But ceramics are also very challenging to manufacture in production quantities. The very properties that contribute to long life in the end product make these materials hard to grind. While they are wear-resistant, ceramics are vulnerable to micro-fracture due to mechanical thermal mishandling. In addition, the yields achieved in production operations are frequently very low. In a typical job shop, 10 pieces are ground to yield three, while the other seven are wasted. This low yield drives up costs.
What’s needed is a systems approach for manufacturing these products cost-effectively. Recent research has indicated that improvements in the design of diamond wheels used to finish ceramics can be one key to meeting this objective.
OD/surface grinding on a reciprocating table is the method used to produce wear components and kiln furniture (typically Crystolon alumina and quartz wafer boats) used in wafer fabs and other electronics industry applications. Materials being ground in these and other OD applications include aluminum oxide, aluminum nitride, chemical vapor deposition (CVD) silicon carbide, silicon nitride and tungsten carbide.
Smaller diamond wheels are used for ID grinding to finish inside diameters in such applications as: ceramic cylinders for diesel engines, bearing races, liners for wear products (e.g., kiln furniture), can dies and necking dies. Another area of promise is wear components, such as washers for faucets, and for water pump motors and rotors.
The second reason, of greater concern here, has to do with the use of grinding wheels with diamond abrasives. Diamond abrasive wheels have commonly been the product of choice when grinding ceramic components due to the extreme hardness of this abrasive. Contributing to the high machining costs cited above is the fact that diamond abrasive is expensive. Further, challenges are posed by the bond system used in these products and the way wheels are prepared for grinding.
The bond is the substance that holds the abrasive particles in place. For the wheel to do its work, the bond must wear away at a controlled rate to expose new cutting edges as the working particles dull. In some cases, the bond is able to conduct heat away from the work area.
The type of bond most widely used with diamond wheels is resin. It is made of a tough polymer formed to hold the diamond particles to the rim of the grinding wheel. Resin-bonded wheels remove material quickly. However, standard resin-bond wheels wear too fast under the pressure of ceramics grinding. They also require frequent truing to hold their form.
A stronger bond type is metal. Metal-bond wheels offer longer life than resin-bond products, and can be operated at the higher speeds used in ceramic grinding. This would be an excellent choice for ceramic grinding, except for the fact that conventional metal-bonded wheels require periodic dressing that is difficult and time-consuming. (See sidebar, “Truing and Dressing Wheels,” for more information on truing and dressing.)
In searching for new grinding solutions such as the one required here, it is best to take a “systems approach.” This method looks at the total interaction of all grinding wheel parameters—including abrasives, bond, pore structure and design—and how they influence the thermal and mechanical conditions in the grinding zone. In this study, critical factors included thermal shock, heat and friction, which can cause fractures in ceramic parts.
Using the systems approach, the joint DOE/Norton effort resulted in the development of a new metal matrix bond.* This metal bond is designed to provide intermediate grinding action between standard resin and metal bonds. Wheels with the new bond require less force than resin wheels, cut more freely, and maintain the long product life typically associated with metal bond products. Not only do they achieve the high wear ratios associated with metal-bond wheels, they also achieve the finer finishes typically associated with resin wheels.
The new bond system has resulted in a wheel that significantly improves G-ratios on such materials as silicon nitride, while attaining high stock removal rates. These wheels provide excellent surface finish, and minimize sub-surface damage because they require less normal grinding force than typical resin-bonded wheels. (A list of materials that have been successfully tested or produced with metal-bond wheels is given in the “Successful Applications of Metal-Bond Wheels” sidebar.)
In one recent test, the metal-bond product was mounted on a Blanchard surface grinder. The goal was to grind 50 silicon carbide components with a 0.645 diameter. The wheel, which measured 12 x 1⁄4 x 17⁄8 in., was run against a conventional resin-bond wheel at 1200 rpm
The results of the test showed that the metal-bond wheel was able to grind at better than twice the speed of the conventional product, with a cycle time of 10.25 minutes compared to 25 minutes. In addition, the new wheel exhibited wheel wear of 0.002 in., compared to 0.008 in. of wear on the standard product. Once again, the metal bond proved to be longer lasting; the conventional wheel ran for seven weeks, while the metal-bond product ran for between 21 and 28 weeks.
Other tests confirm the productivity improvements of the metal-bond wheels. In another silicon carbide application, a conventional resin wheel took 10 hours to complete the grinding application. The wheel with new metal bond completed the job in two hours—an 80% time savings.
In another production operation, a company grinding an aluminum nitride ceramic component that can’t be made to near-net shape was using three machines, three operators and three wheels to achieve the final form. After introducing the new wheel to the system, this manufacturer now runs a single machine on three shifts, going from roughing to finishing with the one wheel.
In tests at Norton’s Higgins Grinding Technology Center, the metal-bond wheel showed substantial performance advantages over resin and vitrified counterparts. The new wheel reached the maximum material removal rate (MRR) of 4.4 in3/min/in at
0.8 in3/min/in, restricted by the G-ratio and surface quality criteria.
While the emphasis here has been on grinding, it is especially important to consider the entire grinding cycle. When ceramic parts fail, they break at the end of the manufacturing cycle; therefore, the loss should be defined as a cost of the entire manufacturing cycle, not just the grinding cycle. It is shortsighted to consider only the cost of abrasives rather than the cost of the entire process.
For long-term success, focus your efforts on yield and throughput, and work as a partner with your abrasives supplier. Viewing your ceramics grinding operation as an entire system is the best way to achieve lower total abrasives costs.
To restore sharpness and freeness of cut to the wheel, bond must be removed from between the diamond cutting points. This step is called dressing. Usually, silicon carbide dressing sticks are plunged, in a manual or automated process, into the face of the wheel to remove the bond (see Figures 2 and 3).
Truing and dressing are necessary parts of any grinding process. However, they are time-consuming. Minimizing the amount of downtime required to true and dress diamond wheels is a key to improving productivity.