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In the early 1990s Brent Bowman managed a shop that, among other things, machined graphite electrodes, lights out. And it didn't run blind, either, but instead maintained quality products and long tool life. (Some tools lasted six months, machining graphite two shifts a day.) He made this happen, he says, by making the machine tool itself perform as a "virtual" operator.
During operation, the system asked itself questions about specs, cutting tools, positioning. If it found a tool was 0.002 inch out of position, it would either compensate for it or, if absolutely necessary, shut down (though this rarely happened).
The automation, he says, hinged on a small piece of technology inside the work envelope: the touch probe.
Machine Tool=CMM
As current owner of Polson, Mont.-based Hi-Tec Industrial, Bowman provides custom setups, programming and software for machine-tool-probe applications. One of the oldest concerns against these probes, he says, centers around one question: How can a machine-tool check its own work?
The concern is warranted when the machine is not calibrated properly. "If you want to inspect with [a machine tool], you have to treat it like a coordinate measuring machine [CMM]."
To use a probe on a machine tool requires four levels of calibration. Most know the first three, Bowman says, but not necessarily the fourth.
Probe. First comes calibration of the probe itself. Inside each are electronic relays that check for accuracy. "It's either 100 percent or nothing," Bowman says. "If anything is off by the slightest amount, the probe goes into a red-light condition and won't allow you to run it."
CNC Machine. Next is the calibration of the machine by using a laser to ensure "the axes are moving where they're supposed to be moving," Bowman says, and to ensure compensation between axes exists, so when the cutter or probe is programmed along all axes, it arrives at exactly the right time and space. "Angular calibration is important, too, especially on multi-axis machines," he adds. "Merely checking is not enough; updating the machine pitch error, backlash and thermal parameters is essential."
CNC and Probe. The third level involves calibrating the machine tool and probe together, including defining the electronic center line of the stylus and orienting it to the center line of the spindle so that they are one and the same. (If they are not, a calibration number is added to the calculation.)
Here also comes the calculation of the "electronic" radius of the stylus. This reveals the measure the control sees, taking into account the errors between it and the stylus—lead screws, servos, macro-processor and the like.
