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The Technique of Lapping

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The Single-Plate Lapping Machine

The single-plate lapping machine is finding a wide and ever-increasing range of applications, and for this reason a detailed description of this machine and its working principles and applications will now be given.

As in all machining processes, the operations are all conducted with specific objectives, in order to maintain control over the different factors influencing the quality. These factors are four in number:

  1. The flatness of the lapped surface is theoretically a copy of the lapping plate, which is constantly kept flat by the conditioning rings.
  2. The parallelism and dimensional uniformity of workpieces is ensured by the flatness of the pressure plate and by interchanging the workpieces.
  3. The size accuracy is achieved by the precisely controlled removal rate per unit of lapping time.
  4. The surface finish is determined by the grit type, lapping fluid (film) and pressure.

In modern lapping machines, the material removal rate is high enough to make lapping a serious competitor to grinding in many cases. In contrast to grinding, the same grit and the same lapping medium can be used for a wide range of materials. The fixtures used for lapping are generally simple to make, and therefore inexpensive. Nowadays, workpieces are commonly delivered for finishing with a machining allowance of 0.2 to 0.5 mm, so lapping can be implemented directly after turning, milling, punching or sintering, except in the case of large area, solid workpieces, as these workpieces require longer machining times (large chip volume). Components with large recesses, such as covers, housings,
flanges with recesses or holes, rings etc. are often provided with large machining allowances, and in such cases lapping is frequently much faster than grinding. There is no “grinding in air”, as the entire workpiece surface is machined at the same time.

Figure 15:
Assortment of workpieces
machined on a single-plate
lapping machine. However,
relatively tall workpieces and
workpieces with complicated
shapes can also be machined.

The main factor contributing to the success of the single-plate lapping machine (Figure 16) is the effect of the conditioning rings, which keep the lapping-plate surface flat during the lapping process. The single-plate lapping machine is in fact the only machine tool in general use which is capable of continuous self-correction for maintenance, or even improvement, of operating accuracy. The accuracy of all other machine tools decreases with each second of use unless the errors are compensated or corrected by means of additional equipment or measuring controls (it is worthwhile to make cost comparisons).

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