Which is the best metal sheet and plate cutting process systems to use for a given application or range of products? The answer is not always simple and does not always involve just one of these processes. By Jim Colt, Application Technology Manager, Hypertherm
Agnes Tong, product marketing manager, APAC, ESAB demonstrates how mechanised oxy-fuel and plasma cutting technologies can complement each other in heavy fabrication applications.
Advancements in cutting technologies have opened the door to more choices in heavy plate fabrication for fabricators and steel service centres around the world. Two of the primary technologies considered for CNC shape cutting systems, oxy-fuel and plasma cutting, can serve as complementary technologies – either as separate processes or in a single, integrated system – to optimise heavy plate fabrication from a capability, quality and cost perspective.
To optimise plate fabrication, start with asking the fundamental questions:
- What is the application?
- What are the cutting needs for the application (i.e., edge quality, production rate, material types and thicknesses, etc.)?
- What restrictions must be assessed (i.e., budget, floor space, etc.)?
In evaluating the cutting needs for the application, consider the traditional capabilities for each technology:
Process Capabilities: Oxy-Fuel Cutting
The oxy-fuel cutting process is simple and relatively inexpensive. Equipment investment is minimal and the consumables used are common, low-cost fuel gases such as acetylene, propane, and natural gas. Generally used to cut mild steel plate 76.2mm or greater, some fabricators cut up to 203.2mm plate using this method. As an oxygen-based process, this technology requires carbon to effectively cut, restricting its use to mild steel applications. Only low carbon steel and some low alloys have oxides with a lower melting point than the base metal, so they can be cut with the oxy-fuel process.
When adjusted properly, oxy-fuel cutting results in a smooth, square cut surface. There is little slag on the bottom edge, and the top edge is only slightly rounded from the preheat flames. This surface is ideally suited for many applications without further treatment.
Fundamentally, oxy-fuel is a slow cutting process. However, in many applications, increased productivity in oxy-fuel cutting is accomplished using multiple torches cutting simultaneously. Generally, system configurations include four to six cutting torches, but high production applications such as steel service centres may use up to eight cutting torches. When cutting thicker mild steel and applying multiple cutting torches, oxy-fuel could easily have a productivity advantage over other technologies such as plasma cutting.
Process Capabilities: Plasma Cutting
Plasma process technology offers greater versatility than oxy-fuel. It can be used to cut any electrically-conductive material, including carbon/mild steel, stainless steel, aluminium, copper and brass. Plasma technology can also be used to gouge, mark (neither of which are processes for oxy-fuel), or cut metal with scale, rust or painted/primed.
Though not necessarily reaching the precision cut quality levels of waterjet or lasers (CO2 and fibre), considering all thermal cutting processes, plasma offers the best cut quality across the broadest range of materials using a range of current ratings, most commonly 30A to 450A with higher current used for thicker stainless steel and aluminium cutting.
Slag/dross, surface finish, cut angle, top edge rounding and heat-affected zone (HAZ) are key when judging gauging cut quality. Plasma technology has progressed to the point of producing a virtually “dross-free” cut. Plasma also has a much smaller heat-affected zone that oxy-fuel, which reduces effects such as plate warping and discoloration.
Add to this plasma’s high cutting speed, and this technology provides an optimal mix of productivity, cut quality and capacity. These capabilities combined with current levels of process automation, make plasma a predictable precision technology, attractive for a host of industries and applications.
While the use of multiple torch configurations in oxy-fuel cutting provides high productivity, the cut quality produced by plasma can, in many applications, eliminate secondary processes like weld prep that may be necessary after an oxy-fuel cut. By eliminating a secondary step and reducing overall process time, a fabricator may be able to justify a slower cutting operation. While it’s feasible to plasma cut with multiple torches at once, the additional cost factor usually limits this to no more than two torches.
Changing Technologies: Art vs Science
Both oxy-fuel and plasma technologies have advanced in the past decade, but plasma has done so at a much faster pace. Traditionally, parts thicker than 25.4mm were cut with oxy-fuel. Today, plasma cutting is able to cut materials up to 76.2mm thick. Inherently a more automated technology than oxy-fuel, plasma cutting is becoming even more automated. Improvements in CNC and CAD/CAM/ nesting software have also helped further these technologies.
It takes a skilled worker to consistently produce a good oxy-fuel cut. An experienced operator can achieve consistent quality cuts by making the necessary process adjustments to gas flow, standoff, and speed.
Plasma cutting is less of an art and more of a configured science, less dependent on operator skill. Plasma cutting systems offer different process levels, part programs are predefined and built-in features and parameters such as pierce cuts, cut speeds, cut heights, standoffs and automatic arc ignition greatly simplify machine operation. The easy-to use nature of plasma processes coupled with technology advancements in a number of areas such as cut-to-cut cycle times and arc voltage sensing for consistent cut quality and increased consumables life have helped fuel its aggressive growth. It can be argued that plasma cutting technology provides cut quality approaching that of laser and waterjet without the maintenance intensity and costs of those systems.
Oxy-fuel technology has realised advances in torch capabilities. Modern oxy-fuel cutting systems are equipped with features such as internal ignition torches with flame sensing, and electronics integrated into the torch for a more “intelligent” cutting process. The electronics allow for and control such features as integrated height sensing, which requires no separate height sensor to keep the correct distance between the cutting nozzle and blank; integrated ignition that eliminates the need for and maintenance of external ignition devices, and quick nozzle change, technology to rapidly change nozzles without tools, thus minimising setup.
Side by side, in terms of market adoption, plasma technology has gained the most ground on a global scale. Demand for this cutting technology continues to grow, especially in emerging markets such as China, South America, Southeast Asia and India. In these regions, oxy-fuel cutting has historically been the thermal cutting method of choice simply because it’s a simple, low-tech process that requires a small investment. Many of these shops are migrating to plasma cutting because of the opportunity of increased productivity, better cut quality, ease of use and increased confidence in the predictable cutting results this technology delivers. This trend is possible as infrastructure to support these technologies has grown with more access to the consumables, gases, as well as to support and service resources.
Driven by the changing landscape of the machine operator, higher operator turnover rates resulting in a diminished experience base at the CNC, and the natural evolution of productivity, both oxy-fuel and plasma processes have become more dependent on the manufacturer or system integrator to create a machine tool that’s easy to use and requires less operator intervention.
Efforts by machine manufacturers to integrate components will continue to enable advancements of the technologies. This has also led to better integration in a multi-process system.
Leveraging Capabilities in Combination
Leveraging the capabilities of each technology is possible in an integrated, multi-process cutting system. Combining technologies offers the ability to choose the strengths of each and augment the weaknesses of the other.
Fabricators or steel service centres who need the ability to cut a wide range of materials will often look to machines equipped with two or more cutting processes. If a shop does not have the work capacity or the floor space to warrant dedicated oxy-fuel and plasma systems but needs the flexibility of both technologies, a combination machine is an alternative. Combining technology provides:
High productivity at lower cost:
A multi-process cutting system gives these fabricators the best of both worlds, even to the point where both technologies can be applied to a single part. An advantage of multi-process cutting is the ability to use the slower, more accurate process for some contours, and then switch to the faster, less costly process for other contours. The result is achieving the required part accuracy at a far lower cost than if the high accuracy process was used to cut the entire part.
Essentially, the combination plasma/oxy-fuel system promises the highest productivity using the tool that’s best for the job. It also gives the fabricator free reign to think outside the box for ways to optimise cutting results with the flexibility to optimise each part to the requirement: users can optimise cut quality, operating cost or productivity, depending on what’s most important to their operation.
More capacity using fewer resources:
Five years ago economic conditions forced many fabricators to reduce production shifts. As business improved, fabricators have chosen to invest in technology such as multi-process cutting systems that provide the flexibility and added capacity without increasing labour.
Advancements in technology and automation for both plasma and oxy-fuel cutting have taken the burden off the operator. In a multi-process system this means a single operator is often capable of running both processes.
A smaller footprint:
A multi-process cutting system also makes better use of floor space, which is a particular concern for fabricators in emerging markets where floor space is at a premium. Combining technologies also offers costs sayings from the simple standpoint that one cutting table is less expense than two.
Strengths and weaknesses:
There are downsides to any multi-process system. While a multi-process cutting system is versatile, generally, it’s not practical to cut concurrently with both technologies. It is choice of either or.
In combining technologies there is a new set of requirements that need to take into consideration table size, machine accuracy and precision for both processes.
A plasma cutting system used to cut thinner materials at fast speeds with high cut quality is not dependent on a large or robust table to hold 101.6mm mild steel plate, but does require precision machine motion (acceleration and deceleration) to achieve a narrow kerf width (the width of material that the process removes as it cuts through the plate) and thus good cutting results.
Because oxy-fuel cutting equipment handles thicker plate materials, it requires a larger, more robust table. The larger kerfs and slower cut speeds of this process make this equipment less sensitive to machine motion and acceleration aspects, so components need not be high precision (ball screw and rack and pinion versus linear drives).
In combining the two technologies, a larger table with higher precision capabilities is needed.
The system integrator plays a vital role in providing the total cutting solution. When considering a multi-process system, look for a manufacturer that is familiar with both technologies and with the complexities that are part of integrating multiple technologies for a single cutting solution. A vertically integrated manufacturer – one that designs the components to work together and then integrates those components to form a complete cutting tool – has the most control over the outcome of the process.
Flexible, Custom-made Solution
Gaining flexibility to match individual plate fabrication requirements using both mechanised oxy-fuel and plasma cutting – either operating in parallel as stand-alone systems or in a combination, multi-process machine – can optimise cutting operations. As these technologies advance, fabricators can produce their parts smarter, faster, with the highest quality and at less cost. The choice to use these technologies in parallel or in combination is as individual as the application requirements.