September 11, 2007
Continued improvements to CNC plasma cutting technology have made these units much more adaptable and user friendly. They have also helped improve consistency and cut quality.
A plasma cutting table is a workhorse in most fab shops. The technology has seen great improvements over the years, and it's worthwhile to take a look at its evolution. Not coincidentally, plasma technology's growth has coincided with technology improvements starting with the CNC.
Computer numeric control (CNC) technology was devised by a collaboration of MIT professors and associates, and refers specifically to a computer controller that reads G-code instructions and drives a machine tool. The introduction of CNC machinery radically changed manufacturing by dramatically reducing the number of steps requiring human interaction. With the increased automation of manufacturing processes brought on by CNC machining, considerable improvements in consistency and quality were achieved, and the frequency of errors (bad parts) was reduced.
Punched tape was the medium for transferring this G-code into the controller for many decades until it was superseded by floppy disks, RS-232, and, finally, Ethernet cabling and networking. As a direct link between computer and machine controller, direct numeric control (DNC) came to be an engineering catchword.
With machine and computer communicating in G-code, the only error that could occur was input error. Seeing the possibilities of reducing programmer input ambiguities, companies began to design software required for these new CNC machines. Parts processing time decreased exponentially with the advent of computer-aided design (CAD). Using CAD, the machine programmer had a method of designing complex parts without having to write individual lines of code. With the programming aspect of the model allowing finer positional points, better control of the machine's motion was then needed.
In the beginning stepper motors, which rotate in increments, or steps, were the standard in motion control technology. The smaller the steps, the more finite the control of the motor's position. Positional accuracy defines the precision with which a system can control the actual placement of the X, Y, and Z axes. Three-axis systems are the norm today, although some machines control five to seven axes of motion. Stepper motors had two distinct advantages—they were fast and they were powerful. However, stepper motor systems are open-loop, meaning there is no return, or feedback, of the actual position. Electrical or mechanical errors are not accounted for.
A servo system is a closed-loop system, meaning that in addition to the position that is output to the motor, feedback of actual position is achieved via an encoder or resolver. A controller or amplifier uses this feedback of actual position to output corrections, which maintain the desired position. Depending on the sophistication of the servo amplifier or drive system, the feedback is used to maintain position more precisely. Even at rest, a servo system is held in the desired position or location. Other innovations such as increased encoder resolutions and sampling rates promise untold accuracy.
The future of motion control lies in feedforward technology. As feedback refers to the ability of a system to gain actual positional information versus desired position from some sort of sensor, usually an encoder, and make the necessary corrections in speed, acceleration, and deceleration to maintain the desired position, feedforward technology predicts discrepancies in positional accuracy before they happen. The controller needs to be programmed with all the pertinent information of the machine mechanics. The gear tooth pitch—and even its expected wear patterns—need to be taken into account on a rack-and-pinion, X-Y-coordinate-type machine. The ball or drive screw and bearing tolerances must be calculated as well for Z-axis calibration. Tolerances are 0.005 in. to 0.015 in. Feedforward machines could achieve tolerances from 0.00010 in. to 0.002 in. with ease in the future.
Another major technology advance currently implemented and growing rapidly is open-architecture control software. In the past the software manufacturer was required to rewrite the program, which was slow and sometimes impossible. Some manufacturers today, and many in the future, will open their proprietary programming to the end user. This will allow the user to tailor a machine's functionality to a specific application, further streamlining production and eliminating extraneous operations that add no value.
These advances will help decrease the need for secondary processing. Dross formation (melted and readheared metal on the bottom of a cut) is reduced when the speed of the torch can be finely controlled. Dross generally is formed when the machine is operating at a speed either too high or too low for the material and process being used. Low-speed dross can be easily removed, but high-speed dross requires grinding. Beveling, which inhibits mating and welding, is usually offset by automatic torch height control (ATHC) in the Z axis. ATHC is commonly achieved by monitoring the voltage differential between the electrode/nozzle and ground as measured by the CNC. Ground is the return path of the electrical circuit designated as the material or plate being cut.
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