Arthur Turner, specialist in micromachining
Waterjet cutting has been around for the best part of half a century. It is generally regarded as a cutting process for large and thick components with quite open tolerances, bringing visions of large steel plates spring to mind, but this is a misconception. The phrase ‘waterjet microcutting’ is very much a part of today’s vocabulary, but its capabilities still need much wider recognition in the micromanufacturing world, as it can be a better machining process to complement existing technologies.
Waterjet cutting or microcutting is used in two ways, namely:
- with high-pressure water for the cutting of softer materials; and
- with high-pressure water containing abrasive for the cutting of harder materials such as tungsten carbide and even bulletproof glass.
Bullet proof glass: Waterjet microcut components in bullet proof glass.
The decision as to which process to use depends on the type of material to be cut and its thickness.
When microcutting, a start hole is required and this is generally produced either by the waterjet itself or a drilling head on the machine. Needing a start hole is something that wire eroding and waterjet cutting have in common but here is where their process capability changes.
The user needs to think about exactly what they want to achieve, as in our 21st century engineering industry, more and more complex materials are being developed. Wire eroding is good for metallic materials and can achieve very tight processing tolerances. Waterjet cutting, on the other hand, can be used on a wider range of materials, from polymers to glass and sapphires, while coping well with ferrous materials.
4 hearts in different materials: Four waterjet microcut hearts in zirconia, mother of pearl, carbon fibre reinforced polymer (CFRP) and aluminium.
A significant benefit of waterjet cutting is its ability to machine ceramics, since they are widely used in the electronics industry. A good example is a 12 mm wide aluminium oxide component, which was produced in 24 seconds at a feed rate of 245 mm/min. This would be extremely difficult to machine by any other method.
Ceramic, aluminium oxide: A 12 mm wide, waterjet microcut component in aluminium oxide.
A key feature of any design is the tolerance applied to each dimension and it is here where waterjet cutting has made massive gains that have enabled it to morph into waterjet microcutting. A typical market-leading machine now offers a positioning tolerance of approx. ±2.5 µm with a cutting process tolerance of approx. ±10 µm. In addition, it will incorporate a smaller focusing tube that affords jet diameters down to 50 µm and 200 µm when cutting with purely water and water containing abrasive, respectively. These advancements give product designers a free hand in producing their components. A couple of impressive waterjet microcut component examples that I have come across are:
- a small 0.18 mm thick stainless-steel component that was produced in only 16 seconds; and
- a Nitinol (shape memory material) spring clip with a complex design that was produced in less than two minutes.
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Nitinol spring clip: A waterjet microcut spring clip in Nitinol that took less than 2 minutes to produce.
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Machining Nitinol clip plate: Waterjet microcutting of a slip plate in Nitinol.
Both components were produced much quicker than with wire eroding and considerably cheaper than with stamping. Furthermore, they could be manufactured in volume from one piece of sheet material, thus eliminating a load/unload time for each individual component.
Naturally, the size of the waterjet influences the feedrate that can be used. Recently, Finepart Sweden conducted tests on a 2 mm thick Inconel component, observing that cycle times of 26 min 53 sec and 10 min 20 sec were achieved using 0.3 and 0.5 mm jets, respectively. Also, in tests on a 2 mm wide aluminium component, the company reported cycle times of 5 min and 2 min 30 sec with the 0.3 and 0.5 mm jets, respectively. Examples of the cutting speeds for stainless steel are shown in the table below with the conditions achieved (Q definitions) on the surface.
The size of the jet also influences the amount of water and abrasive to be used per minute and this needs to be appreciated in the cost of the component, for example, 0.2 l of water and 10 g of abrasive will be used per minute with a 0.2 mm jet, while 0.4 l of water and 25/30 g of abrasive will be used per minute with a 0.3 jet. So, increasing the jet diameter by 0.1 mm more than doubles both the water and the abrasive being used.
Another factor to consider is what the best surface finish that can be achieved is. For waterjet microcutting, an Ra 0.8 value is given but this depends on part thickness, and the thicker the job, the less this is going to be.
Today, and this applies to all machining techniques, 5-axis operation is important for some components. This is offered in two different configurations, depending on the machine. In addition to the usual 3-axis movement, one configuration incorporates a two-axis rotary table (for 4th axis rotary and 5th axis tilting movements), and the other configuration incorporates a single-axis rotary table (for 4th axis rotary movement) and a tilting cutting head (for 5th axis tilting movement). These two configurations offer different manufacturing capabilities.
When machining thicker components on a 3-axis system, some slight tapering can be experienced due to the jet force deteriorating as it cuts through the material. This is overcome with a tilting spindle that can be adjusted (via programme) to ensure a vertical wall is produced.
Material, component size and tolerance challenges ensure that different machining techniques are required, and waterjet cutting is becoming ever more competitive.
Arthur Turner, specialist in micromachining