Principle of Engineering drawings: Drawing Sheet, Sheet Sizes, Designation of Drawing, Title Block, Border and Frame, Grid Reference System ( Zoning), Triming Marks, Scale

Principle of Engineering drawings

Engineering drawings are to be prepared on standard size drawing sheets. The correct shape and size of the object can be visualised from the understanding of not only the views of it but also from the various types of lines used, dimensions, notes, scale, etc. To provide the correct information about the drawings to all the people concerned, the drawings must be prepared, following certain standard practices, as recommended by Bureau of Indian Standards (BIS).

Drawing Sheet

Engineering drawings are prepared on drawing sheets of standard sizes. The use of standard size sheet, saves paper and facilitates convenient storage of drawings.

Sheet Sizes

The basic principles involved in arriving at the sizes of drawing sheets are:

(a) X : Y = 1 : 2 , 

(b) XY = 1

where X and Y are the sides of the sheet.

For a reference size A0 (Table ) having a surface area of 1 square metre, X = 841 mm and Y = 1189 mm. 

The successive format sizes are obtained either by halving along the length or doubling along the width, the areas being in the ratio 1:2.

Drawing sheet formats
Drawing sheet formats





Designation of Drawing

The original drawing should be made on the smallest sheet, permitting the necessary clarity and resolution.The preferred sizes according to ISO-A series (First choice) of the drawing sheets are given in Table. 

Preferred drawing sheet sizes (First choice) ISO-A Series




When sheets of greater length are needed, special elongated sizes (Second choice) are used (Table ). These sizes are obtained by extending the shorter sides of format of the ISO-A series to lengths that are multiples of the shorter sides of the chosen basic format.

Special elongated sizes (Second choice)





Title Block

The title block should lie within the drawing space such that, the location of it, containing the identification of the drawing, is at the bottom right hand corner. This must be followed, both for sheets positioned horizontally or vertically 

Location of title block
Location of title block





The direction of viewing of the title block should correspond in general with that of the drawing. The title block can have a maximum length of 170 mm. Figure shows a typical title block, providing the following information:

(i) Title of the drawing

(ii) Sheet number

(iii) Scale

(iv) Symbol, denoting the method of projection

(v) Name of the firm

(vi) Initials of staff drawn, checked and approved.

NOTE: According to Bureau of Indian Standards, SP-46:1998, ‘‘Engineering Drawing Practice for Schools and Colleges’’, First angle projection is preferred.

Border and Frame

Borders enclosed by the edges of the trimmed sheet and the frame, limiting the drawing space, should be provided with all sheet sizes. It is recommended that these borders have a minimum width of 20 mm for the sizes A0 and A1 and a minimum width of 10 mm for the sizes A2, A3 and A4 (Fig.). A filing margin for taking perforations, may be provided on the edge, far left of the title block.


Details in title block in machine drawing
Details in title block
Drawing sheet layout
Drawing sheet layout




Centering Marks

Four centring marks may be provided, in order to facilitate positioning of the drawing when reproduced or microfilmed. Two orientation marks may be provided to indicate the orientation of the drawing sheet on the drawing board.

Metric Reference Graduation

It is recommended to provide a figure-less metric reference graduation, with a minimum length of 100 mm and divided into 10 intervals on all the drawing sheets (Fig.) which are intended to be microfilmed. The metric reference graduation may be disposed symmetrically about a centring mark, near the frame at the border, with a minimum width of 5 mm.

Grid Reference System ( Zoning)

The provision of a grid reference system is recommended for all the sizes, in order to permit easy location on the drawing of details, additions, modifications, etc. The number of divisions should be divisible by two and be chosen in relation to the complexity of the drawing. It is recommended that the length of any side of the grid should not be less than 25 mm and not more than 75 mm. The rectangles of the grid should be referenced by means of capital letters along one edge and numerals along the other edge, as shown in Fig. The numbering direction may start at the sheet corner opposite to the title block and be repeated on the opposite sides.

Triming Marks

Trimming marks may be provided in the borders at the four corners of the sheet, in order to facilitate trimming. These marks may be in the form of right angled isosceles triangles or two short strokes at each corner (Fig).

Scale

Scale is the ratio of the linear dimension of an element of an object as represented in the drawing, to the real linear dimension of the same element of the object itself. Wherever possible, it is desirable to make full size drawings, so as to represent true shapes and sizes. If this is not practicable, the largest possible scale should be used. While drawing very small objects, such as watch components and other similar objects, it is advisable to use enlarging scales.

Designation

The complete designation of a scale should consist of the word Scale, followed by the indication of its ratio as:

SCALE 1 : 1 for full size,

SCALE × : 1 for enlarged scales,

SCALE 1 : × for reduced scales.

The designation of the scale used on the drawing should be shown in the title block.

Recommended Scales

The recommended scales for use on technical drawings are given in Table. The scale and the size of the object in turn, will decide the size of the drawing.

Recommended scales




Scale Specification

If all drawings are made to the same scale, the scale should be indicated in or near the title block. Where it is necessary to use more than one scale on a drawing, the main scale only should be shown in the title block and all the other scales, adjacent to the item reference number of the part concerned or near the drawings.




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