Micrometer Explained: How to Read and Use a Precision Measuring Tool

What Is a Micrometer?

A micrometer is a measuring instrument that can make extraordinarily precise measurements. Most micrometers are designed to measure within one one-thousandth of an inch! That’s a close fit. Exact measurements like this are necessary when even the smallest of space between objects can cause problems or difficulties.

A micrometer, sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw widely used for accurate measurement of components in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as vernier, dial calipers, and digital calipers.

Micrometers are usually, but not always, in the form of calipers (opposing ends joined by a frame). The spindle is a very accurately machined screw and the object to be measured is placed between the spindle and the anvil.

The spindle is moved by turning the ratchet knob or thimble until the object to be measured is lightly touched by both the spindle and the anvil.

Micrometers are also used in telescopes or microscopes to measure the apparent diameter of celestial bodies or microscopic objects. The micrometer used with a telescope was invented about 1638 by William Gascoigne, an English astronomer.

Micrometer Symbol

A micrometer also called a micron, metric unit of measure for a length equal to 0.001 mm, or about 0.000039 inches. Its symbol is μm. The micrometer is commonly employed to measure the thickness or diameter of microscopic objects, such as microorganisms and colloidal particles.

Parts of Micrometer

A micrometer is composed of:

• Frame: The C-shaped body that holds the anvil and barrel in constant relation to each other. It is thick because it needs to minimize flexion, expansion, and contraction, which would distort the measurement. The frame is heavy and consequently has a high thermal mass, to prevent substantial heating up by the holding hand/fingers. It is often covered by insulating plastic plates which further reduce heat transference.
• Anvil: The shiny part that the spindle moves toward, and that the sample rests against.
• Sleeve, barrel, or stock: The stationary round component with the linear scale on it, sometimes with vernier markings. In some instruments the scale is marked on a tight-fitting but movable cylindrical sleeve fitting over the internal fixed barrel. This allows zeroing to be done by slightly altering the position of the sleeve.[12][13]
• Lock nut, lock-ring, or thimble lock: The knurled component (or lever) that one can tighten to hold the spindle stationary, such as when momentarily holding a measurement.
• Screw: The heart of the micrometer, as explained under “Operating principles”. It is inside the barrel. This references the fact that the usual name for the device in German is Messschraube, literally “measuring screw”.
• Spindle: The shiny cylindrical component that the thimble causes to move toward the anvil.
• Thimble: The component that one’s thumb turns. Graduated markings.
• Ratchet stop: Device on end of handle that limits applied pressure by slipping at a calibrated torque.

Types of Micrometers

There are several types of micrometers that are designed to measure different types of objects or spaces. Most micrometers are available in sets to accommodate measurements of varying size.

1. Outside Micrometer

This type of micrometer is designed for measuring the outside of objects—the outside diameter (OD). They look and move much like a C-clamp, which opens and closes by turning an internal screw.

In a micrometer, the object you wish to measure is clamped between the anvil (the stationary end of the clamp) and the spindle (the moving part of the clamp). Once the object is secured in the clamp, you use the numbering system on the thimble (the handle portion) to find your measurement.

2. Inside Micrometer

While the outside micrometer is used for measuring the outer diameter of an object, the inside micrometer is used to measure the inside, or inside diameter (ID). These looks more like a pen, but with a thimble in the middle that turns.

As the thimble turns, the micrometer expands like a curtain rod would. This then extends until each end of the tool is touching the inside of the pipe. When this happens, you use the numbering system on the thimble to find your measurement.

3. Depth Micrometers

While inside and outside micrometers are both used to measure the diameter of an object or hole, a depth micrometer is for measuring the depth of a hole, recess or slot. Depth micrometers have a base that aligns with the top of the recess that needs to be measured.

The thimble is on a shaft that sticks up from the base. As the thimble turns, a measurement rod comes down from the shaft. You continue to turn until the rod hits the bottom surface of the hole being measured. When this happens, you use the numbering system on the thimble to find your measurement.

When Would I Use a Micrometer?

You would use a micrometer when a very precise measurement is needed. There are several different designs, depending on what needs to be measured. This could be the size of a pipe, tool or object from the outside. This could be the inside width of a pipe, bearing or another hollow object. Or this could be the depth of a hole or recess.

These are the tools you will reach for when accuracy is the most important factor. This is frequently true for machines with moving parts. Parts that move in and out of each other, like a piston, for example, need to remain in a steady, straight line. If these parts have even the smallest bit of sway, they can begin to fail.

This is also true in other applications, such as the use of bearings. Other applications that require the most exact measurement are pipe fittings especially if the pipe will be moving gases with very small and light molecules, like helium. Micrometers are also the preferred tool when measuring the thickness of items like sheet metals.

How Do I Read a Micrometer?

It is important to check if the micrometer is English or metric before using it for measurements. Make sure you are using a tool that has the same unit of measure as whatever you are already working with.

Once the micrometer is rotated into the proper measurement, the measurement can be taken. This requires adding together numbers found on the spindle and thimble which will give you the accurate measure.

How to find the numbers you want will vary depending on the type and design of the micrometer. Instructions on how to read your micrometer will come from the manufacturer with your tool.