DIMENSIONS

DIMENSIONS

What is a leader or pointer line? How a leader should be drawn?

        A leader is a thin continuous line drawn from note of the figure to show where it applies. It is terminated by an arrow head or a dot. The arrow head touches the outline, whereas the dot is placed within the outline of the object. The leader is generally drawn at any convenient angle, usually 30º, 45º, and 60º but not less than 30º.

                                             

Explain the (i) size dimensions (ii) location dimensions.

        Size dimension: The dimensions which indicate the various sizes of the object such as length, breadth, diameter etc. are known as size dimensions. These dimensions are represented by letter ‘S’.

        Location dimension: The dimensions which locate the position of one feature with respect to the other feature are known as location dimensions. Distances between the centre lines of the holes from the edges are given by location dimensions. These dimensions are marked by letter ‘L’.

                                           

What are the different methods of dimensioning?

         Aligned Method: In aligned system, the dimensions shall be placed parallel to and above the dimension lines, preferably in the middle and not by interrupting the dimension lines. Here the dimensions can be read from the bottom or from the right side of the drawing. Fig. (a)

        Uni-directional Method: In uni-directional system dimensions shall be horizontally placed so that they can be read from the bottom of the drawing sheet. Here the dimension lines may be interrupted preferably near the middle for the insertion of dimensions. Fig. (b)

                       

                    Fig. (a)                                         Fig. (b)

What are the general rules of dimensioning?

1) Every dimension must be given, but no single dimension should  be repeated.

2) Dimensions should be placed outside the views.

3) Avoid dimensioning to hidden lines wherever possible.

4) Dimension lines should not cross any other line of the drawing.

5) Aligned system of dimensioning is recommended.

Explain with the help of sketches (i) chain dimensioning

(ii) parallel dimensioning and (iii) combined dimensioning.

Chain Dimensioning – In this system, dimensions are arranged in a straight line.

                                                                          

Parallel dimensioning – In this arrangement, all the dimensions are given from common base line. The smaller dimensions are placed nearer the view and the larger further away so that the extension lines do not cross dimensions lines.

Combined dimensioning – Combined dimensioning is the result of the simultaneous use of chain and parallel dimensioning.

                                                                                               

Explain with simple sketches, the methods of dimensioning of circles?

        Circular features are often used in engineering, for example in cylinders, holes, or a series of holes in a component. Sometimes a component has a circular shape.

        Dimensions are needed to convey information to the person making the part so they make the correct sized cylinder or hole.

         What's needed is the diameter of the circular feature - the distance from one side of the circumference to the other through the centre point.

        Dimensions for the diameter of circular features begin with this symbol – Ø

      Dimensions of circular features can be written in several ways. 

Aligned:  Fig. (a) Where the dimension is shown in the same direction as the dimension line.

Uni-directionally: Fig. (b) Where the dimension is shown parallel to the bottom of the drawing sheet.

From the side view and uni-directionally:

         Note the use of projection lines extending from the object but not touching it. These help clarify the shape, keep the diagram neat and the information readable.

From the side view and aligned:

From the top or end view and uni-directionally:

The diagram shows multiple diameters.

Projection lines extending outside the circle.

Dimension lines that give the dimensioning information uni-directionally. 

For small diameters:

        Here the dimensioning information needs to sit outside the circle. This can be achieved in several ways: 

 (a) uses a leader line and a dimension line

 (b) uses projection lines and a dimension line

 (c) uses a leader line which must be in line with the centre of the circle.

                       

                

Composed by:  R.Satheesh, M.E., Asst Prof., email: rsatheeshemail@gmail.com.