It is the time to construct our yacht. Materials are delivered at the yard and engineers and fabricators make the best of the available resources, to maximise the use of the materials and minimise the environmental impact. The working practices of steel and aluminium are similar and even the tools in many occasions are the same, therefore we’ll investigate them together.
The structural and mechanical design has already been completed, the structural members are defined and expanded, ready for cutting (I am overlooking straightening, sandblasting and primer for another post on painting). It is the stage when nesting takes place, trying to fit as many parts as possible in a plate, with the assistance of software and practical thinking.
Engineers try when designing the components to prepare parts that are rectangular or at least near rectangular to be able to nest most effectively, as presented in the picture below. It is a great example of taking the best of your material. Yet all the pieces onboard are not rectangular, like the girder of the bulbous bow, or the web frame around the bilges. The curvature is prominent and has to be integrated in the plate. Although I am yet to experience a software that can tackle these cases, I have seen engineers to utilise more than 90% of the plate, which is in the end our goal. The metal profiles should also be approached in a similar attitude.
Present economy supports scrap to be marketed at the recycling facilities, but the scrap market is unstable and as profit is not always there and it might be tempting to simply discard. So the goal is always to use the most of the metal.
Nesting is a precursor of cutting and affected by it, as tolerances between parts are defined by the cutting method and its kerf, the amount of material lost from the flame. Presently there are three prevailing technologies for cutting, plasma, laser and waterjet, excluding the mechanical means from this article.
Plasma is ionised gas, the fourth form of matter, along with solid, liquid and gas. As mentioned, it is utilising a gas that can be oxygen, argon, hydrogen or nitrogen and can cut thicker metals. Yet from the three options, has the largest kerf. It also consumes a significant amount of power, which can reach up to 1000Amps depending on the job at hand, although for the majority of works it is around 500Amps.
Its emissions depend on two factors. First on the ionised gas type, taking nitrogen for example, that generates various nitrogen oxides that are harmful and flammable. Secondly on the material cut, looking at stainless steel that can emit hexavalent chromium which is carcinogenic or aluminium that produces ozone. Among the options of used gas, argon and hydrogen are the safer ones, yet hydrogen has residue left over the cutting table and needs to be cleaned at the cavities.
Plasma is also quite noisy, exceeding in some cases the maximum acceptable for working standards. For this reason it is in many cases done underwater, limiting the noise and bringing the fumes down to 10%. Yet there is waste water that needs to be disposed in designated facilities and the table has to be carefully cleaned in this arrangement.
Due to its price, it is the preferred option for small and medium sized organisations.
On the opposite end of the initial investment spectrum lays the laser cutting machines. There are various types, depending on the medium of radiation, simplest being CO2 and nitrogen, moving to crystals and lately fibres. These cutters are precise, have the smallest kerf, are the fastest, and the finished edge is superior to that of plasma. Yet they have a maximum thickness threshold and, apart from the fibres ones, they can’t cut highly reflective material because it may result to mirroring the laser on to the nozzle itself. In respect of emissions, they are responsible for some harmful and toxic gases and in the facility there is dedicated ventilation to protect the working force, yet the gases are released in the atmosphere. Power consumption again depends onto task at hand, but is certainly higher than that of plasma.
Waterjet, as stated in its name, is using a jet of water, pure or enriched with abrasive, to cut. It is the most versatile cutter, can effectively cut most materials, including thick ones at low speeds. It has high precision and the edge finish is superior. Plasma or laser affect the edges because of the heat in the cutting area, harden the metal locally and need to treat the edges before further preparation, which is not needed through waterjets.
The water can be reused, after being distilled from the abrasive elements. The most common abrasive material is garnet that has no reported harmful emissions, yet silica must never been used. It can be though very noisy, way above 100dB and cutting underwater is most common way to limit the noise, that then falls within acceptable levels. It is also usually in an enclosed compartment, sound enclosure. Compared to the other cutters, it has high running costs, because of the abrasive additions, while it is in the middle spot of initial investment.
In an overview of the aforementioned parameters.
- The individual parts have to be design and nested in the most efficient way to maximise the material used;
- Every cutter has its strengths and weaknesses, for an organisation that can have all three types, the advantage would be to make the use of which fits best for the specific job;
- Once again the factor electric power is integrated in the evaluation, denoting the necessity of clean energy from the available means;