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Fig. 1: Imperial scale micrometer with parts labeled. Image made available by Three-quarter-ten at Wikimedia Commons under the CC-BY-SA 3.0 license.

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Fig. 2: Vernier scale (on the top half of the barrel labeled from 2 to 0) on a metric micrometer barrel. The horizontal scale at the bottom is the main scale graduated in 0.5mm units. The scale around the barrel on the right is the micrometer scale graduated in 0.01mm units. The Vernier scale is designed for 0.001mm measurements. Image made available by Glenn McKechnie at Wikimedia Commons under the CC-BY-SA 2.0 license.

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Fig. 3: Simple micrometer made from a metric M6-1.0 machine bolt, nuts and fender washers. A printed paper scale is glued to the top washer. Two pieces of black tape on the edge of the lower washer indicate the measurement reference. In this photograph, the washers are 2.80mm apart.

A micrometer is a device that measures small distances very precisely using a calibrated screw mechanism. The name comes from the Greek micros (meaning "small") and metron (meaning "measure"). It is not to be confused with the unit of length known as a "micrometre", one millionth of a metre, sometimes also called a "micrometer" (or "micron") by those bloody Americans. The micrometer device has been around since at least the 17th century, when it was used in astronomical observations.

The micrometer achieves its high precision using the principle of the screw. A screw with a fine thread pitch requires many turns in order to move the screw an appreciable amount. Since it's easy to measure the number of turns of a screw, and marking the barrel of the screw can allow measurements to a fraction of a turn, the distance the screw moves can be determined to high precision. A common metric micrometer uses a screw with two turns per mm, and a barrel marked with 50 increments, leading to a final precision of 0.01mm. Some micrometers add a Vernier scale to achieve a precision of 0.001mm or better (figure 2). In a micrometer, the screw is usually mounted in a caliper frame (figure 1) so that the screw tightens onto a small object to measure its width.

construction

A good precision micrometer can cost \$30 to \$150, but a poor man's version can be constructed quite easily from inexpensive hardware. You will need:

• a metric M6-1.0 machine bolt (length of your choice)
• 2 nuts to match
• 2 fender washers that fit over the bolt
• epoxy
• rubber cement

The M6-1.0 machine bolt is a standard 6mm-diameter metric bolt with a thread spacing of 1.0mm. This thread pitch conveniently corresponds to a bolt movement of 1.0mm per turn of the bolt. Choose a length that is at least 20mm longer than the longest item you wish to measure to account for the thickness of the bolt head, nuts and washers. The fender washers should be just large enough to fit over the bolt. A standard USS 3/16" fender washer is about 1/4" inside diameter by 1-1/4" outside diameter, and will usually fit nicely over a 6mm bolt — although some particular washers may need to have the inner hole enlarged slightly with a file.

Start by epoxying the two washers on top of the two nuts. Screw everything down onto the bolt while the epoxy dries, but be sure not to get any epoxy on the threads! After the epoxy has dried, screw one nut and washer combination onto the bolt with the washer facing away from the bolt head, until the nut almost touches the head of the bolt. Don't screw it tight up against the head of the bolt; doing so may force the nut to tilt slightly on the bolt. Epoxy the nut in place from the bolt head side. Print the attached file (MicrometerScale.pdf) and cut out one printed scale. Be sure to cut the inner hole out of the printed scale. Use rubber cement to glue the scale on top of the remaining washer. The nut should face upwards, and the inner hole of the scale should just fit over the nut.

Screw the nut and washer with the attached scale onto the bolt with the scale facing away from the head of the bolt. Tighten it all the way until the two washers are touching each other. Put a small piece of tape on the first washer at the location of the zero mark. The tape will be your measurement reference as you move the measuring scale. See figure 3.

troubleshooting

Although the precision of this micrometer is nominally 0.01mm, a number of factors may limit the accuracy to something worse. There is some play in the attachment of the nuts to the bolt, allowing the nut to be tilted slightly on the bolt. For a standard fitting, the play may contribute as much as 0.08 mm error either way. Higher class bolts/nuts with tighter fittings can be used, but these may be hard to come by. See these tech tips from ASM for a description of thread classes. In general, you can reduce the effect of play in the threads by using a larger diameter bolt with a finer thread pitch. A metric M16-1.5 bolt may be the best you can find in standard sizes, but of course you'll need a different scale to account for the fact that one turn is 1.5mm instead of 1.0mm.

Sometimes, two nuts are not mounted exactly parallel on the bolt, or washers are attached perfectly flat against the nuts. When you screw the two nuts together so that washers are touching, look to see if the washers are touching all the way around the perimeter. Sometimes they are not exactly parallel, and you can see a small gap between the washers on one side. In this case make sure you measure your object on the side where the washers are touching. This side is properly described by the reference mark you made with the tape.

further resources

The folks at wikiHow have published a very useful tutorial on how to read thread specifications.

Boltdepot.com posts charts of standard metric and US bolt sizes and thread pitches.