Laser ablation to the rescue on super-hard tools
In addition to carbide, ceramics, and cermet, the drive to create the hardest possible cutting tool materials has given us the alphabet soup of PCD, PCBN, CVD-D, and MCD (polycrystalline diamond, polycrystalline cubic boron nitride, chemical vapor deposition diamond, and mono-crystalline diamond, respectively). These latter materials are among the hardest materials known, natural or man-made, so they’re extremely difficult to shape into usable cutting tool geometries. But new laser technology offers attractive solutions.
Grinding and Erosion Have Their Limits
PCD and PCBN are the most commonly used super-hard cutting tool materials. (“Super hard” referring to a hardness value exceeding 40 gigapascals in a Vickers 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 together 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 finish, grinding can’t produce a perfectly 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 virtually 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 produces 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.
As Ronald D. Schaeffer, PhD, of PhotoMachining Inc. (Pelham, NH) explained, there are three key factors to consider in designing a laser for a given application: the wavelength 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 mechanisms, making them better for applications 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 better for micro-machining.” But though UV photons interact quite well with any form of carbon, including diamond, 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 nanoseconds per pulse), CVD-D, MCD, and natural diamond are effectively transparent. On the other hand, the physics 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 picosecond 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 energy 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 other factors the same, a picosecond laser would be 1000 times more powerful than a nanosecond laser, with peak bursts in the megawatts.
Generally speaking, a picosecond laser is able to cut all the super-hard materials now used for tooling, while a nanosecond laser would be excellent on PCD and PCBN but limited in its applicability to the other materials. To be more specific, Schaeffer said lasers with pulse durations under about 10 picoseconds are “non-wavelength specific,” while above this point you begin to see small differences in absorption in otherwise transparent materials, depending on the wavelength.
A picosecond laser can also sublimate the material, converting it directly from solid to gas, without any transfer of heat energy into the material. Even on PCD and PCBN, a nanosecond laser first heats and melts the material before vaporizing it, albeit on a very tiny scale. In practice such a laser typically heats an area, a plasma forms around this area, and then material evaporates.
Nanoseconds Versus Picoseconds
As Claus Dold, EWAG AG’s head of process technology explained, “You can’t cut pure diamond with a nanosecond laser because it would be transparent. [EWAG is part of the United Grinding group, with its US headquarters in Miamisburg, OH.] But if you were trying to cut MCD-Yellow, for example, there would be a 10–12% absorption using 532-MHz light with a nanosecond pulse. That’s because there is nitrogen and other elements in it that can absorb the radiation. This is just enough to start the process. Once you ablate a little bit of the diamond, you graphitize it. And graphite absorbs all the radiation. So once the process starts you can cut this material. The problem with this is you get thermal damage which can cause defects to the cutting edge.”
The differences are difficult to measure and there is some secrecy among the players. For example, Rollomatic (Le Landeron, Switzerland with US headquarters in Mundelein, IL), won’t say what laser is used in their LaserSmart 500 machine. But their product sales manager for the machine, Sven Peter, said it “achieves a surface finish of 0.1µm Ra on the cutting edge for PCD, MCD, and CVD-D, with a very thin heat affected zone.” He added that the Ra on carbide is only “slightly higher.”
EWAG’s Laser Line Ultra uses a picosecond laser and appears to be equally adept at the super-hard synthetic diamonds and carbide, with the added benefit of removing the material with virtually no heat. “There is some residual heat,” said Dold. “But not enough to graphitize the material. The heat-affected zone is undetectable, which we can prove with Raman spectroscopy. We don’t see any graphite.” Dold said EWAG “guarantees a surface finish Ra of 0.2 microns. Going lower depends on the material.”
So depending on your product mix and tool tolerances you may find a nano-second system acceptable. (EWAG also offers a nanosecond machine, the Laser Line Precision.) As you might expect, picosecond lasers are significantly more expensive, though the price for all laser technology is trending downward and perhaps the cost differential will cease to be a factor.
Tool Grinding Mechanics Versus Laser Mechanics
Of course, actually removing a volume of material to create a cutting tool requires millions of overlapping laser pulses. So these systems have galvanometers that can rapidly move the pulses across a roughly 50 x 50-mm square in the X-Y plane. The scanner can also change the focal point position in Z when needed. Beyond that, the machine orients the workpiece with five mechanical axes. So you could say these are eight-axis machines (three optical and five mechanical).
Unsurprisingly, the two leaders in using laser ablation for super-hard cutting tool manufacturing, EWAG and Rollomatic, are known for building very precise tool grinders. Much of this experience is directly applicable, while some areas enabled, or even required, different thinking. For example, Dold said isolating the optical elements from vibration and designing short, stiff beam paths are critical to ensuring proper beam alignment. The same could be said for designing around a grinding spindle.
Tool grinding experience with kinematics, workholding, temperature control, tool geometries, and software are all essential. But there are distinctions in these areas. For example, although Rollomatic does not otherwise build machines with linear motors, the LaserSmart uses linear motors for X and Y and torque motors for the two rotary axes. Peter said that in addition to superior precision, only linear/torque motors deliver the speed and acceleration needed to efficiently create small radii and similar features. EWAG uses linear/torque motors on all mechanical axes.
In another twist, Rollomatic started with its highly regarded V-block toolholding system on the LaserSmart, but since laser machining doesn’t impart any significant pressure on the tool (there are no grinding forces after all), the company can achieve excellent results with a standard collet system or (more commonly), a standard HSK63 toolholder. EWAG does likewise. And of course, both companies have to remove the vapor coming off the tool, rather than deal with coolant and mist. Evacuating and filtering the vapor prevents laser diffraction as the process continues and maintains a healthy environment in the shop.
In an interesting analogy to grinding with the face of the wheel on the OD, EWAG achieves extremely fine finishes by using the side of the laser beam, essentially using just a line of photons. They call this “tangential machining.” Dold said, “The number of free electrons you have to generate to ablate diamond is 10 to the power of 21 and the same applies to the photons. For a typical cutting tool application, we’d start with roughing, removing quite a bit of material—in the range of 0.1 to 0.2 mm. Then for finishing we would switch to tangential machining in which we are using the side of the beam. The advantage is that you can do very fine surfaces with this. But the material removal rate goes practically to zero.”
Everyone agrees that getting good results in super-hard tooling requires more than just “the laser.” “The key,” said Peter, “is delivering an entire process that combines excellent mechanics with the best means of guiding the beam, controlling the pulses, overlapping the pulses on the tool surface, turning the tool during machining, and so forth.” Because this process is significantly different from tool grinding, Dold said, “EWAG’s strategy is to explain how things work and why some things work better so the customer is able to manufacture whatever he wants. We think the faster people understand what laser can do for them, the faster the market will grow.”
When it’s all put together, the results can be astonishing. Both companies say their machines can reliably deliver a cutting edge as sharp as a one micron radius. While general practice within the cutting tool world would suggest that an extremely sharp edge is brittle and needs to be rounded to achieve satisfactory tool life (referred to as edge preparation), Peter said if you can guarantee a consistent edge (i.e. without any chipping), an extremely sharp tool performs better in many applications.
Just as important and even more challenging, these machines can also control the cutting edge to deliver a specific and accurate edge roundness for those applications where that would perform better. And they can deliver a consistent circular land of any width along the edge. Laser technology is also able to create chipbreakers and other features that were virtually impossible to create in PCD and other super-hard materials with any other method. Profile accuracy is in the range ±3 µm on finished tools. Rollomatic customers have reported accuracies as good as ±2 microns.
Peter said most users are producing the same tool designs with laser they would have made using conventional methods, but that “the superior edge quality and sharpness possible with laser will allow toolmakers to develop new tool geometries with higher performance.” Peter also suggested that laser’s ability to cut almost anything frees the designer to consider entirely new materials and there are rumblings in the industry of just such developments.
Dold said that although the original impetus for laser ablation in cutting tool production was for CVD-D tools, the main market remains PCD. “But this is a new field and everyone is looking into it. There are people who say the thermal damage created by nanosecond lasers in MCD is acceptable as long as the tool lasts and they use this approach. Then again, prices are coming down for both systems. So we’ll see.”
In an intriguing tack, Markus Stolmar, United Grinding North America’s VP, tool division (Miamisburg, OH), said he sees more market potential for laser technology in the carbide market than the super-hard tool market. “The super-hard market will remain small,” he said. “But laser’s ability to create a well-defined edge and special surface finishes, including surfaces that repel chips, may prove to be a bigger seller for carbide tools.” Peter said that “the carbide applications are interesting and multiple. We will see more applications with combinations of diamond and carbide materials being machined on lasers.” So with a laser pun fully intended: Stay tuned!