Because everything you draw must be drawn accurately to some scale, it is essential that you become thoroughly familiar with scale drawing.

A scale is a ruler marked off conveniently to represent proportionally larger or smaller distances. All mechanical drawings are drawn to some convenient scale.

The plans of a house, for example, are usually drawn to the scale of ¼-inch equals one foot. This means that every ¼-inch on the rule represents a foot on the house. If the house is 40 feet long and 20 feet wide, the plan, drawn to the ¼-inch scale, will be 10 inches long and 5 inches wide. The same plan, drawn to the 1/8-inch scale (one-eighth inch equals a foot), would be 5 inches long and 2½ inches wide. In architectural drafting the scales most commonly used are, ½ inch equals a foot, ¼ inch equals a foot, 1/8 inch equals a foot and in some cases a inch equals a foot. These are known as the ½-inch scale, the ¼-inch scale, etc.

In map work the scales are usually so many hundred feet or so many miles to the inch, thus: 1 inch equals 200 feet; 1 inch equals 3 miles; 1 inch equals 50 miles, etc.

In detail work and shop work the scale is denoted as ½ SIZE, ¼ SIZE, or FULL SIZE. In this case ½ inch equals an INCH; ¼ inch equals an INCH; or an inch equals an INCH.

The three scales may be divided into three groups as follows:

1. Architect's plans and elevations:

1 inch, or fraction thereof, equals 1 foot

2. Map work:

1 inch, or fraction thereof, equals so many feet or miles

3. Shop drawings and detail work:

1 inch, or fraction thereof, equals 1 inch

Suppose you had to draw the plan of a house which is 35 feet long and 22 feet wide, and you wanted to get it on this page. What scale would you use? The page is 8½ inches wide and 11 inches deep. If you used the half-inch scale, your length would be 17 ½ inches, which is too long for the page. If you used the quarter-inch scale, your length would be 8¾ inches so that it would fit lengthwise, as the page is 11 inches long. You would naturally choose the quarter-inch scale and work accordingly.

Now suppose that you had to draw a plan of a box 14 inches long and 8 inches wide. If you used the half-inch scale for this, your drawing would be slightly more than a half inch high by a little over a quarter inch wide—far too small. You would use the half-size scale for this. In this scale, a half inch equals an inch instead of a foot. Your plan would be 7 inches long by 4 inches wide on this scale.

If you had a map to make, covering a distance of 300 feet one way and 200 feet the other, it is obvious that architect's scales would be totally useless. Here you would select the engineer's scale of so many feet to the inch. If you use the scale of 50 feet to the inch, your map would be 6 inches one way and 4 inches the other. A few of the more common scales are shown in Figure 6. Scale A shows the one-inch and half-inch scales. Reading the lower numbers from left to right, we get the scale of 1 inch equals a foot, reading the upper numbers from right to left, we get the scale of a half inch equals a foot. Note the space beyond the 0 point. It is divided into 12 parts, each part representing one inch. Now verify the length of the line marked 1' 10" on the inch scale and the line marked 7' 4" on the half-inch scale, remembering to read the upper numbers from right to left. Can you tell what the length of line E is on the inch scale and line F on the half-inch scale?

Scale B shows the quarter-inch and half-inch scales. Reading the lower numbers from left to right, we get the scale of one-quarter inch equals a foot; reading the upper numbers from right to left we get a scale of one-eighth inch equals a foot. Note the space beyond the 0 point. In the quarter-inch scale, it is divided into 12 parts representing inches; in the eighth-inch scale it is divided into 4 parts representing 3 inches each. Now verify the length of the line marked 12' 5" on the quarter scale and the line marked 17' 3" on the eighth scale, remembering to read the upper numbers from right to left. Can you tell the length of line G on the quarter scale, and the lengths of lines H and J on the eighth scale?

Scale C shows an engineer's scale of 30 feet to the inch. Each division is a foot; therefore, you can easily verify the length of the line marked 147' and tell the length of line K.

There are scores of different scales such as ¾ and a inch, 1½ and 3 inch, full size, half size, quarter size, 10, 20, 30, 40, 50, 60, and 100 feet to the inch, and many others. In doing shop drawing an ordinary ruler will do, for if you are working half or quarter-size it is only necessary to divide your reading by that fraction. You may also use the half or quarter-inch scale, reading it in inches instead of feet.

Fig. 7

In using scales, always read the mark directly from above, and use a very sharp needle-point pencil to make the tiny dot. Some draftsmen make little arrows pointing to the particular mark.

In order to do the following problems and all others in scale drawing, you should get a triangular scale (see Figure 12 on page 6). This contains 12 different scales and may be purchased at any art store.

The vital importance of accurate scale drawing cannot be overemphasized, YOU CANNOT HOPE TO DO MECHANICAL DRAWING UNLESS YOU CAN DRAW ACCURATELY TO ANY SCALE ! Be sure that you know how to do this before continuing.

After you have done the few problems in Lesson 5, study the drawings on the three pages following. Note the very crude drawing of the bookcase and the lamp, and the finished scale drawings of them. The bookcase is drawn to the half-inch scale—a half inch equals a foot—and the lamp is drawn to the quarter-size scale—a quarter inch equals an inch. Take the half-inch scale and verify the dimensions on the bookcase, and take the quarter-inch scale (remembering that a quarter inch equals an inch and not a foot in this case), and verify the dimensions on the lamp. Do the same for the drawings on the other two pages. After you have convinced yourself that you understand scale drawing, do the four problems on page 21 in Lesson 6.

As a draftsman, you will receive roughs from your superior. These may resemble the roughs shown on the next three pages. It will be up to you to draw them neatly and accurately to scale.

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