The largest aircraft manufacturer in the world, an emerging fabricator, and a family-operated machine shop all have one thing in common, they must carefully choose the best cutting tool for their next job. Increasingly, those jobs involve work on composites, complex man-made materials that typically combine carbon fibers and diverse lightweight materials to produce maximum strength. Consequently they are generally very abrasive to the cutting tools used to machine them. Composite manufacturers continue to innovate, introducing new composite compositions and, with the help of creative toolmakers, their products continue to revolutionize aerospace, marine, auto, and other industries.
As other fast-developing technology histories have shown, early-stage development tends to encourage a culture of continual innovation, and the field of composites is no exception. As new products continue to be introduced by competing manufacturers, toolmakers are challenged to innovate and improve ways to cut and fashion structural composite material and parts that are diverse in composition and characteristics.
While composites vary in complexity from industry to industry—and even within industries—most are extremely abrasive. This characteristic greatly affects tool life and performance, and its associated costs. Manufacturers and fabricators working with composites must accept the need to continually innovate with custom-made tools, unlike adhering to the standards needed for more traditional materials. While tool choice and geometries are relatively simple when applied to steel or aluminum, the choices required to work composites are complex due to numerous factors that impact the final job or product outcome, such as per-unit tool cost, machining time, resharpening program cost, cost per hole, and more. A wrong tool choice can critically affect quality, turnaround time, and ultimately bottom-line profitability.
There are currently three tool choices available that can withstand the harsh characteristics of advanced composites. They are: solid carbide, diamond-coated carbide (DCC), and polycrystalline diamond (PCD). All three have features that make them marketable to manufacturers seeking the right tool choice for their specific composite-material project.
Solid-carbide tools have been in use the longest, and are by far the most economical and easiest to work with. Conventional solid-carbide tools continue to be the most widely used tools for work on composite materials, including work inside the aerospace industry, because of solid carbide’s capacity for holding precise geometries and its comparatively low cost. In some aerospace applications, however, where long tool life is required, diamond solutions are specified in spite of the higher tool cost.
Diamond is the hardest substance known to man, but which diamond tool performs best and in which situations? When machining composites, the hardness of the tool’s cutting edge is obviously paramount, but unique project considerations often trump edge-hardness alone as the determining criterion for choosing the best tool. As an example, Lockheed Martin began a project using a PCD tool, presumably for the hardness of its cutting edge. But, after an extensive third-party evaluation and testing program, they replaced a PCD tool with a DCC tool. Not only did the DCC tool reduce initial tool cost, it also achieved a significant solution to another problem, namely delamination of the wing skin composite material, which ultimately was of much greater concern than the cost of the tools required for the job (see sidebar).
The PCD tool is a synthetic diamond product produced by ultrahigh pressure and heat applied to a diamond powder with a metal-matrix binder. There are basically two types of PCD tools. One uses a brazing process, inserting a PCD blank into a groove that has been cut into a carbide tool body. The other uses a sintered-vein process that bonds diamond powder and binder that is poured into a groove along the carbide body. Heat and pressure are then applied to create the PCD, and at the same time bond it to the carbide tool body. Both provide hard cutting edges and are extremely wear resistant. However, PCD tool unit cost is high, ranging from 3 to 5× the unit cost of a competitive DCC tool, and as much as 6 to even 10× that of a noncoated carbide tool that may last less than half the tool-life of the PCD tool. Because of the hardness of the PCD cutting edge, it is a good but expensive choice under certain conditions.
CVD Diamond-Coated Tools
The preferred DCC technology, CVD, was commercialized nearly 20 years ago for nonferrous cutting-tool applications. Diamond coating is a well-established technology for machining graphite and hard-carbon material, and is now being used to improve tool performance for carbon-fiber composite applications.
CVD diamond coatings consist of 100% pure diamond crystals that are grown in a vacuum chamber using super-heated filaments to activate hydrogen and methane gases. The resulting vapor mixture bonds to the surface of tungsten carbide in thickness that is controlled within a range of 3–30 µm.
Unlike PCD-tipped tools, diamond coatings protect the entire cutting surface of complex geometry tools. And unlike the cobalt binder in PCD, diamond does not react with composite resins when machining, and the long life cycle of PCD tools overcomes the need for time-consuming resharpening programs. Increasingly, with the right tool geometries and recent advances in multilayer coatings, even extremely abrasive composite materials are being efficiently machined to exact tolerances using diamond-coated solid-carbide tools.
While the more exotic and long lasting PCD and DCC tools are being increasingly specified, especially for high-volume aerospace jobs, uncoated solid carbide continues to be the most widely used tooling for work on composites as well as traditional materials, and has been in use for over 50 years. Their potential for unlimited cutting geometries, coupled with tool price points at a fraction of PCD and still more than half the price of DCC, makes carbide a good tool choice in most cases. They are also widely and immediately available, and can be resharpened many times, further extending their effective life.
Clearly, related factors inherent in any project must be considered when choosing a cutting tool, starting with the end result in mind. Criteria include type of cutting-tool material, setups and support, such as resharpening programs; skill of the workforce; turnaround requirements; even worker training must all be factored in. Compounding these complexities, fabricators face demanding logistic and scheduling challenges that can influence tool choices as well. Because of these considerations, a custom-tool solution approach is common. Custom toolmakers have accumulated applicable expertise covering a wide variety of situations, careful analysis of variables, and material options in choosing the right tool for the job.
F-35 Cutting Tool Shakedown
Lockheed Martin Aeronautics Co. (LMAC), manufacturer of the innovative and advanced F-35 fighter plane, chose DCC tools for two separate projects after testing and discovering both problems and solutions. One of these solutions reduced the number of tools required per wing from 24 to only 2. That is a 92% reduction in tools required—a huge savings for the planned production run of the 2783+ F-35s.
In the first case, LMAC chose to replace a PCD technology tool with a DCC compression router. The original F-35 wing-fabrication process requires post-mold machining of carbon fiber-reinforced wingskins to achieve precise edge shape and size. Because of the high cost of the overall project and long production cycles required to build over 2000 planes, the life cycle and quality of the originally specified PCD tool was at issue.
The tool was lasting only nine linear feet at one-third the total material thickness and, in the process, causing excessive delamination of the composite material. Apart from the high cost of the tool, the delamination caused by the PCD router failed to meet Lockheed’s quality standards, and thus became an even higher priority concern.
Lockheed contracted the National Center for Defense Manufacturing and Machining (NCDMM—Latrobe, PA) to analyze tool cutting surface geometries within the application parameters to discover existing cutting forces. Their approach not only determined the cause of delamination but led to a way to increase tool life, by using the DCC compression router.
Although compression routers have been around for a long time and are widely used in the woodworking industries, the opposing cutting forces of the up and down cuts make it an ideal tool choice for routing composites and eliminating delamination. Instead of the smearing effect that results with the straight or slightly angled flute of the PCD router, the compression router utilizes the advantages of the helical flute and the cutting forces of the up and down cut to cleanly shear the composite fibers, preventing delamination. Amamco was able to take this traditional tool concept and further engineer it with geometries and tolerances specifically designed for composite.
The compression router LMAC specified incorporates a significant new understanding of tool geometry with the benefits of CVD multilayer diamond coating.
Measurable factors of the F-35 wingskin machining process were: Span Time, Tool Cost, Cutting Distance, and Lockheed’s own EOP QARs (Edge Of Part Quality Assurance Reports). A thorough performance analysis revealed several significant savings factors achieved by Amamco’s CVD diamond coated, compression router. Lockheed fabricators can now machine a complete wingskin using only two cutting tools—one to rough and one to finish—instead of the 24 PCD cutting tools previously required.
The F-35 Composite Drill: Lockheed chose another DCC tool for a second F-35 project that required a process change. The process challenge involved the approach to a composite part with the outer mold line (OML) facing down against the tooling fixture. Because of the process change, breakout of the sensitive OML surface was a major concern. PCD drills were producing good holes but at very slow feed rates, and at an extremely high cost per tool.
Tests using a tool especially designed for the specified composite showed an Amamco supplied diamond coated composite drill to increase tool life by a 450% (the CVD diamond-coated tool produced 1200 holes compared to the 275 produced by the PCD drill on the same task). Test results also showed a reduced span time of 75% and increasing feed rates from 2 to 25 ipm with the CVD diamond-coated tools.
While the standard continues to be the solid carbide tool for machining both traditional materials and composites, both PCD and CVD diamond-coating solutions are increasingly available. Tool choices will primarily be made on the basis of available budget and job-floor factors that are many and complex, and often further complicated by material availability and project schedules that are usually far beyond the control of engineers and production managers. However, all things being equal, two primary attributes determine the cutting tool’s effectiveness and efficiency; cutting-edge hardness and cutting geometry.
Speeds and feeds play a major role in the performance of any cutting tool, especially when working with today’s advanced composites. A measurable factor that contributes to the most efficient speeds and feeds is thrust. Keeping thrust to a minimum while maximizing cutting efficiencies can reduce heat and friction created at the cutting edge. The less heat and friction created, the longer a tool will last, and the higher quality of cut it will produce. Cutting-edge geometries contribute greatly to managing thrust over the life of a cutting tool. Both PCD and CVD diamond tool technologies are designed to manage heat at the cutting edge, and continue to be innovative tool choices where either hardness or geometry will make a significant difference in the quality or cost of the project.
Because PCD has limited geometry options, it tends to have a higher initial thrust threshold. Carbide also tends to have a lower thrust threshold because of its ability to have unlimited geometries. However, this advantage can quickly change because of the friction and heat created in the abrasive environment of composites. CVD diamond coatings unite the best of both worlds. Because the coating itself is a true 100% diamond crystal, it can handle the heat and friction created in the abrasive composite environment. And because it is a coating it can be applied to the geometry advantages of solid carbide. Add in the advantage of multilayer diamond coating, and you have a solution for many of the challenges composites present to today’s manufacturers.
In a perfect world, CVD would last forever and PCD would be capable of unlimited geometries. For now, each has its place among choices made by those who face the challenges presented by the structural building material of the future.