Micrometer Measurements

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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.

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" incriments and are accurate to 4 decimal places. The anatomy of an outside micrometer is show below:

http://www.shoutwiki.com/w/images/mindworks/d/d6/Micrometer_caliper.jpg

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 scale, the measurements of the micrometer are labeled in one-tenth of an inch increments with each line on the scale 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 scale to reference the thimble measurement. The last line shown on the thimble below the horizontal scale 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 scale 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 scale. As seen in the above example, the line associated with "3" on the scale 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. Generally, the are available in sizes less than 1" in diameter.

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.

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

http://www.shoutwiki.com/w/images/mindworks/4/4a/Hole_gauge.jpg

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.

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