FINISHING TOUCH: INTEGRATED PLASMA CUTTING
By Jim Colt
As the next natural progression in the evolution of mechanized plasma cutting, many shops are changing the roles of their programmers and the operators by switching to integrated CNC plasma cutting systems that draw more expertise from the CAM database to achieve cut consistency, increased productivity and lower cutting costs.
THE EVOLUTION OF SOLUTIONS
Early mechanized plasma cutting systems were accepted on the industrial shop floor for a couple of reasons. First of all, they cut metal very fast (productivity). Second, they could cut non-ferrous (stainless and aluminum) material better than that other technology (oxy-fuel).
The relative time frame for the commercial acceptance and use of plasma cutting equipment was the early to mid-1960s, when the technology was still in its infancy. The cut quality (by current standards) was horrible, operating cost was very high due to very short consumable parts life, inefficient power supplies and lots of secondary operations (grinding and rework) that were required.
But the process was fast and, man, it could really cut those non-ferrous materials!
Engineering was the limiting factor in the early days of plasma cutting. Making the process actually operate at all during this stage of product design required some great control of high temperature physics inside the plasma torch. A plasma arc was developed inside the plasma torch using DC power at up to 1000 amps, using nitrogen as the plasma forming gas. This high energy arc achieved temperatures exceeding 40,000 deg F, yet was forced through a copper nozzle orifice to control its shape, velocity and energy density.
The very fact that a 40,000 deg F plasma arc passes through a small copper orifice (copper melts at around 1,985 degrees F) and the copper nozzle does not immediately melt is a phenomena that I think about every time a plasma torch fires and cuts metal!
Perhaps the biggest hurdles back in the 1960s with regards to improving all facets of plasma cutting system performance were directly related to the important control factors that affect the process, including efficient cooling of the torch components, accurate gas flow and timing, pure DC power supply output, accurate control of DC current, torch height control and X-Y cutting motion control.
Control of timing and parameters within these systems relied on early pre-computer and microprocessor systems that depended on relay logic, lots of wiring and fixed timing circuitry. Although that was very high tech back in the day, it is extremely crude by modern standards.
Over the years plasma cutting technology evolved based on available electronics control technology. This included timing circuitry that was more advanced, which was later replaced with microprocessor-controlled circuitry that could be hard programmed, which moved on to today’s PC and microprocessor-controlled plasma and CNC machines that are software controlled.
The newest PC-based CNC machines can control every plasma cutting function, resulting in dramatically longer consumable life (more than 10X the life of early plasma) as well as extremely nice cut quality and faster cutting speeds.
Up until just a few years ago, however, there were still some missing links that needed to be developed and improved to further refine the mechanized plasma cutting processes.
For example, if you carefully watch many CNC plasma machines on the shop floor, you will still notice a fair amount of wasted time between cuts. You will also become aware of how the operator spends a lot of time punching keys on the machine control panel. Or observe the automated torch height control system and watch the agonizingly slow cut-to-cut cycle times that are based on the retract and plate sensing steps required between each cut.
In reality, an incredible amount of time can be saved in making all of the major components of a CNC plasma cutting machine operate in a more efficient manner to further improve shop floor productivity – and ultimately lower its operating costs. The major components of an industrial CNC plasma machine are:
The CNC control. State-of-the-art is a PC-based control with an intuitive touch screen operator interface and the ability to digitally interface to all of the other major control systems listed below.
The Plasma system. The best technology is a high definition class plasma with automated gas flow system and a digital interface linking its control functions directly from the CNC control.
Torch height control. This is an often overlooked but highly important component for plasma cutting. Again, state-of-the-art is a height control that intimately communicates with the CNC control through a digital interface. For much deeper technical information on this function, click on Torch Height Control for Automated Plasma Cutting Applications“ (Slice of Advice, 4Q 2010).
Motion control drive systems. Properly engineered and inertia matched to the mass of the machine for best control of following error as well as the ability to meet the plasma systems requirements for constancy of speed and high acceleration rates. The best technology drives are digitally or optically linked to the CNC control.
CAM software. The latest technology, CAM (formerly referred to as nesting software), contains large databases with all of the expert information necessary to control the processes installed on the cutting machine. This data takes the expertise and guesswork out of the operators hands and makes the cutting machine perform with consistency for day to day, operator to operator performance. For a thorough analysis of the CAM function in plasma cutting operations, click on “State of the Art CNC Plasma Cutting Software” (Slice of Advice, 1Q 2012).
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