PCD TOOLING JUSTIFICATION REPORT

The improvement of cutting tool materials took a quantum leap in the early 1970s when General Electric Co introduced a variety of polycrystalline diamond (PCD) cutting tool materials trademarked Compax diamond. This PCD material consists of a layer of micron-sized diamonds integrally bonded with a carbide substrate. The diamond layer's abrasion resistance, coupled with the carbide's strength, produces a cutting tool material that achieves a tremendous increase in machining performance. PCD is primarily used in non-ferrous metalworking applications such as copper and aluminum or to machine plastics, rubber, synthetics, laminates, and woods. GE next introduced BZN Compacts, polycrystalline cubic boron nitride (PCBN) used for machining ferrous materials such as gray cast iron. PCBN is manufactured like PCD, except a layer of cubic boron nitride crystals replaces the diamond.The properties of hardness and abrasion resistance that make polycrystalline tools superior cutting devices also make these tools extremely difficult to grind and finish. Grinding the tools to proper dimensions and configurations is always a challenging task for today's toolmaker. Unlike conventional cutting tool materials, PCD and PCBN require certain considerations to be effectively ground. These include the basic machine design, selection of the grinding wheel type, grinding methods, and a close attention to detail.

Today's technology and machine capability lend itself to straightforward adaptations of PCD and PCBN tool grinding. A key to grinding polycrystalline tools is using a machine that is capable of performing to the level of precision required for polycrystalline tool fabrication. The machine's design is a prime consideration for grinding these materials and should have a number of features to aid in this task.

These features should include a variable speed spindle to handle all types of grinding wheel bond systems. Resin, metal, and vitrified bond systems each grind properly at a different optimum speed. For instance, metal-bond wheels run at slower speeds than resin and vitrified bonds.

The grinder should also be equipped with at least a 2.5 hp motor. Unlike grinding high-speed steel or tungsten carbide, polycrystalline tools require more horsepower to avoid fluctuation in the spindle speed as the wheel's load increases. If a motor lacks the necessary power, the grinding wheel vibrates or creates too much heat and pressure. This leads to chipping on the edge of the polycrystalline cutting tool, as well as damage to the grinding wheel face. The ability to control tool pressure assists in preventing these problems from occurring.

For the most accurate tool configuration and design, the machine should include an optical comparator so radii can be ground without removing the tool from the setup. This eliminates the risk of losing geometric size and form on the tool. Another consideration when grinding polycrystalline tools is the machine tool's rigidity, the stiffness and vibration-free characteristics of its base, which are critical for good edge quality on the finished tool.

Not the same old grind

As technology and knowledge of the grinding process evolved, natural diamond and later manufactured diamond and cubic boron nitride were chosen as preferable cutting tool abrasives for the harder non-ferrous (carbides, ceramics) and ferrous (hardened steel) tool materials. However, the techniques and expertise that toolmakers had developed in grinding conventional tool materials, such as carbide or steel, could not be transferred directly to grinding advanced polycrystalline materials.

 It was concluded that only a diamond wheel with the proper design, bond, and crystal characteristics could effectively grind PCD and PCBN cutting tools. The diamond wheel is durable and can be tailored to have the proper diamond type, crystal size and friability to withstand the forces that are generated during the grinding process.

Grinding wheel manufacturers developed bond structures to take full advantage of General Electric's diamond product lines. RVG Diamond provided them with a range of crystal sizes to use for the rough operations, while MBG Diamond and Micron complemented this operation with crystals suitable for precise finishing operations. Due to the friability, or the ability of the abrasive crystal to fracture and regenerate sharp points, of resin bond crystals (RVG), the resin bond diamond grinding wheels maintained an open, aggressive cutting face. However, the finish quality of the radii on the polycrystalline tools needed improvement. As technologies advanced, metal and vitrified bond systems began to gain prominence.

Metal bond wheels were used predominately in the finish grind operation and achieved mirror-like polish and chip-free edge when magnified at 30X. Grinding wheel performance and quality results now matched the needs of the polycrystalline tool fabricators. Metal bond compositions, however, had some disadvantages. The metal bond wheels acted hard and were slow cutting, requiring frequent dressing to keep the wheel face open. The toolmakers ultimately wanted a wheel that blended the cutting action of the resin bond wheel with the edge quality and polish achieved with the metal bond wheel.

The solution to this problem: vitrified wheels. This aggressive and free cutting bond structure adapts to various spindle speeds with minimal adjustments; porosity minimizes wheel loading during the grinding operation; and its free cutting nature eliminates the time and delays associated with frequent wheel dressing.

The evolution of the vitrified bond system has enabled polycrystalline tool fabricators to choose from a range of products within this bond family. Vitrified diamond grinding wheels meet the needs of the toolmaker primarily concerned with increased productivity (total number of tools produced), as well as the toolmaker influenced more by wheel cost and parts per wheel ratios. The various alterations in abrasive mesh size and bond characteristics allow today's toolmakers to customize their operations to meet their customers' needs and satisfy their own internal production goals. The same technology developed with the resin bond system for using the different abrasive mesh sizes also applies to the vitrified bond systems. Grinding wheel manufacturers became adept at mixing the right combination of ingredients to provide each polycrystalline tool fabricator with the "right wheel for the job."

Treat the wheel right

A diamond grinding wheel will not be as efficient if it is not handled and prepared correctly, even with all the new technology. When a new wheel is mounted, it should be trued with a silicon carbide wheel and brake-type truing device so that it is concentric and presents a uniform surface to the cutting tool. When the procedure is completed, the grinding wheel is smooth and glazed, with no crystal protrusion. Dressing the wheel with a medium-hardness aluminum oxide dressing stick removes the bond material from around the crystals and creates a good crystal protrusion for the grinding operation. The dressing stick should contain particle sizes close to the grinding wheel's mesh size to optimize wheel flatness and eliminate the risk of over-dressing the wheel or chipping tools.

For more information from Superabrasives Inc, Wixom, MI, circle 354. (Originally published in the September 1998 issue  of Tooling & Production.)