48 GEARSJuly2007

In the last issue of GEARS, we looked at how to use a vernier caliper. We discussed the differ-

ent components that make up a caliper, how to zero the caliper, and how to use a caliper to make four different types of measurements. If you’re still unsure of any of these concepts, go back and reread that article before going any further.

Once you’re sure you understand the basics of vernier calipers, you’re ready to learn to read the caliper scales.

As we go through the various cali-per types, we'll compare our measure-ments with a digital caliper (figure 1). A digital caliper will display in standard numerical format, and will resolve down to about 0.0005”, or 0.01 millimeters. That should be more than accurate enough for anything you’re likely to measure. And they switch between Imperial (inches) and Metric at the touch of a button.

And, while cost might have been an issue in the past, today you can get

by Steve BodofskyACCURATE MEASUREMENTS

Reading a Vernier Caliper

Figure 1 — The easiest way to make accurate measurements is with a digital caliper. There’s no need to learn how to read the scale,

and the accuracy is better than you’re likely to need.

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50 GEARSJuly2007

decent digital caliper for just a few dol-lars. I got the one for this article on line for just under 10 bucks, plus shipping. Harbor Freight offers one for about $16 plus shipping. And I found several oth-ers, all for under $30.

So before you risk making a costly error while trying to count those little scratch marks on a caliper scale, get a digital and avoid the whole thing.

Standard CaliperThe term “standard” is a horri-

ble misnomer for this type of caliper, because there’s very little that’s standard (figure 2). Some read in thousandths of an inch; others read in fractions, usually down to 1/128th of an inch. Some have a

metric scale, but not all.If your vernier caliper reads in frac-

tions, the best thing you can do is give it to someone you really don’t like. Using this type of caliper requires not only reading the different scales, but also adding fractions; that means finding a common denominator and adding the numerator. Then you’ll still probably have to convert your measurement to decimals. Not an easy task overall, with lots of steps for a mistake to creep into your measurement.

Standard Caliper; Decimal Imperial Scale

The better choice is a vernier cali-per that measures in decimals; that is,

thousandths of an inch and hundredths of a millimeter. Let’s look at what each scale means, beginning with the Imperial (inch) scale:

The Main or Fixed scale breaks each inch into 10 graduations (figure 3); each graduation equals 1/10th of an inch, or 0.1”. Each of those graduations is bro-ken into four smaller graduations, each equaling 0.025”.

Notice that the adjustment jaw, or vernier, also has graduations: 25 on the Imperial scale (figure 4). Those divisions allow you to refine your measurement down to 0.001”. Let’s measure the width of this air conditioning feeler gauge and see how to use these calipers:

With the feeler gauge held in place,

Reading a Vernier Caliper

Figure 4 — The vernier has 25 gradua-tions on the top (inches) scale. These allow you to refine your measurement. Look for the one that aligns perfectly with a graduation on the fixed scale:

That’s the measurement, down to 0.001”.

Figure 5 — For this measurement, the zero lines up with the third graduation after 0.4”; that’s 0.475”. The gradua-

tion on the vernier that aligns with the graduations on the fixed scale is 21, so we add 0.021” to 0.475”. The total

measurement is 0.496”.

Figure 2 — The standard caliper comes in all different forms and formats, making the term “standard” a huge misnomer. Here are two different types of standard caliper: one that reads in thousandths of an inch and hundreds of a millimeter (A),

and a second that reads in fractions of an inch (B).

Figure 3 — Each inch on the fixed scale breaks into 10 large divisions,

or 0.1”. Those graduations break into four smaller graduations, equaling

0.025” each.

GEARS July 2007 51

the measurement appears to be about half an inch (figure 5). But look closer: The zero line on the vernier is just a tiny bit shy of the half-inch mark. That indi-cates that this feeler gauge is less than half an inch… but how much less?

This is where those extra 25 gradu-ations come into play. Look along those 25 graduations: Find the one that lines up precisely with a graduation on the fixed scale. Look closely: Some will appear closer than others. You should find a range of about five graduations that look awfully close… choose the one in the middle.

In this case, the graduation that lines up the best seems to be 21, or 0.021”. Now we add the two measure-ments:

0.475” — from the fixed scale 0.021” — from the vernier scale 0.496” — actual measurement

So the measurement is 0.496” based on the standard decimal caliper. How accurate is that? Let’s compare it to the digital caliper (figure 6).

Not bad. According to the digital caliper, the feeler gauge is 0.4965”; that’s within 0.0005”. Can’t ask for much better accuracy than that!

Standard Caliper; Metric Scale

Now we’ll look at the bottom scale on the standard caliper: This is the Metric scale (figure 7). Each gradua-tion on the fixed Metric scale is equal to one millimeter. The numbers indicate centimeters, or ten millimeters.

Notice the graduations on the Metric side of the vernier: It has 10

numbered graduations, and each of those numbered graduations is divided into five smaller graduations. The num-bered graduations are tenths of a milli-meter; the smaller graduations are each equal to 0.02 millimeters. These gradu-ations work the same way as the ones on the Imperial scale. Let’s measure the feeler gauge again and see how the Metric scale works:

With the feeler gauge in place, the zero on the vernier scale is just over two millimeters past the 10-millimeter mark on the fixed scale (figure 8). So the feeler gauge is just over 12 millimeters. But how much over?

Just as before, look for which grad-uations on the vernier line up with the graduations on the fixed scale (figure 9). It looks like the graduation at number 6

Figure 6 — A quick check with the digital caliper reveals our measurement was accurate to within 0.0005”; not bad for counting a bunch of lines on a scale!

Figure 7 — Each graduation on the fixed Metric scale is equal to one millimeter. The graduations on the Metric side of the vernier are also broken into 10 divisions, with each of them being broken into 5 smaller divisions. So each small division on the

vernier lets you measure down to 0.02 millimeters.

So the measurement is 0.496” based on the stan-dard decimal caliper. How

accurate is that? Let’s compare it to the digital

caliper (figure 6).Not bad. According to the digital caliper, the feeler gauge is 0.4965”; that’s within 0.0005”. Can’t ask

for much better accuracy than that!

52 GEARSJuly2007

is in perfect alignment with the fixed scale. That means this feeler gauge is exactly 12.60 millimeters across.

How does that compare with our previous measurement? If we divide 12.6 by 25.4, we get 0.4961”; pretty close to the 0.4965” we measured with the digital caliper.

To confirm it with the digital cali-per, we just press the inch/mm but-

ton and measure it again (figure 10). Hmm… 12.60 millimeters. Right on the money!

Standard Caliper; Fractional Imperial Scale

If you’re still insistent on learning how to use a fractional caliper, here’s how to read the scale (figure 11). This caliper has two basic scales: one in

16ths of an inch, the other in 32nds of an inch. The 16ths scale has eight addi-tional divisions on the adjustable jaw, allowing you to refine your measure-ment down to 1/128th of an inch.

Let’s measure that feeler gauge again (figure 12): At first glance, it appears to be an even half inch. But a closer examination reveals that the sec-ond to the last line on the vernier jaw

Reading a Vernier Caliper

Figure 8 — The zero on the Metric scale is just a little over two graduations past 10 millimeters, so the actual measurement is just over 12 millimeters.

Figure 9 — Now when we examine the graduations on the vernier, we see the

graduation at number 6 is in perfect alignment. So the measurement is

12.6 millimeters.

Figure 10 — To measure the feeler gauge in millimeters using the digital caliper, all we need to do is press the “inch/mm” button. Our measurement was right

on the money.

Figure 11 — This fractional caliper provides measurements in 16ths and

32nds of an inch. And the vernier scale on the 16ths side lets us refine

our measurement down to 1/128th of an inch.

Figure 12 — At first glance, it appears that the feeler gauge is 1/2” wide.

But a closer look reveals that the last graduation on the vernier scale is

lined up with the fixed scale, so it’s actually 1/128” less than 1/2”. Several mathematical steps later shows that

it’s actually 0.4922” wide.

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54 GEARSJuly2007

So when measuring, only one of those measurements can relate to a compete rotation of the needle. The other would be only a part of a revolution, and the zero position would continue to change, depending on the size of the item being measured. The only way to allow both measurements to zero at the same point would be to change the gearing in the dial; possible, but a much more com-plex and costly construction.

So, if you insist on going with a dial caliper, you should probably get two: one Imperial (inches) and one Metric.

Now let’s try the dial caliper, and see how it compares with the others:

As you can see, the fixed scale indicates about 0.5” (figure 14). But the dial is at 96; right near the end of the scale. So this measurement is just under 0.5”.

The previous measurement on the fixed scale is 0.4”, or 0.400”. The 96 is 0.096”, so we add 0.096” to 0.400” and get 0.496”; as we said, just under 0.5”.

The digital scale measured the feel-er gauge at 0.4965”, so the dial caliper was within 0.0005”; again, not a bad level of accuracy.

Digital CaliperWe could get into the whole pro-

cess of measuring the feeler gauge again, but to be honest, there’s no real reason. Once you use the procedures

Reading a Vernier Caliper

Figure 13 — The fixed scale on the dial caliper is broken into tenths of an inch, and each graduation on the dial equals 0.001”.

Figure 14 — Measuring the same feel-er gauge shows the fixed gauge right near the half-inch mark. But the dial is at 96, which indicates the feeler is just under a half inch: 0.496” to be precise.

is the one that’s actually lined up with the fixed scale. That means this shim is actually 1/128” less than 1/2”. Now let’s do the math. Remember, to add or subtract fractions, we must first find a common denominator:

1/2 = 64/12864/128 – 1/128 = 63/128So the feeler gauge is 63/128”

wide. Great; but how close is that to our earlier measurement? To figure that out, we have to convert the fraction to deci-mals, by dividing the numerator by the denominator (now do you see why you don’t want a fractional caliper?).

63 ÷ 128 = 0.4922So the gauge is 0.4922” wide.

How does that compare with our earlier measurement? It’s within 0.003”, which isn’t bad, considering what a cheesy caliper it is. And, of course, there’s always something lost when rounding the calculation. Not too shabby, all things considered.

But was it worth the effort? Probably not: especially for a measure-ment that was still off by about 0.003”. For my money, I’d rather go with the digital, and avoid the confusion… and the inaccuracy.

Dial CaliperCAUTION: Dials such as these

are subject to parallax errors; that is, slight errors created by the angle from which you look at the dial. To avoid parallax errors, always keep the dial directly in front of you when reading the measurement; never read it from an angle.

Most dial calipers display either inches or millimeters; only a few dis-play both. That’s because the conversion ratio would create a problem for using them. Let’s look at how to read the dial, and you’ll see why.

The fixed scale is broken into 10 graduations per inch (figure 13). As you move the vernier, the needle on the dial makes a complete revolution for each 1/10th of an inch it travels. And the dial is broken into 100 graduations. So each graduation on the dial equals 1/1000th (0.001) of an inch.

The problem with having a dial caliper that reads in both inches and millimeters is there’s no way for a com-plete rotation to equal both 1/1000th of an inch and 1/100th of a millimeter.

covered in the last article, the measure-ment appears digitally on the display. You want Metric? Press the button, and it’s Metric. Press again and it’s back to Imperial (inches). There’s nothing we can add that you didn’t learn back in 3rd grade.

So, in general, each of these cali-pers provides an accurate measurement, all within about a thousandth of an inch or less of one another (except the fractional caliper, and remember, you’re going to give that to someone you dis-like!) But when it comes to accuracy, ease of use, and price, these days it’s hard to imagine any reason to go any way but digital.

Next time, we’ll take a look at how to use a micrometer, and compare its accuracy to a caliper.

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