Factors to consider during the Lapping Process

Motion Type in the Lapping Process

The perfect movement with single-sided lapping is that a workpiece slides over the lapping plate, the path of the relative movements at any point of the workpiece ought to be composed, forming a continual curving line, and the size of the path from each point should be the same. Motion type in the lapping machine is a crucial factor to ensure processing accuracy, the most common is the ring method, which places workpieces inside the rotating ring (or conditioning ring) set properly between the centre of the lapping plate. By changing the position of the reciprocal movement, and the number of rotations of the workpieces, this method allows a change in the density of the trajectories, and control over the amount of the lapping plate abrasion. They can also condition the shape (concave, convex or saddle) of the lapping plate.

Double-sided lapping, simultaneously laps both sides of the workpieces housed in the carriers (wafer holder) that are placed symmetrically on the lapping plate and held in between the upper and lower lapping plates, allowing the workpieces to have the same trajectories. As lapping pressure works better on thicker workpieces, it has the helpful effect of minimising the thickness variations and parallelism mistakes of the workpieces.

Lapping Surface Plate

One of the main factors in the lapping process is the lapping plate, its surface accuracy and material are critical. Lapping plates are made of different materials, depending on the workpiece being processed. Cast iron lapping plates consist of hard cementite and relatively soft ferrite, this is very beneficial to lapping as unevenness of the abrasive size (particle distribution of the abrasives) can be eased by the ferrite, whereas cementite can boost the cutting actions of the abrasives further. Composite lapping plates are typically made from a mixture of synthetic resins, metal particles and key bonding/hardening. The composite structure of the lapping plates allows for a combination of the strengths of each material, resulting in improved performance and durability compared to cast iron lapping plates.

The Lapping plate surface can be grooved, such as crosscut, concentric circle and spiral. These grooves are very effective not only in artificially creating functions similar to that of the cast iron plate, but also in holding and supplying lapping slurries and discharging cutting chips as well as in deconcentrating the pressure distributions.

Abrasives and Reagents in the Lapping Slurry

Alumina (Al2O3) or silicon carbide (SiC) are used for the lapping abrasives, as both are very hard with an efficient cutting action. Alumina abrasives are round-shaped with high toughness, which makes them hard to be crushed, whereas silicon carbide abrasives are easily crushed due to their sharp cutting edge.

Diamond slurries are often used in precision lapping applications, as they offer several advantages over traditional abrasives such as Alumina or silicon carbide. Diamond particles are harder and more durable than these traditional abrasives, allowing for faster material removal and improved surface finish. They also have a more consistent particle size and shape, leading to more predictable and repeatable results.

Diamond slurries are suspensions of diamond particles in a liquid medium, typically oil or water. The diamond particles in the slurry serve as an abrasive, removing material from the workpiece as it is rubbed against it.

Diamond slurries provide faster material removal, improved surface finish, and more consistent results compared to traditional abrasives.

The average size of the abrasives is proportional to the processing effectiveness and surface roughness of the workpiece. Typically, the size of the abrasives is sequentially reduced during the lapping process. This condition has been logical for ensuring excellent removal of the projected surface topographies and work-damaged layers created in the preceding processes so that the predetermined shape of the workpieces can be obtained.

Reagents in the lapping slurry work to lubricate and evenly disperse abrasives, allowing the rolling of the abrasives and discharge (transferring function) of cutting chips. Oil or oil-based water solutions are used when removal actions of the abrasives need to be subdued, and the coefficient of friction should be controlled.

Mechanical Lapping Conditions

Processing characteristics in lapping include removal rate, lapping friction, surface roughness, and processed-damaged layers, which are impacted by the material of the lapping plates, the abrasives and lubricating fluids, and the relative speed (rotation number, drive distance).

Typically, the following equation (Preston’s formula) is applied:

(Stock removal) = α x (processing pressure) x (relative speed) x (processing time)

The parameter α depends on the conditions such as size, type, and density of the abrasives.

Horizontal lapping friction whilst lapping is related to the mechanical properties of the workpiece. Typically, the harder the workpieces are, the friction tends to be higher, and when processing pressure increases, the processing friction also tends to increase. An increase of the processing friction and pressure occurs due to subsequent increment of the number of acting abrasives, and increasing cutting depth of the abrasives. However, when the processing pressure exceeds a certain level, processing friction becomes consistent as the removal action of the workpiece reaches the limit for crushing.

Below are general points that will help you in the lapping process to achieve the desired finish:

The lapping plate should be softer than the part to be lapped.
The abrasive in a compound ought to be as hard as the metal being lapped. Stronger abrasives will charge or embed in softer materials/metals. A non-embedding or non-charging compound should be used for soft metals as garnet abrasives are used for brass or bronze. The softer the metal, the softer the abrasive. The harder the metal, the harder the abrasive.
High lap speed will increase stock removal. If the pressure weight is too much, it will score the part.
Increase in the pressure on the part against the lap will increase the speed of the cut.
Serrated or grooved lapping plates are best for flat surfaces with large areas, and also for flat areas with holes in the surface.
Laps with no serration or grooves are recommended for cylindrical lapping.
For measuring flat lapped surfaces, a mirror polish is necessary with the use of an optical flat and monochromatic light.
Abrasives when mixed into an oily paste or greasy vehicle will deliver greater results than just a combination of fluid oils and abrasives.
Tungsten carbide is best lapped with diamond paste. Boron carbide abrasive will slowly abrade the metal, but the finish is not as good obtained by diamond paste.
A gray or frost-like surface may be as smooth and accurate as a bright finish. A bright or polished surface does not indicate that it is smooth, but a smooth surface may be bright or polished.
A polished surface is harder to produce than a gray matte finish and will show scratches more readily.
A soft lapping plate will cut faster than a hard lapping plate. It will last longer and give a brighter surface. A hard lap cuts slowly, wears faster, and gives a dull finish, but its accuracy of lapping is greater.
Thin workpieces can be lapped parallel but not necessarily flat.
Every abrasive has a different finishing quality as to brightness or reflective ability. Silicon carbide abrasive will only produce a matte finish.
Final finishes are best obtained with no loose abrasive on the plate. Hence, the embedded abrasive granules and a very thin lubricant are used on the surface of the plate to get the final finish.
When soft metal parts such as brass or bronze running seals are lapped, a non-embedding abrasive should be used.
Different abrasives or grit sizes should not be used at the same time on the same plate. It is recommended to use different plates for roughing and finishing operations.
Diamond paste should be used for lapping tungsten carbide or other metals when the Rockwell C hardness exceeds 64.
Brightness does not indicate flatness. A matte finish can be just as smooth and accurate as a bright finish.
Fine abrasive grit sizes do not mean fine finishes. Abrasives can be very fine. The abrasive should be coarser to abrade or remove the desired amount of stock which causes the abrasive granules to break down into inert-sized particles. These fine particles produce the desired finish.
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