What is Lapping | How it works

What is lapping?

Lapping is an abrading process used to produce geometrically true surfaces, improve dimensional stability, or provide a very close fit between two contact surfaces. It is achieved by rubbing two surfaces with each other by hand or machine with abrasive material as a rubbing agent. This process removes the irregularities caused by the sawing and grinding. In this process, a thin layer of metal from 0.005 to 0.01 was removed.

There are two types of lapping processes. The first process is generally known as grinding. In this, a lapping tool or grinder is moved over the metallic object. It removes the material from the surface and gives a good surface finish. 

In other processes, lap with the ceramic and pitch which are soft are embedding with the abrasives. It is suitable for hard materials. This combination helps to finish hard surfaces and gives better polish.

How lapping works?

In lapping operation, abrasive powder such as emery, chromium oxide, corundum, and iron oxide mixed with oil, water, or pasts with some carrier. Carrier is to separate two workpieces for preventing their edges from damage, and e-clips are to hold these parts.

In this process, shoes called laps are rubbed against the work. Charging of lap is nothing but embedding abrasive particles on the surface of the lap. After charging the lap, there is no need to apply more abrasive particles to the lap. The objective of the lapping is to achieve a uniformly flat and smooth surface. With the hands or machine, the lap moves in a rotary or reciprocating motion.

In the lapping of cylindrical work, the workpiece gets clamps and rotates on a lathe or drill machine and moves lap over the surface of the work. The lapping of the small flat surfaces, with the help of the hand, work is moved against a rotating disc or in an irregular path over a stationery faceplate lap.

In the free abrasive lapping process, abrasive slurries spread directly on the lap plate. It offers minimum damage to the specimen surface. 

Factors to consider

Speed

Lap plate speed is essential during the lapping process. If speed is increases or variation in the speed caused vibration and oscillations, which may cause irregularities on the lapped surface.

Pressure

An increase in pressure may cause the roughness value to differ and wiping. So, for common applications, it is necessary to apply pressure lower than the 2 PSI. 

Abrasive size

In the lapping process, abrasive size is responsible for the surface roughness. According to the type of application and requirement of the surface texture, abrasive particle sizes are selected. 

Material of the workpiece

The selection of the abrasive is depending upon the type of workpiece to be handle. Because it is necessary to select correct abrasives as per the material composition of the workpiece to get a better result.

Vehicle slurry ratio

If film thickness between lap and workpiece is less, then more abrasives come in contact, and transfer of more cutting energy occurs. It gives high material removal rate and gives a good surface finish.

Types of lapping machine

There are two types of lapping machines. The vertical axis machine and centerless machine.

In the vertical axis, the machine is for flat or round surfaces between two opposed laps on vertical spindles. The centerless lapping machine is for producing round parts such as bearing races, valves tapped, and shafts. The principle of a centreless lapping machine is the same as centreless grinding. 

Materials for lapping process

In general, cast iron is the most common material for the lap. Soft steel, brass, copper, and hardened steel are also some alternative materials for the lap. 

Abrasive

Vehicle material for lapping

Machine oil

Greases

Rape oil