Understanding Machine Shop Numbers and Values

It’s not that those math classes you took were wrong; they weren’t. It’s just that in a machine shop, the language of numbers is different. To make parts correctly and size them properly, you need to understand the lingo, especially when it comes to the right side of the decimal point. Here’s what we mean.

In the machine shop, people speak in thousandths, even millionths, of an inch, especially in precision machining, when you’re trying to achieve very tight tolerances with micro-cutting tools. When using measurements in inches, everything is based on 0.001″, or one one-thousandth of an inch, and that’s how you referred to it during math class at school. In the shop, we just say “a thou” or “a thousandth.”

For a measurement of 0.0001″ (or one ten-thousandth of an inch, by the book), we just say “a tenth” in the shop. That is because it is one-tenth of the 0.001″, which is our standard or frequent starting point.

Let’s play out a few examples. 

When we talk about tolerances and say we want to hold “two tenths,” that is 0.0002, or four places over from the decimal. We’re talking about tenths of the thou, or thousandth, of an inch.

Knowing this unique shop-speak about small measurements will have a big impact on your success on the job. As they say, it’s the little things.