Engineering drawings are to be prepared on standard size drawing sheets. The correct shape and size of the object can be visualized 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).
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 (See Table – Preferred drawing sheet sizes (First choice) ISO-A Series) having a surface area of 1 m2, 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 (Refer Figure “Drawing sheet formats”).
- Designation of Sizes – 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 above table. When sheets of greater length are needed, special elongated sizes (Second choice) are used (See Table – Special elongated sizes (Second choice)). 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.
- 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.
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. “Details in title block” figure below shows a typical title block, providing the following information:
(i) Title of the drawing
(ii) Sheet number
(iv) Symbol, denoting the method of projection
(v) Name of the firm
(vi) Initials of staff drawn, checked and approved
- Borders and Frames – 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. A filing margin for taking perforations, may be provided on the edge, far left of the title block.
- Centring 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 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. The numbering direction may start at the sheet corner opposite to the title block and be repeated on the opposite sides.
- Trimming 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.