Cutting of individual plates is the first stage of manufacturing in shipbuilding, any repairs due to cutting inaccuracies will be very costly. In short, the precision of the cut plate is crucial. The objective is to improve accuracy in manufacture by reducing thermal deformation. Following alignment and fitting, and the total assembly procedure, will also be easier.
International competition says that all shipyards must manufacture their products economically. For this reason, there is substantial investment in accurate manufacturing - from block assembly up to grand section assembly.
Nowadays, shipyards worldwide almost exclusively use plasma cutting to cut shipbuilding plate in the thickness range, 8-20 mm. Plasma cutting provides five-times higher cutting speed than oxy-fuel cutting, and thermal deformation is much lower, guaranteeing better dimensional accuracy of cut parts. Laser cutting has also been introduced into shipbuilding. The process is characterised by the lowest heat input and almost rectangular cuts that need no post-weld-machining. As a result, the weld cross-section and the related heat input from subsequent welding are lower.
Today, guiding machines for thermal cutting processes are exclusively CMC controlled. The control consists of a process computer, which not only controls the path movement of the torches, but also calls-up the cutting programmes as well as the machines monitoring devices. Cutting tools are interrogated for their wear and information is transferred to the operating panel and/or to the manufacturing supervision. In this way, a fully automatic cutting process is achieved. The objective is to achieve a continuous quality of cut parts even if the used plates are of different grades of material.
Analysis of a container ship with regard to cutting and welding performance
An investigation of a containership with a total length of 220 m showed the following cutting and welding characteristics:
-Ship weight: 7500 tonne
-27000 cut parts
-27000 profile parts (stiffener elements)
-350 km welded joints
The plate thickness breaks down as:
-08-18 mm: 75 per cent
-30-40 mm: 25 per cent
This means that 75 per cent of the plate thickness range is ideally suited for plasma cutting, justifying the almost exclusive use of plasma cutting in shipbuilding. To increase plate turnover in manufacturing, one must observe
-Removing cut parts
-Cutting programme availability
-Cutting and marking speeds
Today, one assumes that in cutting the cut length, and marking length are roughly the same. Today, up to 20 m/min marking speed is reached.
Status of plasma cutting
In the hunt for increased speeds in plasma cutting, plasma cutting with oxygen has consistently been developed, because it guarantees the best results with regard to the quality of the bevel and the absence of slag. Currents as high as 400 A are now achieved, which results in a considerable increase in cutting speed.
However, with a higher current at the cutting nozzle, heat input also increases, resulting in a more oblique cutting edge. A possible remedy is to use a torch with bevel units, which position the torches at a slight incline. Naturally, cuts for weld edge preparation are carried out with these machines; V and Y bevels with plasma torches, additionally K bevels using oxy- fuel torches.
In plasma cutting, the consumption of wear parts (electrodes and tubes) is a cost factor, which should not be underestimated. Nowadays, considerable life cycles have been achieved by consistent research and development. For example, the life cycle with oxygen cutting used to be considerably shorter than with nitrogen cutting, at the initial development stage. New developments, eg, a silver coating on the hafnium electrodes, have considerably increased the life cycle of wear parts. Modern electronics have also brought about considerable progress, i.e. a controlled increase and decrease of welding current at the start and the end of the cutting sequence, respectively. Today, the life cycle of modern electrodes can exceed four hours arc time.
It is well known that plasma cutting generates airborne particles, such as dust; IR and UV light, as well as high noise levels. For this reason, in Europe, it was mostly performed under water. Water muffles the noise, binds dust, and protects sufficiently from radiation. There are disadvantages, however. Filling the water tables takes time, in comparison with dry tables. In addition, wet plates cannot be welded without loss of quality. Rust may occur on the bevel edge faces, and cutting particles must be cleaned from cut parts after water drainage. This is carried out with high-pressure water pumps, installed on the cutting machine.
There is a tendency, in Europe, for users to return to dry cutting, as this technique offers considerable advantages for productivity, cut quality, charging, discharging. However, noise level is a major problem. Noise protection cabins are provided for operating personnel and, sometimes, machines are encapsulated (modern cutting machines can operate fully automatically).
Status of laser cutting
As mentioned, laser systems have been introduced into shipbuilding, in shipbuilding nations such as the US, Korea, Italy, and Germany. However, the cutting speed is still lower and investment costs are considerably higher than those of a comparable plasma system. On the other hand, wear costs are clearly lower; a precise cost comparison evaluation is not yet available, although in progress. Some arguments in favour of using a laser should not be underestimated include:
·Cut faces can be used without further post- machining
·Noise level of a laser-cutting machine remains inside permissible limits
·A laser system can be operated without personnel, as wear part consumption is very low (approximately 40 hours without operator activities)
·Based on laser power of 5 or 6 kW (currently used for cutting), cutting speeds can be achieved which are regarded as acceptable
·Due to horizontal cutting edges, plates on both side of the cut can be used immediately, reducing the cutting length and partly balancing the lower cutting speed compared with plasma cutting
·Lower heat input causes a lower distortion of the par
·The narrow joint generate; less airborne particles and, additionally, there are material savings
Marking is an important process needed to draw bending lines, lines for stiffening elements and registration codes. Marking time is often calculated to be as long as cutting time. Ink marking, similar to an ink jet printer, is most commonly used as it can draw lines of various width, and letters, at a speed of up to 20 m/min. Powder marking has been almost completely replaced by plasma marking, which is more dependable.
Precision in cut
Within the long chain of actions needed to build a ship, cutting is the first manufacturing step. If the required accuracy is not achieved in this first stage, all subsequent manufacturing stages will be negatively affected. These precision cuts must meet accuracy requirements within millimeters. Modern guiding machines with servo drives having low dimensional tolerances and precision guides match these requirements. Also, the plasma torches and their wear parts (nozzle, electrode, gas diffuser) must be accurately manufactured. To eliminate the typically inclined plasma cut, bevel machines are used which adjust the torch by some degrees, and which are guided tangentially to the contour. As a result, a perfectly horizontal edge is achieved.
One expects a constant cutting quality from modern cutting machines. To realise this requirement, it is necessary to design both the power sources and the cutting tools in such a way that all cutting parameters can be programmed to adapt the machine to different cutting processes.
Control and Process monitoring
Modem, CNC controlled cutting machines allows an operator to control several cutting machines. Laser machines often operate without personnel. In case of a malfunction, however, absence of personnel may lead to considerable damage. To avoid such risks, machines are equipped with monitoring devices that pass information to the CNC control, in the case of failures. Messages can be sent to PCs, fax, or mobile phones to alert maintenance personnel via modems.
Another problem can be infrequent errors, eg, once a day or several times a week. To carry out an error analysis, the CNC control can be connected to the manufacturers development systems via the Internet, to log the machines proceedings. With these Internet connections, troubleshooting is carried out successfully over long distances.
Plasma cutting is widely used in shipbuilding, alongside oxy-fuel cutting for 25 mm plate thickness and upwards - especially for multi-torch operation. Also laser cutting with its specific quality advantages has been introduced.
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|Posted : 8/12/2005|