GD&T Metrology Measurement Tools: an introduction. Calipers and Micrometers

Both large and small companies need accurate and affordable measuring instruments for production, product research and development (R&D), quality control, condition monitoring, workplace safety and more.

From accelerometers and vibration meters in machinery, instruments for measuring hardness of various materials, universal testers, geometrical measurement or electromagnetic measurement, the diversity of measuring instruments and test equipment that we offer in Mexican Yamazen covers a wide range of commercial and industrial applications.

In this article we will focus on handheld measuring instruments, which are useful for dimensional metrology.

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Handheld Measuring Devices: Calipers and micrometers

Calipers

This instrument is used to measure lengths with much more precision than a ruler. If a ruler can accurately measure millimeters, the caliber reaches the tenth and even half-a-tenth of a millimeter. They are also usually known as 'Vernier calibrators' or simply 'Vernier', although this is not strictly correct, since this definition is specific to the measurement scale printed on the cursor; other names in other languages may be “king’s foot” or “nonio” (from the latin Nonius, the name of the inventor of a complex marking set for astrolabes).

There are other types of handheld devices to measure distances between 2 points also called Calipers, but mechanically they are somewhat different from the ruler and cursor caliper we are talking about; some are shaped like a compass or a clamp, but we will not mention them for the sake of simplicity.

Parts of the caliper.

Usually a caliper consists of: one graduated ruler mounted on another sliding called the cursor, which indicates the measured position and contains the vernier scale or nonius as a graduated ruler (or the dial covers or digital screens in such cases). Each ruler has a pair of legs; the two long legs are used to measure the exteriors, the two small legs for measuring interiors (e.g. hole diameters), and a rod that comes out from the rear to measure depths.

How to use:

In order to make a measurement, once the caliber is open, the legs must be adjusted to the measure we require, we will look at the fixed graduated scale (the large one). The ‘0’ mark on the cursor will indicate the measured millimeters, the millimeters being the stripe of the graduated scale that is below the stripe of the ‘0’ of the cursor. If the ‘0’ of the cursor matches with a line of the graduated scale, that would be the exact measure, since it would not have decimals. If it does not match, we will look then at the scale of the cursor. The first stripe of the cursor that matches with a stripe of the large graduated scale will be the tenths of a millimeter or what is the same 0.1mm (gauge with 10 divisions in the cursor) or the half tenths of a millimeter 0.05mm if the gauge It has 20 divisions in the cursor. Let's see an example:

Vernier calipers are acceptably accurate and accessible, in addition to being relatively resistant due to having minimal moving parts. However, they may be susceptible to thermal expansion errors and interpretation error.

Calipers without Vernier scale, dial and digital.

Currently there are other variants of calibrators that do not use the vernier:

Dial Calipers: These calipers have a circular clock-type dial that rotates a complete revolution for each ruler unit, and the lower resolution is marked on the dial divisions.

Advantages: Faster and more accurate readings by not having to visually assess the position of the Vernier scale. Can be “zeroed” to measure distance differences of distances in any direction. A little more delicate than regular Vernier scale calipers. Affordable price.

Disadvantages: being mechanical, they may be susceptible to thermal expansion. They usually do not have a good level of IP protection, so they cannot be used on production lines.

Digital Calipers: Electronic calipers that present the measured values on LED or liquid crystal displays.

Advantages: They usually use glass bars or other materials previously marked with the measuring distances, so their readings are the most accurate and have a variety of measurement options. Lower susceptibility to thermal expansion.  Available with high IP levels, even with the ability to be submerged for a limited time. Some models can transmit data electronically.

Disadvantages: They require much more delicate handling, since the absolute scales from which the readings are obtained are usually more fragile, and if they are damaged they cannot be repaired. Usually more expensive

Common mistakes on using calipers:

1. Parallax effect: when reading the vernier scale, it is important to verify that the position in which we hold the caliper is not oblique to our line of sight.

2. Excess force: Excessive use of force (especially when measuring outer distances) will cause measurement errors due to deformation of the measuring bars.

3. Thermal expansion: it is important to maintain a constant and adequate temperature in the area where the measurement is performed to maintain consistent results.

4. Loss of parallelism at the measuring bars: the inclination between the movable and the fixed tip (due to blows, poor storage or excessive force applied) will cause measurement errors when not conforming to the Abbe principle.

5. Not checking the point of origin: Forgetting to check or set the correct point of origin will cause false readings.

Click here to know more about Mitutoyo calipers.

Micrometer (Screw Gauge)

It is important not to confuse with the unit of length ‘micrometer’ (µm) also called micron, equivalent to one millionth of a meter, or one thousandth of a millimeter. A micrometer (also called screw gauge, or Palmer screw/caliber) is a high precision measuring instrument capable of measuring up to micrometers (µm), and hence its name. When the calibrator is not accurate enough and you need to measure a tolerance of less than 0.002” or 0.05mm, you should use a micrometer. They are widely used by lathe and mill machinists, mechanics, jewelers anybody who wants to measure parts with reliability up to microns, like, for example, to measure plate thicknesses, bearings, gears, nut diameters, etc.

You can see the component parts of a micrometer in the following images:

How to use:

The piece to be measured is placed against on the top of the measuring anvil (the fixed bumper usually at the left of the device) with the left hand while we approach the spindle contact tip by turning the ratchet knob with the thumbs and index finger. Usually the micrometer is attached to a base for ease of use, a special device design to avoid heat transfer by holding the micrometer with your hands and to facilitate the use of both hands on the measurement. In the image it can be seen that we are measuring a ring and a nut, and these are held between the anvil and the spindle.

When the piece to be measured is perfectly held between the anvil and the end of the spindle, keep turning the ratchet knob until you hear 3 clicks. Now we set the measurement using the brake lever (we turn the brake) while the spindle and anvil are holding the piece we want to measure. We withdraw the piece from the already locked micrometer and read the measurement value on the micrometer.

Nowadays, having to take measurements is simplified even more if a digital micrometer is used, since a digital display will directly indicate the size of the piece.

Advantages of a digital micrometer: Greater precision. Ability to start at zero in any position. High IP availability. Better resistance to thermal expansion. There is no need to use the brake so it is easy to remove the work piece.

Disadvantages: They are more delicate and more expensive than analog models.

Common errors in the use of micrometers:

1. Parallax effect: when reading a vernier scale, it is important to verify that the position in which we hold the micrometer is not oblique to our line of sight.

2. Excessive use of force when rotating the drum and ratchet: excess pressure or repetitive ratchet shocks will alter the results and reduce accuracy to the readings by damaging the internal screw.

3. Thermal expansion by manual use: holding the micrometer body by hand transferring heat to the micrometer body.

4. Parallelism of the tips: not complying with the Abbe principle (due to blows, poor storage or excessive force applied) for micrometers not only applies to the parallelism of the tips, but also to the parallelism of its surfaces, so it is important check them with optical parallel interference bands.4

5. Proper use of the support: the support must support the center of the micrometer body to avoid deformations, in addition to being held with adequate force.

Data collection: Mitutoyo U-Wave

No matter how accurate the measurement tools are; it doesn't matter if they are vernier, dial or digital, if the measurement data is not recorded properly all our effort will be in vain. This is why we present Mitutoyo's U-Wave data collection technology.

Mitutoyo U-Wave technology allows digital and automatic collection of Digimatic calibrators and micrometer readings (among many other types of Mitutoyo tool). At the press of a button, the correct value is sent through the U-Wave system to any computer or device configured within the U-Wave network, ensuring that the measurement reading is always correct.

Find out more about Mitutoyo’s U-Wave technologies here.

At Yamazen Mexicana we represent Mitutoyo's measurement tools, market leaders and the most popular measurement tool brand in the country. If you need more information about any measurement solution, please contact your Yamazen representative.

In the next installment we will talk about other types of dimensional metrology tools.

Cheers!