Tech Talk
What is "Lapping"?
Lapping is a process of material removal achieved by applying loose abrasive between the surface of the work and tool, wihtout positive guidance of the work, ususally resulting in a finish of multi-directional lay. By taking a plate "A" (the work), some loose abrasive, and another plate "B" (the tool), and moving "A" over the abrasive in random directions, "A" will end up with some scratches. These scratches represent material removal. This is the basic concept of lapping. Abrasive size, abrasive hardness, abrasive concentration, pressure on the workpiece, etc., affect how much material is removed. 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. Double-sided lapping machines, especially florr-mounted machines, are more complex operationally. They are also more expensive so everyone involved in the lapping process will usually have more training. Single sided machines are very easy to operate, so most operators are not extensively trained. These machines are equipped with a rotating lap plate, a cycle timer, roller yokes, and conditioning rings. To accommodate machine processing, a reliable method to apply loose abrasive was devised. The abrasive was mixed 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 surfacer having all the elements in one plane.
Surface Finish (Roughness) - The finer irregularities of the surface texture, which result from the inherent action of the production process.
Parallelism - The condition of a surface or axis which is equidistant at all points from a datum plane or axis.
Abrasive Compounds typically used for lapping usually fall into four categories:
1. Concentrate - This type of product is loaded with suspended abrasive (at least 40 to 50% by weight). Concentrates used with a delivery system allow the customer to control abrasive usage. This methodology also requires a diluent (vehicle).
2. Premix - The abrasive is suspended into a product that is ready-to-use without any mixing or agitation required.
3. Paste - Abrasive is mixed into a non-flowable product.
4. Agitation/Mixing - The abrasive is added to a vehicle and is constantly agitated to keep it in suspension.
The following physical characteristics are essential in determining how well abrasive compounds work:
1. 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.
2. Lubrication - This characteristic maintains smooth cutting action and helps produce a better surface finish.
3. Wetting - Wetting the abrasive, the parts and the tool helps to prevent dry spots, galling and abrasive agglomeration. Oil and water are not usually wet enough, so additives are required.
4. Suspension and Dispersion - Properly suspended and dispersed abrasive provides uniform cutting rates.
5. 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.
6. 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.
7. Particle Size Distribution - Generally speaking, abrasive powders with a narrower particle size variation provide better surface finishes and stock removal rates.
