Which tool is used to measure clearance between a surface plate and a narrow surface for flatness?

The thickness gauge is the appropriate tool for measuring the clearance between a surface plate and a narrow surface for flatness because it is specifically designed for this purpose. Thickness gauges are used to measure the thickness of materials and can effectively ascertain how flat a surface is by providing accurate readings of clearances and intervening spaces.

This tool typically consists of a set of blades or measuring rods that can fit into narrow gaps, allowing for precise measurements. By inserting the thickness gauge between the surface plate and the narrow surface, one can determine if there is any deviation from flatness. Moreover, thickness gauges are generally easier to maneuver in tight spaces compared to other measuring tools.

Other tools, while useful in different contexts, do not serve this specific function as effectively. For instance, a depth gauge primarily measures the depth of holes or recesses and wouldn't provide the necessary measurements for flatness between two surfaces. A micrometer, although highly accurate for measuring dimensions, is designed for measurements of small distances and is better suited for measuring diameter or thickness rather than evaluating clearance. Meanwhile, a Vernier caliper measures internal and external dimensions, and while it can measure thickness to some extent, it is not specialized for measuring clearances in the same manner as a thickness gauge. Thus