Lapping

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Lapping is a machining operation, in which two surfaces are rubbed together with an abrasive between them, by hand movement or by way of a machine.

This can take two forms. The first type of lapping (traditionally called grinding), typically involves rubbing a brittle material such as glass against a surface such as iron or glass itself (also known as the "lap" or grinding tool) with an abrasive such as aluminum oxide, emery, silicon carbide, diamond, etc., in between them. This produces microscopic conchoidal fractures as the abrasive rolls about between the two surfaces and removes material from both.

The other form of lapping involves a softer material for the lap, which is "charged" with the abrasive. The lap is then used to cut a harder material—the workpiece. The abrasive embeds within the softer material which holds it and permits it to score across and cut the harder material. Taken to the finer limit, this will produce a polished surface such as with a polishing cloth on an automobile, or a polishing cloth or polishing pitch upon glass or steel.

Taken to the ultimate limit, with the aid of accurate interferometry and specialized polishing machines or skilled hand polishing, lensmakers can produce surfaces that are flat to better than 30 nanometers. This is one twentieth of the wavelength of light from the commonly used 632.8 nm helium neon laser light source. Surfaces this flat can be molecularly bonded (optically contacted) by bringing them together under the right conditions. (This is not the same as the wringing effect of Johansson blocks, although it is similar).

Operation

Small lapping plate made of cast iron

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.

Small lapping machine

The second picture is that of a commercially available lapping machine which is needed for this process. 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 eight to ten foot diameter plates are not uncommon and systems with tables 30 feet in diameter have been constructed. 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.

Lapping machine and retention jig

When there is a requirement to lap very small specimens (from 3" down to a few millimetres), a lapping jig can be used to hold the material while it is lapped (see Image 3, lapping machine and jig). A jig allows precise control of the orientation of the specimen to the lapping plate and fine adjustment of the load applied to the specimen during the material removal process. Due to the dimensions of such small samples, traditional loads and weights are too heavy as they would destroy delicate materials. The jig sits in a cradle on top of the lapping plate and the dial on the front of the jig indicates the amount of material removed from the specimen.

Two-piece lapping

Where the mating of the two surfaces is more important than the flatness, the two pieces can be lapped together. The principle is that the protrusions on one surface will both abrade and be abraded by the protrusions on the other, resulting in two surfaces evolving towards some common shape (not necessarily perfectly flat), separated by a distance determined by the average size of the abrasive particles, with a surface roughness determined by the variation in the abrasive size. This yields closeness-of-fit results comparable to that of two accurately-flat pieces, without quite the same degree of testing required for the latter.

Schematic of two-piece lapping

One complication in two-piece lapping is the need to ensure that neither piece flexes or is deformed during the process. As the pieces are moved past each other, part of each (some area near the edge) will be unsupported for some fraction of the rubbing movement. If one piece flexes due to this lack of support, the edges of the opposite piece will tend to dig depressions into it a short distance in from the edge, and the edges of the opposite piece are heavily abraded by the same action - the lapping procedure assumes roughly equal pressure distribution across the whole surface at all times, and fails in this manner if the workpiece itself deforms under that pressure.

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 (280 nm). 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 "Johansson 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" (2.5μm).

optical flats in wooden case

Monochromatic light unit

Another method that is commonly used with lapped parts is the reflection and interference of monochromatic light.^[1] 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 or both sides – 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 one half wavelength 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. In the past the light source would have been provided by a Helium lamp or tube,^{[citation needed]} but nowadays a more common source of monochromatic light is the low pressure sodium lamp.^{[citation needed]} The picture to the right shows a typical monochromatic light unit used in workshops and laboratories.

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

Of roughness

Surface roughness is defined by the minute variations in height of the surface of a given material or workpiece. The individual variances of the peaks and valleys are averaged (Ra reading), or quantified by the largest difference from peak-to-valley (Rz). Roughness is usually expressed in microinches. A surface that exhibits an Ra of 8 consists of peaks and valleys that average no more than 8 microinches over a given distance. Roughness may be also 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 is measured with a profilometer, an instrument that measures the minute variations in height of the surface of a workpiece.

See also

• Surface metrology for a brief description of these devices.

References

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