The inch unit or English scale
You probably have little trouble with the multiples of the inch. You know that there are 12 inches in a foot, and that there are 3 feet in a yard. But when you come to the divisions of the inch, you may have some difficulty. A study of a 6-inch scale will help.
First, think of the scale as divided into 6 parts. Each part is 1 inch long. Select any one of these parts; for example, the inch between 3 and 4. Note that one line, longer than the others, divides the inch into two parts, each one-half inch long.
Next, look for two shorter lines, equal in length to each other, which divide each of these halves into 2 parts. Each of these parts is one-quarter inch long.
These are again halved by lines. These new dimensions are half of one-quarter, or one-eighth inch long. The eighths are next halved to make sixteenths, then the sixteenths to make thirty-seconds, and these to make sixty-fourths.
Note that as the divisions become smaller, the number under the line of the fraction (the denominator) becomes greater. Not only does it become greater, but every time it doubles, the distance it measures is halved.
A much-used scale is one which on one side uses one edge to divide the inch into eighths and the other edge for sixteenths. On the other side of this scale, the inch is divided into thirty-two parts on one edge and sixty-four on the other.
Use of the steel scale
Suppose, using this scale, you proceed to lay off a distance of 3 3/16 inches. First, turn to the edge divided into sixteenths. After the inch marked 3 it is easy to count off 3 one-sixteenth divisions and find your measurement. Suppose, however, you wish to measure a distance of 3 13/16 inches.
It would take too long to count off 13 divisions, so you use another method. You know that there are 4 sixteenths in each quarter, so you count off 3 one quarter divisions and add 1 of the sixteenth divisions. When you deal with smaller divisions, use the same method.
Frequently it is more convenient to subtract than to add a division. For example, suppose you want to lay off a distance of 63/64 inch. You know that a full inch contains 64/64 therefore the division you want will 1/64 inside any full inch. Look at the edge divided into sixty-fourths, count back one division within any inch, and there is the desired dimension.
There is no rule to be learned; after a little practice you add or subtract (whichever is easier) without giving any conscious thought to the selection of method. Most beginners have more trouble remembering how many of the smaller divisions there are in the larger divisions than in reading the scale. If you must stop to think how many sixty-fourths or thirty-seconds there are in a given part of an inch, it will direct your mind away from the reading of the scale.
Time spent in memorizing the number of smaller divisions contained in the larger divisions will greatly shorten the time required to use the steel scale and other rules.
Care of the steel scale
The expert machinist or toolmaker gives his measuring tools the same care that the infantryman gives to his rifle. He knows that the accuracy of his work will suffer from the least impairment of the tools upon which this accuracy depends.
Don't use the end of a scale to scrape foreign material off a surf ace. Don't beat a tattoo with it while you are reading the drawing. In short, never use it for anything except for measuring. When you put it away, wipe it with an oily cloth and lay it down — don't throw it into the drawer. Good habits formed in the early use of tools can make the difference between an expert mechanic and one who is only fair.
Linear measurements requiring a higher order of accuracy. Long before modern measuring devices were invented, machines such as the intricate clocks we see in museums were built. The method used in building these machines was to make the basic parts by eye measurement and then fit the other parts to them and to each other by feel or touch. Great accuracy was achieved in this way.
Modern practice employs this same method of touch or feel to achieve accuracy. It is employed, however, in a different way. Instead of fitting two parts of unknown dimensions to each other, a part of unknown dimension is fitted to a device of known dimension called a gage. In this way, if the fit is accurate, it shows that the dimension is also accurate.
The gage may be of one fixed dimension or it may be adjustable, as in the case of the micrometer and vernier calipers. The construction and use of these tools will be discussed later.
In connection with gages, devices such as parallel bars, buttons, and accurately sized blocks are used. The use of these with the gages permits a high degree of accuracy in linear measurement. In each instance, the setting of the gage is dependent upon touch, and the reading done from a scale which accurately indicates the distance included between the surfaces in contact with the gage.
Straightedge surface testing
A surface may be tested by laying a straightedge upon it and noting whether light passes between the straightedge and the surface. Thin strips of known thickness may be passed between the straightedge and the surf ace being tested at points where light shows through. In this way the amount that the spot is low may be determined. By applying the straightedge at different angles, the entire surface may be tested.
A more accurate way to test a surface is to use the surface plate. These plates are generally made of specially aged cast iron, ground or scraped until they are as flat as skill and care can make them. In use they are coated thinly with a paint such as Prussian blue. The surf ace to be tested is rubbed very gently over the paint. The high spots are indicated by the fact that they are coated by the paint.
Both straightedge and surface plate must be given very careful treatment. It is customary to keep a straightedge clamped to a board to protect its edge. Surface plates are generally kept in a wooden container. After use they are carefully wiped off, coated with a film of neutral oil, and covered.
The metric steel rule
Linear measurements in the metric system are based on the meter as a unit. A metric steel rule comparable to the 6-inch scale we have discussed would be a 15-centimeter scale. If you were to lay it beside a 6-inch scale you would find it a little less than one-tenth of an inch shorter. Instead of being divided into 6 parts, it would be divided into 15 equal parts. Each of these parts is called a centimeter, and contains 10 millimeters.
These millimeters are marked off and divide each centimeter into 10 equal parts. Since each smaller unit in the metric system is equal to one-tenth of the next larger unit, it would seem logical to mark off these millimeter divisions also into 10 equal parts. We do not do this for the simple reason that the human eye could not follow the very narrow divisions that would result. Instead, each millimeter division is marked off into 2 parts, each one-half a millimeter long.
A half of a millimeter is just a little longer than a sixty-fourth of an inch. When it is necessary to work with smaller units than this, the steel scale ceases to be useful and other means are employed.
Using the metric scale
Some older mechanics and draftsmen recall the days when measuring devices calibrated in the metric system were difficult to get. Even when they could be obtained it was difficult to get mechanics to use them. If an order was received to build a machine or part from metric specifications, it was necessary to convert all dimensions to the nearest equivalent in the English system.
Now metric measuring tools are as available as those calibrated in English measure. When a part is to be made from a drawing dimensioned in the metric system, the mechanic works directly from the measurements given, using the metric measuring tools.
When you look at a drawing dimensioned in the metric system you find no units of measurement indicated. All numbers are understood to mean millimeters. The number 470 appearing alone would mean 470 millimeters. Since there are 10 millimeters in each centimeter, you read this 470 as 47 centimeters on your scale. If the dimension were 475, you would add 5 millimeters to your measurement by counting off 5 more divisions to the right of the 47 on the scale.
The Ford decimal scale
In 1932 the Ford Motor Company adopted a system of dimensioning in which the inch is divided decimally. The scale used for working from these dimensions has its inches divided into 10 parts. Each of these parts is divided into 5 divisions. These smallest divisions are then equal to one-fiftieth of an inch.
Dimensions for use with this scale are always given in the decimal form. Thus, five and one-half inches would be written 5.5, not 5 1/2". If closer dimensioning were necessary, another digit would be added to the right; for example, the dimension 5.52 might be used. This second digit to the right of the decimal is never an odd number.
It may be 2, 4, 6, or 8, but never 3, 5, or 7. The use of the odd numbers would require reading to one hundredth of an inch. This would be expecting a little too much of our eyes. By using only numbers divisible by 2 for the second digit to the right of the decimal, we are able to use our finest divisions (one-fiftieth of an inch long) directly. For example, if we were to lay off a length of 3.54 inches, we would find our length to be 5 one-tenth divisions and 2 one-fiftieth divisions to the right of the 3-inch mark.
There are, of course, times when measurements must be made without the use of rules. These measurements can only be approximate, but sometimes they are sufficient and in most cases they are better than no measurement at all. Probably you have at some time paced off a distance, knowing that your shoe is so many inches under or over 12 inches, or one foot, in length. Possibly you know about how long a stride to take to measure off one yard; if you are tall, it may be rather a short step for you, but if you are short it may be rather a long one.
These two methods of approximate measurement can be very useful. Practice them until you are reasonably accurate in them. Try also measuring the width and length of your hand, the distance from fingertips to elbow and shoulder. These measurements can be useful too.
Another means of estimating distances is to recognize the height of familiar objects. Chairs, for example, are generally about 18" high, and tables 30" to 32". You know your own height, and that the average man is a few inches shorter than 6' tall. Learn also the dimensions of objects you habitually carry, such as cards, comb, and wallet. An ordinary index card, 3" X 5", for example, can be a useful and reasonably accurate measuring gage.