GEOMETRICAL CONSTRUCTIONS

          In this chapter, we deal with problems on Geometrical construction, which are mostly based on plane geometry and which are very essential in the preparation of Engineering Drawings.

They are:

1) Bisecting a Line

2) To draw Perpendiculars

3) To draw Parallel Lines

4) To divide a Line

5) To divide a Circle

6) To Bisect an Angle or Arc

7) To Trisect an Angle

8) To find the centre of an Arc

9) To construct an Ogee (or) Reverse curve

10) To construct Equilateral triangles

11) To construct Squares

12) To construct Regular Polygons

13) Special method of drawing Regular Polygons

14) Regular polygons inscribed in circles

15) To draw regular figures using T-square and set-squares

16) To draw Tangents

17)  Lengths of Arcs

18) Circles and Lines in contact

19) Inscribed Circles.

TO DRAW PARALLEL LINE -- To draw a line through a point and parallel to a given straight line

A) To draw a line through a given point, parallel to a given straight line

1. Let AB be the given line and P be the given point at a distance.

2. With centre P any convenient radius, draw an arc CD cutting AB at E.

3. With the same radius, from point E as centre draw an arc cutting AB at F.

4. Point E as centre and radius equals to FP, draw an arc cutting CD at Q.

5. Draw a line connecting PQ, This is the required line parallel to AB.

 

B) To draw a line parallel to, and at a given distance from a straight line

1. Let AB be the given line and R is the given radius.

2. Mark points P and Q on line AB, as far apart as convenient.

3. By taking R as radius draw an arc C from given point P.

4. with same radius draw another arc D from point Q.

5. Draw the line CD, just touching the two arc's, now this becomes the parallel line to the given line AB.

TO DRAW A PERPENDICULAR TO A GIVEN LINE FROM A POINT OUTSIDE IT (AWAY FROM IT)

 (A) When the point is nearer the centre

(i) Let AB be the line and P be the point.

(ii) With centre P and any convenient radius draw an arc cutting AB at C and D.

(iii) Take any radius greater than half the length of CD in compass, and with centres C and D draw the arcs intersecting each other at E.

(iv) Draw a line connecting P and E, this line cuts the line AB at Q. 

(v) Then PQ is the required perpendicular to AB line.

(B) When the point is nearer to the end of line

(i) Let AB be the line and P be the point.

(ii) With centre A and radius equal to AP, draw an arc cutting AB at C.

(iii) with centre C and radius equal to CP, draw an arc cutting previously drawn arc at D.

(iv)  Draw a line joining P and D and intersecting AB at Q.

(v) then PQ is the required perpendicular.

 

To draw a continues curve of circular arcs passing through any number of parts not in straight lines

 1) Let us consider 3 points A, B, C be given points which are not present in straight line. 

2) Draw lines connecting A with B, B with C.

3) Draw perpendicular bisectors to lines AB, BC

4) These two perpendicular bisectors intersect at point ‘O’

5) With O as centre and radious equal to OA, draw an arc passing through points A, B, C

TO FIND CENTRE OF AN ARC

Method 2:

Problem: To find the centre of an Arc

1. Draw an arc AB.

2. Draw a line (Chord) that connects the two points of the arc.

3. Set your compass more than half of the length of line and place compass on one end of the line ( or arc) and draw arc's above and below the arc AB.

4. Keeping the same distance set on your compass, swing arcs from another end of the line, draw arcs above and below the arc AB cutting previously drawn arc's.

5. Draw a line through the two intersection points of the arcs.

5. It will cut the line AB at 'O', This is the centre of the chord.

TRISECTING A Right angle

           An angle Trisector divides an angle into three equal angles. If the angle is 'θ', the three angles made after trisecting will be 'θ/3'.

Problem: To Trisect a Right Angle ABC

1. Let us consider ∠ABC is right angle, B as vortex,

2. Take a compass and draw an arc cutting Horizontal and vertical lines at D and E as shown.

3. With same radious on compass from point E scribe an arc that cuts previously drawn arc at F, and from point D scribe an arc and it will cut the previously drawn arc at point G. 

4. Then join the points BF, BG 

5. These will form the trisector of the given right angle.

Manually Bisecting a Circular Arc

          In Geometry, bisecting an arc is cutting arc exactly in half.

Problem: To bisect an Arc

1. Draw an arc AB.

2. Draw a line (Chord) that connects the two points of the arc.

3. Set your compass more than half of the length of line and place compass on one end of the line ( or arc) and draw arc's above and below the arc AB.

4. Keeping the same distance set on your compass, swing arcs from another end of the line, draw arcs above and below the arc AB cutting previously drawn arc's.

5. Draw a line through the two intersection points of the arcs.

Bisecting a line

          In Geometry, Bisecting a line is cutting a line exactly in half. It may also be referred to as constructing a perpendicular bisector as the line you are drawing will be at a right angle to the original line. A line that passes through the mid point of the line segment is known as the segment Bisector.

Problem: To bisect a given straight line AB

1. Draw a straight line of given dimension and mark start and end points as A, B.

2.  To bisect this line, set your compass to just more than half the length of the line. Keep your compass this size for the rest of the question.

3. Place compass on one end of the line (i.e., point B) and draw an arc that crosses the line AB. Do not adjust the compass and keep the same length.

4. Place compass on the other end of the line (i.e., point A) and draw an arc. It must cut the other arc above and below the line. Mark the point of intersections as C,D.

5. Draw a straight line through where the two arcs intersect above and below the line (i.e., C,D). It will cut the line AB at O. CD bisects AB at right angles. 

6. Measure both sides of the line from 'O' check that they are of equal length, if they are equal we did it correctly.