Lapping Basics: Characteristics and Application Methods

This article introduces the characteristics of lapping and the machinable materials and surfaces, expounds the lapping principle and the machining accuracy that can be achieved by lapping, and lists in detail the composition of the lapping agent, the function and materials of the lapping tool, as well as the lapping trajectory and parameters.

Lapping is a traditional precision and finishing process.

By using the free fine abrasive particles attached and embedded on the surface of the lapping tool, and with the help of certain pressure and relative motion between the lapping tool and the workpiece, the extremely tiny chips are removed from the surface of the workpiece, so that the workpiece can obtain extremely high dimensional and shape accuracy and extremely low surface roughness value, which is widely used in precision and ultra-precision machining.

1. Characteristics of lapping

Lapping has the following characteristics:

1) The surface roughness of the workpiece can reach Ra = 0.006 ~ 0.1 μm, and micro-cutting with Ra = 0.1 μm can be carried out.

2) It can make the mating surface of the couple to get a very precise fit.

3) Lapping is carried out at low speed and low pressure, so the heat generated is very small.

There is no metamorphic layer on the surface of the workpiece, so the quality is better.

4) The structure of grinding device and grinding machine is relatively simple, which is suitable for both single piece manual production and batch mechanical production.

The machining accuracy of manual grinding depends on the accuracy of the lapping tool and the operation technology of the workers, while the machining accuracy of mechanical grinding also depends on the precision lapping tool, reasonable trajectory and correct operation method.

5) In the grinding process, the hard abrasive particles are easy to embed into the soft surface of the workpiece, which affects the service life and optical properties of the workpiece.

6) Lapping efficiency is low.

Generally, the material of the lapping tool is soft and easy to wear, so it should be repaired in time to ensure the accuracy of the lapping tool.

2. Machinable materials and surfaces of lapping

Grinding can process all kinds of metal materials such as steel (including quenched steel), cast iron, copper and cemented carbide, as well as non-metal materials such as ceramics, gems, semiconductors, glass and plastics.

The surface shape of workpiece includes inner and outer cylindrical surface, conical surface, plane, convex concave surface, inner and outer spherical surface, screw thread and gear etc.

3. Lapping principle

When lapping, the appropriate amount of abrasive is added between the lapping tool and the workpiece surface.

Under certain pressure, the compound motion of reciprocating motion and rotation, or the compound motion of rotation and planetary motion, is carried out.

The abrasive particles in the abrasive are made to slide or roll between the lapping tool and the workpiece surface for micro cutting.

Because the abrasive particles are very fine, only very thin material layer can be cut.

The surface of the workpiece can get a very small net movement track, so that the workpiece can get a very high surface quality and machining accuracy.

According to the use conditions of abrasives, they can be divided into wet lapping (i.e., sanding lapping, shown in Figure 1), dry lapping (i.e., sanding lapping) and semi dry lapping (i.e., paste lapping paste lapping).

Fig. 1 Wet lapping process

Fig. 1 Wet lapping process

4. Machining accuracy that lapping can achieve

On the basis of good pre-processing, lapping is to carry out micro-cutting with surface roughness Ra = 0.01 ~ 0.1 μm, and micro feed.

This is difficult to achieve by other machining methods, so the machining accuracy and surface quality can be several times higher than other machining methods.

The precision can reach 0.025 μm and the surface roughness can reach Ra = 0.006 μm.

The general machining method is to follow the “copy processing”, while grinding is the “creation processing” throughout.

In this process, the precision of the lapping tool and the workpiece is improved at the same time, which is higher than the original precision of the lapping tool.

5. Abrasive

The abrasive is composed of abrasive, grinding fluid and auxiliary filler.

According to different grinding methods and workpiece materials, liquid abrasive, grinding paste and solid abrasive can be prepared.

Abrasive is the basic component of abrasive, its performance and reasonable selection directly affect the grinding efficiency and quality.

1) Abrasives

The commonly used abrasives are corundum, silicon carbide, boron carbide, diamond and cubic boron nitride.

In order to further reduce the surface roughness of the workpiece, soft abrasives such as iron oxide, chromium oxide and cerium oxide are also used.

Corundum abrasive is used to grind general steel parts;

When grinding hard and brittle materials such as cast iron, hard alloy, gem and ceramics, silicon carbide or boron carbide abrasive is used;

Diamond abrasive should be used when grinding cemented carbide, ceramics, gemstones and optical glass;

Cubic boron nitride abrasive should be used when grinding high-speed steel and die steel.

The choice of abrasive size and grinding efficiency are directly related to the surface roughness of the workpiece.

Coarse particle size leads to high efficiency and large surface roughness value Ra;

On the contrary, the lower efficiency is, the smaller surface roughness Ra is.

The grain size is W0.5 ~ W40, and the surface roughness Ra is 0.006 ~ 0.4 μm.

2) Lapping fluid

The lapping fluid plays the role of cooling and lubrication in the process, and makes the abrasive particles evenly distributed on the surface of the lapping tool.

When rough grinding, N15 full loss system oil is used;

When finishing grinding, one part of N15 total loss system oil, three parts of kerosene, a small amount of turbine oil or spindle oil, and light mineral oil or transformer oil are used.

When grinding cast iron, kerosene is used;

When grinding copper, animal oil and a small amount of spindle oil or vegetable oil is used;

When grinding quenched steel and stainless steel, vegetable oil, turbine oil or emulsion is used;

When grinding diamond, olive oil, roundness oil or distilled water is used;

When grinding cemented carbide, gasoline is used;

Alcohol is used when grinding gold, silver and platinum;

Water is used when grinding glass and crystal.

3) Auxiliary fillers

In the grinding process, the auxiliary fillers play the role of adsorption and improving processing efficiency.

Commonly used types are concocted by stearic acid or oleic acid, fatty acid and industrial glycerin as major components in different proportion.

4) Preparation of abrasive

In wet lapping, kerosene, mixed grease and abrasive powder are used as liquid abrasives.

The ratio is not strict and the concentration (mass fraction) is about 30% ~ 40%.

When the powder is fine and the machine supplies it automatically, the concentration decreases.

In dry grinding, 15g of abrasive powder, 8g of mixed fat, 200ml of aviation gasoline and 35g of kerosene were used with soaking for one week;

The grinding paste contains 20%~50% abrasive powder, 25%~30% oleic acid and 18%~30% mixed grease, and sometimes about 15% Vaseline is added when grinding very fine powder;

Solid abrasive (abrasive soap) is used to improve the surface gloss of workpiece.

Its formulation is chromium oxide 57%, paraffin 21.5%, beeswax 21.5%, stearic acid mixed grease 11% and kerosene 7%.

6. Functions and materials of lapping tools

1) Functions of lapping tools

The lapping tool is the forming “model” of lapping, which copies its own geometric accuracy to the workpiece to a certain extent.

It is the carrier of abrasives for coating and inlaying abrasives.

In its relative motion with the workpiece, the workpiece is processed to obtain the correct machining accuracy and surface quality.

The lapping tool must have grooves to store abrasives and prevent the accumulation of excess abrasives;

It must have certain geometric accuracy, enough stiffness, compact material without impurities and uniform hardness.

2) Materials of lapping tools

Cast iron is suitable for grinding various materials;

Mild steel is suitable for grinding small diameter thread and small hole;

Brass and red copper are suitable for rough grinding and gem grinding;

Hardwood is suitable for grinding copper and soft metals;

Tin and lead are suitable for improving the surface quality of workpieces because they are too soft to change the shape of workpieces.

7. Lapping trajectory and parameters

1) The lapping trajectory is shown in Fig. 2, and it has different forms such as straight-line reciprocating cycloid.

Fig. 2 The lapping trajectory

Fig. 2 The lapping trajectory

2) Lapping pressure

Wet lapping pressure is 10-25 Mpa, dry lapping pressure is 1-15 Mpa, fine lapping takes small value.

3) Lapping  speed

The lapping efficiency is directly proportional to the lapping speed.

The lapping speed of wet lapping is 20-100 m/min, and that of dry lapping is 10-20 m/min.

If the accuracy of the workpiece is high and the material of the workpiece is soft, the smaller value is taken.

4) Lapping  allowance

The inner hole is 0.01-0.03 mm (diameter allowance), the outer circle is 0.005-0.01 mm, and the plane is 0.005-0.01 mm.

5) Problems needing attention

The abrasives must be well preserved to avoid dust falling in;

When changing the abrasive with different particle size, the original abrasive on the workpiece and the lapping tool must be cleaned with kerosene to avoid scratching the surface of the workpiece;

The lapping environment should be dust-free.

8. Conclusion

Lapping is a kind of precision and ultra-precision processing technology, the surface roughness value is very low.

With the rapid development of science and technology, the requirements for product quality are becoming higher and higher.

In the field of local finishing, lapping has the irreplaceable advantages of other cutting methods, which shows its special significance in the processing of high precision and high surface quality workpiece.

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