USE OF THE DRAFTING SCALEAccuracy in drawing depends
to a great extent upon correct use
of the scale in marking off
distances. You should place the edge of the scale
parallel to the line being measured (fig. 3-13). To
eliminate shadows cast by your body or hands, point
the desired scale face away from you for horizontal
measurements and toward your left for vertical
measurements. With a sharp pencil, mark
off short dashes at right angles to the scale
at the correct distances, aligning the mark carefully
with the scale graduation. Have your eye
approximately over the point being measured,
Figure 3-13.-Use of the drafting scale.
and make light marks to denote the point of measurement.
When setting the compass to a given radius or when setting divider points, never place the sharp points of these instruments on the scale. Lay out the desired radius or distance on a straight pencil line by using the scale in the manner described above. Then adjust the compass or dividers to the indicated length by using the measured line. A scale surface marred by pinpricks is difficult to read and is unsuitable for accurate work.
In making successive measurements along the same line, make as many measurements as possible without moving the scale. If a number of distances are to be laid out end to end, hold the scale in one position and add each successive measurement to the preceding one. If the scale is moved to a new position each time, slight errors in measurement may accumulate. For example, four successive measurements of 1 5/8 in. each should give an overall length of 6 1/2 in., not 6 9/16 in. Therefore, make as many measurements as you can without changing the reference point. This will avoid cumulative errors in the use of the scale.
Note that your pencil touches the scale only for the purpose of marking a point on the paper. Never use a scale as a straightedge for drawing lines. A typical office ruler has a metal edge; it is a scale and straightedge combined. But a draftsmans measuring scale is for measuring only; it is not a ruler. A scale properly used will last for decades, but a scale used as a straightedge will soon have the graduations worn away.