The micrometer, also called Palmer calibers or screws of Palmer- are used in various industries: metalworking, plastic, bulonería, automotive, etc.. It is a measuring instrument that allows measuring lengths with an accuracy of up to one-thousandth of a millimeter.
As with the Vernier caliper or caliper, there are different types of micrometers, either to measure the exterior, the interior, heights, depths, and so on. They are also divided into analog and digital. We will see below the different types -according to this last criterion- and the correct way to operate them.
Analog and digital micrometers
There are basically two types of micrometers available on the market: analog and digital, and each of these types is available for the different micrometer models according to the measurement to be made: exterior and interior parallels, exterior and interior cylinders, special cylindrical parts -like wicks, strawberries and calizuares- and also the height and depth measurements.
Mechanical or analog micrometers
They are the most traditional micrometers and preferred by many professionals and technicians, because reading at the foot of the machine, for someone experienced, is much simpler and more reliable than digital ones, and the use of batteries is not required. In return, some novice operators may have difficulties at first, since the reading is not immediate and requires some practice to do it fluidly.
This type of micrometer – when working in the metric system – has inside it a screw whose pitch is half a millimeter. Thus, a complete revolution of the drum will cause the stop plates to be separated exactly half a millimeter. Using this principle, it is possible to resolve up to one-thousandth of a millimeter (0.001 mm), although not all micrometers have the necessary vernier; in those cases, its nominal precision is one-hundredth of a millimeter (0.01 mm).
There are different micrometer models, according to their use: from general use to highly specific ones (such as the depth measurement micrometer, shown below). However, the operating principle is similar: they all have a screw, an approaching drum and cricket to achieve the support of the stop plates.
It is important when working with this type of micrometer, to take special care not to force the drum, and to approach the stops making exclusive use of the cricket or ratchet. This device is prepared to adjust the stops against the piece to measure with the necessary pressure; once secured, the mechanism causes the ratchet or cricket to rotate in false-hence its name-and prevents damage to surfaces or the instrument.
There are some micrometers equipped with a dial with a needle, which indicates the measurement in a way perhaps more directly to the operator, although most of the time these micrometers are limited to a precision of one hundredth. Due, also, to the complexity of the mechanism necessary to move the needles, these instruments are somewhat bulky, which makes their use practical only in the laboratory or technical office environments.
A micrometer with three contacts, special for measuring cylindrical pieces, including strawberries, wicks, bits and calizuares.
In this case, the operating principle is similar to its analog pairs, although an encoder is used as an interface for the measurement, which makes the measurement easily readable on a digital display. The reading, thus, is much simpler than in its analog equivalents, although, at the cost of sacrificing reliability, since if the instrument is powered with defective batteries, a false measurement can occur.
A great advantage over analog micrometers is that, due to their digital nature, they can instantly convert measurements to inches, and arbitrarily set a zero in the measurement.
Interior of a micrometer or Palmer caliber
Whether analog or digital, exterior or interior, height or depth, the interior of exterior micrometers does not change much. The micrometer screw is usually manufactured with a length of 25 mm, so that each instrument can make measurements within that range, starting, of course, with an arbitrary zero measurement; This is standardized in 25 mm steps: from 0 to 25 mm, from 25 mm to 50 mm, from 50 mm to 75 mm, and so on.
Then, according to the application or the work to be done is the type of micrometer you should buy. Also, according to the user that the micrometer will receive, the body or arch is modified, making it larger or replacing it with a base, a tripod, and so on. The majority of the micrometers of general use have an insulating plate that allows them to take it without affecting, with the corporal heat, the measurement using the expansion of said arch.
In general, the stop plates are interchangeable; this is because, with use, the rectified and a cemented surface of said plates tend to wear down.
In some specific measurement models -such as micrometers for threads, gears, or slots- it will be necessary to replace these stops to match their geometry with the type of thread, size of the slot, or module of the gear to be measured.
Special micrometer to measure grooves. Note the top plates with a wedge-shaped edge, and the part used to calibrate the instrument.
When the stop plates are changed, it is necessary to verify that the instrument is correctly calibrated. For this, they are usually accompanied by small calibration pieces like the one that can be seen in the photograph above.