M is for Micrometer

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M is for Micrometer
For most jobs, I use a tape measure. For some specialty measuring jobs I use an interior folding rule. For other specialty jobs, I sometimes use a micrometer.

Micrometers aren't just for establishing very small distances with great accuracy. Like any shop micrometer, mine (pictured to the left) can measure in thousandths of an inch or in fractions of a millimeter. Since mine is a purely mechanical micrometer, it does this by means of a finely machined ruled scale (two, actually: imperial on top, metric on the bottom) and another finely machined slider. In taking a reading, the zero marker for the slider will probably fall between one of the markings on the rule. You simply count up the slider until you find the slider marking which is precisely aligned with a scale marking and, presto, that's your fractional measurement.

In practice, you can discriminate proportional half-shadings between "a big bit off" vs. "a tiny bit off" vs. "zeroed" (1). A "tiny bit off" corresponds to 0.025 millimeters, i.e. 25 microns, the smallest gradation that can be measured by a trained eye in any practical sense. To be honest, I wouldn't try to put a spacecraft on Mars using measurements THAT fine, but it's useful for measuring wear on certain kinds of contact parts.

I don't generally do woodworking that requires thousandths of an inch. Since wood expands and contracts with the weather, the time of year, how sunny the room is, what kind of finish you've applied, how the piece is used, etc., there's not a lot of point in measuring beyond a sixteenth of an inch (2). Finer distances than that are done by feel.

No, where I use this micrometer most is in determining the sizings for round parts. See, putting a tape across the end of a dowel will sometimes give you a bad reading if you're not cutting directly across the center. A micrometer does a three-point alignment with the perimeter, so it always gives you a true diameter.

The lower jaws are for measuring exterior diameters, the upper prongs are for measuring interior diameters. You stick the prongs in the hole, open the mic until it stops. If you're a clumsy sort of person, you can tighten the set screw so that the reading won't be changed by a hand bump or dropped mic, but who among us has ever done anything so dumb? (3)

When the mic opens, the centerline probe extends from the end. This isn't just part of the mic's sliding mechanism. It's used to measure depths, especially of blind holes were a normal tape measure can't reach. Again, since I work mostly with wood, not metal, I rarely need to know the depth of a hole to a thousandth of an inch. While I used this a lot back when I did a lot of engine repair work, for woodworking I often use less precise methods.

Micrometers nowadays are digital and much easier to read than mine, with onboard memory that records multiple readings. However, since I don't have much call for such precision, this one is fine for me.

1. The machinists who taught me how to use this micrometer referred to these as, respectively, "off by a CH", "off by an RCH" (4), and "dead nuts". Machinists are a foul-mouthed bunch.

2. Imperial measurements go in binary fractions down to thirty-seconds and sixty-fourths of an inch. Below that, it switches to decimal fractions as thousandths of an inch. If you want a bunch of foul-mouthed machinists to laugh at you and call you a "dumb-ass college boy", suggest that this is a more cumbersome system than metric. Go ahead - it's a formative experience.

3. Me. Of course, once you drop a precision measuring device like a micrometer, it's no longer a precision measuring device. It has become what machinists call "a worthless, lying piece of shit, you dumb-ass college boy".

4. According to the link cited above (5), a real RCH was actually measured with great precision and found to be 30 microns. Those machinists knew what they were talking about! 

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  1. I was wondering what kind of woodworking would call for a micrometer. Now I know! (I've always used a screwdriver for hole depth.)

  2. I used to have one of these as a kid, though I spent most of my time pretending it was a rifle.


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