What is a Micrometre Used For?

Precision measurement tools are a vital part of making accurate and functional parts and maintaining the machines needed to do so. A micrometre is just one type, most commonly used by machinists, but it will often be found in the toolboxes of fabricators, engineers, and more.

In this article, we’ll look at what a Micrometre does, the different types, and how to use one.

What is a Micrometre Used For?

A Micrometre is used to precisely measure between two external faces, typically between 1” and 6”, although larger and smaller versions do exist. A micrometre can typically measure to a precision of 10 thousandth of an inch for imperial micrometres or 0.001mm for metric ones.

They are a commonly used tool by machinists and engineers and allow the accurate measuring of square, cylindrical and spherical parts.

Parts of a Micrometre

There are 7 main components to a micrometre:

• Frame, this holds the anvil and spindle concentric
• Ratchet stop, this is used to limit the torque applied when measuring
• Thimble, used to spin the micrometre open, and show the whole units
• Sleeve, displays the vernier graduations of units
• Spindle, extends to measure the object
• Anvil, provides a hard and parallel surface

What Types of Micrometres Are Available?

Like most tools, there are specialized versions for different types of work, in the case of micrometres there are different types for measuring differently shaped objects:

Standard or Vernier Micrometre

This is the simplest and most common type of micrometre, it uses a vernier scale to accurately and quickly show the measurement. The benefits of standard micrometres are that they don’t require a battery and are typically cheaper than digital ones.

Digital Micrometre

As the name suggests, these have a digital LCD that indicates the measurement. They are often faster to read, especially for less-experienced users, and tend to have a higher level of precision.

Naturally, they are more expensive than a comparable standard micrometre and require periodic changes of the batteries.

Ball Micrometres

Instead of the flat surfaces on the anvils, ball micrometres have spherical ends, this allows for easier measurement of round parts, specifically involving internal diameters or radii.

Ball micrometres can be used to measure flat objects, but are more delicate and expensive than standard ones, so should ideally be kept for their intended use.

Thread Micrometre

Used for measuring threads, these have a point on the spindle and a double-v shape on the anvil, this allows them to index on the thread of a part. With some referencing to the appropriate tables, the thread’s pitch can be ascertained.

Internal or Bore Micrometre

These are used for measuring inside diameters, they lack the frame of the typical micrometre and instead have an anvil at each end. They are inserted into the ID to be measured and extended.

Depth Micrometre

Similar to the bore micrometre, a depth micrometre does not feature a traditional anvil and frame, instead there is a wide flange at the zero position. This allows the spindle to protrude into a hole, with the anvil resting on the top surface.

This allows for extremely precise measurement of depths, such as in holes, slots, rebates and grooves.

How to Use a Metric Micrometre?

First: Check the micrometre is not damaged or mis-adjusted. Wind the sleeve all the way to zero (use a gauge block if the lower end is not zero) and check the vernier graduations line up at zero – this helps to confirm the micrometre is properly set up.

1. Place the part in between the anvil and spindle, and wind the ratchet stop in, ensuring that the anvil face is parallel to the face of the object.
2. When the ratchet stop clicks, read the whole unit number, indicated by the bottom graduations of the sleeve measurement
3. Note if there are any half-mm graduations showing on top of the sleeve markings, if so add this to the whole-unit number
4. Finally, read the vernier scale on the thimble, add this to the numbers so far.

How to Maintain a Micrometre

Micrometres should be kept dry, and ideally at a consistent humidity and temperature, failing to do so can cause them to lose accuracy over time.

Oil the mechanism as described in the manual, and ensure if very accurate measurements are needed, you calibrate the micrometre against a gauge block periodically.

Are Micrometres Better than Calipers?

There are a few key differences between micrometres and calipers, typically the latter are used to measure larger items, with the minimum measurement range being 0-150mm, and 0-300mm being common too.

Calipers typically measure down to 0.02mm, which is many times less precise than a micrometre. Another disadvantage is the physicality of the tool too, whereas the micrometre places the object to be measured directly in line with the measuring scale, a caliper is offset.

Whilst this can be useful in certain situations, such as where a caliper can be easily used and a micrometre can’t, it can introduce inaccuracy, a perfect example of the Abbe principle.