Greg Blackman on the benefits of laser machining tool bits or production moulds, including cutting diamond tooling and laser polishing metal moulds
Making moulds and tooling for manufacturing can be an expensive prospect, which in itself can be a reason for turning to laser processing as there are no mechanical tool bits to get worn down and need replacing. Additive manufacturing is the big hope for the future of tool making, as it means highly complex tool designs or those that would otherwise be too expensive to make can be printed on-demand. In the meantime, however, there are areas where lasers can be a benefit in the mould or tool making process, such as when producing tools made from hard materials or in polishing tool bits.
Diamond tooling is one example of a superhard material where there are definite benefits to using a laser to sharpen the material rather than working it mechanically. ‘You can cut and sharpen these materials mechanically, but the huge advantage of shaping these tools with a laser is that it allows for more complicated contours and can provide a finer edge finish,’ explained Dr Dirk Müller, director of marketing at laser provider Coherent.
Cutting mechanically removes grains of materials, because the metal cutting tool and the polycrystalline diamond might both have a certain granularity. That leaves a jagged edge on the microscopic scale. ‘If you do it [sharpen] with a laser, rather than removing granules, very thin layers of material are removed,’ Müller added. ‘Therefore, you can get a very sharp and very precise edge.’
Diamond tooling prolongs the tool life, which means the end user doesn’t have to change the tool bit as often. This results in higher machine utility and less downtime. A diamond-based tool bit might be two or three times more expensive than a standard one, (even standard tool bits are not cheap), but the diamond tool has a much longer life. It then becomes an economic question of whether a diamond tool bit is a worthwhile investment.
Coherent has supplied ultrafast lasers for machine maker Ewag for its Laser Line Ultra tooling production system. The laser machine can cut polycrystalline diamond and tungsten carbide, and also shape the cutting edge with the laser.
Ultrashort pulsed lasers – in the case of the Ewag machine, pulses of picosecond duration – are necessary to cut diamond. ‘Materials like diamond or tungsten carbide are not so easy to machine with any laser,’ said Müller.
Most laser processes are based on the fact that the laser melts the material, which evaporates locally or is forced away from the cut. High melting point materials such as diamond or tungsten carbide are less easy to ablate by melting. In the case of diamond there is the additional challenge that diamond is transparent to most laser wavelengths and hence the laser energy can’t be coupled into the material.
Ultrafast lasers, on the other hand, use a different mechanism to ablate material. ‘They [ultrafast lasers] can machine virtually any material, because the basis of the ablation is at the atomic level,’ explained Müller. The electric field from an ultrafast laser strips electrons from the surface of the material. What’s left behind are positively charged surface ions, which repel each other. ‘The laser doesn’t melt the material, but breaks the molecular bonds at the material surface. This mechanism applies to any material. Ultrafast lasers are ideal for processing brittle, hard to machine materials like diamond, sapphire or glass,’ he commented.
Polished to perfection
It’s not just very hard materials where processing with a laser can have benefits; laser polishing of metal tools or moulds provides a good surface finish without ablating material.
‘Conventional polishing uses abrasives and removes material to get a smooth surface,’ explained Dr Edgar Willenborg at Fraunhofer Institute for Laser Technology ILT. ‘Laser polishing doesn’t remove material, but redistributes it by melting.’
Fraunhofer ILT has been working on polishing metals for around 10 years, and, last year, Dr Willenborg organised Fraunhofer ILT’s first conference on laser polishing. The basic principle of laser polishing is a remelting process, where a thin surface layer of material is melted and the surface tension redistributes the material for a smooth finish.
There are two process variants: macro-polishing with continuous wave (CW) lasers and micro-polishing with pulsed lasers. For macro-polishing the remelting depths are between 20µm and 80µm, noted Dr Willenborg, commenting that ‘standard milled, turned and EDM-processed surfaces with an initial surface roughness of a few micrometres Ra can be polished in this way’.
Laser micro-polishing is used with very good pre-processed surfaces like grinded surfaces. Here a pulsed laser with a pulse duration of a few hundred nanoseconds is used to smooth the micro-roughness. The process, however, can’t remove milling marks on a surface, noted Dr Willenborg. The remelting depth is typically around 1µm.
‘Laser polishing is suitable for a medium quality finish. Typical roughness after laser polishing is Ra 0.1µm to 0.3µm, depending on the material,’ commented Dr Willenborg.
YAG lasers with a wavelength of around 1µm are typically used to polish metals. A round laser spot of a few hundred micrometres is used depending on the initial roughness – larger spot sizes generally polish rougher surfaces, while small beam diameters give a fine processed surface, said Dr Willenborg. The laser scans the surface in a meandering pattern to give an even polish.
‘Many polishing tasks are still a manual process. When you have complex 3D parts like in tool and mould making, you often manually polish these parts. The laser can automate this process,’ stated Dr Willenborg.
‘There are also cost advantages of using a laser, not for all applications, but already for some,’ he continued. ‘For other automated solutions like tumbling or slide-grinding, it’s often a question of cost, although the laser does provide other advantages.’
Tumbling, where the parts are placed in a vibrating container along with abrasive polishing stones to produce the finish, is very cost-effective and a lot of 3D parts can be polished. However, polishing parts to high manufacturing tolerances, or when component edges shouldn’t be rounded, then tumbling cannot be used. In these cases, said Dr Willenborg, the parts will be polished manually and, again, a laser can automate this process.
A further advantage of the laser is that it’s a clean process; it doesn’t need any abrasives or produce any dust. Therefore, no post-cleaning is required. With medical parts, for example, lasers do not need polishing lubricants – chemical agents cannot be introduced when manufacturing implants, for instance.
‘Macro-polishing with a CW laser takes between 10 and 60 seconds per square centimetre – nickel and titanium alloys take 10 seconds per square centimetre, while iron-based alloys or steels take 30-60 seconds per square centimetre,’ said Dr Willenborg. A typical value for micro-polishing is 3.3 seconds per square centimetre, he noted.
‘In the last few years, the laser polishing section at Fraunhofer ILT has concentrated on 3D laser polishing, to achieve the results we’ve had for flat surfaces on 3D surfaces,’ remarked Dr Willenborg. ‘The main topics here are process development, machine development, and also software development. We now have the machine to polish complex shapes and so we are turning back to the basics and focusing on enhancing the speed of the process, reducing the roughness further, and looking to applying the laser polishing to different materials.’