Lapping

Lapping is a precision finishing process that uses loose abrasive particles between a lap and a workpiece to improve flatness, surface finish, dimensional accuracy and fit. It is slower than many machining operations, but it can produce very fine surfaces with low thermal and mechanical damage.

• Key takeaway: Lapping removes small amounts of material using free abrasive grains in a slurry or compound.
• Key takeaway: The process is used when flatness, parallelism, surface finish and close mating contact are more important than fast material removal.
• Key takeaway: Common applications include ceramics, glass, brittle materials, seals, valves, gauges, optical parts and precision mechanical components.
• Key takeaway: Abrasive size, lap material, pressure, speed, slurry chemistry and time control the final surface quality.

Table of Contents

• What is lapping?
• Types of Lapping
• Advantages of lapping
• Disadvantages or limitations of lapping
• Important lapping process variables
• Lapping vs polishing
• FAQs
• References

What is lapping?

Lapping is a type of finishing process which is carried out with the help of loose abrasives. In this finishing process, surface smoothening is achieved by the abrasive grains which are supported by the lap, which functions as a cutting tool shank. Ceramic machining, Brittle material machining, and Glass machining are some of the applications of the lapping process.

The principle of lapping is based on the cutting power of either a free abrasive grain in a carrier or a fixed abrasive particle within a composite lapping plate matrix. The lapping plate on which a slurry medium is applied moves under the workpiece/job surface while the workpiece/job rotates on the lapping plate. The uniform abrasive layer forming between the lapping plate and the workpiece gradually removes material from all the contacting areas simultaneously, putting very little stress into the workpiece. Ultimately when the whole workpiece has been lapped, the shape of the lapping plate is imparted on the workpiece (as in Figure 1). For this reason, flat lapping machines are capable of generating incredibly flat workpieces.

It seems as if the abrasive grains are working in a free movement which is not entirely true. Depending on the lapping process used, the abrasives are either freely rolling or sliding between the workpiece and lapping plate. In diamond lapping, the abrasive particles become fixed within the machined medium. The material removal takes place slowly due to the movement of the lapping plate with respect to the workpiece.

Figure 1: Lapping Operation

Lapping is a process that requires precision, hence it takes quite a bit of time. In order to speed up the process, the two-step operation can be used. The first step is to remove material and create flatness and the second step is to produce a specific surface roughness (Ra value). This is only necessary when our desired Ra is below 0.05µm and we need to remove a lot of material.

With conventional lapping, abrasives such as aluminum oxide or silicon carbide are applied in a carrying medium (e.g Oil) onto a hard-worked surface (e.g. cast iron). The particles can’t be pressed into the surface and remain secured there, so they roll and displace freely in all directions. They hammer small particles out of the workpiece, imparting deep deformations. This phenomenon takes place as the free-moving abrasive particles are not able to produce a real scratch on the component surface. Instead, it hammers out small pockets.

Typically the lapping process is used to:

• Produce a fine surface finish
• Produce sealing surfaces
• Produce sharp cutting edges
• Produce flat stress-free surfaces
• Produce a datum face
• Improve the wear-resistant properties of surfaces

In general, the abrasives used are in suspension (slurry medium) and applied to the platen during the process or are embedded in a film that is adhered to the platen. There is a wide variety of abrasives when selecting a lapping or polishing process. Selecting an abrasive is dependent upon the job hardness, targeted surface finish, desired removal rate, lifetime, and price. There are four types of abrasives that are used in this process:

SiC: It is hard and has a needle or blocky-like structure. It is used in applications where rough lapping is required. It is used for applications that require smooth surface finishes.

Al2O3: Alumina is generally used where fine surface finishes are required since it breaks down over time & gives excellent surfaces during lapping. It is relatively inexpensive than other abrasives.

B4C: It is harder than most other abrasives (excluding diamond) and has a blocky crystal structure. It provides excellent removal rates and is typically used when fast removal with moderate surface quality is needed.

Diamond: It is the hardest material known and has a sharp, angular-like structure. It has high removal rates and surface finishing properties. It has the ability to produce excellent surface finishes combined with high removal rates.

The incredible video below demonstrates a few lapping methods and the kinds of geometry the lapping produces:

Types of Lapping

According to the rubbing action, the lapping process can be divided into two types:

1. Hand Lapping: In this process, the workpiece is rubbed over the lapping plate manually. First, the plate is coated with the abrasive material which is in the form of paste. Now, the workpiece is rubbed over the plate by applying a small pressure manually.
2. Machine Lapping: It is used for finishing large quantities of similar parts. This machine involves a rotating lapping plate and conditional rings. The conditional rings provide sits for the workpiece. This ring rotates and rubs the workpiece against the lapping plate.

Lapping plates or working plates grades can be separated into 4 categories:

• Soft working plates:- For e.g.: paper, cloth, plastic, wood, tin, aluminum alloy, copper, etc.
• Hard-working plates:- For e.g.: cast iron, mild steel, soft ceramic, etc
• Hardened working plates:- For e.g.: hardened cast iron, hardened steel, hard ceramic, etc., up to 500 HB
• Multi-metal plates:- For e.g.: sintered metal powder or a combination of 2-3 different metals.

Advantages of lapping:

• The dimensions are accurate
• It corrects minor imperfections in shape.
• Excellent refinement of surface finish.
• It produces a close fit between the mating surfaces
• It doesn’t require the use of holding devices; therefore, no workpiece distortion occurs.
• Generally, less heat is generated in lapping than in most of the other finishing operations. This minimizes the possibility of metallurgical changes occurring in machined parts.
• When both sides of a flat workpiece are in lapping simultaneously: extreme accuracy in flatness and parallelism. Also, the relief of inherent stresses can be achieved.

Disadvantages or limitations of lapping:

• It is a slow & gradual process
• The process seems simple but it is an expensive process

References

1. https://www.kemet.co.uk/blog/lapping/what-is-lapping
2. https://www.stahliusa.com/stahli-publication/the-technique-of-lapping/the-lapping-process/
3. https://lnf-wiki.eecs.umich.edu/wiki/Lapping
4. HUNSAKER, J. Friction and Surface Finish. Nature 146, 138–139 (1940). https://doi.org/10.1038/146138a0

Important lapping process variables

Successful lapping depends on controlling the abrasive action at the interface. The same nominal process can produce very different surface finishes if the abrasive, lap plate, pressure or slurry delivery changes.

Lapping vs polishing

Lapping and polishing are both finishing processes, but they are usually used for different goals. Lapping is mainly used to improve geometry such as flatness and parallelism while also improving surface finish. Polishing is usually focused more strongly on reducing roughness and improving appearance or optical quality.

In practice, a component may be lapped first to correct form and then polished to achieve a finer or more reflective surface. The correct sequence depends on material, tolerance, surface roughness target and whether subsurface damage must be minimized.

Where is lapping used?

• Mechanical seals and valves: flat mating faces reduce leakage.
• Optics and glass: controlled abrasive finishing improves flatness and surface quality.
• Ceramics and brittle materials: low-stress material removal helps avoid cracking.
• Gauges and precision tooling: accurate flatness and fit are critical.
• Semiconductor and microfabrication work: lapping can thin or planarize delicate substrates.

FAQs

What is lapping?

Lapping is a precision finishing process in which loose abrasive particles remove small amounts of material between a lap and a workpiece.

What is the main purpose of lapping?

The main purpose of lapping is to improve flatness, parallelism, dimensional accuracy, surface finish and the fit between mating surfaces.

Is lapping the same as polishing?

No. Lapping primarily corrects geometry and surface finish using abrasive grains, while polishing is usually focused on producing a finer, smoother or more reflective surface.

What abrasives are used in lapping?

Common lapping abrasives include aluminum oxide, silicon carbide, boron carbide and diamond, selected according to workpiece hardness and finish requirements.

Why is lapping slow?

Lapping is slow because it removes only a small amount of material in a controlled way to achieve high precision and avoid excessive heat or distortion.

See also

• Abrasive Wear
• Friction
• Wear
• Surface Roughness
• Plastic Deformation at Microscale

Last updated: May 2026. Reviewed topic: lapping process, precision finishing, loose abrasive machining and surface finish control.