What is the purpose of a ratchet in a screw gauge?

A screw gauge (micrometer) measures very small thicknesses and diameters with sub-millimetre precision. The ratchet—the clickable cap at the end of the thimble exists to enforce a constant, limited torque on the spindle. Without this safeguard, operators might over-tighten the spindle against the anvil, compressing soft samples (e.g., wires, foils, plastics) and even damaging the instrument’s threads. Over-tightening introduces systematic error and poor repeatability. The ratchet slips once the preset torque is reached, producing a series of clicks; at that point the contact pressure is correct, and further rotation won’t move the spindle closer to the anvil.

Practically, this torque-limiting action makes measurements operator-independent. Whether a beginner or an expert is holding the gauge, the contact force at measurement is consistent, which is essential when resolution is 0.01 mm or better. In quality control and toolroom settings, technicians use the ratchet for the final approach: first bring the spindle close with the thimble, then switch to the ratchet until it clicks 2–3 times, and finally note the reading from the sleeve and thimble (and the vernier, if present). This habit minimizes parallax, thread wear, and elastic deformation of the workpiece key for reliable, audit-ready data.

Highlight Key Statics:

• Prevents excess pressure and deformation of samples.
• Delivers consistent torque for repeatable results.
• Extends instrument life by reducing thread wear.