Grinding is the removal of a stock material, or "work," by abrading against a harder material - usually called a "stone."
Lapping is a process where a slurry containing a hard grain is trapped between the work and a lapping or pressure plate.
Technically, the use of pellets is a grinding process with lapping equipment. Results using pellets approach those of lapping in terms of flatness because the plate containing the pellets can be kept true or flat, like the lapping plate. The efficiency of the pellet process as measured by the work-stock removal rates approaches that of grinding because the cutting points are held firmly in place, and each point is used until it begins to dull.
The engineer/supervisor over the lapping process must consider several factors when selecting a system of pellets, just like selecting any other grinding or lapping abrasives. A proper selection of pellet parameters will affect rate and quality of the grinding, as well as the useful life of the grinding pellets. Consider the following:
The hardness of the material to be lapped is an important consideration. Glass would require a different pellet selection than would ceramic or carbon steel. It should also be noted that the durability for pellets made for hard materials will be less than that for pellets designed for soft materials.
The surface finish of the material at the start and the desired finish at the conclusion of the lapping are also important. Constantly lapping overly rough materials, or skipping a grade in diamond particle size, will lower the life expectancy and the efficiency of the grinding pellets.
The chemical/mechanical action of any coolant or lubricant in the lapping process must be considered. For most materials there is a selection of desired lubricants, and the pH is often critical for both the pellet and work.
As the work is ground away by the pellets, the pellets are also ground away by the work, but desirably at a much lower rate. There are two actions on the pellets of which the operator must remain aware. The pellet matrix is being worn away exposing new diamond points, and simultaneously some existing diamond points are being worn flat.
Under normal usage, the two actions are in equilibrium. However, if the pellets are used on a harder material than that for which they were designed, the matrix will not wear as fast as it should. The outcome is that the points will become dulled. The operator will know this is happening because more pressure will be required to achieve the same results.
As an example, when the pellets which contain 50 micron diamond particles are first applied to the surface generated by the pellets which contain 75 micron diamond particles, grinding effectiveness will appear large at the beginning and slowly decrease. This occurs because the rougher surface wears the pellet matrix faster, exposing a constant supply of new diamond points. As the work surface becomes smoother, effectiveness will decrease due to lack of wear on the matrix.
This diamond-point-flattening can also occur when a pellet is used for too
long on a given work piece. Because this can occur
in normal operations, a loose grain may be applied to the grind surface to wear
away pellet matrix and thus expose sharp diamond points. However, the applied
grain must be a specific match to the diamond particle size in the pellet in
use. Doing this occasionally is not harmful to the pellets, but care must
be used to apply the correct loose abrasive to the matching pellets for the sake
of the pellet and the work.
Two different types of diamond are used to make pellets: friable and hard.
A metal binder (matrix) is used to encase the hard diamond, and an organic/carbonaceous binder is used to encase the friable diamond. For many materials, either is a good choice. The metal bonded pellet is a more aggressive grinder (tolerates higher pressure), but it does create more subsurface damage. The friable diamond in the organic pellet provides less aggressive grinding action.
A process could start with metal bonded pellets and, when needed, finish with organic bonded pellets - all prior to polishing. Very sensitive optics, such as those used with lasers, might require the use of organic bonded pellets throughout.
The same pellet system used to lap glass will not effectively lap ceramic, and vice versa.
When pellets are used on material softer than that for which they were designed, the matrix (binder) will erode away too fast reducing grinding efficiency (sometimes dramatically), and causing excessive pellet wear.
When pellets are used on material harder than that for which they were designed, the diamond points will wear flat and thereby greatly reduce the effectiveness of the pellets.
The standard lapping plate is flat. The pellets are glued or epoxied to the plate in any reasonable design. A layout design may depend on how pressure is applied to the plate and how the work rotates against the plate. The layout of pellets may, for example, be evenly distributed or slightly more dense nearer the center.
Theoretically, the maximum population density of cylindrical pellets on a flat plat will be about 91%, but this would not allow for an even flow of the lubricating fluid. If the pellets are spaced too far apart, the likelihood that one of them would catch an edge on the work, causing it to be popped off, is increased. A density of sixty percent, plus or minus ten percent, seems to work fine.
Lapping plates with concave or convex surfaces are possible. Please contact Gator Diamond for more information.
The pellets are supplied either mounted onto a plate (customer’s plate or a
plate supplied by Gator Diamond) or as loose pellets ready for the customer to
mount onto his own plate. An epoxy kit can be supplied with the pellets if
the customer desires to mount the pellets.
The information in this section is brief and non-technical in nature in order to give a quick overview of the factors that affect pellet selection.
Gator Diamond, Inc. has prepared a Technical Bulletin which provides detailed information about the pellet grades and configurations that are available, plus in-depth descriptions of the many factors which affect grinding/lapping operations.
To receive a free copy of this informative booklet, contact us at the address provided on our Contact Us page.