The Technique of Lapping
The Lapping Powder
The lapping powder comes in the form of silicon carbide, aluminum oxide, boron carbide, diamond powder (also see Page 38 “Lapping with Diamond”) or similar products. The quality criteria are the particle size, size distribution, hardness, shape and number of edges. The particle size is chosen according to the surface finish required, and naturally also according to the rate of material removal judged necessary. Of the lapping grits classified in Figure 45, the most common are sizes 400, 500 and 600. Many types of grit are commercially available, the most effective for giving a good surface finish being those with the least variation in size. This is particularly the case for fine grit and slurried grit. For any given grit size the maximum variation should not exceed 20% of the nominal. Oversized particles cause scratching, and undersize ones are simply carried along with the mass, without doing any useful work.
A scratch on the workpiece surface can cost many times the saving made by
preferring a poor-quality lapping medium to a higher-quality one. The powder should mix easily with the fluid, forming no lumps and remaining suspended for a considerable time before settling, and should contain no water (if mixed with oil bases). Slow settling is important to avoid separation before the medium is pumped into the lapping zone. Mixing ratios range from 1:10 to 1:5 or from 100 to 200 g powder to 1 liter of fluid. When using water and anticorrosion additives, and lapping over the entire plate, the ratio can be as high as 1:3 to 1:2 or approx. 300 to 500 g per liter. when changing from one grain size to another, it is recommended to rinse the working area thoroughly at least.
A change from conventional grains to diamond grains has become apparent in the field of lapping (also see Pages 38-40 “Lapping with Diamond”). The use of new, suitable lapping fluids is opening up more and more new lapping possibilities. Even soft materials are being lapped successfully with diamond. New types of working plates and their surface design allow use of this machining method without significant extra costs per workpiece. Diamond powder, which is still expensive, frequently results in shorter machining times and cleaner surfaces.
Virtually every grit particle is used in an optimum manner when lapping with diamond. This is the only explanation why, by comparison with conventional lapping, the same or even better machining performance can be obtained with so few grains. A comparison of consumption for machines of average size (diameter 700-1000 mm) resulted in the following values on the basis of lapping oil:
- With diamond lapping, the consumption is 224 Carats or 0.4-0.8 g/h
- With conventional lapping with Si-C, the consumption is 200-1000 g/h
With water-based lapping fluids, the consumption increases to 5 times this
amount, or 1000-5000 g/h. The consumption ratio “diamond:Si-C” therefore is 1:5000, which initially seems unbelievable.
Let us now briefly consider the grain count:
1 Carat, i.e. 0.2 g diamond powder with a mean particle size of 15 microns, contains approximately 3 million particles or 15 million particles/g (according to Spring).
1 g Si-C 500 with a mean particle size of 13 microns contains 10 – 15 million particles/g or approximately 10-15 billion particles/1000 g (10,-15,000,000,000). This is an incomprehensibly large number in most cases. This quantity is exceeded x-fold with smaller particle sizes.
The utilization of conventional grit (Si-C, Al2 03 etc.) is therefore relatively poor.
It can be concluded from this that many particles are not used on the working plate or break so easily that rolling does not take place (also see Page 1-4 by Martin and the author) . There is much work for research and development here.
It is already known that with water-based lapping, the majority of the particles are required for establishing a grit layer instead of an oil film. The rounding of the workpiece edges also tends to confirm this (according to Prof. Spur and Dr. Sabotka, Ceramic Machining).
Lapping powder classifications in accordance with FEPA Standard
|94% Value |
|3% Value |
Figure 45: Table of lapping grits in accordance with FEPA Standard