Printers and prepress people often need to know things like the thickness of a sheet of paper, or the thickness of a blanket.
We make calculations for packing, for folding, for the spine of a book, and more, using measurements of tiny things, and for that we use a micrometer. There are several variations of this instrument, but the most common is the simple Outside Micrometer, an instrument that affords amazing precision while requiring no batteries!
This is a common Outside Micrometer. Mine is a simple model made by L.S. Starrett of Athol, Massachusetts. A micrometer like this costs about $100 today. You can spend a little less or a lot more, depending on features.
The micrometer has a history that parallels the development of industry. A microscopic measuring tool was drawn by Leonardo da Vinci in one of his famous sketchbooks, but there is no evidence that he built the device.
In France, in 1639 W. Gascoigne invented a micrometer caliper comprising jaws and a vernier scale. The previous year he had used screw threads to measure the angles of the stars, and this probably represents the first application of the technology employed in a modern screw-thread micrometer.
In 1772, James Watt invented a table-top micrometer. He is more famous for his invention of the steam engine. Other machinists and mechanics followed in their footsteps.
The common micrometer is an essential tool for making small measurements, but for some it’s a mystery. Let’s remove the mystery from the micrometer!
This image shows the micrometer after reading the thickness of a piece of paper, which measured just less than six thousandths of an inch. The correct reading is 0.0055 in.
It’s a simple machine consisting of a screw spindle inside a sturdy frame. The threads of the spindle are the key to its precision. Each turn of the spindle represents 0.025 inch, or twenty-five hundredths of an inch. Four turns of the spindle represents 0.1, or one-tenth inch.
To measure the thickness of a piece of paper, one inserts the sheet between the end of the spindle and the anvil (some micrometers need calibration, requiring the spindle to be closed, then the barrel – on the right – to be set to zero.) One then gently closes the gap between the spindle and the anvil, with the paper in between.
A typical sheet of paper is between 0.002 and 0.008 in. The thickness is called its “caliper.”
In this example, the micrometer is reading exactly 0.012 inch. This is the thickness of an offset printing plate I read to determine the packing on a printing press.
Plates are typically 0.012 in. (which is spoken as “twelve thousandths”) or slightly more than 1/100 inch.
The nice thing about an Outside Micrometer is that you can easily measure to about half of its basic precision by estimating the mid-point between two of the lines. This means that a micrometer which is precise to 0.001 is actually capable of making readings that are 0.0005 in. This is an incredibly precise measurement.
In this example the opening is very wide, at exactly 0.10 inch. It takes four complete turns of the thimble (on the right) to get to 0.10 inch; each turn is 0.025 inch. Notice that there are four vertical ticks visible to the left of the arrow; the sum of these is 0.10. The capacity of this instrument is one inch – 40 turns of the thimble.
So, when the measurement falls between two lines on the Reading Line, you can say that a sheet of paper is 0.0045, or “45 ten-thousandths” of an inch. Most people who use micrometers would say “Four and a half thousandths.”
We often hear newscasters referring to things in football-field units, and very small things a “the thickness of a human hair.” What, exactly, does that mean? How thick is a human hair? Mine is 0.0023 in. My cat’s hair is much thinner: 0.001 in. I have measured human hairs that are much thicker than mine, so it’s not a reliable unit of measure except in a metaphorical sense.
U.S. currency is 0.004 in. thick, or the thickness of two human hairs. But, let’s not split hairs over this.
Now there is no mystery to the micrometer.
