Table of Contents Lapping The Lapping Process Some Figures on Lapping Hand Lapping The Single-Plate Lapping Machine Conditioning Rings and Functional Principle The Working Plate Pressure Plates Workholders Speed of the Working Plates Machine Types Centerless Cylindrical Lapping The Lapping Oil The Lapping Powder Surface Finish Quality Accuracy Aspects of Production Lapping Idle Times The Polishing Table Unit Price Calculation Measuring Laser Flatness Measuring Instrument Checking the Working Plate What Workpiece Types are Lapped? Diamond Lapping
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The machining process of lapping involves two surfaces that are rubbing together with a lapping medium (lapping fluid and lapping grit) trapped between them. While the rotating lapping plate is moving, stock removal of 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. In contrast, workpiece flatness can be measured by using an optical flat.
If there is prevention from the abrasive grain rolling, either by a corresponding selection of cutting tool like the carrier plate, pressure, or the fluid, this will cause the particles to stick. If the abrasive grain particles stick to the cutting tool 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 of the cutting tool.
For a long time, it was supposed that in all cases the loose grit cut the surface during the machining process to produce flat surfaces holding tight tolerances. While 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 description is in this article (also see Page 22).
A simple way to observe the action of abrasive 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 abrasive particles 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 high-quality surface finish, polishing with extremely fine abrasive paper or with a polishing machine can alter the appearance of the flat surface.
Cutting generally occurs when &#;soft&#; rotating lapping plates are in use on a lapping or polishing machine. The cutting tool uses a soft plate and fine grit particles, a mirror finish is relatively easy to obtain depending on the amount of material that needs removal. In contrast, the use of micro-particle diamond powder allows applications to be considerably wide. The development of new cutting tools like carrier plates and their surface design, e.g. special grooves, etc., as well as special fluids as an abrasive grain medium, have resulted in further significant-high-quality production increases during the last decade. The micro-particle diamond powder is continuing its success of stock removal in grinding technology and in lapping and polishing technology.
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Lapping plates or working plates grades can be roughly separate into 4 categories:
A lap plate hardness value between 140 and 220 HB has been found to give optimum results. The rotating lapping plates with these hardness values can be continually corrected by means of conditioning rings in single-plate lapping machines, allowing high precision of the lapped workpiece, flatness, parallelism, stock removal, and surface finish criteria required. The lapping process always requires a medium, which is manufactured with additives. Oil-based 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 (lapping agents). The term &#;lapping agent&#; generally refers to a ready-to-use agent (product), i.e. fluid and grit (abrasive grain size) during the lapping process.
Fig. 10: Lapping agents and lapping tools
Diamond or boron carbide is needed for extremely hard materials such as sapphire, carbides, and some ceramics. Silicon carbide can be lapped on medium-hard materials such as softer metals and some aluminas. Because of its softer cut and lower quality, aluminum oxide is commonly used in the glass and silicon industries. It is important to remember that the abrasive charges the surfaces of your lap plates. It is almost certainly easier to make the plates smoother than the substance being lapped, while the abrasive can be as hard as, if not harder than, the work bits.
Last night, my old friend insomnia sat on the edge of my bed all night and kept me company.--Probably had something to do with many futile attempts to get my Atkinson engine started and not fully succeeding. I am going to pull the head off and lap the valves again. I find that the arthritis in my hands makes it somewhat difficult to rotate the valves back and forth with the lapping compound as I normally do. Not only do you have to rotate the valve back and forth against the seat, but you also have to apply a certain ammount of pressure between the valve face and the seat while doing the rotating, and every so often you have to lift the valve away from the seat to recharge the lapping compound, as it all gets squeezed out of the interface during the lapping procedure. I was taught long ago to never use a power tool such as an electric drill to do this, as it will quickly damage the valve and the seat. The rotating must be back and forth--at least 360 degrees one direction, then 360 degrees the other direction, repeated many times. Last night, I thought of a machine which would do exactly what is needed, that is hand powered, very simple to build, and best of all, I have all the material to build it right here---as in "Don't have to buy anything". It is a very simple machine, and I will post drawings, models, and pictures as I proceed with it. this will not be a long term build, but I THINK its a good idea.
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