Micrometer Measurements

Micrometer History & Uses
A micrometer is a measurement device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades. The first micrometer was developed by Jean Laurent Palmer of Paris in 1848. Throughout the late 1800's and 1900's micrometers led to increased standards of quality across all manufacturing fields and have played a vital role in complex mechanical systems for the past 100 years.

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Outside Micrometer
An outside micrometer is used to measure external surfaces of shafts, blocks, spheres and various common shapes in mechanical engineering. These micrometers are the most common micrometers used in manufacturing and many can be found in the Mechanical Engineering Shop in Gauss Johnson. Outside micrometers are available in the shop in 1", 2", 3", 4", 5", 6", 7", 8", 9", 10", 11" and 12" sizes. Outside micrometers measure 1" increments and are accurate to 4 decimal places. The anatomy of an outside micrometer is show below:

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How to Operate: Micrometers measure various dimensions by spinning the thimble until the spindle makes contact with a surface. Generally, once contact is made with an external surface, the user will turn the ratchet knob and slide the micrometer back and forth on the surface. The micrometer should drag on the surface but transverse movement should still be achieved. Once this is obtained, the user will engage the lock nut and remove the micrometer from the surface. This procedure is the same for all external micrometers.

How to Read: In the shop, students will primarily find micrometers measuring in English units.

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The English micrometer above is similar to those available in the shop. On the sleeve, the measurements of the micrometer are labeled in one-tenth of an inch increments with each line on the sleeve indicating 25-thousandths of an inch. As indicated in the above picture, the last exposed line is the measurement that is recorded. In this case, .15" is the last line exposed. It is beneficial to the user to write these numbers down until one is familiar with reading a micrometer. Next the user will direct his/her attention to the thimble measurement. Use the horizontal line on the sleeve to reference the thimble measurement. The last line shown on the thimble below the horizontal sleeve line is the number in ten-thousandths that will be added to the dimension. In this example, the number is .011". If the number on the thimble aligns exactly with the horizontal sleeve line, then the measurement is exactly .011". However, in this example, the thimble is turned slightly passed .011" and therefore the user must direct his attention to the line the aligns most closely with the small numbers printed around the sleeve known as the vernier. As seen in the above example, the line associated with "3" on the vernier aligns perfectly with a line on the thimble. This measurement is your one ten-thousandth measurement. In this example the measurement is .0003". If this micrometer is a 1" micrometer the measurement would be .1613". If it were a 2" micrometer the measurement would be 1.1613". Each micrometer measures 1" precisely and is labeled by its maximum measurement. Therefore, typically you subtract 1" from the designated number of inches the micrometer is and add that value to your measurement. For example, if you have a 5" micrometer, add 4" to your reading. If you have a 12" micrometer, add 11" to your reading.

Hole Gauges
Hole gauges are used to measure the diameters of slots and holes. They are designed for use with an outside micrometer and function well for measuring existing holes or slots in purchased parts. Hole gauges come in a variety of sizes and there are plenty in the shop to fit your application.

How to Operate: Hole gauges consist of a tapered threaded shaft that twists inside of a hollow shell with a spherical end. As the shaft threads into the shell, the shell expends to meet the diameter of the given hole or slot. Once in contact with the edges of the hole, the hole gauge will feel snug inside the hole. Ensure that the hole gauge drags on the inside of the hole but is not firmly in place nor slides out easy. Other hole gauges use a spring to release to prongs that fit to the inside diameter of a hole or slot. Once these are sprung out, use the locking mechanism to lock the diameter for measurement.

How to Measure: Once the hole gauge is removed, use an outside micrometer to measure the diameter of the hole gauge. See picture:

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Depth Micrometer
Depth micrometers are excellent for determining the depth of small holes, extruded cuts, or odd geometries unable to be measured by inside micrometers or outside micrometers. These micrometers only come in the 1" variety in the Gauss Johnson shop. Here is a picture of the anatomy of a depth micrometer:

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How to Operate: Depth micrometers have a base that is placed flat against a surface and the thimble is twisted to extend the rod. The rod contacts the bottom of the hole or slot and tends to lift the base. When this occurs the user is to retract the rod until the base is returned to its flat position and the user will twist the cap until it clicks. Common practice is to move or slide the depth micrometer along the flat surface to ensure there is drag along the bottom of the hole or slot. Once this occurs the user will engage the lock nut and remove the depth micrometer for measurement reading.

How to Read: Depth micrometers are read in the same fashion as the outside micrometer. However, the numbers are placed in the opposite order as those displayed on an outside micrometer. For example the following depth micrometer reads .3765":

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The numbers read the same, but are placed in opposing order. This can be disorienting to those familiar with outside micrometers. Don't over-think the measurement, it is read the same as an outside micrometer.

External Links and Videos
How to read a Micrometer video: http://www.youtube.com/watch?v=i_jygJkJujE

Wikipedia "Micrometers"" http://en.wikipedia.org/wiki/Micrometer

Interactive Micrometer reading tool: http://www.linnbenton.edu/auto/day/mike/read.html