Set Squares

The set squares are triangular in shape with one of the angle as right angle and made of wood, tin, celluloid or plastic. Set squares made of transparent celluloid are the most satisfactory ones as the lines underneath them can be seen quite easily. Set squares of different sizes are available in the market. Set squares are used for drawing all straight lines except the horizontal lines which are drawn with T-square or mini drafter. Generally, two types of set squares are in use. They are:

(i) Thirty-Sixty degree (30⁰-60⁰) set square and

(ii) Forty-five degrees (45⁰) set square

The 30⁰-60⁰ set square of 250 mm length has three angles with the measures of 30⁰, 60⁰ and 90⁰ respectively. Similarly, the 45⁰ set square of 200 mm length has three angles with the measures of 45⁰ and 90⁰ respectively. Those made of transparent celluloid or plastic are commonly used as they retain their shape and accuracy for a longer time. Set squares sometimes lose their accuracy due to internal strains. So they should be tested periodically. Sometimes set squares have French curves.

Fig. 1 45⁰ Set Square

Fig. 2 30°-60° Set Square

Uses

i) Set-squares are used for drawing all straight lines except the horizontal lines which are usually drawn with the T-square. Vertical lines can be drawn with the T-square and the set-square.

ii) In combination with the T-square, lines at 30° or 60° angle with vertical or horizontal lines can be drawn with 30°- 60° set-square and 45° angle with 45° set-square. The two set-squares used simultaneously along with the T-square will produce lines making angles of 15°, 75°, 105° etc. as shown in Fig. 3 and 4. In general, set-squares are used to draw angles of 15°, 30°, 45°, 60°, 75°, 90°, i.e, any multiples of 15°.

Fig. 3 Drawing lines at 30°, 60° and 90°

Fig. 4 Drawing lines at 15°, 75° and 105°

iii) Parallel straight lines in any position, not very far apart, as well as lines perpendicular to any line from any given point within or outside it, can also be drawn with the two set-squares.

iv) A circle can be divided in six, eight, twelve and twenty-four equal parts by using set-squares and T-square.