Lapping

The first type of lapping (traditionally often called grinding), 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, jeweller's rouge, optician's rouge, emery, silicon carbide, diamond, etc., between them.

Due to the dimensions of such small samples, traditional loads and weights are too heavy as they would destroy delicate materials.

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.

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.

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.

The easiest method for measuring flatness is with a height gauge positioned on a surface plate.

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-neon lamp or tube, using the neon 632.8 nm line,[citation needed] or mercury vapor green line but nowadays a more common source of monochromatic light is the low pressure sodium lamp.

[citation needed] Today, laser diodes and LEDs are used, both being inexpensive and narrow-band light sources.

The individual variances of the peaks and valleys are averaged (Ra value), or quantified by the largest difference from peak-to-valley (Rz).

A surface that exhibits an Ra of 8 consists of peaks and valleys that average no more than 8 μm over a given distance.