Lapping is a low-speed, low-pressure abrasive finishing process used to obtain extremely flat surfaces and very fine finishes (often below 0.05 μm Ra), which is exactly what a gauge block requires because it must serve as a dimensional reference. In lapping, fine loose abrasive particles suspended in a carrier fluid (the lapping compound) roll and slide between the workpiece and a soft lap plate (usually cast iron). Material is removed a few atoms/particles at a time, so the process corrects micro-geometry gradually rather than by aggressive cutting.

Because material removal is so gentle and cumulative, uniform pressure and uniform relative motion across the entire surface are the single most important factors to control:

• Every point on the surface must receive the same amount of abrasive action; if pressure or motion is heavier in one zone, more material is removed there, and the block ends up convex or concave instead of perfectly flat.
• A figure-8 or randomised motion is often used precisely to distribute wear evenly over the lap and the workpiece and to prevent grooving.

The other choices actually degrade the result rather than help it. Applying excessive force speeds removal but causes uneven, localised removal (gouging) and destroys flatness — the opposite of the goal. Coarse abrasives remove material faster but leave a rough, scratched surface, defeating the purpose of a fine-finish operation. High temperature and fast lap speed introduce thermal expansion and distortion in the block, so dimensions measured afterward will not be accurate; lapping is deliberately kept slow and cool. Thus the controlling factor is uniform pressure and movement across the surface.