The Technique of Lapping
In earlier days, hand lapping and polishing frequently resulted in chipping at the workpiece edge, at holes or recesses, especially when the lapping medium was carried by textile or hardwood surfaces. The removal of soft areas in a surface structure could produce a dimpled finish like an orange skin. For modern, high-grade material removal, and for the very hard materials now widely used, traditional lapping media (corundum, silicon carbide, boron carbide) cannot meet the requirements of particle size constancy and material removal performance. This led to the development of diamond lapping agents and associated methods.
This has made possible:
- Brilliantly polished surfaces with high flatness values and without any associated disadvantages, even for materials with very different microstructure hardness values.
- Reduced lapping and polishing times, and therefore low wage and machine costs.
- Lower lapping medium costs in many cases.
- Improved automation possibilities for lapping machines.
- New possibilities with the use of fixed-grit plates.
- Nothing to prevent “clean lapping”.
It is not easy for the uninitiated to classify the many different diamond powders available on the market, or to select the right powder for the task in hand. Frequently, a randomly selected powder is used for reasons of time and costs, without previous comparisons. Manufactures of lapping and polishing machines have a wealth of experiences in this field and offer diamond agents that have proven their worth in practical use.
Some information on the manufacture of diamond powder is given below:
- Natural diamonds are one source of diamond power. Waste products from jewelry quality diamonds and diamonds with visual defects are broken into the desired qualities, sifted, washed or sedimented. The grain shape is monocrystalline, i.e. the grains feature in a single plane as with granite blanks, into smaller, sharp-edged pieces.
- The first synthetic diamond produced from graphite (originally by means of the “Du-Pont explosion method) was polycrystalline. This type fractures in various planes, i.e. multi-block.
- Synthetic diamonds, both monocrystalline and polycrystalline, are nowadays produced under extremely high pressure. The grain shape and strength can be influenced to a great extent using a special technique.
Wheather monocrystalline or polycrystalline diamond is used when lapping or polishing depends on the workpiece material, the carrier (working plate), the carrier medium and the working process. Of crucial importance is good collaboration, adequate wetting (no lumps or agglomerates), long-term suspension in the medium (floating) as well as the lubricant-film behavior on the carrier or in the working gap. The quantity, type of carrier, removal of abraded material (chip flow), etc. must be intermatched. Successful lapping with diamond also depends to a great extent on the operation of machines. the best diamond agents and the most refined technical equipment are useless if the knowledge, experience, and attitude of the operating personnel do not meet the high demands placed on them. Successful use of the lapping technique is ensured id these technical and human requirements are fulfilled.
Diamon mediums are applied by specially developed spray devices (Figure 75) which ensure that the fluid uniformly mixed and precisely metered, time-adjustable quantities are supplied to the working plate. If various grit sizes must be used, only the container, hoses and nozzles have to be replaced.
In theory, these micro-grit diamond media can be used on any type of lapping machine. However, there are great differences with respect to good material removal performance and the corresponding polishing method. Only robust and massively constructed lapping and polishing machines are suitable here. Working plates used for job lapping consist of steel, cast iron or copper in most cases and use special profile shapes. Good results can be achieved by using spiral-shaped multi-metal plates. Steel, copper, aluminum, tin, wood, and fabric plates are used for polishing.
In polishing, the amount of diamond medium consumed is relatively low. Depending on the volume of material removed, the figure is only 6-8 carats/day for a lapping machine with a plate diameter of 400mm. The machining time compared with the use of silicon carbide can be shorter by a factor f 10. Today, diamond powder is very widely used, allowing much shorter lapping and polishing times, greatly increased accuracy, better surface finish and reduced costs by comparison with the use of other media. The microstructural properties of the workpiece material can also favor choosing diamond powder. Diamond is now used to machine extremely hard materials such as hardened steel, special alloys, glass, silicon, carbide metal, ceramics, sapphire or even diamond itself. Diamond is used for micro-fine lapping or polishing of softer materials, e.g. steel of all types, nonferrous metals, graphite, ferrite, and even plastics.
Examples of such workpieces include: Sliding rings, sealing rings, plungers, ferrite cores, contact, molybdenum parts, titanium parts, surface plates, gauge blocks, optical flats, sapphire glass, reversible carbide tips, carbide metal cutters, hard ceramics, silicon nitride and tungsten nitride rings, diamond tools.
In this connection, the special additions below may be of interest to the user:
“Diamond for lapping and polishing highly accurate components” /JDR 20 (1986, No. 4
“Lapping with diamond on 2-plate lapping machines” / JDR 24 (1990), No. 1.
The foregoing articles should give an idea of the very promising future which can be expected of the lapping and polishing process. The industry is increasing acknowledging lapping as a cost-effective, precision machining method. University studies and seminars are contributing a fuller understanding of its possibilities. A.W. Stahli AG regularly offers its own practical seminars. These seminars are also oriented towards beginning users or operating personnel of users. As a complement to this article, films in three languages for schools and production management are available for the author.
A further publication has been produced on the topic “Dual-plate Lapping” and is available from Lapp-Technik, A.W. Stahli AG