A drawing of an object is prepared to define its shape and to specify its size. The shape description is based on projection and the size description on dimensioning. Every drawing must give its complete size description stating length, width, thickness, diameter of holes, grooves, angles, etc. and such other details relating to its construction. To give all those measurements and information describing the size of the object in the drawing is called dimensioning.
Placing of Dimensions
Dimensions should be placed on the view, which shows the relevant features most clearly. The two recommended systems of placing the dimensions are:
- Aligned System. In this system, all dimensions are so placed that these may be read from the bottom or the right-hand edge of the drawing sheet. All dimensions should be placed above the dimension lines. (Refer Fig. 1)
- Unidirectional System. In this system, all dimensions are so placed that these may be read from the bottom edge of the drawing sheet. In this system, there is no restriction controlling the direction of dimension lines. This system is advantageous on large drawings, where it is inconvenient to read dimensions from the right hand side. In this method, all dimension lines are interrupted, preferably near the middle for the insertion of the dimension value. (Refer Fig. 2)
General Principles of Dimensioning
- As far as possible, all the dimensions for one particular operation shall be specified in one view only, such as diameter and depth of drilled hole, or size and depth of a threaded hole, etc.
- Normally dimensions should be placed outside the views, Fig 3 but if it is not possible it may be placed within the view as shown in Fig 4. However, dimensions should not be placed within a view unless drawing becomes clear by doing so. Dimensions should not be placed too close to each other or to the parts being dimensioned.
- Dimensions are to be given from visible outlines, rather than from hidden lines, Fig 5. Dimensions are to be given from a base line, a centre line of the hole, a cylindrical part, important hole or a finished surface which may be readily established, based on design requirements and the relationship to other parts. (Refer Fig 6 & 7)
- Dimensions for different operations on a part, for example, drilling and bending, should be given separately as in Fig 8, if permissible by its design.
- An axis or a contour line should never be used as a dimension line but may be used as a projection line. (Refer Fig 9)
- The intersection of dimension line should be avoided as far as possible if, however, the intersection of two dimension lines is unavoidable, the lines should not be broken. Dimension lines may be broken for inserting the dimension in the case of unidirectional dimensioning. (Refer Fig 2)
- Overall dimensions should be placed outside the intermediate dimensions. Where an overall dimension is shown, one of the intermediate dimensions is redundant and should not be dimensioned. (Refer Fig 10).
Drawing of very big objects cannot be prepared in full size because these would be too big to accommodate on the drawing sheet. Drawings of very small objects also cannot be prepared in full size because these would be too small to draw and to read. A convenient scale is chosen to prepare the drawings of big as well as small objects in proportionately smaller or larger size. Therefore, scales are used to prepare a drawing at a full size, reduced size or enlarged size.
The ratio of the size of the drawing to the size of the object is known as the representative fraction. It is denoted as RF.
The Representative Fraction (RF) when given in terms of ratio is known as Representative Ratio (RR). If the length of an object is one meter and it is represented on the drawing by a line one centimeter long, then
RF = (1 cm) / (1 m) = (1 cm) / (1X 100 cm) = 1/100
In terms of Representative Ratio, RR = 1:100
The scales recommended for use in engineering drawing by IS: 10713-1983 are as follows:
- Full Size Scale. When the size of the drawing and the object is the same, then it is known as full-size scale, i.e. 1:1.
- Reduced Scale. When the drawings are smaller in size than the actual objects, the reduced scales are used. Recommended scales are 1:2, 1:5, 1:10, 1:20, 1:50, 1:100, etc.
- Enlarged Scale. When the drawing to be drawn is larger than the actual objects, the enlarged scales are used. The recommended scales are 50:1, 20:1, 10:1, 5:1, 2:1, etc.
Layout of Drawing Sheet
- Sheet Sizes. The preferred sizes of the drawing sheets recommended by the Bureau of Indian Standards (BIS) are given below as per SP: 46 (1988).
Table 1 Size of Drawing Sheet
|Sheet Designation||Trimmed size (mm)||Untrimmed size(mm)|
|A0||841 X 1189||880X1230|
The layout of the drawing on a drawing sheet should be done in such a manner so as to make its reading easy. Fig. 11 and Fig. 12 show an A1 size sheet layout. All dimensions are in millimeters. A full size drawing paper is normally of 565 mm x 765 mm size.
- Margin is provided in the drawing sheet by drawing margin lines (Ref Fig 11). Prints are trimmed along these lines. After trimming, the prints would be of the recommended sizes of the trimmed sheets.
- Border Lines. Clear working space is obtained by drawing border lines as shown in Fig 11. More space is kept on the left-hand side for the purpose of filing or binding if necessary. When prints are to be preserved or stored in a cabinet without filing, equal space may be provided on all sides.
- Borders and Frames. SP: 46 (1988) recommends the borders of 20 mm width for the sheet sizes A0 and A1 and 10 mm for the sizes A2 to A5. Frame shows the clear space available for the drawing purpose.
- Orientation Mark. Four centering marks are drawn as shown in Fig 12 to facilitate positioning of the drawing for the reproduction purpose. The orientation mark will coincide with one of the centering marks which can be used for the orientation of drawing sheet on the drawing board.
- Grid Reference System (Zone system). The grid reference system is drawn on the sheet to permit easy location on the drawing such as details, alterations or additions. The rectangle of grid along the length should be referred by numerals 1, 2, 3… and along the width by the capital letters A, B, C, D, etc. as shown in Fig 12.
- Title Block. Space for the title block must be provided in the bottom right-hand corner of the drawing sheet as shown in Fig 6.3and Fig 4. The size of the title block as recommended by the BIS is 185 mm x 65 mm for all designations of the drawing sheets. Fig. 5 shows the simplest type of a title block. All title blocks should contain at least the particulars as shown in table 2.
Table 2 Title Block
|1||Name of firm|
|2||Title of the drawing|
|4||Symbol for the method of projection|
|6||Initials with dates of persons who have designed, drawn, checked standards and approved.|
|7||Sl No. of sheet and total number of sheets of the drawing of the object|