1. ENGN103 Engineering Drawing
Lecture 3

2. SCALE
When the object is bigger than the size of drawing sheet, it is drawn at reduced scale.
Therefore, scale is the ration of actual object and the size shown on the drawing. The scales are designated as:
SCALE 1: X for reduced scale (X is the ratio)
SCALE X:1 for enlarged scale
SCALE 1: for full size
Standard recommended ratios are multiples of 2, 5 and 10 as:
Enlargement: 50:1, 20:1, 10:1, 5:1, 2:1
Reduction: 1:2, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500, 1:1000, 1:2000, 1:5000

6. Geometrical Construction
To bisect a line
To divide a line into a number of equal parts
To divide a line in a given proportion
To bisect an angle
To find the center of an arc
To inscribe a circle in a triangle.
To draw the circumscribing circle of a triangle.

7. To bisect a line
Draw the given line AB, with centers A and B and radius R greater than half of AB, draw arcs to intersect a t C and D. Join CD, when E will be the mid point of the line. Also CD will be perpendicular to AB

12. To divide a line into a number of equal parts
From one end of the given line (say A), draw AC at any convenient angle. Using dividers or a scale, mark off from A on AC the required number of equal parts, making them of any suitable length. Join the last point to B on the given line, and through the other points draw parallels to this line to cut the given line. This construction makes use of the properties of similar triangles.

21. To divide a line in a given proportion
Suppose the proportion to be 2 : 3. using the previous construction, proceed as if to divide the line into 5 parts (2 plus 3) but only draw a line through point 2 on AD. Then AB will be divided in the required proportion.

27. To bisect an angle
Draw the given angle ABC and from the apex B draw an arc of radius R to cut AB and CD at D and E. R may be any convenient radius. With D and E as centers and radius R’, draw two arcs to meet F. Again, R’ may be any convenient radius. Join FB to bisect the angle.

33. To find the center of an arc
Select three points, A, B and C on the arc and join AB and BC. Bisect these lines and produce the bisectors to meet at O. O is the center of the arc.

44. To inscribe a circle in a triangle.
Draw the given triangle ABC and bisect an two angles. Produce the bisectors to intersect at O, which is the center of the inscribe circle.

52. To draw the circumscribing circle of a triangle.
Draw the given triangle ABC. Bisect any two of the sides and produce the bisectors to intersect at O. O is the center of the circumscribing circle.