The properties of the lapping disc are determined by both geometrical factors and the working material. When machining workpieces with large surfaces, disc grooving is necessary in order to ensure that there is an even supply of grains at all points on the lapping disc [SMIT83].
The abrasive materials mainly used in lapping processing are silicon carbide (SiC), corundum (Al2O3), boron carbide (B4C) and, increasingly, diamond.
The lapping disc produces a rotary movement and serves as a carrier for the lapping slurry, workpieces and dressing rings. The rotational speed must be measured so that impermissibly high centrifugal forces are avoided. The lapping disc is generally made of special, fine-grained, perlitic cast iron materials or of a hardened steel alloy.
Among the various physical and chemical properties of the lapping disc material, the penetration depth of the lapping grains is very often used as the decisive value. Three ranges of hardness are distinguished: soft (< 140 HB), medium hard (140 to 220 HB) and hard (> 220 HB) [STAE76]. A low disc hardness favours the sticking of abrasive grains into the disc surface and leads to a chip formation on the workpiece [DAVI73]. Harder discs, however, tend to cause a rolling of the grains in the active gap [KASA90]. In addition to grain engagement behaviour, the disc hardness also determines disc wear and the attainable amount of removal. In general, using harder discs leads to less disc wear and a greater amount of removal, although the dressing rings have an inferior corrective effect [FELD90].
In order to remove the process heat, the lapping disc may be fitted with a liquid cooling system underneath the surface. In the case of rough lapping, cooling is indispensable because otherwise, after long machining times, the temperature of the lapping disc can rise to up to 50 °C above room temperature. This would result, in turn, to an undesired heating of the workpieces, changes to viscosity and the increasing evaporation of the carrier fluid. The avoidance of heat and its removal also play a decisive role, for example, in the finishing of workpieces with exacting tolerances, since the extreme accuracy requirements involved demand the inspection of the geometry and flatness, as well as the parallelism of the workpieces at the usual reference temperature of 20 °C.
In order to achieve level workpiece surfaces, it must be ensured that the lapping disc, subject as it is to wear over the course of processing, retains its level shape. This is one of the main tasks of the dressing rings. For certain processing cases, it may be required that the lapping disc have a slightly convex or concave shape.
In addition to the lapping disc as active partner, the carrier fluid also determines the engagement of the grains in the process. Through the formation of a fluid film on the lapping disc surface, the carrier fluid both prevents direct contact between workpiece surface and tool surface and facilitates the distribution of the grains on the lapping disc as well as the grains’ mobility in the active gap between the workpiece surface and the disc surface [DEGN79]. The viscosity of the carrier fluid is the most essential influence on the process. It is even generally assumed that high load capacity and viscosity can have the result that the lapping film thickness exceeds the grain size, thus preventing an effective material removal. Therefore, low-viscosity carrier fluids must be used in the case of small grain size [DEGN79].
Given a large quantity of lapping grains, the specific granular stress can be so greatly reduced that their cutting effect is lessened as a result of increased sliding and rolling movements.
That surface quality improves with small concentrations can be explained by the fact that high granular stress increases grain breakage, which reduces the effective grain size [DAVI73].
In rough lapping, the use of coarse lapping powder is expedient for attaining high removal rates. Impermissibly high roughness values must then be reduced in a second operational step with finer grains.
The nature of the machining task dictates not only the kind of lapping powder to be used, but also the necessary mixture ratio between grains and carrier fluid.
Typical reference values for rough lapping are 80 to 100 g lapping powder to 1 l carrier fluid and, for ultra-precision lapping, 65 to 80 g lapping powder to 1 l carrier fluid when oil-based; when water-based, the values increase three to four times.
grains are too large, impermissible scratches result, whereas small grains no longer participate in material removal.
The choice of lapping powder and carrier fluid is not arbitrary. Criteria for the quality of the lapping slurry are, among others, sufficient miscibility (no agglomeration, no premature settling). Highly viscous oils or composite media made of oil, paraffin, petroleum and other additives are typically used as carrier fluid. These must not lubricate, must ensure a secure transport of the chips from out of the active zone and have good cooling properties. The gradually forming fluid film may not become too thick during machining, since this would prevent an effective material removal. Breaking the film can lead to damages resulting from cold welding of workpiece and lapping disc.
The type of lapping powder used is determined, among other things, by the workpiece material to be processed. The combinations of lapping powder and workpiece material given in table 8-1 have proved to be effective.
Table 8-1. Favourable combinations of lapping powder and workpiece material