Lapping is a high-precision machining process that produces very high surface quality, dimensional accuracy and narrow tolerances on machined workpieces. It can be used for nearly all materials that do not plastically deform. Even very hard materials like glass, ceramics or cemented carbides can be lapped.

In the lapping process, the workpiece and an opposing surface rub together with a lapping lubricant in the gap between them. Metal is removed by abrasive lapping powders dispersed in the lapping oil rather than being bound in a tool surface.

The abrasive grains roll and slide on the workpiece, essentially kneading the surface to create microcracks. The cracks then link together to break small particles out of the part, leaving very smooth, planar surfaces. Therefore, lapping is not a cutting process like grinding or honing, but a means of removing material by fatigue.

The lapping plate is usually made of hardened cast iron and has radial slots that drain the used lapping medium and metal removed from the surface. The plate rotates and carries conditioning rings that rotate with it in a kind of planetary drive system. In this way, the direction of rotation for every workpiece changes continuously, resulting in almost exclusively nondirectional machining marks and a matte finish.

Pressure plates push the workpiece onto the lapping plate. Pressure is usually kept low at the beginning of the process, then gradually raised to increase the material removal rate. Toward the end of the process, pressure is reduced to optimize the roughness of the lapped surfaces.

Lapping powders comprise abrasive grains of silicon carbide, aluminum oxide, boron carbide or diamond powder. The powders produce huge differences in surface quality, depending on particle size distribution, hardness, shape and number of edges. Particle size controls both surface finish and material removal rate.

The lapping medium is a mixture of the lapping powder and lapping oil. Mixing ratios range from 80 to 120 grams of powder to 1 liter of lapping oil for roughing and 65 to 80 g/L for finishing. The lapping medium is usually stirred continuously in a special tank to avoid separation of oil and powder.

Lapping Oils

The lapping oil carries the lapping powder and distributes the abrasive powder evenly over the lapping plate. It helps prevent grooves and scratches on the workpiece and reduces the temperature of the process. The oil also transports removed material into the lapping plate slots or to the edge of the plate.

Oil viscosity is critical to the efficiency of the process. Too viscous an oil creates a thick lubricant film between lapping plate and workpiece, and the lapping powder may lose contact with the surfaces to be machined. Too low a viscosity creates a narrow gap that may be too small for the lapping powder to roll, producing grooves and scratches on the workpiece.

Usually, lapping oil viscosity is adjusted to the grain size of the lapping powder. Larger grains require higher viscosity, smaller grains a lower viscosity. Critical properties of the oil to produce high-quality machined surfaces, low surface roughness and high productivity include:

Good dispersing capacity to prevent fast sedimentation of the lapping powder and to prevent agglomeration of the grains.

Good wetting for fast and easy preparation of the lapping medium and to provide homogeneous distribution on the lapping plate.

Sufficient lubricity to enable the lapping grains to roll.

Optionally anticorrosion properties to prevent flash rust on workpieces,

Compatibility with workpiece and machine materials.

Dispersing Capacity: Lapping powder should mix easily with the oil and should remain suspended for some time before settling. Slow settling is important to avoid separation in the feed lines and on the lapping plate that would produce a nonhomogeneous surface finish.

The lapping medium is usually stirred continuously to avoid separation; however, if the stirrer stops, sedimented powder should remix easily. Settling speed can be reduced by using higher viscosity lapping oil; however, this usually has a negative influence on material removal rate and, because of poorer flushing properties, on surface quality.

Agglomeration: When lapping powder agglomerates in the oil, the agglomerates do not roll but rather stick in the gap between the lapping plate and workpiece, where they can cause broad and deep grooves on the surface. Depending on the kind of powder, lapping oil additives must be selected carefully to avoid agglomeration, separation or sedimentation. In particular, highly polar additives like some organic acids can form concrete-like sediments that are hard to redisperse.

Agglomeration results from attractive London-van der Waals forces between the particles. To stabilize the medium against agglomeration and fast separation, repelling forces must be established between the particles by adding detergent/dispersing additives that adsorb on the particle surface. These additives keep the particles apart by electrostatic repulsion or steric stabilization and reduce the tendency to agglomerate.

Among the many types of detergents/dispersants available, calcium sulfonates show very good dispersing efficiency for different types of lapping powders. Adding calcium sulfonates helps avoid the formation of agglomerates and also extends the settling time of the grains. Any sediment formed usually is soft and easy to redisperse even after a number of days. Because they also work as corrosion inhibitors, calcium sulfonates help protect machined surfaces from flash rust.

Wetting: Fast and homogeneous preparation of the lapping medium requires that air and moisture at the particle surface be displaced and replaced by the lapping oil. In addition, the lapping medium must form an even and consistent film on the lapping plate. Surface active agents increase the lapping oils wetting properties, enabling the medium to spread more homogeneously on the lapping plate and workpiece to provide a constant film thickness and avoid the accumulation of particles.

Lubricity: Insufficient lubrication prevents the abrasive particles from rolling in the gap between the lapping plate and workpiece. Rather, they slide and rub the surfaces, causing damaging scratches. Particle sliding can be avoided by increasing oil viscosity. However, increased viscosity can lead to increased oil film thickness, and the smaller powder particles may lose contact with at least one surface, reducing material removal rate.

Polar organic additives containing heteroatoms like oxygen, nitrogen or other atoms with an electronegativity higher than that of carbon or hydrogen can optimize lubricating properties without increasing oil viscosity. These additives adsorb to the metal surface, effectively reducing friction.

Corrosion: Depending on the kind of metal being machined, the surfaces can be sensitive to corrosion. Even when the parts are stored indoors, flash rust can appear after a few days. This happens much faster in high-humidity atmospheres. To prevent rust, a corrosion inhibitor compatible with the other additives in the formulation can be added.

Improving the Process

To improve the productivity of lapping media, Rhein Chemie Additives evaluated a variety of additive combinations in the lab and in field trials. The silicon carbide lapping powder used in the tests had an average particle size of 12 micrometers in a concentration of 100 grams per liter of lapping oil. The oil was an API Group I paraffinic mineral oil with a kinematic viscosity of 25 square millimeters per second at 40 degrees C. The lapping medium was prepared in a vessel equipped with a continuous stirrer.

The first test used a combination of calcium sulfonates to ensure optimal dispersion and to avoid agglomeration. The fine distribution of the abrasive grains improved maximum surface roughness by 19 percent. However, material removal rate dropped by 12 percent, and some small scratches were detected on the lapped surface, indicating insufficient lubricity.

The second test combined calcium sulfonates with an additive based on sulfurized esters and hydrocarbons. This additive has a highly polar structure that allows it to adsorb on metal surfaces, forming a shear stable friction reducing layer. Because of its active sulfur content, this additive was expected to increase material removal rate, which it did. Material removal rate increased by about 96 percent, compared to the reference, while surface roughness improved by 8.6 percent. Also, the workpieces showed no scratches.

The third test oil was based on a sulfurized olefin with high active sulfur content in addition to calcium sulfonates and a sulfur carrier. The objective was to examine the influence of active sulfur on the lapping process.

Active sulfur chemically reacts with metal surfaces to form metal sulfide layers. Because of their ionic bonds, these layers have much lower elasticity and plasticity than the original metal. In the lapping process, the more brittle metal sulfide was expected to break easily into small particles that would increase metal removal rate.

As estimated, removal rate increased by 105 percent with the active sulfurized olefin. Surface roughness was approximately 3 percent better than with the reference oil. Again, the lapped surfaces were free from scratches.

Increasing the concentration of active sulfurized additive further increased metal removal rate by 121 percent compared to the reference. However, the quality of the lapped surface was significantly decreased. Apparently, the higher concentration of active sulfur generated a thicker layer of metal sulfides. As a result, the particles cracking out of the surface were likely larger, leaving a more irregular surface with higher roughness.

Finally, in contrast to the commercially available lapping oil, the test oils prevented flash rust formation.

Wilhelm Rehbein is senior manager application technology at Rhein Chemie Rheinau GmbH, located in Mannheim, Germany. Contact him at wilhelm.rehbein@lanxess.com.