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Lapping Process: Definition, Working Principle, Types, Material, Advantages, Application [Notes with PDF]

Hello reader, Hope you are doing good! In this article, we are going to learn Lapping Process and its sub-topic like Definition, Working Principle, Types, Material, Advantages, Disadvantages, and Application in detail.

And PDF you can download at the end of every article.

So let's start,

What is a Lapping Process?

Lapping is a low-pressure machining process used to increase the dimensional accuracy and surface finish of the workpiece. The lapping process uses fine Abrasive particles to perform cutting action on the surface of the workpiece.

The workpiece is rubbed against the lapping plate with the help of a machine or by hand, with powdered Abrasive particles or Abrasive paste in between.

The lapping process smoothes the surface of the workpiece, this increases the functionality and aesthetics of the workpiece. It is also used to ensure the correct fit between two workpieces and correct minor imperfections. 

Principle of the Lapping Process:

The simple working of a lapping process involves three elements. A workpiece, a lapping plate, and Abrasive particles.

Abrasive particles are poured over the lapping plate and the workpiece is rubbed on it.

The Abrasive particles remove microscopic material from the workpiece when a force is applied during the relative motion between the plate and the workpiece.

The Abrasive particles tend to reduce or flatten the microscopic peaks and valleys present on the surface of the workpiece. 

Characteristics of the Lapping Process:

The following characteristics of the Lapping Process are:

  • The size of Abrasive grit used in the lapping process varies from 120 to 1200 mesh depending upon desired surface smoothness. 
  • The Abrasive used can be in powdered form or can be mixed with some suitable oils or greases called vehicles. 
  • A very small amount of material is removed in the lapping process that is about 0.025mm.
  • The lapping process can be done manually or with the help of machines. 
  • Materials commonly lapped are hard materials like steel. 
  • The pressure applied during the lapping process is low when compared to other surface finish processes. 

Types of Lapping Processes:

There are two types of lapping processes that are further divided into their respective domains:

  1. Hand Lapping and
  2. Machine Lapping Process

Hand Lapping Process:

As the name suggests the process of lapping in all the forces are applied manually is called hand lapping. There are two types of hand lapping systems involved in improving the surface quality of the workpiece. 

#1. Flat surface lapping:

The lapping plate here is made of gray cast iron. This contains some groves which are used to collect the metallic chips and waste Abrasive particles in it.

Abrasive grits of the desired size are sprinkled over the lap. Then the workpiece is rubbed carefully on the lap.

More amounts of Abrasive particles of the same mesh are added when required. Use of suitable fluid can be done if required. The process must be carried out by a skilled operator.

Once the lapping is done, the lap must be cleaned for the next usage. 

Note: Using an excess of Abrasive particles may damage the lap. This may result in improper finishing of the job and lap failure. 

#2. Cylindrical Lapping:

The laps used for the lapping of internal and external surfaces of cylindrical workpieces are made of mild cast iron, copper, or brass.

For lapping of the internal surface of a cylinder, the laps are made to have a close fit with the internal diameter. The laps move in random axial motion w.r.t the workpiece. This increases the surface finish of the workpiece as well as the laps.

The Abrasive particles are mixed with oil or grease in the process. The laps used for the lapping of the external surface of a cylinder are called ring laps. It is provided with a screw to adjust with different diameters.

The workpiece to be lapped is held in a lathe to provide rotational motion. The ring lap is then reciprocated on the rotating workpiece. Abrasive particles mixed with certain oils and greases can be used. 

Machine Lapping Process:

When lapping operation is done using machinery it is called machine lapping. Listed below are various machine lapping processes used for different types of workpieces. 

#1. Vertical Lapping Machine:

A vertical lapping setup consists of a lower lap, an upper lap, and a work-holding device. The workpiece is kept on the lower lap and is positioned in a job holder. The upper lap is used to load and unload the workpiece.

Abrasive particles mixed with paste-type vehicles are applied on the laps before the operation begins. In the case of lapping a flat surface, the lower lap is held stationary, while in the case of cylindrical lapping the lower lap is allowed to move.

Various jobs are lapped using vertical lapping machines, for example, precision gauge blocks, rolls of roller bearing, and ball bearing balls.

Another type of vertical lapping setup consists of an Abrasive cloth fixed to the lower lap. Identical workpieces can be lapped by moving about the work holder randomly. This is called transposing. 

Note: The Lower lap must be fixed while machining a flat surface as it may topple. 

Vertical Lapping Process

#2. Centreless lapping machine:

A centreless roll lapping consists of a regulating roller, a lapping roller, a workpiece, and a V notched fiber stick. The regulating roller is powered by a motor to provide rotational motion to the setup.

A lapping roller is used to lap the workpiece from the other direction. The diameter of the lapping roller is twice the diameter of regulating the roller. Both the rollers are made up of cast iron, due to their stability and low cost.

The rollers are bonded with Abrasive particles. V notched fiber stick is used to hold the workpiece to be fed between the rollers. The workpiece is fed into the roller at a very slow rate with a reciprocating motion.

Material removal takes place due to the action of Abrasive particles of the workpiece. A very fine finish of 0.05 microns can be obtained with the help of centreless lapping.

The material removal is very slow in the case of centreless lapping. 

Centrless Lapping Machine

#3. Spherical lapping machine:

The lapping machine used to lap spherical objects has a different construction. Two spindles are used in the case of spherical lapping.

The workpiece to be lapped is attached to the first spindle which gives a rotating motion to the workpiece. The other spindle is used to hold the tool to a certain angle w.r.t the workpiece.

Pressure is also applied on the workpiece by the tool spindle. As soon as the rotation motion is given to the workpiece the material removal takes place due to the contact between the Abrasive tool and the workpiece. 

Spherical lapping Process

#4. Belt lapping:

Belt lapping uses a belt made of Abrasive clothes used for the lapping process. The belt is driven with the help of a motor. There are two wheels used to drive the belt.

The workpiece is manually touched to the belt over the wheel. Material removal takes place after the contact of the workpiece and the moving belt.

This is a convenient method of lapping for low-precision jobs. There are no requirements for clamping the workpiece. 

Belt Lapping Process

Abrasive used for the lapping process:

  • Different types of Abrasives are used for lapping different material hardness. 
  • Abrasives used for lapping hard materials generally are silica carbide and fused alumina. 
  • Abrasives used for lapping soft materials are chromium oxide, unfused alumina, and garnet. 

Vehicles used for the lapping process 

Vehicles used for lapping processes are generally water-soluble oils, vegetable oils, mineral oils, petroleum jelly, or greases. Vehicles must have the following properties. 

  • The particles must be uniformly distributed when mixed with the vehicle. It must not contain lumps. 
  • The viscosity of the vehicles should not be affected by temperature changes. 
  • A vehicle should not evaporate at high temperatures. 
  • It should not promote Corrosion for metal surfaces. 
  • It should be non-toxic in nature. 
  • It should not stick to the workpiece and can be easily removed by simple cleaning. 

Materials that can undergo the lapping process:

There are various elements originally compounds that can undergo a lapping process. Some of those are Cast iron, Ceramics, Tungsten, Plastics, Carbon compounds, Stainless steel, Bronze, Rubber, and Silicon.

Lapping process Parameters:

The following parameter is:

  • Lapping speed: The lapping speed for an ideal lapping operation must range from 1.5 to 4m/s
  • Lapping pressure: Lapping pressure depends upon the hardness of the material and the Abrasive particles used. Lapping pressure varies from 10 to 30kPa for soft materials and up to 70kPa for hard materials. 
  • Lapping allowance: Lapping allowance is directly proportional to the lapping time. As the lapping allowance increases, lapping time also increases. The lapping allowance also depends on the material being lapped. 
  • Cast iron - 0.2mm
  • Aluminum alloy - 0.1mm
  • Soft steel - 0.05 to 0.1mm
  • Hardened steel - 0.005 to 0.02mm

Advantages of the Lapping Process:

It increases the surface finish of the workpiece by reducing peaks and valleys on the material surface. It also provides aesthetic touch to the workpiece. 

Lapping process also increases the life of the mating parts by reducing the friction between the Mating parts.

It increases the geometry and the dimensions of the workpiece. It also ensures a perfect fit between the workpieces. 

No thermal stresses are developed inside the surface of the workpiece due to the minimum heat involved in the process. 

The process can be carried out by unskilled labor as well. This is due to the simplicity involved and the absence of computerized controls. 

It increases the parallelism between two parts kept parallel to each for various joining operations. 

Disadvantages of the lapping process:

Lapping is an expensive process as the Abrasives loss is more. This result is increased use of  Abrasive particles for polishing operations, which in turn gives rise to the production cost. 

Another disadvantage of the lapping process is that the process is very slow. The metal removal rate in the case of the lapping process is very less. This makes the processing time-consuming which results in decreased production rate. 

Waste management becomes a problem in the case of a lapping process. A large amount of Abrasive waste is produced in case of a lapping process. This makes it difficult to dispose of the waste. 

Some of the Abrasive compounds may get embedded in the surface of the workpiece which results in later damage of the workpiece. 

The process is not generally automated; these results increase labor-intensive and increase the cost of production. 

Applications of the Lapping Process: 

The lapping Process is used for polishing hard and brittle metals which are difficult to be polished using other methods. 

It is also used for machining materials like glass due to its slow material removal rates. 

The lapping process is widely used for making specimens for metallurgical testing. 

The lapping process is used for valve manufacturing due to its accurate uniform material removal feature. 

Automotive industries also use lapping processes for various polishing operations. 

More Resources:


Media Credits:


  • Image: By Saubhik Roy- Design Team Learn Mechanical.
  • Feature Image: Modified by Author

References:


So this is all about Lapping Process, I hope this session was beneficial for you. Do let me know if you have any doubts regarding this topic, I will try to answer your quires within 24 hours!

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