Profile cutting options

Citation

Kirkpatrick, I. (1999), "Profile cutting options", Aircraft Engineering and Aerospace Technology, Vol. 71 No. 5. https://doi.org/10.1108/aeat.1999.12771eaf.002

Profile cutting options

Profile cutting options

Keywords: Cutting, ESAB

The art of profile cutting is an ever evolving process centred on the four methods of oxy-fuel cutting, plasma cutting, laser cutting and water-jet cutting. Modern developments are constantly improving these processes in terms of better cut quality, higher cutting speeds, lower operating costs and integration into automated production methods.

Oxy-fuel cutting is the oldest and most commonly used cutting method -- primarily used for unalloyed steel and larger dimension work-pieces. The process involves preheating the plate to its ignition temperature with a mixture of fuel gases (e.g. oxygen and propane or oxygen and acetylene) and then using oxygen as the cutting gas.

The process can be easily mechanised by mounting one or more cutting torches on a cutting gantry, which moves across the work piece. Because of the comparatively low cost of the cutting heads, multiple cutting torches are used to provide simultaneous cutting of identical parts at an economical cost.

Because the oxy-fuel gas process is based around the chemical reaction between the oxygen and the iron, the cutting speeds are restricted to how quickly this chemical reaction proceeds. Recent developments by ESAB with their Cooljet torches have increased this cutting speed by 30 per cent. However, the speed is still below 1m/min.

In spite of these cutting speeds, oxyfluel gas cutting offers a number of advantages when considered as a method of cutting profiles from mild steel. The capital cost of the equipment is relatively low, power requirements are low and maintenance costs are low too. From a practical point of view, oxygen, propane, natural gas and acetylene (the gases used) are cheap and easy to come by. The oxy-fuel process also covers a wide thickness range from 3mm to 500mm.

Oxy-fuel gas cutting is, however, not suitable to all sheet or plate cutting requirements as the process is generally limited to cutting mild or low alloy steels. The success of oxy-fuel cutting is also dependent to some degree on the surface condition of the material to be cut while its high heat input can result in a wide heat affected zone on the cut edge of the material. On thinner material below 3mm, some deformation and buckling of the material may occur.

Over recent years, the use of plasma cutting has increased. This cutting process is suitable for conductive metals of all types and produces a higher cutting speed in material of up to approximately 50mm compared with oxy-fuel cutting. The sectional area is also better. This method is ideal for bevelling materials which are going to be joined by welding. Underwater plasma cutting produces significant environmental benefits in the form of reduced noise, gas and smoke.

However, the most commonly used plasma systems are based on low-amp air plasma (using air as the plasma gas). Since their introduction in the 1980s, these systems have benefited from many developments which have been targeted at making the process more reliable and easier to use. As such, plasma cutting has been universally accepted as a valuable tool in all segments of the modern metal working industry.

It satisfies a range of applications from around 1mm thickness mild steel plate up to an economic maximum of 25mm plate. It also has the ability to handle most mainstream fabrication materials, and is less costly and faster than laser or water-jet cutting.

These plasma systems have a low capital cost, a low running cost and can be used either in manual mode or in conjunction with an automated profiling system. Indeed, the ability of plasma systems to cut sheet metal at speeds of 6m/min and more has resulted in the development of high-speed CNC profiling machines that can take advantage of the process.

Although the use of air as the plasma gas is acceptable for general-purpose cutting applications where a high quality finish is not paramount, specific gases do offer improved finishes. For example, when cutting mild steel, oxygen is usually the plasma gas that is used. Cutting this material with oxygen rather than air also offers a better finish, less dross, better weldability and higher cutting speeds.

Another approach to higher quality production cutting sees water injection plasma systems used, as radial or vortex water constricts the arc more effectively than a gas injection type. The water also cools the electrode and nozzle, which results in longer life for consumables -- and having a cooler nozzle means that materials such as ceramic can be used to prevent double arcing, which again increases the nozzle's working life.

The most recent development in plasma cutting relates to high definition systems that have extended the application of plasma cutting into the areas of precision light to medium gauge applications. Indeed, these fill a gap in the market with a technology that costs significantly less than laser, yet is far more precise than conventional plasma while still offering similar flexibility.

High definition plasma systems work by concentrating the cutting arc in a very small diameter, and they use a much smaller nozzle aperture than conventional plasma systems. Various means are employed to avoid the premature burnout of nozzle and electrode components. This results in a beam energy density that is three to five times greater than that achieved with conventional plasma, which in turn results in a shallower temperature gradient in the arc.

As a consequence, both edges of the cut are square, both the cut width and heat-affected zone are narrower and, because the overall energy input is lower than with conventional plasma, the heat-related distortion of thin material is much less. However, an important consideration with regard to high-definition plasma applications is the need to maintain a consistent torch/work piece distance. For whereas conventional air plasma systems can operate successfully with an arc voltage variation tolerance of þ5V, high-density plasma needs a much tighter þ1V if it is to operate property. To satisfy this requirement, ESAB has developed a fast-response Z axis that is integral with the torch mounting.

The laser cutting process is relatively new and produces high precision, extremely narrow sections and high cutting speeds in thin material. It is suitable for most materials -- metals, plastic, glass, wood, insulation material and so on. However, its main application is for cutting steel up to 25mm thickness.

The word LASER is an acronym of the description of the way a beam is produced: light amplified by stimulated emission of radiation. The material to be cut is heated by the focused laser beam to ignition, melting or evaporation temperature and is blown out of the kerf by the cutting gas.

On the way to the cutting head, the diameter of the collimated light beam is increased in order to minimise the light wave loss on the way to the cutting surface. At the same time, the laser beam is reflected once more in ESAB's laser cutting systems in order to ensure a consistent laser cutting quality performance over the entire working surface.

Today's laser systems are much improved on earlier versions, as industrially hardened designs have been developed which are better able to cope with shopfloor conditions. Lasers are also suited to cutting a wide range of materials from plastics and timber through to stainless steel and high temperature alloys and, as such, they are unique among high-temperature cutting processes. Laser cutting also offers high accuracy, as the tightly focused beam produces a narrow cut with a high quality edge, virtually no kerf and a very narrow heat-affected zone.

The cutting capacity of commercially available laser profiling systems is up to about 25mm steel plate, though the high cost of laser sources tends to limit its application to high value added applications rather than general fabrication work. Indeed, it appears that fabricators are currently favouring high definition plasma, while sheet metal formers are opting for laser.

However, recent developments by ESAB on the laser beam delivery system and guiding system, now enable fabricators to obtain the laser benefits of accuracy and quality on larger and thicker plate. Plate sizes up to 20m long and up to 5m wide can now be processed.

It is accepted that on outside profiles there is invariably little to choose between high density plasma and laser, but the latter certainly offers distinct advantages where internal profiles, intricate work and fine holes are required. As for speed, there is little difference between the performance of high definition plasma and a 3kW laser, though the higher the power, the greater the cutting speed.

Multi-axis laser bevel cutting heads are now also available. Depending on the nature and thickness of the material, cutting at angles of up to 45ú is possible, eliminating the need for preparation of the weld seam.

The multi-axis bevel cutting head works with the high precision and the multi-axis module ensures a smooth cutting process. The result is a high quality laser cut.

The principal advantage of water-jet cutting is extremely low thermal effect and a narrow heat-affected zone. Cutting can take place with water and with water in combination with a cutting material, such as quartz sands. Pure water cutting is used to cut soft materials, such as insulation material, jigsaw puzzles and frozen food. What is known as abrasive cutting, with water and quartz sand, for example, is used to cut metal, stone and other hard materials.

There are lots of materials where thermal cutting processes are not possible and not economical. Typical materials are brass, copper, aluminium, glass, ceramics, wood and composites. For this, water jet cutting is a suitable process, which operates with a high pressure water jet. The process offers the benefits of high cutting accuracy, high surface quality, dross-free cutting surface, no heat-affected zone, no further finishing operations, a clean cut surface and the maintenance of the structural integrity of the material. This means that in many applications water-jet cutting can provide a finished part without the necessity for machining or a secondary operation.

Suitable for most metals, water jet cutting does provide an excellent quality of cut and there is no heat input and, therefore, no distortion. The method involves using a very high pressure water jet up to 4,000bar, which is passed through a constricting nozzle. When cutting metals, an abrasive such as silica or garnet has to be added to the water stream. This is partly a disadvantage as the cost of the equipment to handle abrasives can be high. There is also a high capital cost involved in enclosing the process and providing the necessary safety back-up.

Further details are available from ESAB Automation Limited. Tel: + 44 (0) 1264 332233; Fax: + 44 (0) 1264 332074.

Ian KirkpatrickGeneral Manager, ESAB Automation Limited