Compass and Divider

Compass

Compass is used for drawing circles and arcs of required diameter. It consists of two metal legs hinged together at its upper end by means of joint known as knee joint. An adjustable or fixed needle is fitted on to the end of one of the legs whereas the other leg is provided with an attachment which can be fitted with a pencil lid or pencil depending upon the nature of attachment. There are different types of compass available in the market depending upon their sizes, such as

i) Large Size Compass

ii) Small Size Compass

Circles up to 120 mm diameters are drawn by keeping the legs of compass straight. For drawing circles more than 150 mm radius, a lengthening bar is used. It is advisable to keep the needle end about 1mm long compared to that of pencil end so that while drawing circles, when the needle end is pressed it goes inside the drawing sheet by a small distance (approximately 1mm). Curves drawn with the compass should be of the same darkness as that of the straight lines. It is difficult to exert the same amount of pressure on the lead in the compass as on a pencil. It is, therefore, desirable to use slightly softer variety of lead about one grade lower in the compass than the pencil used for drawing straight lines, to maintain uniform darkness in all the lines. As a rule, while drawing concentric circles, small circles should be drawn first before the centre hole gets worn.

Fig. 1 Compass

1) Large-Size Compass with Interchangeable Pencil and Pen Legs

It consists of two legs hinged together at its upper end. A pointed needle is fitted at the lower end of one leg, while a pencil lead is inserted at the end of the other leg. The lower part of the pencil leg is detachable and it can be interchanged with a similar piece containing an inking pen. Both the legs are provided with knee joints. Circles upto about 120 mm diameter can be drawn with the legs of the compass kept straight. For drawing larger circles, both the legs should be bent at the knee joints so that they are perpendicular to the surface of the paper.

To draw a circle, adjust the opening of the legs of the compass to the required radius. Hold the compass with the thumb and the first two fingers of the right hand and place the needle point lightly on the centre, with the help of the left hand. Bring the pencil point down on the paper and swing the compass about the needle-leg with a twist of the thumb and the two fingers, in clockwise direction, until the circle is completed. The compass should be kept slightly inclined in the direction of its rotation. While drawing concentric circles, beginning should be made with the smallest circle.

Fig. 2 Large-Size Compass with Interchangeable Pencil and Pen Legs

2) Lengthening Bar

Circles of more than 150 mm radius are drawn with the aid of the lengthening bar. The lower part of the pencil leg is detached and the lengthening bar is inserted in its place. The detached part is then fitted at the end of the lengthening bar, thus increasing the length of the pencil leg. It is often necessary to guide the pencil leg with the other hand, while drawing large circles.

Fig. 3 Lengthening Bar

3) Small Bow Compass

For drawing small circles and arcs of less than 25 mm radius and particularly, when a large number of small circles of the same diameter are to be drawn, small bow compass is used.

Fig. 4 Small Bow Compass

Divider

Dividers are used to transfer lengths to the drawings either from scales or from the drawing itself. Dividers are similar to the compass and are made in square, flat and round forms. However, unlike compass, a divider is provided with two needles on both the legs. Similar to the compasses, two sizes of dividers are used in technical drawings. One large divider and the other small spring bow divider. They are very convenient for setting-off points at equal distances around a given point or along a given line. The dividers are used for the following purpose.

1. To divide curved or straight lines into desired number of equal parts,
2. To transfer dimensions from one part of the drawing to another part
3. To set-off given distances from the scale to the drawing.

Fig. 5 Divider

1) Large-Size Divider

It is used to transfer dimensions and dividing lines into a number of equal parts. The divider has two legs hinged at the upper end and is provided with steel pins at both the lower ends, but it does not have the knee joints. In most of the instrument boxes, a needle attachment is also provided which can be interchanged with the pencil part of the compass, thus converting it into a divider.

2) Small Bow Divider

The small bow divider is adjusted by a nut and is very convenient for marking minute divisions and large number of short equal distances.

Bow Instruments

Bow pencil and bow pen compass are used for drawing circles of approximately 25 mm radius. Bow divider is used for marking or dividing smaller spaces. There are two types

(i) Integral legs with spring action

(ii) Two legs held with a curved spring on top with handle on it.

Bow instruments may have adjusting wheel and nut. To draw circles, it is better to mark the required distance separately and set the instruments and check. Then only the circles or arcs should be drawn on the drawing. Adjustment should be made with the thumb and middle finger. The instrument is manipulated by twisting the knurled head between the thumb and finger.

Fig. 6 Bow Instruments

Drop Spring Bow Pencil and Pen

Drop spring bow pencil and pen are designed for drawing multiple identical small circles. Example: Rivet holes, Drilled/reamed holes etc. The central pin is made to move freely up and down through the tube attached to the pen or pencil unit. It is used by holding the knurled head of the tube between thumb and middle finger while the index finger is placed on the top of the pin. The pin point is placed on the centre point of the circle to be drawn and pencil or pen is lowered until it touches paper. The instrument is turned clockwise and the circle is drawn.

Fig. 6 Drop Spring Bow Instruments