laser ablation
test.) Both can be ground or eroded, so laser isn’t the only option, but the earlier methods have limitations. As their name implies, PCD and PCBN cutting tools are made with thousands of tiny crystals held to- gether in a binder, generally cobalt. The toughest grinding wheels (the “superabrasives”) are also made with diamond or PCBN crystals. As you might expect, grinding diamond with diamond is inherently slow and difficult to control. And although the most sophisticated solutions to these problems can produce tools with an extremely good surface fin- ish, grinding can’t produce a per- fectly sharp cutting edge in PCD or PCBN. That’s because the grinding wheel pulls away the PCD/PCBN crystals at the edge, so the edge can’t be any smoother than the grain size of the PCB/PCBN. It’s also virtu- ally impossible to grind the kind of tiny radius that is often desirable on the cutting edge. Erosion can shape PCD and PCBN tools much faster, but pro- duces an inferior surface finish and does no better at the edge. That’s because neither PCD nor PCBN (or the other super-hard materials) are conductive. Erosion uses tiny electrical sparks to burn away material, but it’s the conductive cobalt binder in a PCD/PCBN tool that is being removed, leaving a relatively pitted surface. Thus the surface finish quality is limited to the grain size. By the same token, producing high-quality PCD/PCBN tools with erosion requires fine grain PCD/PCBN.
Laser Cuts Right Through Diamond for a Better Edge and Finish
Laser changes all this. A laser beam can cut PCD and PCBN crystals (not just the binder), creating a sharper edge and a superior finish. Laser can also cut other super- hard materials like CVD-D and MCD that are beyond the capabilities of grinding and erosion. But not just any laser.
LF12
AdvancedManufacturing.org
Á 3D laser ablation is the result of millions of overlapping pulse shots. A laser pulse can be targeted at a spot as small as the wavelength, or between about 500 and 1000 nanometers, and the lasers discussed herein have repetition rates in the hundreds of kHz up to 1 MHz.
As Ronald D. Schaeffer, PhD, of PhotoMachining Inc. (Pelham, NH) explained, there are three key fac- tors to consider in designing a laser for a given application: the wave- length of the light, the repetition rate of the laser pulses, and the duration of each pulse. It turns out that the last factor is the critical determinant in whether or not the laser can cut all the materials in question. Schaeffer said, “In general infrared lasers have a long wavelength and long pulse duration and impart their energy to the material through heat mecha- nisms, making them better for appli- cations like welding. The photons in ultraviolet [UV] light interact with the electronic bonds in most molecules, enabling ablation without heating the material. This makes them bet- ter for micro-machining.” But though UV photons interact quite well with any form of carbon, including dia- mond, UV lasers are too slow to be cost-effective and they can only be used on very thin material. What’s more, the other wavelengths would ordinarily not interact with CVD-D, MCD, or natural diamond at all.
Short Pulses Result in Peak Power Intensity At “normal” pulse lengths (roughly 50 or more nano- seconds per pulse), CVD-D, MCD, and natural diamond are effectively transparent. On the other hand, the phys- ics changes for ultra-short-pulse lasers (i.e., lasers with a pulse duration under about one nanosecond). And the shorter the better, because the peak power for a laser pulse is inversely proportional to the length of the pulse. A nanosecond equals 1 x 10-9 seconds and a picosec- ond equals 1 x 10-12 seconds, so the numbers become a bit mind-boggling. A nanosecond laser with a nominal power rating of 50 watts would deliver kilowatts of en- ergy with each tiny pulse. (The exact wattage would also depend on the pulse frequency and the intensity would vary depending on the size of the focal spot.) Leaving all
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