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.

 

PLACING OF DIMENSIONS

          It is necessary to indicate the size, and other details of the object for the construction and function, using lines, numerals, symbols, notes etc, we indicate the details in a drawing by proper dimensioning. Dimensions are written either above the dimension lines or inserted at the middle by breaking the dimension lines.

Normally dimensions are placed according to either of the following two systems:

1) Aligned System and

2) Unidirectional system

1) Aligned system:

          In Aligned system, the dimension is placed perpendicular to the dimension line in such a way that it may be read from the bottom edge or the right-hand edge of the drawing sheet. The dimension should be placed near the middle and above, clear of the dimension lines.

2) Unidirectional system:

          In unidirectional system, all the dimensions are placed in one direction such that they can be read from the bottom of the drawing. In this system, the dimension lines are broken near the middle for inserting the dimensions. This system is mainly used on large drawings -- as of automobiles etc., where it is inconvenient to read dimensions from the right-hand side.