DECIDING ON THE RIGHT SHEET AND PLATE CUTTING PROCESS

When investing in new CNC sheet or plate contour cutting equipment, which process will best cover all of the requirements? The answer lies in knowing how to use plasma, oxy-fuel, laser, or abrasive waterjet processes most effectively.

The most often repeated question I get from many metal fabricators that are in need of new CNC sheet or plate contour cutting equipment is this: Which process will best cover all of my requirements? In some cases, the answer is relatively easy because the particular user’s requirements fit one of the process capability ranges quite well. However, in many cases more than one of the cutting technologies is needed for the best result.

The prime reason for my travels to end user sites over the years has often been to help solve a cut quality issue, or a cut part tolerance mismatch, or even a cost per part dilemma. As the long-time associate of a manufacturer of plasma cutting equipment, I have often been expected to help bend the laws of physics that define the capabilities of the plasma cutting process in order to ensure that the parts being cut meet the specifications of the end product . . . and, in some cases, it simply cannot be done!

In the interest of full disclosure, I must note that I have, fortunately, spent my career working with a company that believes in recommending the correct technology for customer needs – even at the expense of losing an order.

While there are a lot of variations in performance between the major contour cutting processes, as well as differences in the results produced by the different brands and models of the same processes, I think it will be most helpful to outline the distinct differences in the capabilities between plasma, oxy-fuel, laser and abrasive waterjet systems in terms of productivity, cost per foot of cut, cut part accuracy, metallurgical implications, materials compatibility, ease of use, etc.

Keep in mind that this is a rough comparison of the different processes. Each manufacturer may take exception to some of these comparisons, as all of the processes have areas where their capabilities may overlap. I have been in quite a few contract fabrication shops that have all of the above listed equipment at their disposal and, in most cases, the right equipment is being used based upon the end user requirements for cut parts. That being said . . .

MATERIAL COMPATIBILITY
Plasma will cut all electrically conductive metals. The major strength here is with carbon steels in the range of gauge to 2 in thickness using a high-definition oxygen-based plasma system. Good quality results can be achieved on stainless and aluminum in the range from gauge to over 1 in, with maximum piercing on stainless and aluminum to 4 in thick, and edge start cutting to 6-1/4 in.

There are a variety of different plasma processes that use different gases, power levels and levels of arc constriction. These process variations provide different edge/metallurgical results based upon the heat affected zone and chemical effects.

Laser cutting technologies are available with a wide array of power ranges. The most common lasers are CO2-based, with new technology in the form of diode-pumped fiber lasers rapidly entering the marketplace. Lasers will cut a variety of different materials. Some CO2 beams encounter reflectivity issues on certain highly bright surfaces, such as some grades of aluminum and copper, but newer fiber lasers can cut reflective materials quite well. Lasers are particularly strong on cutting metal materials ¼ in and thinner, but higher-powered CO2 systems can cut up to 1 in steel and ¾ in stainless materials with respectable tolerances.

Oxy-fuel technology is limited primarily to cutting steel. This process is relatively simple and low-cost as capital equipment investment. Using multiple oxy-fuel torches to cut simultaneously on a single CNC cutting machine is relatively common, making heavy plate cutting quite productive with this process. The best material thickness range for this process is from about ¾ in to over 15 in. Oxy-fuel torches are often mounted on the same CNC machine as plasma systems – a technique that dramatically expands the thickness capability of these cutting machines.

Abrasive waterjets have been in use for a lot of years in various forms. By far the most common applications are for cutting softer materials, such as plastics, woods, carpeting, even food products. By increasing the pump output pressures and injecting a garnet-based abrasive, almost any hard material – granite, steel, aluminum, stainless, etc. – can be cut with very good precision, but at relatively low speeds. For cutting metals, the best advantage of an abrasive waterjet is the lack of a heat affected zone (HAZ). So the strength of a waterjet is that it cuts virtually any material.

COMPARISON OF PROCESS CAPABILITIES
Each of the process manufacturers will claim different cut speeds (of course), but each of the processes have their own comparative advantages and disadvantages regarding cutting speed. Often, the process with a lower speed will have an advantage in terms of cut quality or edge metallurgy. Cut speeds that are high, such as laser speeds on gauge thickness materials and plasma speeds on middle thickness ranges, will have a large impact on the cost per foot of cut or cost per part, because speed is one of the largest factors affecting cut costs.