ACCURATE MEASUREMENTS Reading a · PDF file54 GEARS October 2007 on the sleeve, or 0.0250”. The thimble is divided into dif-ferent sized graduations, too. The longest graduations

52 GEARSOctober2007

Choosing the Right Size Mic

The first step in using a microm-eter is choosing the right size mic. Micrometers come in various sizes, but are only designed to measure within a range of about one inch. For example, the set shown in figure 1 includes three micrometers:

• Zero to 1 inch• 1 to 2 inches• 2 to 3 inches

The thing to remember is that the first digit in your measurement will always be the smaller of the two mea-surements in the range. That is, if you’re using a zero-to-one inch mic, the first digit will always be a zero:

0.xxxx"If you’re using a 1-to-2 inch mic,

your first digit will always be a 1:1.xxxx"How do you choose the right size

mic? That’ll be pain-fully obvious the first time you try it: The

right mic is the one that fits over the object you’re measuring, and adjusts tight enough to provide a measurement. If the thimble rotates past the sleeve measurements, and the part you’re mea-suring doesn’t fit snuggly between the anvil and spindle, you’re using the wrong mic.

Reading the Scale Imperial (Inches)

Now we’re ready to look at the scale. This is the part of using a mic that’s fairly consistent, regardless of what type of mic you’re using.

Start by looking at the sleeve (fig-ure 2): It has a number of different-size graduations. The largest graduations are numbered, from zero to 9. Those each indicate a tenth of an inch, or 0.1000”.

Each of those graduations is divided into four equal parts. So those smaller graduations are each equal to 0.0250”, because 4 x 0.0250” = 0.1000”.

Now look at the thimble (figure 3). Each full rotation of the thimble is equal to one of the small graduations

In the last issue of GEARS, we looked at micrometers: What they are, basic terminology, and how to

use them. In this issue, we’ll discuss how to read the scale on a micrometer, or mic. While there are many differ-ent types of mics, the scales are pretty much the same.

As with the caliper, micrometers are available in a digital format, and can be had for a fairly reasonable price if you’re willing to shop around. For that matter, a digital caliper will prob-ably provide you with adequate accu-racy and resolution for most of your measurement needs.

But assuming you’re willing to spend the time necessary, and want to be sure of the most accurate measure-ment possible, a micrometer may be the tool for you. Let’s look at how to read them, starting with a micrometer that’s calibrated for Imperial (inches) measurement.

by Steve Bodofsky

Reading a MicrometerACCURATE MEASUREMENTS

Figure 1: Micrometers come in a wide range of sizes, with a limited range of measurement. In virtually every case, the first digit of your measurement will be the lower range of

the mic.

Figure 2: Each of the numbered graduations on the sleeve is equal to a tenth of an inch. Those are divided into four smaller

graduations, each equal to 0.0250”.

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54 GEARSOctober2007

on the sleeve, or 0.0250”. The thimble is divided into dif-ferent sized graduations, too. The longest graduations are numbered by fives; each of them is equal to 0.0050”.

Those graduations are divided again; how many times depends on the resolution of your micrometer. In this case, the larger graduations are each equal to 0.0010”; the small-er equal 0.0005”, or five ten-thousandths of an inch.

That’s pretty good, but it’s not the limit of our resolu-tion. Take another look at the sleeve. See those extra divi-sions, running perpendicular to the first set we discussed (figure 4)? That’s called the vernier scale; each graduation on the vernier scale is equal to 0.0001”. So this micrometer can provide measurements that resolve down to one ten-thousandth of an inch! The trick to reading the micrometer is to add the readings on each scale, to provide a single, complete measurement of the object.

TIP: Not all micrometers have a vernier scale; in fact, not all mics have a scale that reads down below 0.001” (figure 5). In that case, all you can do is estimate the ten-thousandths measurement, based on the position of the thimble in relation to the spindle.

Measuring with a Mic Imperial (Inches)

Now let’s see how to read the micrometer scales to measure an object. For this measurement, I’ve chosen a small adjustable wrench. To make the measurement (figure 6):

• Zero the micrometer.• Open the micrometer enough to allow the item to

fit between the anvil and spindle.• Close the micrometer against the area you want

to measure, using the ratchet to adjust it against the object without overtightening it.

• Lock the micrometer spindle using the locknut.If you’re unsure of any of these procedures, read

the Accurate Measurements article in the last issue of GEARS.

Now we’re ready to read the measurement. To make it easier to read and explain, I’ve decided to cheat just a bit: I’m using a flattened drawing of the micrometer scale, adjusted exactly the way the micrometer displayed its measurement (figure 7). This just makes it easier for you to read the display all at once, without having to rotate the micrometer to see all the different scales. Think of it like

Figure 3: A full rotation of the thimble equals one graduation on the sleeve. And the thimble is divided into smaller gradua-

tions; in this case, down to 0.0005”.

Figure 4: The vernier scale allows you to refine your measure-ment even further; down to just 0.0001”.

Figure 5: Not all mics have a vernier scale. If yours doesn’t, you can estimate the difference between the zero line and the

graduations on the thimble.

Figure 6: The procedure for measuring an object with an outside mic is always pretty much the same. If you’re

unsure of the procedures, read the Accurate Measurement column in the last issue of GEARS.

Reading a Micrometer

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Figure 7: This is a flattened version of a micrometer scale; like using a world map instead of a globe. The advantage is it lets you examine all of the scales at once, without having to rotate

the mic to see the rest of the scale — a big benefit in print!

56 GEARSOctober2007

using a map of the world instead of a globe; everything’s right there in front of you, instead of rotating it to see what’s on the back.

Since the micrometer we used was a zero-to-1 inch mic, the first digit will be a zero:

0.xxxx”

Next we look at the large, numbered graduations on the sleeve. The thimble is back far enough to reveal the third numbered graduation, so the measurement is at least three tenths of an inch. Here’s what we have so far:

0.3xxx”

A closer look at the sleeve reveals at least one smaller graduation is showing; maybe two. So which is it, one or two? It’s one; here’s how we know:

Look at the graduations on the thimble (figure 8). Notice we’re close to the 20; that is, we’re near the end of its scale. Remember, each rotation of the thimble is equal to one small graduation on the sleeve. So, since we’re near the end of the scale on the thimble, we’re just short of the sec-ond graduation on the sleeve. That’s why we can see a little of the next graduation, even though we haven’t reached the second graduation yet.

Okay, so we have one small graduation showing on the sleeve; that’s another 0.0250”:

0.3000”+ 0.0250” 0.3250”

Next, look at the graduations on the thimble (figure 9). The zero indicator line on the sleeve is lined up just a bit shy of 21. 21 would be 0.0210”; since this is less, it’d be 0.0200”. But there’s another, short graduation, between the 20 and 21. That’s 0.0005”, so the thimble reads 0.0205”. That’s how much we add to the formula:

0.3000” 0.0250”+ 0.0205” 0.3455”

But wait; there’s more! Notice that the zero indicator is just a tiny bit past the 0.0005” mark. This is where the ver-nier scale comes in. Look at the vernier scale, and find the graduation that lines up with a graduation on the thimble (figure 10). In this case, it’s the 3. That’s another 0.0003” to add to the total. So our final measurement is:

0.3000” 0.0250” 0.0205”+ 0.0003” 0.3458”

And that’s the final measurement in inches: 0.3458”.

Reading a Metric MicSome of you aren’t forced to fight with measuring

length in inches, feet, yards and miles. Some of you get to work in simple decimal equivalents, known as the Metric

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Reading a Micrometer

Figure 8: Are we seeing one graduation or two past the 3 on the sleeve? It has to be one, because we’re too close to the

high end of the scale on the thimble.

Figure 9: The graduations on the thimble tell you how much to add to your initial measurement. In this case, you’ll be

adding 0.0205”.

Figure 10: To refine your measurement even further, see which graduation on the vernier scale lines up with a graduation on the

thimble. Then add that number to the 1/10,000ths column.

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58 GEARSOctober2007

system. For you, there are specially designed micrometers, calibrated in millimeters.

One of the first things you might notice about this Metric micrometer is that it doesn’t have a vernier scale (figure 11). It only measures down to hundredths of a milli-meter. Why aren’t Metric micrometers calibrated to resolve as well as an Imperial micrometer?

Actually, there isn’t much difference between the two. Remember, millimeters are a lot smaller than inches: It takes 25.4 of them to equal just one inch. A hundredth of a millimeter equals just under 0.0004”. So the Metric micrometer provides almost the same resolution as the Imperial one, even without a vernier scale. And you can still approximate more precisely, simply by paying atten-tion to how close the graduations are to the zero line.

NOTE: While there may be some Metric mics with a vernier scale, I couldn’t find one. I presume it’s because of the inherent resolution in the Metric mics, eliminating the need for it. If anyone has a Metric mic with a vernier scale, I’d be happy to use it to add a section to the upcoming booklet for the ATRA web site.

The next thing to notice is the scale on the sleeve (figure 12). There are two sets of graduations: one above the zero line and a second below it. The set above the line is the measurement in millimeters: Each graduation equals one millimeter. The set below the line is calibrated in half-millimeters, or 0.5 mm.

And the scale on the thimble is divided into 50 gradua-tions. Each rotation moves the thimble a half millimeter, so each graduation on the thimble equals 0.01 mm.

Let’s use the Metric micrometer to measure the same wrench, and see how close the two measurements are. We’ll use the same technique to measure the wrench as before, and then read the scale.

The sleeve shows 8 graduations above the zero line, so it’s more than 8 mm, but less than 9 mm (figure 13). And there’s one additional graduation showing below the line, so that’s another half millimeter. So far, the wrench is at least 8.5 mm.

Now look at the thimble. The zero lines up almost exactly with the 28th graduation (figure 14); that’s 0.28 mm. So we add the two measurements:

8.50 mm+ 0.28 mm 8.78 mm

So the wrench is 8.78 mm thick, according to the Metric micrometer. How does that compare with the mea-surement in inches? 8.78 ÷ 25.4 = 0.3457” — just 0.0001” difference in the measurements… and that difference could have been cause by position of the mic, pressure, tempera-ture… any number of things that are beyond our control.

That’s all there is to using a micrometer. There are no real tricks… nothing special to memorize. It’s mostly about taking the time to measure carefully, and to add the measurements together.

Or you can get a digital, and leave the math to some-one else!

Reading a Micrometer

Figure 11: I couldn’t find a single Metric micrometer with a vernier scale. The reason is probably because the Metric mic

resolves low enough without needing it.

Figure 12: There are two sets of graduations on the sleeve of the Metric mic: The one above the zero line is in millimeters.

The one below is half-millimeters.

Figure 13: The thimble lines up just past the graduation on the sleeve that’s halfway between 8 and 9 millimeters. That puts

the measurement at 8.5 mm… so far.

Figure 14: The zero line on the sleeve lines up with the 28th graduation on the thimble. That means you have to add 0.28 mm to the earlier measurement. So the wrench is 8.78 mm

thick.

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