or backlash with a gearbox.

"It depends on what type of system you have," Harrington says. Yet with a linear scale, "even if you go through a gearbox or belt system, your actual position feedback is going to be from what the slide does."

So in this case, improved electronics can improve the machine's mechanical properties.

Editor's Note: Artwork courtesy of CNC Engineering. For more information, visit http://www.cnc1.com/ or write 914 on the Free Product Info card.

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