The principle of peripheral external cylindrical lapping is illustrated in Fig. 8-4c. A slit in the tool enables the widening and narrowing of this sleeve-shaped tool design. This allows the intended working pressure and the desired dimensions to be adjusted. In order to improve the shape in terms of cylindricity and roundness, a rotational and an axial movement are superposed as in corresponding honing processes.
Workpiece and lapping shaft should have the same length. The lapping slurry comes to the contact surface sporadically. Disc-shaped workpieces are compacted into bundles.
Appropriately shaped lapping arbors make it also possible to lap internal cylinders and holes. The arbors can be widened and are thus adaptable to changes in diameter.
Lapping ball surfaces or spherical surfaces is still possible through appropriate tool design. The tool to be implemented has the corresponding negative form of the component to be produced (Fig. 8-4d). The desired shape is achieved under constant change in the direction of movement. The goal of machining is achieving very high accuracy in the final dimensions, which is made possible by the oscillating movements of the tool.
Ultrasonic lapping is cutting with loose grains which, evenly distributed in a fluid or paste, receive impulses through a shaped piece vibrating in the ultrasonic range. These impulses give the grains their cutting ability (Fig. 8-4e).
We must differentiate between ultrasonic lapping (also referred to as ultrasonic polishing), which is used merely to make surface improvements, and the ultrasonic machining of hard and brittle materials, with which method threedimensional shapes can be inserted, for example, into engineering ceramics or glass. Whereas in the first case material is removed as in conventional lapping, i. e. through the rolling of the grains in the lapping slurry, in the latter case the abrasive grains are propelled onto the workpiece surface, thus effecting material removal. Because of its functional similarity to other methods involving material removal, this process, which has become more and more important in recent times because of the increased use of ceramic components, will be discussed in detail in volume three of this series.
So-called press lapping is used primarily for polishing, deburring and rounding. In this process, grinding paste is pressed in closed chambers through breaches and holes in the workpiece or on the entire surface of the parts. Repeated through-flow of the lapping paste improves the abrasive effect and allows the machining of blind holes and other inner surfaces using correspondingly adjusted cores and auxiliary tools [GOSG75]. The grains material used in the process are silicon carbide or diamond.
Precision polishing fulfils special demands for workpiece quality. Contrary to planar lapping, the special cast iron lapping discs are replaced either with polishing discs composed primarily of copper, tin or plastic or with polishing cloth. The latter can also be impregnated with synthetic diamond powder. For a polishing slurry, a mixture is made of diamond powder and a carrier fluid soluble in water, oil or alcohol together with other additives, e. g. antirust agents.
In diamond slurries prepared for lapping and polishing, even coarser diamond grains remain evenly distributed in the fluid over a long period and refrain from precipitating, even if the carrier fluid has a lower viscosity [STAE76, SABO91]. Spray devices guarantee an optimal supply of lapping slurry. The grains used range from superfine (0.25 to 1 qm) to very coarse (20 to 40 qm). The polishing times achievable using diamond powder are well below those using conventional lapping methods. For that reason, in spite of the higher price for the lapping grain, the use of diamond allows for economical, high-quality processing of both soft and hard materials.
In polishing-lapping, dressing rings and workpiece-receiving rings are generally composed of ceramic or plastic in order to avoid adhesion of the polishing disc through undesired material sedimentation. They simultaneously redistribute the diamond grains which penetrate the disc, remaining stuck in place.
The rules and action mechanisms that apply to the normal lapping process also apply to precision lapping. Logically, the components to be polished should be pre-lapped beforehand. The relatively low cutting speeds allow the machining of very thin components, such as those used in the electronics industry or in precision engineering.
The planar polishing of glass can be seen as a special lapping process. The polishing discs are made in this case of a pitch mass or a special plastic, the polishing slurry is a mixture of metal oxides and distilled water. With this method, workpieces of up to 500 mm in diameter can be planar-polished on machines with disc diameters of over 1.5 m.