Did you know that a **machinist’s inch**, while still an inch, makes measuring small parts easier? How does it do this? By breaking an inch up into ten sections instead of the traditional eight you see on a ruler or yardstick. As a result, measurements given in machinist’s inches are actually decimals!

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To make sure a replacement part fits a broken machine exactly, a machinist must be very precise in her measurements. If the part measures a whole number of inches, that precision comes easily. If the part’s measurement falls somewhere *between* inches, things get a little dicey. That’s because fractions such as \begin{align*}\frac{1}{8}\end{align*}, \begin{align*}\frac{3}{8}\end{align*}, and \begin{align*}\frac{7}{8}\end{align*} enter the mix.

To avoid all these fractions, the machine trades decided years ago to divide the inch into 10ths, 100ths and 1000ths instead of eighths. (You can see that the bottom row of tick marks on the ruler above divides each inch into 10ths.) This enabled them to express their measurements in decimals instead of fractions. Working in this way, a machinist can obtain more precise measurements and get the job done right!

See for yourself: http://www.craftsmanshipmuseum.com/Shoptools.htm

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Check out the videos below to learn more about the craft of machining. The first clip below shows you what it’s like to work at a company as a machinist. The second video is an old career guidance film that dives into what a machinist actually does.

http://www.youtube.com/watch?v=yQzlZBj2JMA

http://www.youtube.com/watch?v=egBezwsUJg0