The Technique of Lapping
The Lapping Process
In lapping, two surfaces are rubbing together with a lapping medium (lapping fluid and lapping grit) trapped between them. Removal of the material happens by the countless loose particles between the carrier surface (lapping plate) and the workpiece it’s pressing against. The particles rolling to a greater or lesser extent according to the hardness and porosity of the carrier surfaces, thus performing a kind of kneading, material-removing action on the workpiece surface. If there is prevention from the grit rolling, either by a corresponding selection of the carrier plate, pressure, or the fluid, this will cause the particles to stick. If the particles stick then it will lead to a more abrasive material deformation process in a similar manner to grinding or honing. This will, therefore, tend to push up the material to the front or sides.
For a long time, it was supposed that in all cases the loose grit cut the surface. The rolling effect with its kneading action was only discovered by means of research with the electron microscope (Prof. Dipl. Ing. K. Martin). The process with bound grit particles is described in this article (also see Page 22).
Comprehension of Lapping Process
A simple way to observe the action of particles as illustrated above is to strew coarse lapping powder (about 150 my). On a finely lapped aluminum plate and to cover with a scratch-free plate of perspex, which is then sliding sideways slightly under light pressure. Figure 3 shows, at high magnification and the traces left by the rolling lapping particles on the fine surface. With the continuous lapping process, the continuous kneading effect causes the material to break out of the machined surface, as shown in Figures 6 and 7. Figure 8 shows the result after an extended lapping time of 2 seconds with x grit particles.
If the hardness and microstructure of the carriers or lapping plate (or carrier) are such that rolling cannot occur because the particles become stuck, as already mentioned, the result is cutting or sliding action. Surfaces lapped by rolling appear matte (Figure 8), while pushing, cutting grit produces bright and even shiny surfaces (Figure 5). High magnifications of matte surfaces show rough material particles (Figure 9). In comparison, if it’s necessary for a reflective surface finish, polishing with extremely fine abrasive paper or with a polishing machine can alter the surface.
Polish Finish vs. Matte Finish
Cutting generally occurs when “soft” lapping plates are in use. With a soft plate and fine grit particles, a mirror finish is relatively easy to obtain. In contrast, the use of micro-particle diamond powder has allowed applications to be considerably widened. The development of new carrier plates and their surface design, e.g. special grooves, etc., as well as special fluids as a grit medium, have resulted in further significant production increases during the last decade. The micro-particle diamond powder is continuing its success in grinding technology and in lapping and polishing technology.
Grading of Working Plates
Lapping plates or working plates can be roughly graded into 4 categories:
- Soft working plates,
e.g.: paper, cloth, felt, pitch, plastic, wood,
tin, aluminum alloy, copper, etc.
- Hard-working plates,
e.g.: cast iron, mild steel, soft ceramic,
- Hardened working plates, e.g.: hardened cast iron, hardened steel, hard
ceramic etc., up to 500 HB
- Multi-metal plates, e.g.: a combination of 2-3 different metals or
sintered metal powder (see also Figure 34).
A working plate hardness value between 140 and 220 HB has been found to give optimum results. Plates with these hardness values can be continually corrected by means of conditioning rings in single-plate lapping machines, allowing high precision of the dimensional, flatness, parallelism and surface finish criteria required. Lapping always requires a medium, which is manufactured with additives. Oil-base mediums generally refer to as lapping oils or oil-soluble lapping fluids. Carriers based on water or a similar fluid are called water-based lapping fluids. The term “lapping agent” generally refers to a ready-to-use agent (product), i.e. fluid and grit during the lapping process.
Fig. 10: Lapping agents and lapping tools