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Waterjet aids innovation and cuts costs

04 September 2017

Whether as a designer or manufacturing engineer, abrasive waterjet may not be the first cutting method you think of, but it deserves a close look to see its many advantages over other processes, especially as it now offers advanced 5-axis 3D forming as well as intricate 2D profiling, all on one machine plus other benefits.

Bystronic 3D waterjet cutting an impeller from aluminium plate

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If, for learning purposes, you wanted to cleanly cut a smartphone in half, minus battery, to show what’s inside, what process would you use? In principle, only one – the cold-cutting abrasive waterjet - might enable you to do that. 

Less dramatically, most designers and manufacturing engineers know the range of methods applicable to cut or shape parts out of sheet, plate and block material. But how many immediately think of waterjet?

Becoming a practical tool back in the 1930s, the technology was mainly confined to cutting soft materials. Thanks to impetus from industries such as aerospace, wanting to cut exotic materials like titanium, developments stepped up, especially in recent decades. 

For many years only 2D profile cutting was possible, but now 3D shaping has broadened the scope of possibilities. Through 5-axis computer numerical control (CNC) and advanced rules-based software to optimise cutting speed and hasten throughput, complicated 3D forms may be produced, providing an alternative to conventional milling. 

To reduce material handling, increase productivity and boost efficiency, today’s technology enables two cutting heads to be operated simultaneously, together with 2D and 3D processing if required, all on one machine. Due to its flexibility, quick set-up time and no need for special tooling the process is ideal for rapid prototyping and bespoke production. All these developments have placed waterjet technology firmly alongside established cutting and forming processes.

Pure and abrasive cutting

There are two types of waterjet – pure and abrasive. Pure means water alone is pumped through a small circular orifice at high pressure, but will only cut a number of soft materials. Abrasive waterjet is another matter. Here, a fine cutting medium, such as garnet sand, is added to the water. Pumped at 60,000 to 90,000psi - supersonic velocity - the resulting jet will cut practically any material, thick or thin, accurately and quickly. 

Materials include all ferrous and non-ferrous metals, plastics and composites including carbon fibre, Kevlar and GRP, wood, most ceramics, un-tempered thin or thick glass, quartz and stone. Cutting MDF particle board is not recommended and generally, any material affected by water absorption is not ideal for waterjet. At the other extreme, the process will cut tricky-to-machine metals such as titanium and super-alloys. To machine these metals is costly and requires special care, but using waterjet may overcome difficult issues.

A key feature of the process is its flexibility in handling a wide range of thicknesses, from thin sheet, to thick plate and even thicker block material, for example, 200mm thick granite. Apart from the building and construction industry, cutting granite may be a specialised requirement. However, to exploit the feature, think about stacking a number of sheet materials together, cutting one shape and getting many parts as a result. Multiply this by economically cutting many nested shapes and you have big numbers. 

Key advantages

Bystronic waterjet in action

The waterjet kerf is particularly narrow, 0.8mm diameter is typical, but smaller jet sizes are available. A narrow kerf means fine details are possible, including sharp right-angle corners and holes of small diameter, plus slots with precise semi-circular or square ends.

A narrow kerf also means less waste, both in cutting and nesting. Through an automatic software function, parts may be nested or packed very closely together, squeezing more parts out of a sheet or plate and extracting greater value, especially if the material is costly. 

Cutting accuracy is in the order of plus or minus 0.003” (0.0762mm), not as high as wire EDM or milling, but comparable to fibre laser and much better than other cutting methods. Cutting speed and throughput depends on material and thickness, but is generally much faster than wire EDM, with fibre laser quicker on thin materials

Don’t forget, speed isn’t everything. Heat-based cutting methods may generate heat-affected zones (HAZ). Cost saved on faster speed may have to be spent on fettling or time-wasting secondary processing – expensively removing the oxide. In comparison, because waterjet is a cold cutting process, the cut or edge quality is very clean and no HAZ occurs. Any heat generated, when each micro-particle of abrasive removes workpiece material, is blasted away in cold water exiting the cut.

Crucially, cold cutting means no risk of heat-induced structural or molecular change, or hardness formation, or distortion or mechanical stress concentrations forming as a part cools. What’s more, no micro-fissures are created (always a risk with milling). Fissures may propagate into larger cracks, ultimately causing failure of the part. The cold cutting advantage may be especially appealing to designers of high-stress components for high-risk applications, for example in the aerospace, automotive or rail transport sectors.

Additionally, from a ‘green’ perspective, the process is environmentally friendly, no hazardous waste products are produced. 

Case studies prove the point

A customer wanted a better, less costly way to make three sizes of plain brass bearing. These were being machined from three sizes of bar. After looking at the sizes and potential for exploiting the narrow kerf and depth of cut possible with waterjet, it was seen each bearing could be accurately cut out of just one piece of bar, each nesting within the other. 

No other machining method could do this, apart from maybe EDM, but that would be far too slow and costly. Once cut by waterjet, the bearings were precision machined to create finished parts. Done in one phase on one workpiece, there is now none of the waste produced before. This achieved major time and cost savings.

The process offers a flexible, low-cost and efficient method of cutting and forming a variety of materials, with far more scope than many others. Studying its capabilities and characteristics may spark new ideas - for a designer, discovering fresh possibilities for form and function. Equally, compared to a current and costly cutting method, with waterjet, a production engineer may be able to obtain a bought-in part at less cost or better quality. 

Whatever the case, it’s always worth talking to an expert with experience in the many applications of the technology. Better still, go and see your sample being made. And you can always ask the question, what does happen if you waterjet a smartphone?

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