Conventions make the drawing simple and easy to draw. But it is difficult for untrained eyes to understand it easily. Drafting time should be reduced to cut drafting cost. Time must be saved in drafting. It will take a lot of time to draw the actual shape, hence, some conventions are standardized and used in the drawing to save the time. In 1935 the American Standard Association issued the first American standards, entitled “Drawing and Drafting Room Practice”. This standard advocated conventions in many ways, e.g. partial views, half views, symbols, lettering, lines, hatching lines, etc. These have been adopted as a standard convention by the Bureau of Indian Standards also.
Types of Lines
The basis of any drawing is a line. The use of a right type of line results in a correct drawing. The Bureau of Indian Standards has prescribed the types of lines in its code IS-10714-1983 to be used for making a general engineering drawing. Each line is used for a definite purpose and it should not be used for anything else. The various types of lines and their uses are described below:
- Outlines (A) – Lines drawn to represent visible edges and surface boundaries of objects are called outlines or principal lines. These are continuous thick lines.
- Margin Lines (A) – These are continuous thick lines along which the prints are trimmed.
- Dimension Lines (B) – These lines are continuous thin lines. These are terminated at the outer ends by pointed arrowheads touching the outlines, extension lines or centre lines.
- Extension or Projection Lines (B) – These lines are also continuous thin lines. They extend by about 3 mm beyond the dimension lines.
- Construction Lines (B) – These lines are drawn for constructing figures. These are shown in geometrical drawings only. These are continuous thin light lines.
- Hatching or Section Lines (B) – These lines are drawn to make the section evident. These are continuous thin lines and are drawn generally at an angle of 45 to the main outline of the section. These are uniformly spaced about 1 mm to 2 mm apart.
- Leader or Pointer Lines (B) – Leader line is drawn to connect a note with the feature to which it applies. It is a continuous thin line.
- Border Lines (B) – Perfectly rectangular working space is determined by drawing the border lines. These are continuous thin lines.
- Short-Break Lines (C) – These lines are continuous, thin and wavy. These are drawn freehand and are used to show a short break, or irregular boundaries.
- Long-Break Lines (D) – These lines are thin ruled lines with short zigzags within them. These are drawn to show long breaks.
- Hidden or Dotted Lines (E or F) – Interior or hidden edges and surfaces are shown by hidden lines. These are also called dashed lines or dotted lines. These are of medium thickness and made up of short dashes of approximately equal lengths of about 2 mm spaced at equal distances of about 1 mm. When a hidden line meets or intersects another hidden line or an outline, their point of intersection or meeting should be clearly shown.
- Centre Lines (G) – Centre lines are drawn to indicate the axes of cylindrical, conical or spherical objects or details, and also to show the centers of circles and arcs. These are thin, long, chain lines composed of alternately long and short dashes spaced approximately 1 mm apart. The longer dashes are about 6 to 8 times the short dashes which are about 1.5 mm long. Centre lines should extend for a short distance beyond the outlines to which these refer. For the purpose of dimensioning or to correlate the views these may be extended as required. The point of intersection between two centre lines must always be indicated. Locus lines, extreme positions of movable parts and pitch circles are also shown by this type of line.
- Cutting-Plane Lines (H) – The location of a cutting plane is shown by this line. It is a long, thin chain line, thick at ends only.
- Chain Thick (J) – These lines are used to indicate special treatment on the surface.
- Chain Thick Double Dashed (K) – This chain thin double dashed is used for outline for adjacent parts, alternative and extreme, position of movable part, centroidal lines, initial outlines prior to forming and part suited in front of the cutting plane.
Comparative Thickness/Grades of Lines
The thickness of lines are varied depending on whether the drawing is drawn by ink or pencil.
- Ink Drawing – The line group is designated according to the thickness of the thickest line. For any particular drawing, a line-group is selected according to its size and type. All lines should be sharp and dense so that good prints can be reproduced.
- Pencil Drawing – For drawing finalized with pencil, the lines can be divided into two line- groups. It is important to note that in the finished drawing, all lines except construction line should be dense, clean and uniform. Construction line should be drawn very thin and faint and should be hardly visible in the finished drawing.
Thick and Thin Lines – There are only two types of lines used in drawing, e.g. thick and thin lines. The ratio between the thick line and thin line should not be less than 2:1. If the thickness of thin line is 0.25 mm, then the thickness of the thick line will be 0.5 mm. Similarly the distance between two parallel thin lines (Hatching lines) is twice the thickness of the heaviest line. If the thickness of the heaviest line is 0.7 mm, then the distance between two hatching lines will be1.4 mm. The thickness of lines depends upon the size and type of drawing.
Conventional Breaks and Symbols
Long parts such as bars, shafts, pipes, etc, are generally shown broken in the middle by conventional breaks to accommodate their view of whole length without reducing the scale. The shape of the broken section is indicated either by a revolved section or more often by a same pictorial break line. The breaks used on cylindrical metal are often referred to as “S” breaks and these are drawn partly freehand or partly with irregular curves or compass. Breaks of rectangular metal and wood sections are always drawn freehand.
Conventional symbols are also used in the drawing to indicate many details such as knurl, flat surface, chain, rolled shapes, electrical apparatus, etc. Symbol of two crossed diagonals are used for two distinct purposes, first to indicate on a shaft the position of finish for a bearing and second to indicate that a certain surface is flat usually parallel to the picture plan.
The surface obtained by casting, forging or moulding operations on the work piece is rough. It is to be finished by machining operations. The surface finish or the surface texture is the amount of geometric regularity produced on the surface or a work piece. In high speed machines to withstand severe operating conditions with minimum friction and wear, a particular surface finish is essentially required. An engineer or designer must learn to note and read surface finish on the drawing. He is responsible for specifying the correct surface finish for maximum performance and service life at the lowest cost. By proper surface finish, friction and hence the wear of the two mating parts is reduced. Bearings, journals, piston pumps, cylinders, gears, sliding parts, etc are the objects which require good surface finish. Smooth finish is essentially required on high precision pieces, such as gauges. Surface finish is also important to the wear service of certain pieces subject to dry friction, such as machine tool bits, threading dies, stamping dies, rolls, clutch plates, brake drums, etc. For rack and pinion, chain-sprockets, gear meshing, etc., surface finish is required to ensure quiet operation. Smoothness is also important for the visual appearance of finished products. The degree of surface finish is a factor of cost during manufacturing.
Surface Finish Characteristics – It is not possible to produce absolutely smooth surface. All surfaces have irregularities which can be controlled during manufacturing. The characteristics of surface finish are roughness, waviness, lay and flaws. All smooth surfaces have finally spaced irregularities, in the form of peaks and valleys, called roughness. Waviness irregularities are the longer roughness variations on the surface. Lay is the primary direction of the surface pattern made by machine tool marks. Flaws are infrequent irregularities occurring at random places on the surface.
Symbols for Indicating Surface Finish
The quality of surface finish on a metal surface produced by any production method other than machining is indicated on the drawing by tick symbol. This basic symbol consists of two legs of unequal length inclined at approximately 60 to the line representing the surface to be machined with the vertex touching it. If the surface finish is to be obtained by removing the material by any of the machining processes, a horizontal bar is to be added to the basic symbol converting it into equilateral triangle. If the surface finish is to be produced without the removal of the material, or when a surface is to be left in the very state resulting from the preceding manufacturing process, whether this state was achieved by removal of the material or otherwise, a circle is inscribed in the basic symbol.
If the usual manufacturing process by themselves ensure the acceptable surface finish, the specification of the surface finish is unnecessary, hence need not be indicated. To fully define the quality of surface finish, it is necessary to indicate the different characteristics of surface roughness such as, roughness values or grades, production method, surface treatment or coating, sampling length, direction of lay, machining allowance, other roughness values along with the surface finish symbol. Therefore it is very essential to indicate the exact place for each of these characteristics in the surface finish symbol. The exact place and the method of indicating of these different characteristics in the surface finish symbol are detailed below.
Use of Symbols and Abbreviations
Symbols and abbreviations are intended for saving time and space. Some symbols and abbreviations used in Engineering Drawing are given in Table 5.6 for reference.
|Approximate||APPROX||Not to Scale||NTS|
|Chamfered||CHMED||Serial Number||Sl. No.|
|Counter Bore||C’BORE||Spot face||SF|
|Machined||M/CD||Tee (Structural section)||T|