Lapping is an abrasive machining operation.

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.

Operation

By way of example, a piece of lead may be used as the lap, charged with emery, and used to cut a piece of hardened steel. The small plate shown in the first picture is that of a hand lapping plate. That particular plate is made of cast iron. In use, a slurry of emery powder would be spread on the plate and the workpiece simply rubbed against the plate. Usually in a figure "Eight" pattern.

The second picture is that of a commercially available lapping machine. The lap or lapping plate in this machine is 30 cm (12") in diameter. For a commercial machine that is about the smallest size available. At the other end of the size spectrum, machines with six foot diameter plates are not uncommon. Referring to the second picture again, the lap is the large circular disk on the top of the machine. On top of the lap are two rings. The workpiece would be placed inside one of these rings. A weight would then be placed on top of the workpiece. The weights can also be seen in the picture along with two fiber spacer disks that are just used to even the load.

In operation, the rings stay in one location as the lapping plate rotates beneath them. In this machine, a small slurry pump can be seen at the side, this pump feeds abrasive slurry onto the rotating lapping plate.

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 (average roughness in micrometers or microinches).

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.

As a side note: Two parts that are lapped to a flatness of about 1HLB will exhibit "Wringing-in" or "Jo Blocking." A phenomenon where the two parts will cling to each other when placed in contact. The name "Jo-blocking" comes from the fact that gage blocks - sometimes called "Johansen blocks" after the manufacturer - can be made to stick together in this manner.

Measurement

Of flatness: The easiest method for measuring flatness is with a height gage positioned on a surface plate. Flatness is also measured with a co-ordinate measuring machine. But neither of these methods can measure flatness more accurately than about 0.0001".

Another method that is commonly used with lapped parts is the reflection and interference of monochromatic light. A monochromatic light source and an optical flat are all that are needed. The optical flat - which is a piece of transparent glass that has itself been lapped and polished on one side - is placed on the lapped surface. The monochromatic light is then shone down through the glass. The light will pass through the glass and reflect off the workpiece. As the light reflects in the gap between the workpiece and the polished surface of the glass, the light will interfere with itself creating light and dark fringes. Each fringe - or band - represents a change of 1HLB in the width of the gap between the glass and the workpiece. The light bands display a contour map of the surface of the workpiece and can be readily interpreted for flatness.

For a more thorough description of the physics behind this measurement technique, see interference.

Of Roughness: Surface roughness may be measured by comparing the surface of the workpiece to a known sample. Calibration samples are available usually sold in a set and usually covering the typical range of machining operations from about 125 Ra to 1 Ra.

Surface roughness may also be measured with a profilometer.

See also

Surface metrology for a brief description of these devices.