Manufacturing: Surface Finishing

Lapping -

Lapping is an abrasive machining operation which utilizes a rough chemical-mechanical-polishing (CMP) process, where a sample (such as a metal, ceramic, plastic, glass, or silicon substrate) is machined, smoothed, and planarized to a high degree of refinement or accuracy using a rotating, serrated, cast-iron-alloy circular plate and an abrasive slurry grit in water suspension applied to the plate in a controlled fashion.. Typically, a soft material - called a lap - is charged with an abrasive. The lap is then used to cut a harder material - the workpiece. The abrasive embeds within the softer material which then acts as a holder for the abrasive and permits it to score across and cut the harder material.

Accuracy and Surface Roughness -

Lapping can be used to obtain a specific surface roughness; it is also used to obtain very accurate surfaces, usually very flat surfaces. Surface roughness and surface flatness are two quite different concepts. Unfortunately, they are concepts that are often confused by the novice.

A typical range of surface roughness that can be obtained without resort to special equipment would fall in the range of 1 to 30 Ra.

Surface accuracy or flatness is usually measured in Helium Light Bands, one HLB measuring about 0.000011 inches (11 millionths of an inch). Again, without resort to special equipment accuracies of 1 to 3 HLB are typical. Though flatness is the most common goal of lapping, the process is also used to obtain other configurations such as a concave or convex surface.

High Speed Lapping -

High-speed lapping, also known as fine grinding or flat honing, was actually introduced to the United States in the 1990s, awareness of the process remains surprisingly low and is mostly limited to large manufacturers with in-house component production.

Traditional lapping is a painstaking process using a wet abrasive slurry to slowly finish parts to precise flatness, parallelism and surface finish. The abrasive slurry requires extensive, time-consuming cleaning of both parts and equipment. Even with the cleaning, abrasive particle are often ground into the part itself causing "grit impregnation" that can hurt the part's performance-particularly in fluid sealing applications.

In contrast, high-speed lapping uses a fixed abrasive wheel in place of the wet slurry. This dramatically reduces the lengthy cleaning process and eliminates grit impregnation. Advanced machine controls combine the rapid stock removal rate of traditional grinding with the precision finishing of traditional flat lapping. Automated systems to load and unload part further speed up the process.

The results are impressive. High-speed lapping can finish parts up to 20 times faster than traditional flat lapping. This yields significant cost savings, often up to 40 percent versus traditional lapping. In fact, high-speed lapping can even cost less than traditionally low-cost grinding methods, since simple inexpensive parts carriers replace costly fixtures.

High-speed lapping is ideal for all types of metals, including steel, brass, aluminum, phosphorus bronze, tungsten carbide, cast iron, and powder metals. The process is also used for plastic, ceramic, glass, carbon, and other materials.

The high-speed lapping process is well-suited for parts with either a regular or irregular shape and a wide range of sizes. Some very thin parts may be difficult to finish due to limitations of the carriers that hold the parts in place during the process. Flatness and parallelism can be maintained within single digit micron tolerances. Surface finishes of 1 Ra micrometers can be routinely achieved.

A key commercial consideration is the number of parts to be finished. The process is generally used for parts with annual volumes of one million or more. Companies with sufficient demand can purchase the equipment or contract with outside companies that specialize in high speed lapping.