TORCH HEIGHT CONTROL FOR AUTOMATED PLASMA CUTTING APPLICATIONS

With the level of technology involved in today's high-definition class plasma cutting systems – highly refined torch and consumable designs, extremely accurate gas flow delivery systems, reliable, accurate power supplies, as well as motion control and software that ensure high productivity and accuracy in plate and sheet cutting applications – an often neglected component of any plasma cutting operation is Torch Height Control.

Accurate control of the torch-to-work distance during all phases and types of plasma cutting operations can be the key to dramatic improvements in cut quality, consumable life, as well as productivity and throughput of the cutting machine.

Even the simplest of plasma cutting applications – a hand held plasma torch – often use a method of torch height control. Many brands of hand-held plasma systems have an exposed nozzle (some call it a "tip") that must be held roughly .062 in to .100 in off the plate being cut . . . often some type of standoff device is used to maintain this critical distance.

If an exposed nozzle on a hand-held plasma torch cutting at 30 amps or higher contacts the plate even momentarily, damage can occur to the nozzle orifice that affects the shape of the plasma arc, affecting cut quality. Newer technology hand torches use shielded torch technology, an electrically isolated copper shield that allows the torch to drag on the plate, maintaining the correct distance between the nozzle and the plate.

In high productivity mechanized cutting applications, dragging the torch on the plate is generally a non-acceptable means of controlling torch height. Since the mid-1970s, plasma system manufacturers have been using arc voltage feedback torch height control as a fairly effective means of controlling torch-to-work distance. Unfortunately, controlling torch height using this technology can be rather complicated, and puts another dozen (or so) setup parameters on the CNC machine operators list of tasks that are required to produce plasma cut parts with the best possible quality, with high productivity and low operating cost.

HOW DOES AN ARC VOLTAGE HEIGHT CONTROL WORK?
Plasma cutting uses a high temperature electric arc that is developed by applying DC current to an internal electrode (negative) and to the plate (positive). Gas flowing around the electrode is heated to its ionization temperature, which increases its electrical conductivity, allowing the DC energy to form a superheated electrical arc from the electrode to the plate.

The arc is forced through an orifice in the plasma torch nozzle, which controls the shape and velocity of this plasma arc, effectively controlling the energy and cut quality. A DC voltage can be measured between the negative electrode and the positive plate during steady state plasma cutting . . . this voltage varies depending on the length of the arc between the electrode and the plate – longer arc reads higher voltage, shorter arc reads a lower voltage. In theory, this is a very effective method of gauging torch to work distance.

A typical plasma system height control has a reference voltage that is set by the machine operator, this voltage is provided by the plasma torch manufacturer and is different for every type of material, every thickness, and every plasma process power level. Once the plasma process starts cutting, the THC (torch height control) looks at the actual arc voltage (measured electronically between the electrode and the plate) and compares it to the operator set reference voltage, then the THC adjust the motorized Z-axis (up/down mechanical axis that holds the plasma torch) until the reference and actual voltage are the same, thus controlling the torch to work distance. Any warpage or flatness issues of the plate on the plasma cutting table are automatically adjusted by this height control process.

LEVELS OF COMPLEXITY
The plasma THC sounds relatively simple, unfortunately there are a lot of things that can affect the controlled distance between the plate and the torch. Here are a few of them:
• Cutting speed change. When the machine de-accelerates to cut a small hole or an intricate part of the cut program, the plasma torch continues to cut at a given power output. The slower speed effectively lengthens the arc (wider cut kerf, plasma arc has to extend to reach the plate), which causes the THC to move the torch closer to the plate to maintain the proper arc voltage . . . often the torch dives and contacts the plate.
• Gas pressure fluctuation. Since the arc follows a stream of ionized gas, any changes in gas pressure or flow will affect the resistance of the arc, which can cause the torch to either rise or lower to compensate for a voltage/resistance change.
• Kerf crossing. Kerf crossing at the lead-in, lead-out point near the end of the cut will cause a momentary rise in arc voltage (the plasma arc reaches down through the already cut kerf to find metal), which will cause an end of cut dive.
• Timing issues. If the arc voltage is still active at the end of the cut when the cutting machine motion de-accelerates, the arc voltage will rise and the THC will tend to dive the torch into the plate.
• Electrode wear. As more time and cut cycles are recorded on the torch electrode, a wear pit forms in the hafnium emitter. Since this part of the electrode is the point that the arc attaches to, and the arc voltage is measured from . . . expect the wear in the electrode to cause the torch to operate closer to the plate as the consumable parts wear. An .080 in pit inside the torch electrode will cause the torch to operate .080 in closer to the plate, potentially causing poor cut quality and plate collisions.

IMPORTANT HEIGHT CONTROL SYSTEM FEATURES AND CAPABILITIES
Arc voltage control of steady state cutting height is critical to cut quality, however it is just as important for a THC to have the ability to accurately find the surface of the plate before each cut cycle, and to be able to retract to the torch manufacturer's recommended pierce height (different for each material type, thickness and power level) before firing the plasma arc. After the arc fires, and the pierce is complete (different timing for each material type, thickness and power level) it is important that the torch indexes rapidly to the cut height (again, different for each material type, thickness and power level) in order to allow the plasma arc to stabilize before it gets out of the scrap lead in and on the actual cut profile. At the end of the cut cycle it is critical for the torch height control to retract rapidly to avoid collision with a potential "tipped up" previously cut part on the plate when traversing to the next cut.

CRITICAL FEATURES FOR PROPER TORCH HEIGHT CONTROL
• Accurate sensing of the surface of the plate before each cut cycle. This should be done at high speed to minimize cut-to-cut cycle times, and should work accurately on thin plate as well as thick plate.
• Ability of the height control to index rapidly to the recommended pierce height, then fire the torch.
• Rapid index to recommended cut height after the pierce is complete.
• Activation of AVC (arc voltage control) as soon as the cutting machine achieves programmed cut speed.
• Ability to recognize kerf crossing to minimize diving.
• Ability to communicate with CNC/part program to eliminate diving during cornering or intricate feature slowdown.

"NICE TO HAVE" FEATURES
• Interface with plasma, CNC, and cutting software for automatic parameter upload (ease of use for the operator)
• Built-in diagnostics
• Automatic height control compensation for consumable wear, speed changes
• Communication with nesting software to avoid collisions with potential tip ups.
• No operator settings . . . all height control settings are automated by CAM nesting software.
• Rapid retracts and initial height moves to minimize cycle times.
• Skip retract feature that will minimize time consuming moves when cuts are close together.
• Collision sensing feature to stop all motion in the event of torch to work collision.

So after re-reading what I have written above, I am reminded that a typical CNC plasma cutting machine operator – in order to get the best performance out of his plasma cutting machine – must be quite knowledgeable in regards to all of the settings and parameters in order to achieve repeatable performance on a day to day basis. I couldn't count the number of times I have been on the shop floor of a large plate processing facility that had complaints in regards to cut quality, cut part accuracy and/ or consumable parts life . . . where the problem was ultimately solved by intense operator training in regards to ensuring that the torch pierce height, pierce time, cut height and arc voltage height were always set correctly.

There are height controls available today that are almost a completely transparent part of a CNC plasma cutting machine. In many cases the operator does not have any interaction to adjust any aspect of torch height control, yet the THC system is accurately set every time it is used. These newest systems interact directly with the plasma system, the cutting machines CNC control, as well as with the CAM nesting software, interfacing all of these components with a simple Ethernet style cable. Advantages of this new technology include longer consumable life, better cut quality, higher throughput, as well as making the operators job easier.

Even if you do not have the latest plasma and THC technology on your machine, pay close attention to your existing Torch Height Control system, make sure your operators understand its features, how it works, and where to find the proper settings for each type and thickness of material being cut. It will be worth the effort.

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Jim Colt is a strategic account manager for Hypertherm, Inc., Etna Road, P.O. Box 5010, Hanover, NH 03755, www.hypertherm.com, jim.colt@hypertherm.com.