Engineering drawing consists of a number of geometrical constructions. A few methods are illustrated here without mathematical proofs.
1. To divide a straight line into a given number of equal parts say 5. construction (Fig. 1)
1. Draw AC at any angle e to AB.
2. Construct the required number of equal parts of convenient length on AC like 1,2,3.
3. Join the last point 5 to B
4. Through 4, 3, 2, 1 draw lines parallel to 5B to intersect AB at 4′,3′,2′ and 1′.
2. To divide a line in the ratio 1 : 3 : 4. construction (Fig. 2)
As the line is to be divided in the ratio 1:3:4 it has to be divided into 8 equal divisions. By following the previous example divide AC into 8 equal parts and obtain P and Q to divide the line AB in the ratio 1:3:4.
3. To bisect a given angle. construction (Fig. 3)
1. Draw a line AB and AC making the given angle.
2. With centre A and any convenient radius R draw an arc intersecting the sides at D and E.
3. With centres D and E and radius larger than half the chord length DE, draw arcs intersecting at F
4. Join AF, <BAF = <PAC.
4. To inscribe a square in a given circle. construction (Fig. 4)
1. With centre 0, draw a circle of diameter D.
2. Through the centre 0, drwaw two diameters, say AC and BD at right angle to each other.
3. ,Join A-B, B-C, C- D, and D-A. ABCD is the required square.
5. To inscribe a regular polygon of any number of sides in a given circle. construction (Fig. 5)
1. Draw the given circle with AD as diameter.
2. Divide the diameter AD into N equal parts say 6.
3. With AD as radius and A and D as centres, draw arcs intersecting each other at G
4. Join G-2 and extend to intersect the circle at B.
5. Join A-B which is the length of the side of the required polygon.
6. Set the compass to the length AB and strating from B mark off on the circumference of the circles, obtaining the points C, D, etc.
The figure obtained by joing the points A,B, C etc., is the required polygon.
6. To inscribe a hexagon in a given circle.
(a) Construction (Fig. 4.6) by using a set-square or mini-draughter
1. With centre 0 and radius R draw the given crcle.
2. Draw any diameter AD to the circle.
3. Using 30° – 60° set-square and through the point A draw lines AI, A2 at an angle 60° with AD, intesecting the circle at B and F respectively.
4. Using 30° – 60° and through the point D draw lines Dl, D2 at an angle 60° with DA, intersecting the circle at C and E respectively. By joining A,B,C,D,E,F, and A the required hexagon is obtained.
(b) Construction (Fig. 7) By using compass
1. With centre 0 and radius R draw the given circle.
2. Draw any diameter AD to the circle.
3. With centres A and D and radius equal to the radius of the circle draw arcs intesecting the circles at B, F, C and E respectively.
4. ABC D E F is the required hexagon.
7. To circumscribe a hexagon on a given circle of radius R construction (Fig. 8)
I. With centre 0 and radius R draw the given circle.
2. Using 60° position of the mini draughter or 30-60set square, circumscribe the hexagon as shown.
8. To construct a hexagon, given the length of the side.
(a) contruction (Fig. 9) Using set square
1. Draw a line AB equal to the side of the hexagon.
2. Using 30° – 60° set-square draw lines AI, A2, and BI, B2.
3. Through 0, the point of intersection between the lines A2 at D and B2 at E.
4. loin D,E
5. ABC D E F is the required hexagon.
(b) By using compass (Fig. 10)
1. Draw a line AB equal to the of side of the hexagon.
2. With centres A and B and radius AB, draw arcs intersecting at 0, the centre of the hexagon.
3. With centres 0 and B and radius OB (=AB) draw arcs intersecting at C.
4. Obtain points D, E and F in a similar manner.
9. To construct a regular polygon (say a pentagon) given the length of the side. construction (Fig.11)
1. Draw a line AB equal to the side and extend to P such that AB = BP
2. Draw a semicircle on AP and divide it into 5 equal parts by trial and error.
3. Join B to second division
2. Irrespective of the number of sides of the polygon B is always joined to the second division.
4. Draw the perpendicular bisectors of AB and B2 to intersect at O.
5. Draw a circle with 0 as centre and OB as radius.
6. With AB as radius intersect the circle successively at D and E. Then join CD. DE and EA.
10. To construct a regular polygon (say a hexagon) given the side AB – alternate method. construction (Fig. 12)
1. Steps 1 to 3 are same as above
2. Join B- 3, B-4, B-5 and produce them.
3. With 2 as centre and radius AB intersect the line B, 3 produced at D. Similarly get the point E and F.
4. Join 2- D, D-E, E-F and F-A to get the required hexagon.
11. To construct a pentagon, given the length of side.
(a) Construction (Fig. 13a)
1. Draw a line AB equal to the given length of side.
2. Bisect AB at P.
3. Draw a line BQ equal to AB in length and perpendicular to AB.
4. With centre P and radius PQ, draw an arc intersecting AB produced at R. AR is equal to the diagonal length of the pentagon.
S. With centres A and B and radii AR and AB respectively draw arcs intersecting at C.
6. With centres A and B and radius AR draw arcs intersecting at D.
7. With centres A and B and radii AB and AR respectively draw arcs intersecting at E. ABCDE is the required pentagon.
(b)By included angle method
1. Draw a line AB equal to the length of the given side.
2. Draw a line B 1 such that <AB 1 = 108° (included angle)
3. Mark Con Bl such that BC = AB
4. Repeat steps 2 and 3 and complete the pentagon ABCDE
12. To construct a regular figure of given side length and of N sides on a straight line. construction (Fig 14)
1. Draw the given straight line AB.
2. At B erect a perpendicular BC equal in length to AB.
3. Join AC and where it cuts the perpendicular bisector of AB, number the point 4.
4. Complete the square ABeD of which AC is the diagonal.
S. With radius AB and centre B describe arc AC as shown.
6. Where this arc cuts the vertical centre line number the point 6.
7. This is the centre of a circle inside which a hexagon of side AB can now be drawn.
8. Bisect the distance 4-6 on the vertical centre line.
9. Mark this bisection 5. This is the centre in which a regular pentagon of side AB can now be drawn.
10. On the vertical centre line step off from point 6 a distance equal in length to the distance 5-6. This is the centre of a circle in which a regular heptagon of side AB can now be drawn.
11. If further distances 5-6 are now stepped off along the vertical centre line and are numbered consecutively, each will be the centre of a circle in which a regular polygon can be inscribed with sice of length AB and with a number of sides denoted by the number against the centre.
13. To inscribe a square in a triangle. construction (Fig. 15)
1. Draw the given triangle ABC.
2. From C drop a perpendicular to cut the base AB at D.
3. From C draw CE parallel toAB and equal in length to CD.
4. Draw AE and where it cuts the line CB mark F.
5. From F draw FG parallel to AB.
6. From F draw F J parallel to CD.
7. From G draw GH parallel to CD.
8. Join H to 1. Then HJFG is the required square.
14. To inscribe within a given square ABCD, another square, one angle of the required square to touch a side of the given square at a given point construction (Fig 16)
1. Draw the given square ABeD.
2. Draw the diagonals and where they intersect mark the point O.
3. Mark the given point E on the line AB.
4. With centre 0 and radius OE, draw a circle.
S. Where the circle cuts the given square mark the points G, H, and F.
6. Join the points GHFE. Then GHFE is the required square.
15. To draw an arc of given radius touching two straight lines at right angles to each other. construction (Fig 17)
Let r be the given radius and AB and AC the given straight lines. With A as centre and radius equal to r draw arcs cutting AB at P and Q. With P and Q as centres draw arcs to meet at O. With 0 as centre and radius equal to r draw the required arc.
16. To draw an arc of a given radius, touching two given straight lines making an angle between them. construction (Fig 18)
Let AB and CD be the two straight lines and r, the radius. Draw a line PQ parallel to AB at a distance r from AB. Similarly, draw a line RS parallel to CD. Extend them to meet at O. With 0 as centre and radius equal to r draw the arc to the two given lines.
17. To draw a tangent to a circle construction (Fig 19 a and b)
(a) At any point P on the circle.
1. With 0 as centre, draw the given circle. P is any point on the circle at which tangent to be drawn (Fig l6a)
2. Join 0 with P and produce it to pI so that OP = PP1
3. With 0 and P1 as centres and a length greater than OP as radius, draw arcs intersecting each other at Q.
4. Draw a line through P and Q. This line is the required tangent that will be perpendicular to OP at P.
(b) From any point outside the circle.
1. With 0 as centre, draw the given circle. P is the point outside the circle from which tangent is to be drawn to the circle (F ig 16b).
2. Join 0 with P. With OP as diameter, draw a semi-circle intersecting the given circle at M. Then, the line drawn through P and M is the required tangent.
3. If the semi-circle is drawn on the other side, it will cut the given circle at M1. Then the line through P and M1 will also be a tangent to the circle from P.