Used Lapping Machines for Sale — 218

Lapping is regarded as the oldest process of obtaining fine finish. Lapping process can be defined as a cutting process with loose abrasive grains dispersed in a paste, which is guided on the lapping tool with non-directional paths. Lapping produces smooth, flat, dimensionally accurate surfaces. The major difference between lapping operation and other finishing processes like Grinding & Honing is the usage of loosened abrasive particles instead of bonded abrasives as cutting edges. Also a high-quality surface finish of up to 0.1 µm and a very tight tolerance, as small as ±0.00005 mm is achieved in lapping compared to the ±0.0025 mm achieved during grinding.

Lapping can be performed manually by hand as well as by machines but lapping machines accomplish the process with greater consistency and efficiency. Also hand lapping is rarely used as it is expensive and requires high level of skill. The major parts of a lapping machine are: lapping plate, lapping table, abrasives and the lapping fluid or the vehicle. In lapping, two surfaces are rubbed together with a lapping medium (lapping fluid and lapping grit) trapped between them. Material is removed by countless loose particles between the carrier surface (lapping plate) and the work piece pressed against it.

Lapping Plates and Abrasives

Lapping plates, or simply laps, can be classified into four categories:

• Soft working plates (Paper, Cloth, Wood)
• Hard working plates (Cast iron, Steel, Soft ceramics)
• Hardened working plates (Hardened cast iron, Hardened steel, Hard ceramics)
• Multi-metal plates (Combination of 2-3 metals)

Abrasive particles in the form of loose grains are suspended in a fluid thereby forming a slurry. Oil is the most preferred vehicle which carries the abrasive grains. Grease are also used at times. The fluid should be chosen in such a way that when large abrasive grains are used, a thick film should be selected and similarly small grains are suspended in thin films. And abrasives belong to the following four categories: Aluminium oxide, Silicon carbide, Cubic Boron Nitride (CBN) and Diamond. Abrasive grit must be uniformly graded to be effective in lapping. Soft materials are lapped with Aluminium oxide and hard materials are lapped with Silicon carbide or Diamond. Particle size, distribution, hardness, shape and number of edges are the quality criteria to be considered. The particle size is chosen according to the necessary rate of material removal and surface finish required.

Working with a Lapping Machine

Lapping machines, such as valve lapping machine, back lapping machine, flat lapping machine, double sided lapping machine, center lapping machine or benchtop lapping machine, are fairly simple pieces of equipment consisting of a rotating table, the lapping plate, and three or four conditioning rings. Standard machines have lapping plates from 12 in. to 48 in. in diameter. The lapping plate is most frequently made of high-quality soft cast iron, though some are made of copper or other soft metals. The work is held in the conditioning rings which rotate. The rotation of the conditional rings plate distributes the wear so that the lapping plate stays flat for a longer time. Secondly it holds the work piece in place. The speed at which the plate turns is determined by the job being done. In doing very critical parts, 10 to 15 rpm is used. Work pieces must be pressurized by about 3 pounds per square inch (psi). Sometimes their own weight is sufficient. If not, a round, heavy pressure plate is placed in the conditioning ring. Also parts to be lapped should be placed securely in the condition rings, otherwise movement among them may lead to damages on the edges. Lapping time depends on the amount of stock to be removed, the abrasive used and the quality required.

Advantages and Disadvantages of Lapping Machines & Lapping Tools

Lapping procedure can be performed on any type of material and of any shape as long it is flat. Any type of material from diamond to plastics can be lapped. The type of material only affects the rate of material removal and the surface quality obtained. And as the parts are not clamped and no heat generation is involved, there is no warping which is a major advantage. Even though it is a material removal process, there are no burrs produced instead it removes those left in the primary processes. The major limitation of lapping machining is that it can be performed only on flat surfaces. Parts having very complex geometries cannot be lapped. Lapping is an art as a lot of process variables are involved and therefore highly experienced operators with sufficient skills can only handle the machines. Proper training and education should be imparted to the operators before working on the machine.

Lapping Machine Manufacturers

A few notable manufacturers of lapping machines are: PR Hoffman, Harrisons Engineering Ltd., Peter wolters, Metrolap, Rolap and Engis.