Good product design is as much about understanding the capabilities of the manufacturing processes as it is about fulfilling the product's functional requirements. Designers therefore need to appreciate how processes are evolving, and one that is developing fast is waterjet cutting and the related process of abrasive waterjet cutting.
From the early 1970s, when waterjet cutting was first used commercially on paper-based materials and honeycomb materials for the aerospace industry, development progressed rapidly until the 1980s, when abrasive cutting was commercialised. Modern waterjet and abrasive waterjet cutting equipment is robust, reliable and versatile. Typically an ultra-high-pressure intensifier (pump) delivers water at up to 4150 bar (60,000 psi) such that it exits the cutting nozzle at Mach 2-3 (680-1020 m/s). Diamond or sapphire is usually used for the nozzle to maximise the cutting life. Where used, the abrasive material is usually garnet or olivine. While most machines cut two-dimensional (2D) profiles, others use a cutting head mounted on a multi-axis manipulator to enable three-dimensional (3D) shapes with angled sides to be cut from thicker sheet. It is also possible to mount a cutting head on a five- or six-axis robots to enable, for example, complex mouldings to be trimmed.
Waterjet and abrasive waterjet cutting are remarkably versatile. On the one hand they can cut thin paper-like materials or soft materials such as foam, rubber and food products; on the other hand, they can cut very hard and brittle materials, including hardened steel and granite. They can also cut composite materials, meat products, and carpet. Very thin materials can be cut with ease, and abrasive waterjet cutting can be used to cut up to 180 mm of concrete or 400 mm of steel. Clearly the hardness and thickness of the workpiece have an impact on the cutting speed, as does the surface finish required.
One application for which abrasive waterjet cutting is proving popular is the processing of brick and stone for decorative architectural features. Manchester Brick & Precast, one of the largest brick cutting companies in the UK, has used the process for several years and has recently upgraded the intensifier on its existing waterjet cutting machine, enabling thicker materials to be cut, delivery times to be reduced, and less time and money to be spent on maintenance. After many years of hard use, the original 25 HP (18 kW) pump, cutting head and abrasive handling system were reaching the end of their useful life. These were all replaced with a 50 HP (37 kW) Classic ultra-high-pressure intensifier, an Autoline cutting head and abrasive handling system, all from KMT Waterjet Cutting Systems.
One of the drawbacks with waterjet and abrasive waterjet cutting used to be the short life available from the nozzle and the seals in the cutting heads and intensifiers. However, developments in these areas have resulted in considerably longer lives. Intensifier seals today can easily last 1700 hours (compared with 40-50 hours for previous generations) and cutting head seals can be expected to last 2000 hours (compared with 60-80 hours). Similarly, developments in abrasive mixing units - such as the latest KMT Feedline IV metering system - help to make the cutting process considerably more cost-effective by reducing the maintenance requirements and the costs associated with machine downtime.
Something else that helps to minimise downtime is improved diagnostics. Intensifiers such as the KMT Streamline SL-V 60 offer a touch-screen control with guided maintenance procedures, as well as remote diagnostics via the internet (Fig. 3). Compared with its 50 HP (37 kW) predecessor, this intensifier is rated at 60 HP (44 kW), which means that it can serve either one 0.35 mm orifice or two 0.25 mm orifices at 4100 bar. KMT also manufactures Streamline intensifiers in power ratings up to 100 HP (75 kW); these bigger units are often used with multiple cutting heads, thereby helping customers to minimise the investment needed to operate a machine with multiple cutting heads or multiple waterjet or abrasive waterjet cutting machines.
The three-dimensional cutting capabilities of waterjet and abrasive water jet cutting have already been mentioned, but this subject is worth exploring further. Flow International Corporation has a video on its website that illustrates the potential of a five-axis robot, with examples of plates being cut while held at an angle to the machine bed, complex aperture profiles being cut into curved surfaces, a thin-walled truncated cone being cut form thick material and, probably the most impressive, a fan with 15 curved blades. The UK's Nottingham University is another organisation at the forefront of waterjet cutting. As well as working on ways to use a six-axis abrasive waterjet cutting machine to create pockets in aerospace components, the Nottingham researchers have recently developed a way to cut thin double-curvature polycarbonate with pure water.
Another way in which waterjet and abrasive waterjet cutting are starting to move away from the traditional two-dimensional applications is in the forming of blind holes. It is well known that the two cutting processes are capable of starting a cut away from the edge of the workpiece by first piercing through the material, but some users and researchers are starting to create blind holes by stopping the flow before the hole has penetrated to the far side of the material. Currently the challenge is to control the shape and depth of the hole, but progress is being made in this area.
For product designers, it could be argued that the most significant development in waterjet and abrasive water jet cutting in the last five years relates to improved machine productivity and, therefore, a more cost-effective process. Nevertheless, the use of five- and six-axis robots also creates new opportunities, and the pocketing functions currently being developed are likely to make waterjet and abrasive waterjet cutting even more versatile in the near future.
Aerospace technology solves Easter egg problem
After developing an innovative spraying technology for decorating chocolate Easter eggs, Thorntons realised it did not have a reliable way to cut spraying masks from 1.5 mm thick polycarbonate, as most cutting technologies either melted or damaged the material. The company therefore approached the UK's Nottingham University.
Twelve months previously experts in the university's School of Mechanical, Materials and Manufacturing Engineering had joined forces with Rolls Royce, the East Midlands Development Agency and the Midlands Aerospace Alliance to establish what was believed to be Europe's first waterjet machining technology centre. The €1.4 million (£1.1 million) centre houses equipment including a six-axis computer-controlled waterjet machine. One of the most advanced of its type in the world, the machine can carve cavities and cut almost anything.
Philip Shipway, Professor of Engineering Materials, said: "The attributes of waterjet technology make it the method of choice in certain circumstances, particularly when heat and high forces need to be avoided. When we were approached by Thorntons, we could see immediately that we had a technology that fitted the bill. That is not to say there were not challenges to be overcome, but a combination of ingenuity and hard work by all involved has enabled us to deliver a first-class solution. Thorntons benefits, and we have developed our know-how; everyone wins."
David Brealey, a chocolatier at Thorntons, added: “We have been working with a spraying company on developing a machine that will allow us to spray chocolate as a decoration; the difficulty we experienced was when we tried to make stencils for our Easter eggs. Trying to cut 1.5 mm polycarbonate over the curvature of an Easter egg was causing the development team a real headache. We had tried pretty much every method of cutting available and found water jet cutting to be the most successful. The problem was getting the cut to follow the curve of the egg. Luckily for us our local university had this state-of-the-art six-axis cutter and initial trials proved very successful. Without the technology and support of the university, the potential of the spraying system would have been very limited.”