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Figure 1: The EWAG Laser Line Ultra is a system for fabricating diamond tools.
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Figure 2: Edge quality comparison for cutting tools composed of polycrystalline diamond (ILJIN CXL-II) with 2 µm – 40 µm granularity embedded in tungsten carbide. The laser produced edge is smoother than the one prepared using traditional diamond grinding, with virtually no chips on the cutting edge.
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Frank Gäbler, Director of Marketing, Coherent Inc.
Diamond makes a superb choice for a cutting tool. Yet, its extreme hardness makes it difficult to machine the diamond tool itself and prepare the precise geometry of its cutting edge. To meet this need, the technique of choice for precision micro machining of diamond tools is increasingly the industrial ultrafast laser. This article reviews the advantages of ultrafast laser processing for diamond tools, and shows how it has been implemented by a manufacturer.
Diamond Tool Background
To incorporate diamond into a cutting tool, the synthetic diamond is partially formed (by one of several means – see chart) on a substrate which is then soldered or glued on to a cutting tool insert or rotating tool body. The diamond surface is then prepared, and, in the case of a cutting tool insert, the tool is attached to the main cutting tool body.
Traditional approaches for forming the cutting face are mechanical grinding and electrical discharge machining (EDM). Grinding is performed with diamond abrasives, since this is the only material that can shape another diamond. But grinding introduces undesirable microcracks into the workpiece which weaken it. Also, when preparing a polycrystalline diamond (PCD) tool, which consists of diamond grains embedded in a matrix of cobalt and nickel, the abrasive tool can excise individual grains, leaving voids. Manufacturers typically use a small grain size to minimize the size of these voids. However, larger grains hold more securely in the binder material, meaning that they produce a tool with a longer lifetime. There are inevitable compromises involved to maximize the longevity of a PCD diamond tool.
EDM is an effective means for producing cutting edges on hard materials, but it generally can’t be used with non-conductors. This means it can’t be applied to MCD and CVD-D diamond tools. For PCD it can cut the matrix, but not the diamond grains themselves. As a result, the finished cutting edge consists of grain breakouts and voids which are typically half the grain size, which is completely unacceptable when working with larger grain sizes. Even in cases where EDM can process the cutting edge, it doesn’t enable creation of additional tool geometries like chip breakers or chamfers.
Ultrafast Laser Processing
The mechanism and main benefits of ultrafast laser processing have been described previously in this magazine (see “Microelectronics Packaging – As Features Get Smaller, the Role for Lasers Gets Larger,” in March, 2015, Vol. 8, No. 2). Briefly, the very short (<10 ps) pulse durations of ultrafast lasers produce peak intensities which are sufficiently high to cause non-linear absorption and drive a process called photoablation. Here, material is removed in a relatively cold process by which the laser energy directly breaks the molecular or atomic bonds which hold the material together. In contrast, traditional, longer pulsewidth lasers mostly remove material by intense localized heating until it essentially boils away.
The other key characteristic of ultrafast processing is that the laser energy is applied so briefly that the material’s crystal lattice does not have time to respond. Again, the result is that little or no laser energy is transferred into the bulk material, and therefore doesn’t cause significant heating.
This kind of cold processing enabled by ultrafast lasers is particularly advantageous for micromachining diamond tools. This is because intense heating of diamond can cause it to convert from its ultra-hard, tetrahedral, all covalently bonded lattice configuration into the form known as graphite. The layered, planar structure of graphite is not nearly as hard as the diamond configuration. Thus, it is highly desirable to avoid the formation of graphite if the goal is to produce a hard, highly durable cutting edge.
Laser Processing Diamond Tools
EWAG AG (Etziken, Switzerland) is a manufacturer of high-precision tool grinding machines. Their EWAG LASER LINE ULTRA is a complete, integrated system for fabricating tools, such as cutting tool inserts, rotary tools, mill- and drill-bits. It incorporates a Coherent Hyper Rapid 50 ultrafast laser (which produces up to 50 watts of average power, and can also be configured for 100 watts).
The major advantage of this ultrafast laser-based system over previous technologies is that in can create virtually any geometry in nearly any material. Plus, it can produce the entire tool geometry (cutting edge, rake face, flank face, chip breaker structures, etc.) in a single clamping. Creating complex tool shapes using grinding or EDM usually requires reclamping the tool several times, which introduces additional time and manufacturing costs.
Furthermore, these complex tool shapes can be sculpted with high precision using the ultrafast laser, enabling superior optimization of the tool for its specific intended use. For example, small radius (1-3 µm) cutting edges can be fabricated for mill bits intended for wood or plastic, while edge radii in the 4 µm – 7 µm range can be produced for tools for aluminium, and much larger radii can be manufactured for tools intended for large scale material removal. In contrast, achieving such small edge radii with traditional cutting technology typically requires additional polishing or other secondary steps.
When processing PCD, in particular, the problem of creating voids is completely eliminated with the laser; it is a non-contact process which has no tendency to pull grains loose from the matrix. This enables the use of larger diamond grains, and hence, tools with longer lifetime.
The Coherent laser was chosen for this system for several reasons. First, this particular type of ultrafast laser delivers full power at all repetition rates, which isn’t true of disk lasers for example, and this enables more consistent results. Also, the Coherent architecture supports a unique “burst mode,” in which a rapid string of pulses are produced. Burst mode enables the removal of larger amounts of material for quickly “roughing in” a shape.
In summary, ultrafast laser machining is an excellent, universal method for creating virtually any diamond tool edge shape, reducing the number of separate processes and hence operator costs. Unlike earlier techniques, it allows complete optimization of the final tool edge, no matter what type of diamond is being used.