PRECISION MACHINING (LAPPING/HONING)

The term precision machining mainly refers to production methods involving cutting that are used in order to manufacture high-quality workpieces. The quality can refer to both the surface quality and the dimensional accuracy, i.e. the permissible dimensional deviation from the setpoint. Precision machining includes honing and lapping.

Honing

Honing is a metal-cutting precision machining process for almost all materials, and is the final manufacturing process during production. The aim is to improve dimensional and shape accuracy, and primarily to have a positive influence on the tribological properties (friction, wear, lubrication). The most well known applications are the piston running surfaces on the cylinders of combustion engines and hydraulic components.

Honing jobs can be done on the usual types of lathes and vertical drilling machines. On a honing machine, a rotating tool also known as the hone is guided into the hole being machined, inside which it is both rotated and oscillated (longitudinally back and forth). The material is removed via geometrically undefined cutting edges by an abrasive medium fused into honing tools (also called honing stones) (bound grain consisting of silicon carbide, corundum or diamond). These are incorporated into adjustable and spring-loaded heads, and are pressed against the hole with a defined, variably adjustable pressure. A honed surface is recognisable by the cross grinding (cross hatch). This is characteristic of honing. It improves the sliding properties, as it allows oil to accumulate in the small grooves.

Lapping

Lapping is a production process performed mechanically (or if necessary also manually) for smoothing surfaces while maintaining narrow tolerances. A mixture of lapping powder (aluminium oxide or silicon carbide) and water or lapping oil is used for lapping. In contrast to honing, in which the abrasive grain is securely bound, lapping employs loose (rolling) grain. Due to the small amount of material removal, this allows very high surface qualities to be attained, even with relatively coarse grains. It is important for the grain size of the grinding material to be almost homogeneous, because otherwise single larger grains will produce relatively deep score marks. Lapping is mainly used for machining hard surfaces, e.g. where dimensional and shape tolerances of a few micrometers may be required.

In lapping, the material is removed by means of two processes taking place at the same time:

• The lapping grains press simultaneously into the lapping plate and the workpiece - the surface is cut by the relative movement.
• The grains roll off between the lapping plate and the material surface, the grain tips deform and strengthen the workpiece surface.
  
  
  "(Source: Wikipedia)"