Micrometers, often called “mics”, are likely to be the most utilized precision measuring instrument in any machine shop. As they typically measure in .0001” increments (or .01mm for metric models), micrometers provide extremely accurate measurements and machinists often prefer them to other devices such as calipers.
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Anatomy
Image credit: megadepot.com
As you can see in the image above, a traditional micrometer is generally composed of several different parts. Here are some important things to consider when choosing a micrometer:
- ANVIL: Together with the spindle, the anvil is the part that will be in constantly in contact with the part and therefore the most likely to chip. For this reason, good models usually feature a carbide-tipped anvil that will allow for longer tool life.
- FRAME: Frames come in a number of different sizes and shapes. Some shapes are designed for specific uses. The hub micrometer in the image below, for example, is designed to get into really small areas. When choosing a micrometer, always consider the type of part you are going to measure.
- LOCK NUT: A spindle lock is handy to keep track of a measurement and also to lock the spindle for small lot gaging. Some micrometers have a lock nut (as shown), whilst others may have a locking lever. We recommend choosing the lever type as it seems to require less attention over the years and it’s easier to operate
- SLEEVE SCALES: While all micrometers feature the main index scale on the sleeve, which measures in increments of .025”, the most accurate models also have a Vernier scale for more accurate measurements. Vernier scales measure in .0001”.
- RATCHET SPEEDER: Many mics have a sort of slipping clutch mechanism that prevents over-tightening and aids the user to apply a constant measuring force to the spindle, helping to ensure reliable measurements. It’s not a life-changing feature, but it’s something to consider, especially important in production situations, with different operators, but one QC spec.
Digital vs analog
Newer models, the digital micrometers, have a digital display that makes it easier for you to read measurements. In terms of digital vs. an analog micrometer, there is not much difference in accuracy. Digital mics, however, offer two significant advantages: resolution and quick scale conversion.
The displays on most digital electronic micrometers, in fact, resolve to 0.00005″ and can be quickly converted from imperial to metric and vice-versa.
Since there’s not much difference between the two models, we usually recommend going with the most convenient option, which is generally the analog one.
If you are considering buying a digital mic, this is our recommendation.
Types of micrometers
As already mentioned, mics come in different sizes and shapes according to their intended use. Here are the three main types of micrometers that you will encounter when working in a machine shop:
Outside micrometers
Brown & Sharpe 599-1-31-9 Chrome Framed Outside Micrometer
The most widely used type if micrometer. It’s used to measure the distance between two external points of either round materials if you are working on a lathe, or square materials if you are working on a milling machine.
Inside micrometers
Mitutoyo 139-006 Tubular Vernier Inside Micrometer
Designed to take inside measurements, such as the ID of bushings or bearings. The most common models of inside mics look like outside mics without a frame and anvil, but other types, more similar to calipers, are also available. Unlike outside mics, the read on the sleeve increases as the sleeve expands.
Depth micrometers
Starrett 449 Vernier Depth Gauges, Micrometer Type
Depth micrometers allow for precise measurement of depth for features like holes, slots, recesses of keyways. The tool has a hardened ground and a lapped base attached to the micrometer head. Like inside mics, the measurement increases as the rod extends into the hole.
What type of micrometer should you buy?
If you are just approaching the trade and thinking about where you should start investing to build your personal toolbox, we definitely recommend starting with the best 0-1’outside mic you can afford, as it’s most likely the tool that you are going to use most. Later on, you might fill in with additional micrometers, like a 0-6” set, but you’ll likely use them less often so we recommend investing more in the first one and settle on cheaper models for the add-ons.
As already mentioned above, mics vary depending on the intended use so the best advice we can give is to identify the type mic that you will be using most (outside, inside or depth) and invest in that.
Best models
OUTSIDE MICS
Our favorite model.
- Carbide anvil
- .0001” accuracy
- Locking Lever
- Friction thimble
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A600 Mics Process 2 Manual Pdf
- Carbide anvil
- .0001” accuracy
- Locking Lever
- Friction thimble
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A great quality mic, only third in our list due to the lock nut type.
- Carbide anvil
- .0001” accuracy
- Lock Nut
- Friction thimble
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INSIDE MICS
Tubular inside mic. It comes with interchangeable rods that allow you to measure ID up to 12”.
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More similar to a caliper, the frame is specially designed to enable the micrometer to provide extremely accurate small internal measurements.
A600 Mics Process 2 Manual User
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DEPTH MICS
Depending on the model, you can measure up to 12” in depth. It comes with a ring-type knurled lock nut, ratchet, and speeder for stability and to apply uniform pressure and make quick adjustments for precise measurements.
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Mitutoyo 129-127 Depth Micrometer
Includes interchangeable 0.157″ diameter measuring rods that can be adjusted in 1″ increments for flexibility up to 4”.
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A600 Mics Process 2 Manual Transmission
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