Every industry has a jargon of its own and dimensional metrology is no different. The reason is ease of verbal communication, but that communication can be confusing at times for the uninitiated. For example, all measurement systems, whether they are old or new, have units of measurement that are ignored for all practical purposes. An example of this in precision engineering is where ‘inches’ prevail over ‘feet’ in the Imperial system. The metric system has similar orphans as well.
Awareness of the practicalities of both systems makes it easier to understand the evolution of the jargon so let’s take a look at both of them.
A thousandth of an inch is written as: .00l”, and in casual conversation is called a ‘thou’. If you divide it by ten it is written as .000l” or a ten thousandth of an inch or as gage makers refer to it, a ‘tenth’. All pretty logical stuff. The next step in the progression would be to divide that ‘tenth’ by ten and you end up with a hundred thousandth of an inch or .00001” which is a mouthful that the British used to shorten to a ‘hundredth’. North Americans don’t want to know about that one and skip it and go to the next level down which is millionths of an inch or ‘millionths’, .000001”. Since ten millionths of an inch is the same as a hundred thousandth of an inch and is a lot easier to say, you can see why millionths is used. To make matters more interesting, they are often referred to as micro-inches and written as µ”.
The metric system is based on ten with a different name for the units at each level so each can be treated as a whole unit. For example, the basic unit of length is the meter so a millimeter is a thousandth part of a meter so that system’s ‘thou’ is much bigger than the corresponding inch term. Their ‘millionth’ is a millionth part of a meter or a micrometer but it’s a tad smaller than forty microinches. If the original French spelling is used i.e., micrometre, there is no confusion with the instrument called a micrometer but I have used the U.S. spelling which is recognized by those who make the rules for the metric system.
In the beginning—1879—the term ‘micron’ was used for this increment and it was written using the Greek symbol: ‘µ’ on its own by some, while others expressed it as ‘µm’. It was removed as an officially recognized designation in 1967/68 for—I believe—theoretical considerations. Despite this change, the micron lives on in precision dimensional metrology because it saves a lot of decimal places when it is written and is easier to verbalize.
This column is not about promoting one system of measurement over the other. This is about the practicalities involved on a communication basis and some of the problems that can arise that you should be aware of, one of which results when terms common to one system are used in the other.
For example, it is common in the coating industry to hear the term ‘mils’ being used to describe a coating or plating thickness. It is not uncommon to assume millimeters are being referred to but it is a term used in North America where it refers to thousandths of an inch.
One of our customers is a multinational, European based maker of high precision products and, as you can appreciate due to their origins, all of their products are designed using the metric system. But in discussions with their manufacturing and quality staff in North America, they will refer to ‘thous’ and ‘tenths’—terms usually associated with the Imperial system. What they are referring to is a thousandth or a ten thousandth of a millimeter. Similar expressions are used by precision instrument and gage block makers.
I hope this guide to the jargon will help you navigate within each system—or both. If in doubt, use the correct full name and when writing, use the inch symbol or the metric designation with each use and you’ll avoid embarrassing moments later.