Designing an Efficient Ceramic Finishing Process
November 27, 2002
Understanding the differences between lapping, polishing and fine grinding can help ceramic manufacturers design a precision finishing process that meets modern production needs.
Precision finishing of flat ceramic surfaces can be accomplished using lapping, fine grinding and polishing processes. However, while these processes have been used for years, many machine shop owners and engineers do not understand the capabilities of each method, their optimal applications, or the other process parameters that influence their results. The primary factors that should be used to select a finishing process are 1) its ability to consistently meet part tolerances and 2) its ability to provide a low cost per part. In precision machining, the part tolerances are surface finish, flatness, parallelism and thickness. By understanding how the different finishing methods can meet these criteria, ceramic manufacturers can design a precision finishing process that meets modern production needs.
Similarities and DifferencesLapping, fine grinding and polishing are similar in that they all use “lapping kinematics” to remove material (see Figure 1). The work pieces are held in a sprocket carrier and guided between the upper and lower wheels by a motor-driven inner pin ring and a normally stationary outer pin ring. This action imparts an epicyclical path of the part over the wheel surface, which helps maintain even wheel wear and repeatability of the part tolerances. Precisely controlled pressure is applied to the system to force the abrasive into the work piece surface. The applied force combined with the wheel speed (measured in meters/second [m/sec]) control the material removal rate.
However, while the basic principle of material removal is the same, several major differences exist between these three finishing techniques. Lapping of ceramics is typically performed with loose boron carbide or silicon carbide abrasive, a liquid vehicle and cast iron lapping wheels. Diamond slurry is also used but is generally too expensive to be practical. Fine grinding is a relatively new process that was developed as a cost effective alternative to lapping.* Fine grinding is similar to conventional grinding in that it uses diamond-grinding wheels and water-based grinding fluid to flush chips from the wheel surface. Polishing uses fine diamond compounds and slurries with a soft metal lapping plate. A comparison of the typical operating parameters for lapping, fine grinding and polishing is presented in Table 1.
Both fine grinding and lapping produce a precision-machined surface that meets most needs. If a higher level of flatness and/or finish is required, then a post-polishing operation can be used. The typical tolerances achieved by each process are presented in Table 2. All three finishing techniques generate heat during operation and require cooled wheels to maintain tight tolerances. Compared to lapping, fine grinding uses higher pressures and greater wheel speeds, allowing it to achieve much higher material removal rates. Fine grinding also gives a more precise surface because no roll-off occurs on the part edges, and it is typically three to five times faster than lapping. Another benefit of fine grinding is its ability to be partially or fully automated. For high-speed production processes, fully automated fine grinding systems can drastically reduce the overall grinding time and process cost. A total process cost savings of 50% can be realized due to less consumable costs, elimination of ultrasonic cleaning and significantly less machining time. Only a few special applications exist where lapping is the required process, and these involve finishing very thin parts at low process pressures. When the part thickness drops below about 0.2 mm, fine grinding cannot be used due to the low strength of very thin carriers. In these cases, lapping at low pressures and minimal frictional forces is the recommended method. Low-pressure lapping is also used to machine bow or warp from thin parts. With fine grinding, thin parts deflect under the force of the grinding wheel, and the warp or bow remains. Thicker parts can be finished by either method. Polishing is a very slow process but yields the best precision and finish. Using diamond imbedded in a soft lapping plate gives the best flatness. With polishing, only about 5-15 microns of material are removed, so the preparation of the pre-polished surface is important. Both lapping and fine grinding are used to prepare surfaces for polishing.
Table 3 contains a summary of when to use fine grinding, lapping and polishing.
Design ConsiderationsIn addition to the type of finishing method used, the capabilities of the ceramic forming and sintering operations also influence the final tolerances that can be achieved. The following process design considerations can be used to improve precision finishing operations.
- Reduce the incoming part-to-part dimensional variability. The less variation in thickness of the parts before finishing, the less stock removal is required to achieve precision tolerances. A guideline is to multiply the amount of part-to-part variation by a factor of 10 to estimate the amount of stock required to grind out the variation.
- Use a large working wheel. The larger the wheel width compared to the diameter of the part, the better the flatness and parallelism that can be achieved. In all three finishing processes, the surface of the finished part will mirror inconsistencies in the wheel surface. The smaller the diameter of the part, the less wheel surface it is exposed to at any one time, and the better the results.
- Maximize the number of parts processed per load. The more parts in a machine load, the better the capability to overcome incoming part-to-part thickness variation and total thickness variation (TTV). Since there are limits on the maximum amount of part surface area that can be processed with a given wheel size, maximizing the size of the wheels will allow the maximum number of parts per run.
- Use fine grinding for roughing. Another advantage fine grinding offers over lapping is that most modern fine grinding machines can be programmed to rough and finish the parts in the same operation. This reduces the total processing time by eliminating a handling and cleaning step.
Testing is KeyThe information presented in this article can be applied to most ceramic lapping, fine grinding and polishing applications. As with all processes, a variety of factors can affect the performance of the finishing operation. The best way to design a finishing process is to first understand the differences between the various finishing methods, as well as the design considerations that can influence the final tolerances. Finally, test the parts using the machinery being evaluated. Some machinery suppliers offer testing facilities to aid in this process.
For More InformationFor more information about lapping, polishing and fine grinding, contact Peter Wolters of America, 14 High St., P.O. Box 1585, Plainville, MA 02762; (508) 695-7151; fax (508) 695-7154; e-mail sales@USA.peter-wolters.com; or visit http://www.peter-wolters.com.
SIDEBAR: Single or Double?Single- or double-side methods can typically be used with any finishing technique, depending on the geometrical requirements of the part. An example of both types of machinery is shown in Figures 2 and 3.
Double-side finishing provides the plane parallel surfaces needed for products such as wafers, seals, pump parts, bearings, blades and lenses. Although this article presents information taken primarily from double-side processes, the concepts are equally applicable to single-side operations.