WHAT IS LAPPING?
Lapping is a process of material removal achieved by applying loose abrasive between the surface of the work and tool, without positive guidance of the work, usually resulting in a finish of multi-directional lay.
“A” is the work. “B” is the tool.
Moving “A” randomly over the abrasive will cause visible scratching on both pieces. These scratches represent material removal.
Since manual lapping would be impractical for mass production, machines were developed for this application. The two basic types of lapping machines are single-sided and double-sided.
Single-sided machines are equipped with a rotating plate and conditioning rings for carrying the work. Conditioning rings also maintain or correct plate flatness.
Double-sided machines are equipped with two rotating plates, and are reconditioned with truing gears.
To accommodate machine processing, a reliable method to apply loose abrasive was devised. The abrasive was mixed in with oil (vehicle) so this compound could be easily dispensed while the machine was rotating. The use of a vehicle addresses numerous issues:
- Heat dissipation
- Particle suspension
- Particle dispersion
- Waste removal
- Lubrication
The mechanics of lapping are aimed at imparting the following characteristics to the work:
- Flatness
The condition of a surface having all elements in one plane.
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Convex |
Concave |
- Surface finish (Roughness)
The finer irregularities of the surface texture, which results from the inherent action of the production process.
The portions of the profile below the centerline within the sampling length “L” (A) are inverted and placed above the centerline (B); Ra is then the mean height of the resulting profile (C). Mathematically, Ra is the arithmetic average value throughout the sampling length.
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Geographical Derivation of Ra A – Profile with centerline. B – Lower portions of profile inverted. C – Ra is the mean height of the profile. |
- Parallelism
The condition of a surface or axis which is equidistant at all points from a datum plane or axis.
LAPPING MACHINES TYPES
This illustration shows the different parts of a typical single-plate tabletop lapping machine. This machine is made to run small parts batches to a fine final finish. Shown without the pressure mechanism installed.
This illustration is of a double-sided lapping machine. Notice the upper lapping plate on top of the work pieces. The upper lapping plate is mounted on the bottom of the pressure piston, which controls the force being applied to the work pieces.
WHAT DO WE NEED TO START LAPPING?
- Lapping Machine with Dispensing Equipment
Unless production quantities are very small, you need a lapping machine. The shape of your parts plus the production quantities will determine the size and type of machine for your application.
- Lap Plate and Conditioning Rings
The lap plate is mounted to the drive mechanism of the lapping machine, and the conditioning rings mount on top of the lap plate. The lap plate and the conditioning rings are typically cast iron, with some manufacturers using hardened steel.
Conditioning rings serve two purposes:
- They keep the lap plate flat.
- They carry the work plus any tooling required by the process.
Conditioning rings are usually serrated or solid. Lap plates are either radially serrated or crosshatched, and under special circumstances, they can be solid.
VEHICLES
Oil base vehicles were the dominant vehicle, but with increased environmental constraints, more companies are changing to water base lapping.
Whether you require oil base or water base vehicles, Madison Chemical's exceptional formulation and research capabilities have put us in the forefront of this technology in several areas.
ABRASIVES
Several types of abrasives are used for lapping, but we will only list those typically used.
- Aluminum Oxide
The two primary particles used for most applications are:
- White Calcined Alumina: Each particle has a platelet shape which allows the particles to impart finer finishes than other conventional abrasives.
- Optical Aluminum Oxide: Each particle has a blocky, dense, angular shape which provides consistent stock removal, tight particle distribution, and excellent surface finish.
- Silicon Carbide: Each particle has a blocky shape and is harder, therefore more aggressive than aluminum oxide. This abrasive can cut 20 – 30% faster than the same size aluminum oxide, depending on the material being lapped. Faster cutting also produces a scratchier looking surface. Silicon carbide is more expensive than aluminum oxide.
- Boron Carbide: This abrasive is harder than silicon carbide, so it is more aggressive and more expensive.
- Diamond: This abrasive is harder than boron carbide. Monocrystalline diamond usually has the highest impact and abrasion resistance so it is the diamond of choice for free abrasive lapping.
DOWN PRESSURE
Although it is possible to remove material from any part placed into a lapping process, efficient stock removal requires sufficient down pressure. The abrasive grains will not provide maximum cut rates if they cannot penetrate the work. For most applications, 3 PSI is satisfactory.
METROLOGY
The lapping process requires the proper gauging for checking, lap plate flatness, stock removal rates, and surface finishes.
Obviously, additional metrology might be required depending upon individual applications.
LAPPING CYCLE TIME/ STOCK REMOVAL
Obviously, several issues affect how quickly stock is removed from a part. In our opinion, there are two ways to approach stock removal.
- Lap to Clean-Up
Place parts into a conditioning ring with the appropriate tooling and accessories. Set the machine cycle timer until all of your parts are completely lapped. This method always accommodates the worst case condition on your incoming parts, so you lose control of your productivity.
- Stock Removal Rate Per Minute
First, measure all your parts and record the data. Place all of the parts into a conditioning ring with the appropriate tooling and accessories. Set the timer for 120 minutes, and measure the parts to determine the average stock removal rate per minute. This method allows setting the machine cycle time for a known condition. If the cycle suddenly increased, you would have a reference point for analyzing the problem.
The following physical characteristics are essential in determining how well abrasive compounds work.
- VISCOSITY – The resistance of a fluid to a change of form (flowability). The inference is that coarse particles work best in heavy vehicles, and fine particles work best with light vehicles.
- LUBRICATION – This characteristic maintains smooth cutting action, and helps produce a better surface finish.
- WETTING – Wetting the abrasive, the parts and the tool, helps prevent dry spots, galling and abrasive agglomeration. Oil and water are not usually wet enough, so additives are required.
- SUSPENSION AND DISPERSION – Properly suspended and dispersed abrasives provide uniform cutting rates.
- ABRASIVE CONCENTRATION – The proper amount, by weight, of the abrasive in any vehicle is important for forming a uniform abrasive layer between the part and the tool.
- FILM FORMING – This is a characteristic, of any good vehicle, required to establish the abrasive layer. Vehicles with proper abrasive concentrations included with sufficient lubricating and wetting qualities are a prerequisite for film forming.
- PARTICLE SIZE DISTRIBUTION – Generally speaking, abrasive powders with a narrower particle size variation provide better surface finishes and stock removal rate.
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Large Size Variation
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Small Size Variation
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Telephone: 812-273-6000
Madison, Indiana, USA.
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