Introduction

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Commonly called the “mike” the basic outside micrometer is one of the first calipers to allow direct reading accuracy within .001. Micrometers are made in various shapes and sizes, depending on the purpose for which they are to be used. One of the best ways to see large varieties of micrometers is to review the latest machine shop tool and accessory catalog.

The Frenchman, Jean Palmer in 1848 designed and patented the present micrometer now used throughout the world. Two American manufacture of clocks, watches, jewelry and silverware, Brown and Sharpe, saw this device at the Parisian Exposition in 1867. Ten years later in 1877 they obtained the patent rights from Palmer to produce the tool

Palmer’s and today’s mikes have a range of only one inch of measurement; this is true for any size outside micrometer. There were earlier designs of the micrometer however. Around 1772 James Watt designed a measuring instrument that also had screw rotation but the main difference is this micrometer has two graduated dials for reading measurements in place of the sleeve and thimble that are used in Jean Palmer’s micrometer.

Henry Maudslay also developed a micrometer around 1800s and named the tool “Lord Chancellor.” The Lord Chancellor has a large measuring range, a feed screw with a thread pitch of 0.1 and a dial marked in 0.001 increments.

Patents for the outside micrometer have long run out and now many manufactures worldwide are producing the device in a wide range of prices. It is possible to place the least expensive with a costly mike side by side and it would be difficult to visually tell them apart. More than likely the least expensive micrometer will measure accuracy within .001 and better.

For the casual user this maybe adequate, for the professional the instrument must have reliability and longevity.

The heart of the micrometer is the spindle and the spindle nut. The nut and spindle which has 40 threads per inch should be made from tool steel for longevity and the thread should be ground for accuracy.

Inexpensive spindles maybe machined from more economical steel that is softer which is easier to machine keeping production cost down. Through constant use the threads will become worn and inconsistent and no longer maintain accuracy throughout the one inch range.

Micrometers are used to measure a variety of materials. Oftentimes the material being checked is harder than the spindle of the mike. For example to determine the size of a carbide end mill the anvil and tip of the spindle are rocked over the sharp edges of the cutter. Over a period of time this rocking and material that is abrasive can cause the two surfaces to become uneven.

To avoid the damaging of the anvil and the spindle end carbide tips are applied. Whether its tools steel or carbide tips the two measuring faces must be parallel and flat.

The thimble can be a solid piece with a knurl gripping surface or in two parts with a friction thimble that slips when the mike is clamped lightly between the anvil and spindle.

The proper way to hold the mike when taking a measurement is to hold the workpiece in the left-hand and the mike in the right hand. The frame of the mike should be placed in the palm of the hand, the index finger and thumb gripping the knurl surface of the thimble, the ring finger holding in the center and the little finger set inside the frame.

The work is placed in position on the anvil and the thimble is rotated until it clamp slightly between anvil and spindle adjusting the part to assure alignment. For micrometers not having a friction thimble or ratchet a learned “feel” must be used to ensure consistent, accurate measurement. The index finger and thumb rotate the thimble until it clamp slightly, the fingers must then slip so no additional clamping will occur.

Not all work can be measured using this method and a second technique must be used for large or work held in a machine. The tool is brought to the piece holding the instrument at the bottom and by the thimble. Once set to the part the thimble is rotated and clamped in the same process as above. 2nd Sample Image

A micrometer is an instrument that must be applied to the work carefully; too much pressure when taking a measurement will give a false reading and can spring the frame or strip the fine threads on the spindle.

Micrometers are often preset to a dimension, locked to that setting and fit to the part as a “Go” or “No Go” gage as being oversize, correct size or undersized.

Tapered Frame: One of the features that should be looked for in a micrometer is a tapered frame on the anvil side. This taper allows reading to be made in hard-to-reach places.

Cleaning the Micrometers Tips: Before taking a measurement the tips should be cleaned by clamp slightly on a clean sheet of paper and pulling the paper from the tips. Then check for accuracy by closing the spindle gently and note whether the zero line on the thimble coincides with the zero on the spindle.

Rapid opening and closing of the Spindle: Damage to the spindle threads can also be made by "twirling" the frame to move spindle a considerable distance. Opening and closing can be performed by roll the thimble on the arm or in the palm of the hand.

Care of the Micrometer: After using the mike it should be wiped down with an oily cloth to prevent corrosion and a drop of light oil on the screw thread.

When not stored in the tool box like all precision tools should be laid in a clean area safe place. At no time should other tools be placed on one another.

Micrometer reading is like reading a scale (rule). It is simply a more accurate scale than the linear type you may be used to.

The main scale on a micrometer is on the sleeve and divides an inch into ten main segments numbered 0 through 9. Each of these ten segments represent .100 (one tenth of an inch). Each segment of .100 is divided into four parts, each one representing .025.

The rotating scale on the thimble represents .025 of movement for each full revolution. The line down the middle of the sleeve is used to line up the thimble reading. To read a measurement you simply see how many .100 increments are visible then add the number of .025 increments visible then add the reading on the thimble.