Designing an Efficient Ceramic Finishing Process
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.
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.