The Technique of Lapping
Checking the Working Plate
The working plate on the machine must naturally also be checked for flatness, as this is one of the most important factors influencing the accuracy and cost-effectiveness of the lapping process.
Checking should be carried out once or two times per day, and the conditioning ring adjustment is entered in a log. The first and most simple inspection method is a precise straight edge laid across the working plate. Sliding the straight edge from side to side indicates the location of the rotation point, or whether the working plate is convex or concave (Figure 69).
The second inspection method is a measuring beam with several dial gauges that were previously adjusted on an exact base surface (granite table) (Figures 31 and 32).
The third inspection method is a brass test block lapped on the machine. Its
flatness error closely approximates the error on the working plate in most
cases. It must be borne in mind here that a convex test-block surface
indicates a concave working plate (Figure 69).
In practice, the straight-edge test should always be done first. Only then
should an additional test be performed with the test block. Inspection errors are avoided if this sequence is observed.
The test piece should copy only that part of the lapping plate surface which
it occupied within the conditioning ring, i.e. only in the center of the
conditioning ring. If the working plate or test block is excessively convex a
relatively long time will be required until the final pattern is obtained
(wobbling of the test block on the convex working plate). The test block
method has been largely superseded by the use of measuring rulers (Figure 31).
The amount of material removed from the plate should be such as to correspond to an over-correction of the work-piece flatness. If the error detected is still outside the permissible limits, the machine may be allowed to run for some time with no work-pieces in the rings. If necessary, extra pressure can be applied or medium-sized work-pieces can be lapped as an aid to material removal during correction.
Within the 100 to 400 mm workpiece diameter range for example, flatness to the quality of l interference band (0.0003 mm) can be obtained only if the lapping plate is at least as flat or slightly convex (also see Page 16 “The working plate”). Only a single-plate lapping machine working with conditioning rings can be relied upon to maintain this level of accuracy over a prolonged period. The accuracy achieved must be documented by measuring the working plate. Practical experience has shown that a certain workpiece can also lead to a certain state of the working plate. The next time identical or similar workpieces are machined, the user can benefit from the previously recorded measurement and correction values (Figure 32).