Video Transcript: This is a machinist’s steel rule. Steel rules can be marked in a variety of ways. Some steel rules are marked in the same fractional parts of an inch that you would find on an ordinary ruler or tape measure, but this one is different. This machinist’s rule is marked or “graduated” in tenths of an inch. This type of ruler can actually be a lot easier to read than an ordinary ruler.
Let’s first get a sense of scale with this rule. Here is the one inch mark. You can see that my thumb is about an inch wide.
Now let’s look at the individual marks. Each mark on this rule is numbered, so reading it will just be a matter of understanding what each number represents. Notice that there are ten divisions along the inch, and each one is numbered one through nine. The last mark is the one inch mark, and it is identified by the large number one. The ten divisions are tenths of an inch. The “tenths” label on the rule identifies the divisions as such.
The first mark is labeled “one,” so this is the one-tenth mark. Using tenths means that this ruler actually works better with decimals than it does with fractions. One tenth of an inch is one divided by ten. If we type that on our calculator, we see that it is point one inches. Each of the other marks works the same way. Two tenths is point two, three tenths is point three and so on. Let’s look at an example.
This measurement is one point three inches long. It falls between one and two inches, and it is point three inches longer than an inch.
I’ll let you try one on your own. Pause the video and try to figure out what this reading is.
The reading is two point three inches in length.
Now let’s look at another machinist rule. This one may look complicated at first, but with a little practice and a secret trick, it will get easier to use. This rule has each inch divided into one hundred marks. Each mark is one one hundredth of an inch. Again, if we divide one by one hundred we can see that it makes a nice decimal measurement. One divided by one hundred is point zero one inches.
Machinists use a simple trick to help them keep track of decimal places. Machinist’s don’t often use tenths or hundredths of an inch. They typically use thousandths. Thousandths are tiny, ten times smaller than the smallest division on the rule. A piece of paper is about three thousandths of an inch thick. Now, our rule doesn’t measure down to the thousandths, but if we think in thousandths it will make the decimal place much easier to keep track of. Let me show you what I mean.
We already saw that one tenth of an inch is point one, if we add a couple of zeros after the one, it looks like this. We can read this number as one hundred thousandths. If we are always thinking in thousandths of an inch, we can think less about the decimal place, and more about the numbers themselves. Let’s look at an example. The middle mark is labeled five. This would be a half inch or point five. If we add two zeros, it looks like five hundred. So a machinist would call this mark the “five hundred” mark. They mean that it is five hundred of the tiny thousandths of an inch. Let’s look at another. A machinist would call this number two hundred fifty thousandths. If we think in terms of thousandths, we can count the smallest marks by ten. Watch this.
The point two is two hundred. Then we count by tens. Two hundred ten, twenty, thirty, forty, and fifty. The reading is point two five, or two hundred fifty thousandths. Let me let you try one and see if you are getting it. Pause the video and see if you can make out the reading. The reading is point three three. That would be written like this, but we could read the ruler in thousandths to easily get the decimal point right. We would call three “three hundred” and add ten, twenty, thirty thousandths. Three hundred thirty. It is important to note that we shouldn’t write down the last zero, or people might think we had a more precise measuring instrument than we do, but thinking in thousandths is a good mental trick to make these rules easier to read.
Let’s try another one. Pause the video and think about this one. The answer is one point zero six. See that we have one inch and then ten, twenty, thirty, forty, fifty, sixty thousandths. We drop off the last zero and we are left with one point zero six.
Let’s do one more. This reading is nine hundred twenty thousandths, or point nine two inches. It seems strange to use divisions that are smaller than the ones on our ruler, but you will see that it makes keeping track of the decimal point a lot easier. In other videos we will look at other measuring instruments. The ones that are commonly used by machinists are generally easier to use when you learn to think in thousandths of an inch.