Grinding Polycrystalline Diamond & CBN Tools
Methods For Tool Fabrication Reach New Levels Of Sophistication
By Charles Halprin, President
The introduction of harder and more abrasive workpiece materials and bi-metal components have led to the improvement of a finished part's physical properties, such as overall strength, better fatigue characteristics, and weight reduction. These advances allow these improved materials to be used for components in more critical applications like gears, drive trains and engines. However, these harder materials require cutting tools that are capable of shaping and forming the material into a finished part expeditiously, cost effectively and with high quality.
Metalworking tools (inserts, shanks, etc.) have evolved to match or surpass the hardness and abrasion resistance of these materials they machine, as well as improve in both composition and design. Carbon tool steel, high-speed steel, cast alloys, cemented carbides, ceramics, cermets and other variations all are used to machine these harder materials.
The improvement of cutting tool materials was not complete until the early 1970's when General Electric Company 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 nonferrous metalworking applications such as copper and aluminum or to machine plastics, rubber, synthetics, laminates and woods. GE's BZN* Compacts, polycrystalline cubic boron nitride (PCBN) is used for machining ferrous materials such as gray cast iron. PCBN is manufactured like PCD expect a layer of cubic boron nitride crystals replace the diamond. The very properties of hardness and abrasion resistance that make polycrystalline tools superior cutting devices also make these tools extremely difficult to grind and finish. Grinding these tools to proper dimensions and configurations is an ever challenging task for today's toolmaker.
Unlike conventional cutting tool materials, PCD and PCBN require certain considerations in order to be ground effectively. These include the basic machine design, selection of the grinding wheel type, grinding methods and a closer attention to detail.
Machine Design Paramount
Today's technology and machine capability lend itself to straightforward adaptation 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 in order to accommodate all type of grinding wheel bond systems. Resin, metal and vitrified bond systems each have a different optimum speed at which they grind properly. For instance, metal bond wheels are run at slower speeds than resin and vitrified bonds. Therefore, a change in the grinding wheel's bond material requires a spindle that is capable of making the necessary adjustment. The grinder should also be equipped with at least a 5hp 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, leading to chipping on the edge of the polycrystalline cutting tool along with damage to the grinding wheel face. The ability to control tool pressure will assist in reducing 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 tools' rigidity - the stiffness and vibration-free characteristics of its base. A machine with a cast-iron base and a tolerance of accuracy to .0001 inches indicate that the machine tool builder had precision in mind. Rigidity and high tolerance of accuracy 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 nonferrous (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 the advanced polycrystalline materials.
While some toolmakers initially tried to use conventional abrasive wheels and standard resin bond diamond wheels to grind polycrystalline tools they were confronted with a number of performance and quality issues. Conventional wheels had excessive wheel wear which resulted in rapid degradation of the wheel face contour. Continual redressing and truing was needed to keep the wheel performing at even a low efficiency. This was time consuming and troublesome. Standard resin bond diamond wheels had some success but resulted in high tool costs. The O.D. cutter grinders and surface grinders being used were not adequately equipped to handle this type of grind and the end results were wheel usage costs that escalated.
Toolmakers were forced to discard familiar parameters of speeds, feeds and grinding techniques, and learn new procedures for PCD and PCBN tool grinding and finishing. A modern generation of wheels and machines has resulted from this need. The toolmaker now has become an artisan and technologist as new procedures are learned to properly finish these modern materials.
Improved Diamond Wheels Deliver Superior Results
It was quickly concluded that only diamond could effectively and efficiently grind PCD and PCBN cutting tools. A diamond grinding wheel with the proper design, bond and crystal characteristics delivers the necessary precision and wear resistance for grinding polycrystalline 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 around General Electric's RVG diamond product line, which provided them with a range of crystal sizes to utilize for the rough operations. GE's MBG product line complimented this by furnishing product for the finer finishing operations. Due to the friability (ability of the abrasive crystal to fracture and regenerate sharp points) of resin bond crystals (RVG) the resin bond diamond grinding wheels continually maintained an open, aggressive cutting face but the quality of the radii on the polycrystalline tools were poor. As technologies advanced and the need for polycrystalline tools intensified, metal and vitrified bond systems began to take over.
This push toward improvement strengthened the use of metal bond diamond wheels. Metal bond wheels were used predominately in the finish grind operation and achieved a mirror-like polish and chip free edge when magnified at 30X. Matching the needs of the polycrystalline tool fabricators. But this bond structure had disadvantages, too. The metal bond wheels acted hard and were slow cutting, requiring frequent dressing to keep the wheel face open. The toolmakers 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 answer - vitrified. And the advantages of vitrified bond systems became apparent quickly. This aggressive and free cutting bond structure adapts to various spindle speeds with minimal adjustments; porosity - minimizes wheel loading during the grinding operation; and a 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 exist to supply the toolmaker concerned primarily with increased productivity (total number of tools produced) as well as the toolmaker concerned more with 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 utilizing the different abrasive grain sizes also applies to the vitrified bond systems. Grinding wheel manufacturers are adept at mixing the right combination of ingredients to provide each polycrystalline tool fabricator with the “right wheel for their 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 the wheel flatness and eliminates the risk of over-dressing the wheel or chipping tools.
Your Keys To Success
There are several keys to the success of your operation and each plays a crucial role. The polycrystalline manufacturer, the machine tool builder and the grinding wheel manufacturer all individually and collectively influence your operation and the quality of your finished cutting tool. The selection of these suppliers is significant.
Find a manufacturer of polycrystalline diamond and CBN that can offer you the technical knowledge and experience to support their product and your operation. Draw upon their knowledge and understanding of the characteristics of their product to learn about the differences in grades of PCD and PCBN and how they apply to different applications.
There are several well-known machine tool builders that have grinders designed specifically for fabricating polycrystalline tools. When selecting a machine tool builder find one that understands the complexity of manufacturing polycrystalline tools and has the experience with their machinery in producing these tools. Be sure the machine tool has all the features and options necessary for proper tool fabrication and that these features coordinate with your production operation. Technical support is a key issue that can not be overlooked.
The diamond grinding wheel is a critical component and the selection process can be difficult since there are many grinding wheel manufacturers to chose from. The key element in finding the right wheel manufacturer rests with their experience, technical abilities and support programs. The grinding wheel manufacturer must understand your needs and requirements and be able to deliver a product that will satisfy. Do they understand how to grind polycrystalline? Can they support you with technical knowledge of their product and how it will perform? Will they work with you to supply the best grinding wheel for your unique operation? The answers to these questions may assist in your selection.
Whatever your choices are, you are the final element that ties all of the above together. Utilize the experience and knowledge of your suppliers to develop a quality operation and be confident in your ability to succeed.
Superabrasives, Inc. is a US based manufacturer of diamond and CBN grinding products in resin, metal, vitrified and electroplated bond systems. Founded in 1977, they are well established and have solid technical experience and industry knowledge to support all your superabrasive needs. Superabrasives, Inc . 28047 Grand Oaks Ct., Wixom , Michigan 48393, Phone (248) 348-7670, Fax (248) 348-8037, Toll Free (800) 341-4426.
Acknowledgements: Thank you to Dwight Dyer/Program Technician/Application Development Operations/GE Superabrasives for technical background information. *Registered trademark of General Electric Company, USA.