ACCURATE MEASUREMENTS Using a Micrometer€¦ · We’ll look at how to read both types of scale. The vernier scale is the important one: Some mics have them; others don’t (figure

42 GEARSSeptember2007

When it comes to accurate measurement, the first tool most people think of is a micrometer. A standard micrometer allows you to measure

accurately down to as small as 1/10,000th of an inch, using a simple technology that’s been around for as long as anyone can remember.

When we use the term “micrometer,” we’re actually talking about a wide range of devices used to take precise measurements. There are outside micrometers and inside micrometers; depth micrometers and bore micrometers; and all sorts of specialty micrometers. Some have interchangeable anvils, to offer a wider range of measurements. They have micrometers that measure in Imperial units (inches) or Metric, and are even available in digital formats. But when most of us think of a micrometer, we think of the standard, outside micrometer, or mic (pronounced myk); the one that looks like a C-clamp (figure 1). It’s designed exclusively to measure thicknesses. In this article, we’re going to cover the basics of using an outside mic. While the actual handling of other mics may vary, the method of reading the mic itself is usually pretty much the same.

Parts of a micrometer To discuss how to use a micrometer, it’s first helpful

to have the component parts defined, so we can speak the same language (figure 2):

A. Anvil B. Spindle C. Frame D. Locknut E. Sleeve, Barrel or Hub F. Thimble G. Ratchet or Friction Screw H. Vernier I. Standard Rods or Standards

Holding your micrometer Here’s the correct way to hold a micrometer for use (figure 3). This is important for two reasons:1. It enables you to adjust the micrometer with one hand, while holding the object you’re measuring in position with the other.2. It gives you the opportunity to act all superior and condescending when you see someone who doesn’t know the right way to hold it. (“Where did you learn how to use a micrometer? Sesame Street?”) Okay, maybe there’s really only one good reason, but you gotta admit, you’re going to be thinking about that second one the next time you see someone holding a mic the wrong way!

by Steve Bodofsky

Using aMicrometer

ACCURATE MEASUREMENTS

Figure 1This is what we think of when someone says “micrometer”: an outside mic used to measure thickness. There are many other types of mic, but most are read in the same manner.

Figure 2The parts of a standard micrometer.


GEARS September 2007 43

Zero the micrometerWhile a micrometer tends to be a bit more stable than a caliper when it comes to size, it’s still important to zero your mic before you use it. In most cases, you’ll only have to zero it once at the beginning of your measurement session, as opposed to a vernier caliper, which you should check before each measurement.

To zero the micrometer1. Open the mic, then slowly close it all the way. If you aren’t well versed in handling a mic (which you probably aren’t, or you wouldn’t be reading this article) use the friction screw to close it. Important: Always rotate the shaft slowly when closing the mic. Even though you’re using the friction screw, the momentum of the shaft can be enough to allow it to overtighten if you turn it too quickly.2. Once you have the mic closed all the way, check the scale: the zero on the

Figure 3The correct way to hold an outside mic is to hold it in one hand like this, leaving your other hand free

to control or adjust the object you’re measuring.

Figure 4Once you have the mic closed all the way, the zero on the sleeve

should line up precisely with the zero on the thimble.



thimble should line up with the zero on the sleeve (figure 4).3. To zero the mic, use the spanner wrench that came with the mic to rotate the sleeve (figure 5). Then recheck it. You may have to try this a few times to get your mic zeroed properly.

Tip: Rotating the sleeve a small amount can be tricky; it can take a lot of force to get the sleeve to start turning, and once it starts it may turn too far. To get it zeroed, you may find it easier to rotate the sleeve farther out, and then rotate it back to zero.

Some mics don’t havean adjustable sleeve. If yoursdoesn’t, you won’t be able tozero it; in that case, make anoteofhowfarit’soff,andthenadd or subtract that from yourmeasurement. Some mics don’tclose. For example, mics thatmeasure between 1" and 2" or2" to 3" don’t close all the way(figure6). To zero those mics,you’ll need a standard rod, orstandard. Standards are steelrods that are exactly a certainlength (figure 7). In this case,wehave a1" standard and a2"standard.Tozerothe1"–2"mic:1. Slip the 1" standard betweentheanvilandthespindle(figure8).2. Close the mic gently againstthestandard.3.Adjustthemictozero.To zero the 2"–3" mic, use the2"standard.

Measuring Thickness To measure thethickness of an object with anoutsidemic:1.Zerothemic.2. Open the mic far enough toallowtheobjectyou’remeasuringto fit easily between the anvilandthespindle.3.Gently close themic, until itfitsfirmlybetweentheanvilandthespindle(figure9).Wiggleorrotatetheobjecttomakesureit’s

sittingsquarelybetweenthetwomeasuringsurfaces.4. Once you have the micadjusted to the object, lock thespindlewiththelocknut. Now you’re ready to readthemic.

Reading the Mic Virtually all micrometers arereadthesameway.Theonlyrealdifferencesbetweenthemare:•Maximumsizeorscale•Unitsofmeasure•Vernierscale By maximum size or scale,we’retalkingabouttherangeofthemic. If themicyouneed tousemeasuresbetween2"–3",thefirst digit of the measurementwillusuallybea2.Ifit’sazero-to-one inchscale, the firstdigitwillusuallybeazero. Units of measure is eitherImperial (inches) or metric.We’ll look at how to read bothtypesofscale. The vernier scale is theimportant one: Some micshave them; others don’t (figure10). The vernier scale is anadditional scale that allowsyouto refine your measurement,down to 1/10,000th of an inch,or 1/1000th of a millimeter.Our Imperial (inches) mic hasavernierscale,sowe’llgooverhow to use it in the next issueofGEARS. Ifyourmicdoesn’thaveavernierscale, you can approximate thefinermeasurementbylookingatthe position of the scale on thethimble in relation to the zeropoint on the sleeve. But youprobablywon’tneedto;that’sanawfully precise measurement…more so than you should everneed,unlessyou’reamachinist. If you do need to resolveyour measurement that far,you should probably get a micthat’s either digital or offers avernier scale. They aren’t thatexpensive if you know whereto look: Here’s a set of threethat I bought for the pictures

Using a Micrometer

Figure 5To adjust the mic, use the spanner wrench that came with it to rotate the sleeve, until the zero

lines up with the zero on the thimble.

Figure 6Some mics never close all the way, so you can’t zero them the same way you would a small mic. You’ll need something to fill in the space… you’ll

need a standard.

Figure 7Standards are metal rods that are calibrated in size

to allow you to zero your micrometers. The 1" standard is for zeroing the 1"–2" mic; the 2"

standard is for zeroing the 2"–3" mic.



in this article (figure 11). They cost just $36 including shipping, from JTS Machinery & Supply Company; www.jtsmachine.com. Next time we’ll look at how to read the scales on a typical mic and try a few measurements, using both an Imperial (inches) mic and a Metric one.

Figure 8Hold the standard in place, just as

though you were measuring it. Once the standard is firmly between the

anvil and spindle, zero the mic.

Figure 9Gently close the mic around the object you’re measuring, while

wiggling the object between the measuring surfaces to make sure it’s

sitting squarely in place, and isn’t cocked or out of position.

Figure 10Some mics have a vernier scale;

others don’t. The vernier scale allows you to resolve your measurement down to 1/10,000th of an inch, or

1/1000th of a millimeter.

Figure 10I bought this set of three mics for

the pictures in this article; they were just $36 including shipping.


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Documents

ACCURATE MEASUREMENTS Using a Micrometer€¦ · We’ll look at how to read both types of scale. The vernier scale is the important one: Some mics have them; others don’t (figure