Accessories used in lapping are dressing rings and workpiece receivers, as well as devices used to weigh down or remove weight from the workpieces and supply equipment for providing lapping slurry. The dressing rings located on the machining disc are supported by lateral guide arms with rollers and receive rotary motion through friction locking when the table rotates.
The dressing rings have several functions to perform in lapping:
• receiving the workpieces,
• continuously dressing the lapping disc during machining to ensure the maintenance of its geometric accuracy,
• the even distribution and formation of the lapping slurry,
• the removal of chips directed towards the edge of the lapping disc or into the radially running grooves in the lapping disc designed for this purpose,
• the drive for an additional movement component of the workpieces and
• removing heat [STAE76].
Geometrical changes to the lapping disc surface can be compensated to a great extent by shifting the dressing rings towards or away from the centre. Workpieces with round outer forms are frequently placed in the dressing rings without any additional devices. They attach themselves during machining to the inner ring surface and are thus set into rotary motion (internal-toothing principle).
However, angular workpieces are placed in templates or holders made of plastic, metal or wood. Their contours are adjusted to the dressing rings and the workpieces. This prevents a collision of the parts in the ring. For purposes of mass production, non-production times can be greatly shortened through the use of additional feeding devices. Individual parts whose reception in dressing rings is not possible are fixed on the lapping disc by special mounts and set in rotary motion.
Lapping can only be implemented economically given a certain amount of bearing pressure upon the surface to be processed. Otherwise, the material removed is reduced owing to the floating of the workpiece on the lapping slurry. This is a risk when machining parts with low weight and bodies whose surfaces are large in proportion to their mass. In such cases, an additional load must be added. For this purpose, either load plates are used or, for example, pneumatic loading devices that take over the raising and lowering of the load plates. The workpiece may not, however, become deformed or distorted. Also, excessive pressure would lead to the breakage of the fluid film and to pressure welding.
In the case of very uneven and rough workpieces, elastic intermediate layers are placed between the loading device and the workpiece in order to achieve the most even distribution of pressure possible on the surface to be machined. If, on the other hand, too much pressure is applied to workpieces with a high weight and a small support surface, pressure on the workpieces must be reduced through lifting tools or pneumatic devices.
Production machines are equipped to continuously supply the lapping slurry. The lapping slurry is homogeneously mixed in one or more storage containers with stir-mixing or turbo-mixing devices in order to ensure a steady consistency. The contact pressure has an influence, since a constant fluid film has to be ensured at all times. The average amount of lapping slurry consumed for planar lapping, for example, is 1 to 10 l/h for machining disc diameters from 400 to 1500 mm.
Used lapping powder is not reusable because the grains are blunted or broken due to wear. Nevertheless, the lapping mixture is partially reused, since the carrier fluid can be prepared again after the grains and chipped material settles.