Water jet cutting steel anvil

Cutting metal without touching

Alternative cutting technologies, which let you cut metal and other materials non-mechanically using lasers or water, are becoming increasingly powerful and accurate.

In workshops around the world, the clatter and whine of tool against metal has been supplemented - or in some case supplanted - by new noises, in particular the roar of high-pressure waterjets and the crackle and hiss of lasers instantly vapourising metal.

Now these non-mechanical cutting technologies are changing too, acquiring new capabilities, higher resolutions and better efficiency.

If you doubt water's ability to cut hard materials, just think of the Grand Canyon. Granted, it was on a longer turnaround than the average cutting job, but by using a thin high-pressure jet - initially of water alone, but then including abrasive powder - the process can be made to work rather faster.

Cutting machines of this type are now widely used. An inevitable consequence of employing water is of course that it has to go somewhere, so a waterjet can usually be recognised by the large tank that absorbs the leftover power of the jet once it has cut through the workpiece. The latter is mounted upon an array of vertical slats sitting in the tank.

'Waterjet cutting is generally viewed as more efficient in energy terms than machining,' says Jeff Day, a cutting specialist who is now sales manager at US-based waterjet developer WardJet. 'The abrasive is inert, the materials are generally not an issue, though you might have water disposal issues if you're cutting lead or beryllium, say. And the waste is typically in chunks, which get a higher recycling price than the swarf from machining.'

They also enable parts to be closely nested to maximise material use, and eliminate potentially hazardous airborne dust particles, smoke and fumes. Water usage is relatively low - a few litres per minute - and the used water can either be recycled or filtered and sent down the drain.

Jeff Day says that while a machine can get through significant quantities of abrasive, which is usually powdered garnet, this too can now be dried and reused: 'There is a fair amount of interest in recycling abrasive - we manufacture a recycling system and have a calculator that shows when it is worth doing. It balances the cost of the downtime to dig out the tank, and the cost of disposing of the used abrasive, versus the recycling cost.'

He adds that there are many different types of abrasive, and each recycles at a different rate. With one type you might get 30 per cent back and with another 50 to 60 per cent, while the very finest powder goes through the screen and can usually be sent to landfill.

Five-axis CNC

One of the most significant recent developments in waterjet technology has been the shift to five-axis CNC cutting, driven by advances in control technology. To the normal XYZ axes this adds an A angle-from-perpendicular axis, allowing bevels to be cut, and a C clock rotation axis, turning around the Z height axis.

This makes the machine far more flexible and capable, but it also makes it more challenging to operate. 'With the extra axes you could potentially tilt it 90 degrees from vertical, so you have to pay more attention to safety than with simple vertical cutting where it goes straight into the tank,' Day explains. 'You also need to avoid cutting the tank. So we often ask the customer: 'It can go 90 degrees, but realistically what do you need?'. Then we put appropriate limits on the machine.'

He adds: 'You also need a height sensor for five-axis cutting. If you're cutting an irregular material it's not too bad when you cut vertically, but when you're cutting at an angle, a bow in the material can dramatically affect your tolerances. So a height sensor rings the nozzle and follows the material surface.'

Waterjet pump pressures are also increasing, notes Martin Engel, communications manager at Swiss machinery developer Bystronic. He says this is particularly useful at the macro scale, cutting metals over 150mm thick, but it also speeds up cutting in general. The downside, he adds,'is that 'The stress on the pump is not linear; it goes into incredible numbers. So, while some companies see high power as a selling point, others argue that a moderate pressure - perhaps with more cutting heads in parallel - is more efficient for most applications.'

'There is certainly a lot of interest in cutting with higher pressure,' agrees Jeff Day. 'The debate is whether it's worth the extra cost, for example a 90kpsi nozzle is four times the cost of a 60kpsi nozzle. So, sure, it cuts faster, but it's more downtime - both to replace nozzles, and because it cuts your slats quicker too, so they need replacing sooner. For a 200-300mm constant cut, high pressure might be justified; for a job shop doing variable cuts, probably not.'

Laser cutting too is growing in power. CO2 gas lasers can go from 1kW to 30kW or more, with 4kW or 6kW being typical, and the speed of the cut makes it faster than waterjet cutting. Feed rates of nine metres a minute or more are feasible for thin sheet, for example.

The biggest shift though is probably the arrival of solid-state lasers. These come in two forms, either disk or fibre - the difference is in the shape of the active gain medium, which provides the optical gain within the laser. As well as cutting, fibre-guided solid-state lasers of either type can be used for welding, even of dissimilar metals.

'Disk or fibre' is like a religious debate - for example Trumpf goes for disc and we go for fibre,' says Bystronic's Engel. He adds: 'Solid-state is particularly good for thin sheet, especially up to 4mm using 2kW lasers.

'It is very fast - faster than a gas laser and better quality, because it is a different wavelength. It is still UV, but with a much shorter wavelength, closer to the visible spectrum. It can cut up to 21mm, but here there is less of an advantage.'

He notes that, with the light that much closer to the visible spectrum, machines using solid-state lasers need to be encapsulated and fitted with doors to protect the operator. Laser cutting also produces smoke and particulates from the vapourised metal, which must be removed.

Laser vs water

'In general it is easy to answer the question of laser versus water - whenever you are able to use laser, it is the best price option,' says Engel. He warns, though, that the laser will typically yield slightly lower quality - in particular it can burn the edges of the workpiece,'with oxides forming on the cut surface. By comparison, an abrasive waterjet is unlikely to heat its target past 50C.

Laser's cost-effectiveness is in large part down to speed - it is many times faster than mechanical cutting, and although it cannot match the speed of a punch press, it is faster and cheaper to set up. 'Versus a punch press, it depends on the run size,' Engel notes. 'For five parts, don't bother tooling, but for 100,000, tool! Where you have very large lot sizes, laser is too slow.'

However, you can only cut certain materials with a laser - or, for that matter, with a punch press - whereas a waterjet can cut almost anything. Engel estimates that a laser can cut steel up to 25mm, stainless steel to 20mm and aluminium to 15mm; other industrial laser cutters can also handle titanium, paper, wax, plastics, wood, and fabrics.

Today's common CO2 metal cutting lasers are not suitable for copper and brass, due to the laser wavelength and the surface reflectivity, but the newer solid-state lasers produce a different colour of light and can cut these materials.

Where waterjets take over is for deeper cuts - for example, laser is price-competitive for cutting aluminium up to eight or 10mm thick, then from 10 to 15mm you could use water - and a wider range of materials. As well as metals, waterjets are commonly used to cut rubber, foam, plastics, composites, food, stone and tile - even carpet tiles. Waterjets can also be used outdoors, for example to cut metal or concrete in civil engineering, and mobile versions have been developed for bomb-disposal work.

The list of materials that cannot be cut by waterjet is relatively short; notable examples are tungsten carbide, tempered glass, certain ceramics, and diamond.

Development in both technologies is continuing, of course. Both are shrinking in size - even schools and small workshops can now consider a laser cutter, and while the need for a tank underneath does limit how small a waterjet cutter can go, a 1.2m by 1.5m model can still be very useful.

The two have also been combined to create a water-guided laser which eliminates laser's burning problem. This uses a fine waterjet as the waveguide for the laser, and it has been used to cut and drill silicon wafers, brass sheet and titanium alloy, among other materials.

It will be a long time before milling machines and the like are finally replaced, if indeed they ever are. In the meantime though, alternative ways of cutting and working all sorts of materials may not always be cheaper, but for sure they can offer a whole lot of advantages of their own.

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