The Micrometer - 3: Spindle
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
The heart of the micrometer is the spindle.
The spindle is the part of the micrometer that makes contact with the part being measured (along with the anvil). The spindle has a portion which is threaded at 40 threads per inch. This threaded portion is mated to an identical inside thread on the frame.
By rotating the spindle one turn it moves closer or further from the anvil at an exact rate of .025 per revolution (40/1000). The thimble is attached to the spindle and rotates with it.

The spindle should be made from tool steel for longevity and the thread should be ground for accuracy. Inexpensive spindles may be machined from more economical steel which is easier to machine and keeps production cost down. Through constant use these spindle threads will become worn and inconsistent and will no longer be able to maintain accuracy throughout the one inch range.

Again there is a difference in design and nomenclature. The thimble shown makes reference to "friction" thimble which- as will be shown later - is used the same way as a "ratchet screw" (previous page). Also note the "carbide tip". It is usual to have carbide attached to the tip of the spindle and the anvil.
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label: micrometer, history, anvil, thimble, spindle, frame, barrel, ratchet, adjustment, holding, handling, reading, vernier