Mensuration – the pursuit of accuracy

The need for accuracy in engineering

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Henry Maudslay by Grevedon

Western civilisation, and Britain in particular, can be credited with creating the World as we know it.

The Industrial Revolution was the transition to new manufacturing processes in the period from about 1760 to sometime between 1820 and 1840. This transition included going from hand production methods to machines, new chemical manufacturing and iron production processes, the increasing use of steam power, the development of machine tools and the rise of the factory system.

The Industrial Revolution began in Great Britain, and many of the technological innovations were British.

By the mid-18th century Britain was the world’s leading commercial nation, controlling a global trading empire with colonies in North America and Africa, and with some political influence on the Indian subcontinent, through the activities of the East India Company.

The development of trade and the rise of business were major causes of the Industrial Revolution.

None of this could have happened without the increase in accuracy of the measuring tools, starting with standardising measurements and making tools capable of measuring to a fine tolerance and to ever increasing definition.

Prior to the Industrial Revolution measurement was based mainly on an arbitrary system derived from the length of certain body parts!! (behave, you lot!)

Thus, the yard was the length of an average man’s stride, the foot was the length of his foot and the inch was the length of the top joint of his thumb.

Now, obviously, this led to enormous differences in what a yard, foot and inch could actually measure, but until the Industrial Revolution this really didn’t matter in the grand scheme of things. If you needed a latch making for your door or a bolt for your cart the local blacksmith would just knock something up which fitted.

The implementation of standards in industry and commerce became highly important with the onset of the Industrial Revolution and the need for high-precision machine tools and interchangeable parts.

Henry Maudslay developed the first industrially practical screw-cutting lathe in 1800. This allowed for the standardisation of screw thread sizes for the first time and paved the way for the practical application of interchangeability (an idea that was already taking hold) to nuts and bolts.

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Maudslays screw cutting lathes

Before this, screw threads were usually made by chipping and filing (that is, with skilled freehand use of chisels and files). Nuts were rare; metal screws, when made at all, were usually for use in wood. Metal bolts passing through wood framing to a metal fastening on the other side were usually fastened in non-threaded ways (such as clinching or upsetting against a washer). Maudslay standardized the screw threads used in his workshop and produced sets of taps and dies

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Taps and Dies – a Tap cuts an internal thread, a Die cuts an external thread


that would make nuts and bolts consistently to those standards, so that any bolt of the appropriate size would fit any nut of the same size. This was a major advance in workshop technology.

Linear measurement, too, became more accurate, first with the standardisation of the inch, then with the introduction of steel rules (NOT rulers – those are Kings and Queens!). As machinery developed, and as the existing accurate tools enabled machines to make rules even more accurate the inch was split into even smaller divisions. Those of a certain age will remember the old wooden rule used in the classroom, this had the inch sub-divided into quarters, halfs, eighths, and sixteenths. Some even had sixths and twelfths markings too.

For normal, day to day, use this was sufficient, and indeed, with failing eyesight and poor lighting one sixteenth was probably at the limit of most peoples needs. However, the increasing need for accurate measurement in industry demanded that the inch be sub-divided into even smaller units, thus was born the micrometer.

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An Imperial micrometer

(The micrometer in this picture reads to an incredible one ten thousandth of an inch.)

Reading a micrometer, to the uninitiated, seems impenetrable.

On the face of it it looks like some sort of clamp but in fact, in skilled hands it can render measurements of unbelievable accuracy.

How to read a micrometer

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Parts of a standard Imperial micrometer


First, an explanation of how the micrometer works,

The threaded barrel of the Imperial micrometer is 40 threads per inch, so one revolution of the barrel moves the spindle 1/40th inch, or .025 (twentyfive thou. In engineering terminology).

The barrel has 25 divisions around its circumference, therefore each division on the barrel is one thou. (.001).

The sleeve is marked with a line at every .025 (twentyfive thou.) and is numbered at every fourth line .01 (one hundred thou, or a tenth of an inch).

OK – now onto a practical example

Place the item you want to measure between the anvil — the stationary measuring face attached to the micrometer’s frame — and the spindle

Rotate the ratchet knob — the handle of the micrometer — until the object is finger tight between the anvil and the spindle. The knob is a ratchet and that determines how tight you need to turn it.

Count the number of major numbered divisions revealed on the sleeve. Remember, each one of these represents a tenth of an inch (0.1″).

Count the number of minor unnumbered divisions revealed after the closest whole number. Each one is twenty-five thousandths of an inch (0.025″).

Read the line on the thimble that lines up with the data line on the sleeve. This gives thousandths of an inch (0.001″).

Add everything together. For example, if the sleeve reveals the data line up to three minor divisions past the “2” mark, and lines up with the “13” mark on the thimble, you have two-tenths (0.2″) plus seventy-five thousandths (0.075″) plus eleven thousandths (0.011″), for a measurement of 0.286 inches.

The micrometer, and measuring tools like it, helped engineers create the world as we know it, helped put a man on the moon and the world into cars.

It helped create everything that you see around you, unless you are a resident of the Amazonian rainforest.

Sadly, fewer and fewer people are reading the micrometer in their everyday working lives as engineering is being lost to these shores. It is my belief that everyone should learn how to use one of these instruments, and indeed, if you are interested, buy one from one of the auction sites. As the world shifts to the metric system the Imperial micrometer is being discarded; they are available for very little money, usually under £10– go and pick up a bargain and keep alive a valuable piece of our industrial past.

All hail the mighty micrometer – the world’s unsung hero!

(The author would like to declare that he personally owns 17 micrometers of both the Imperial and Metric variety – collecting becomes an obsession)

Much of the substance of this post has been digested from Wikipedia and engineering websites under the CC useage. The author would like to attribute the pictures used herein to those sources.

 
© Grimy Miner 2018