The first option is to have a plasma cutting and CNC programming expert on staff who can optimize parameters for every step of every cutting routine.
The second option is to use an integrated system and a CNC controller with the intelligence of a plasma cutting and programming expert built within it, along with a suite of software tools that enable machine operations to optimize consumables life with point and click simplicity.
For making heavier cuts, such as in 1-in.-mild steel and thicker, new consumables technology can also increase arc starts to 900 at 400 amps or 1,500 at 300 amps per set of consumables (cuts were 11 seconds in duration). In any event, a quick review of the causes of consumables wear is in order.
Why consumables wear
The primary causes of electrode wear are the number of arc starts and stops, cutting time, material thickness and poor gas flow and/or quality.
The tip of an automated plasma electrode features an insert made from hafnium, which forms a molten puddle when the arc is in operation. The wear mechanisms of hafnium are directly related to its molten state. Imagine a spinning bucket of water; spin it quickly enough, and some of the water spills over the edge of the bucket. This basically represents what happens with molten hafnium. Fortunately, manufacturers can control hafnium wear by controlling the gas and current around the plasma arc.
While O2 plasma gas provides weld-ready precision cuts, starting the arc in a pure oxygen environment is quite oxidizing. It creates an excessively thick oxide layer on the electrode that eats into the electrode body and causes premature wear. Regardless of the plasma gas for cutting, most advanced cutting systems use less oxidizing gases, such as nitrogen or air, during arc start to extend electrode life.
Ramping the current and gas in a coordinated manner also minimizes the thermal shock. This is like slowly opening the floodgates of a dam instead of waiting until the dam bursts. A gradual rise (a time still measured in milliseconds) is far superior to instantly putting hundreds of amps through a cold electrode with layers of oxide and other components left over from the previous cut. Instead of the layer melting slowly, the instant energy rise causes the layer to crack and disintegrate, leading to premature electrode wear.
Controlling torch height during piercing is an especially critical component of extending consumables life. Note that a responsive torch lifter paired with a computer control is essential to preserve consumables life and ensure cut quality. Controlling torch height at various phases of piercing a 1-in.-thick, mild-steel plate provides a good example.
The ideal starting torch height will be about 0.5 in. The CNC will start the power source痴 pilot arc to fire and ramp current to ionize the plasma gas. Next, the torch goes through a pierce retract or elevation height adjustment. The system holds that height until the plasma stream pierces the metal (a pierce delay). The torch then drops to its cutting height. See our video.
If at any point during this process the torch height is off by a millimeter or the current and gas ramp timing are off by a few milliseconds, any number of undesired consequences can occur. This includes molten metal splashing on the torch (shield cup wear, tip orifice wear), a pilot arc that continuously tries to ignite (tip wear), too long of an arc (tip wear), an incorrect pierce delay that will gouge the metal (leading to rework or scrap) and a poor cutting height (which largely controls cut quality).
This is where the value of a CNC controller begins to shine, as the system will automatically set all of the variables after the operator enters material type, material thickness and plasma and shielding gas types.
Speaking of gas, good gas control also contributes to cut quality and consumables and probably more than people realize. For example, incorrect plasma and shield gas settings entirely change the arc characteristics. If the plasma pressure is too high or too low, the arc may be harder to start. That consumes the electrode much faster, as will dirty, oily or moist air, which emphasizes the need for a filter and dryer (see Fab Shop’s April 2013 issue for more information on proper air control for a plasma system).
While manual gas control boxes are available, the industry is largely favoring automatic gas controllers matched to a CNC as part of an even larger trend toward integrated systems. Because the CNC can instantly set and control gas pressure, it provides a better level of quality control and eliminates the changeover time associated with manual controls.
Time to step up?
Many fabricators initially purchase an air-air plasma system (air for both the plasma and shielding gas, with an air-cooled torch). However, when cutting table utilization reaches a certain threshold, it makes good economic sense to step up to a dedicated automated plasma system that features a liquid-cooled torch and other advanced technology. Consumables will cost $45 or more, but they last much longer and ultimately lower costs. Even a basic system can provide 1,200 arc starts per set and lower cost per cut.
Plasma system owners grumble about consumables costs, but don’t realize that much of a plasma system’s capabilities result from the thousands of engineering hours that go into consumables and torch design. Further, manufacturers continue to make strides to extend consumables life.
For example, Victor Thermal Dynamics has developed a new type of consumable for cutting at 300+ amps that uses multiple hafnium inserts (see Fig. 2) instead of a single insert. Tests proved that the parts life can be up to four times higher, which results in considerably lower running costs.
Tip (nozzle) technology has also improved recently with cups that cool all the way to the orifice (see Fig. 3). Better cooling extends consumables life when piercing at a higher amperage, a more common occurrence as plasma systems become prominent for cutting metal in the 0.750 in. to 2 in. range.
As the numbers in this article show, investing in the right technology up-front is one key to extending consumables life and ultimately lowering the cost per cut. In today economy, or any economy, that makes good business sense.
Victor Thermal Dynamics