Fig. 60. The T Square
The T square is used for guiding the pencil or pen when drawing horizontal lines.
The two triangles t t ([Fig. 59]) are used for drawing vertical and oblique lines, and a pair of compasses is needed for circles and arcs of circles. Each triangle contains one right angle, the one on the left being known as a thirty-sixty triangle because the two remaining angles are thirty degrees and sixty degrees respectively. The one on the right is called a forty-five-degree triangle because it has two forty-five-degree angles.
The position of T square and triangle when drawing vertical lines is that shown in the sketch, the line being drawn from the T square upward. Horizontal lines are drawn from left to right.
The rule used in mechanical drawing is called a scale, and should not be used for drawing lines. Its purpose is measuring.
Fig. 61. The Triangles used in Mechanical Drawing
In making a drawing the first step is to determine the spacing. The size of the paper may be measured, the number of views are known, and also the size of each. The views should be so arranged that the spaces between will be in good proportion. It is a good plan to make first a free-hand sketch, putting on dimensions and figuring the spaces before beginning actual work on the mechanical drawing. [Fig. 62] at a shows a free-hand sketch of a single dovetail joint, and b the mechanical drawing complete.
