Outsourced waterjet cutting for hovercraft builder
According to Chris Arnold of ICEE Managed Services, UK hovercraft manufacturer Griffon Hoverwork has found that outsourcing its metal plate cutting has brought a range of benefits.
At its Southampton factory Griffon Hoverwork makes class-leading hovercraft for government, defence and commercial markets around the world. Quality assurance is key, especially where vessels are built to Lloyd’s Register standards. Consequently, precise management of materials, marking, documentation and hence traceability are vital.
A bill of materials (BoM) for a mid-range Griffon hovercraft comprises over 100,000 items, a larger craft more than 500,000. Besides off-the-shelf products, a multitude of bespoke parts and panels represents a major percentage of each BoM. In general, these items are cut from 2mm thick sheet up to 20mm plate, mostly 5083-0 grade aluminium and 304L or 316L grade stainless steel.
The parts range from small, non-standard washers to strengthening gussets and formed brackets, and in contrast many large flat panels featuring holes, slots and simple or complicated profiles. Panels are welded together by the Griffon to form the hull and superstructure of the vessel, and to fit out the interior.
Griffon Hoverwork finds it more economic to outsource the cutting of parts and panels, rather than manufacture in-house. To save significant costs in preparing parts for welding, waterjet cutting is specified wherever possible, with fibre laser cutting where appropriate. The work is done by ICEE Managed Services, a Hampshire-based manufacturing services contractor with marine industry experience. Besides supplying parts and panels, the company also provides valuable additional services in stringent quality assurance, including marking parts and supplying related documentation.
HUNDREDS OF STAGED WORK PACKAGES
Building a large hovercraft may take up to twelve months and Griffon Hoverwork requires cut parts to be supplied in staged work packages. In planning production, it breaks down supply of cut parts into hundreds of work packages. In general, a package equals one person’s parts supply for a week.
If the order book is at full capacity, work package orders may be sent to ICEE on a weekly basis, comprising as many as ten packages. Each package may contain between a hundred to a thousand components to be cut, depending on the build stage.
As part of its added-value service, ICEE supply the work packages for particular tasks, together with all associated documentation. This highly specific packaging and zoning approach makes it far easier for the customer to distribute in-bound work packages straight to the right parts of the factory.
LLOYD’S REGISTER WORK
The value of the comprehensive service is even greater where Griffon Hoverwork builds craft to Lloyd’s Register compliance standards. Each part cut requires unambiguous marking by ICEE, clearly identifying it in relation to associated quality assurance documentation.
Normally, the hovercraft maker specifies 3.1 grade material, which means provenance certification is supplied by the rolling mill. Craft built to Lloyd’s Register standards require double certification of material, especially for the passenger market. This means documentation not only from the rolling mill, but also from an independent standards authority, a far more demanding 3.2 level of compliance.
In these cases (and normally), precise documentation from the cutting contractor is key to prove compliance and provide a traceability record. Thus Griffon Hoverwork will always be certain 1) a part is properly certified before building it into the vessel and 2) can show written evidence later, should proof be demanded a certified material was used.
For ICEE, pressure to ensure perfect documentation and traceability for Lloyd’s Register purposes may be complicated further because the material required is often much thinner for a lightweight hovercraft than other marine vessels. Availability of this kind of material from approved mills is challenging and may require testing from an independent certifier to achieve 3.2 level ratification.
“Managing all this reliably and consistently speaks volumes for ICEE’s competence, expertise and ability to offer more than simply a panel cutting service,” said Griffon Hoverwork’s senior detail designer, Christopher Walden-Bevan. Clearly, this higher level of service saves the customer much time and hence money.
AVOIDING COSTLY HAZ
But there is another time-saving feature that significantly reduces costs on the construction side. It sounds simple - the avoidance of heat-affected zones, or HAZ, on aluminium and stainless steel panels to be welded.
If a panel is cut by fibre laser (or another method that cuts metal by melting it) the intense heat required forms a layer of oxide on the cut metal surface – top, bottom and edge - localised along the length of the cut.
If the oxide is not removed before panels are welded together the risk is it will contaminate a part or the whole weld. Oxide may get entrapped, weakening the weld’s strength. Over time – with wear and tear stresses – a crack may propagate leading to weld breakage. This could result in catastrophic failure of a hull structure or superstructure, a risk the hovercraft maker takes every precaution to eliminate.
If a heat-based cutting process were used Griffon Hoverwork would have to painstakingly remove oxide from cut edges prior to welding, meaning an operator spending hours doing so with a grinder, file or emery cloth. It would also demand a degree of skill and time to check results met required standards.
So the answer is also simple - use a cold-cutting process where HAZ is not produced. Waterjet cutting provides the ideal solution – for aluminium and stainless steel. It results in a clean and accurate edge, wasting little material and on thick profiled plate, no wandering or ‘tapering’ of the cut, so the edge is always square and true for welding.
“Obviously, we have to remove oxide or HAZ after we weld panels to form the hull, superstructure and internal fit-out, but we only have to do it once,” says Griffon Hoverwork’s Christopher Walden-Bevan.
“Combined with ICEE’s help in producing and documenting compliant work packages, the benefits of their cold-cutting waterjet halves the cost of HAZ treatment, so altogether it adds up to big cost savings for us.”
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