Micrometer - Torque Repeatability Via Torque-limiting Ratchets or Sleeves

Torque Repeatability Via Torque-limiting Ratchets or Sleeves

A micrometer reading is not accurate if the thimble is overtorqued. A useful feature of many micrometers is the inclusion of a torque-limiting device on the thimble—either a spring-loaded ratchet or a friction sleeve. Without this device, workers may overtighten the micrometer on the work, causing the mechanical advantage of the screw to squeeze the material or tighten the screw threads, giving an inaccurate measurement. However, with a thimble that will ratchet or friction slip at a certain torque, the micrometer will not continue to advance once sufficient resistance is encountered. This results in greater accuracy and repeatability of measurements—most especially for low-skilled or semi-skilled workers, who may not have developed the light, consistent touch of a skilled user.

A micrometer closing on the workpiece presents an exaggerated analog to an interference fit (press fit). Just as it is very easy to press a fit on an extremely small diameter allowance (say, a shaft just one tenth larger than its hole ), it is very easy when measuring with a micrometer to squeeze the workpiece smaller (or expand the micrometer frame larger) just one tenth, albeit in completely elastic fashion, with only a small amount of overtorquing of the thimble. This concept can seem counterintuitive to beginning students of machining and metrology, because our everyday concept of the rigidity (stiffness) of materials such as steel or aluminum is trained since our childhoods by the fact that, in everyday experience, the deformability of such rigid materials is entirely negligible—so close to zero that our brains are used to thinking of it as equal to zero. But in the context of micrometer measurements, its nonzero nature becomes noticeable (via amplification), in a way that students sometimes find surprisingly odd (counterintuitive).