Lapping is a form of polishing or grinding used to produce components that are flat to extreme tolerances, often with very highly polished surfaces, says Paul Kingscott, sales director of Engis (UK)
The best examples of lapping that people see everyday are the optical lenses used in spectacles and the precious stones found in jewellery.
The process of lapping is actually an ancient one, with its roots traced back to prehistoric man grinding a sharp point on the end of a stick to create a spear.
Obviously the process has developed over the years and is now used in industries where great accuracy is required, such as the aerospace and medical sectors, or the manufacture of lenses for astronomical telescopes and lighthouses.
In engineering the main difference between lapping and similar methods, such as grinding, is that instead of having fixed abrasives like a grinding wheel, in lapping the abrasives are suspended in a liquid, known as a slurry.
Normally the slurry is made of silicon carbide or aluminium oxide, which is the abrasive content, mixed with a water or oil based liquid carrier.
The slurry is then fed on to a rotating metal plate, called the lap, where it forms a fine coating.
It is the action of the slurry between the rotating lap plate and the component that grinds away the material.
Lapping produces a finish of uniform flatness and parallelism to extremely fine tolerances, with accuracy of 0.0000254mm for flatness and 0.000254mm for parallelism.
Not only does lapping deliver greater accuracy and finer tolerances than traditional grinding methods, the process itself is much less invasive than other machining procedures, which results in advantages both in terms of the kind of finish that can be achieved and the type of components that can be handled.
Usually in lapping components are simply held in place using weights, or for more delicate and complex jobs, using a specially designed jig.
This reduces the machining stresses that materials can be exposed to with other methods and eliminates the possibility of distortion caused by clamping or heat.
As a result, lapping is ideal for the machining of very thin or irregularly shaped parts, as well as non-magnetic components.
Of course lapping can be achieved on a very basic level with a hand lapping plate, but commercial lapping equipment is much bigger, boasting lapping plates of more than a metre in diameter with sophisticated systems for the delivery of the abrasive slurry.
Standard lapping will produce a grey matte finish, but the equipment can also be used to polish surfaces to a shiny or reflective surface, depending on the material being polished and the abrasive used.
With this process, instead of employing a standard cast iron lap plate, a softer lapping plate of compressed metal and resin is used.
The abrasive, usually diamond particles, is again fed onto the lap plate, but this time they become embedded in the plate.
This time, however, the lapping plates create tiny scratches on the components that are so small that the surface finish appears shiny.
For an even finer finish the next step is pad polishing, where a soft pad is applied to the lap plate and a slurry of diamond, aluminium oxide or other abrasives are used to produce a truly mirror-like finish.
Polishing can produce a finish with a surface roughness of as low as 0.1 nm Ra.
This process is often used for the manufacture of components used in advanced applications, such as precision optics, semiconductors, compound semiconductors, data storage devices, technical ceramics and medical prosthetics.
The advantages are similar to traditional lapping in that there are reduced machining stresses and distortion, plus the capability to produce components to fine tolerances.
In addition, polishing can be used to machine extremely hard materials such as ceramics and tungsten carbide.
One of the telltale signs of lapping and polishing, and a testament to their accuracy and surface finish capability is the tendency for lapped surfaces to cling together when placed in contact.
Often called wringing-in, the phenomenon reveals just how perfectly flat the surfaces are, because when placed together the pieces are virtually impossible to prise apart.
To achieve the finishes and tolerance that lapping and polishing offer, it is important to get expert advice to find the right combination of slurry, pads, lap plates and lapping machine.
Different materials need specific slurries and the right kind of pad to ensure the desired accuracy and finish.
Engis (UK) is part of the Engis Corporation, a world-wide organisation which manufactures and markets superabrasive finishing systems for operations that demand precision surface polishing and close tolerance requirements.
Engis provides products, services and technological advances in several key areas including: diamond flat lapping/polishing, diamond and CBN-plated tools, bore finishing tools and machines, tool room products and accessories and R+D and technical support.