TECH'MECH

Introduction

1. 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

2. 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. Table 1 shows the types and thickness of lines used for various purposes. Each line is used for a definite purpose and it should not be used for anything else. (Refer Fig. 1). The various types of lines and their uses are described below:

(a) Outlines (A). Lines drawn to represent visible edges and surface boundaries of objects are called outlines or principal lines. These are continuous thick lines.

(b) Margin Lines (A). These are continuous thick lines along which the prints are trimmed.

(c) 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.

(d) Extension or Projection Lines (B). These lines are also continuous thin lines. They extend by about 3 mm beyond the dimension lines.

(e) Construction Lines (B). These lines are drawn for constructing figures. These are shown in geometrical drawings only. These are continuous thin light lines.

(f) 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 45⁰ to the main outline of the section. These are uniformly spaced about 1 mm to 2 mm apart.

Table No. 1 Types of Lines

(g) 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.

(h) Border Lines (B). Perfectly rectangular working space is determined by drawing the border lines. These are continuous thin lines.

(j) 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.

(k) Long-Break Lines (D). These lines are thin ruled lines with short zigzags within them. These are drawn to show long breaks.

(l) 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.

(m) 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.

(n) 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.

(o) Chain Thick (J). These lines are used to indicate special treatment on the surface.

(p) 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.

Fig. 1 Types of Lines

Comparative Thickness/Grades of Lines

3. The thickness of lines are varied depending on whether the drawing is drawn by ink or pencil.

(a) Ink Drawing. The thickness of lines of various groups is shown in table 2. 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.

Table No. 2 Thickness of Lines (Ink Drawing)

(b) Pencil Drawing. For drawing finalised with pencil, the lines can be divided into two line- groups as shown in table 3. 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.

Table No. 3 Thickness of Lines (Pencil Drawing)

Line Group mm	Thickness	Lines
0.2	Medium	Out lines, dotted lines. Cutting plane- lines
0.1	Thin Line	Center-lines, section-lines, dimension-lines, extension lines, construction lines, leader lines, short break lines and long break lines

4. 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

5. 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 (Refer Fig. 2) 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, (Refer Fig. 3) Breaks of rectangular metal and wood sections are always drawn freehand.

6. 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. Fig. 2 Conventional Breaks

Fig. 3 Approximate Method of Drawing “S” Break

Surface Finish

7. 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.

8. 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. (Ref Fig. 4)

Fig 4 Different Surface Finish

Symbols for Indicating Surface Finish

9. 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 as shown in Fig.4. This basic symbol consists of two legs of unequal length inclined at approximately 60⁰ to the line representing the surface to be machined with the vertex touching it.

10. 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 as shown in Fig 5. .

11. 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 as shown in Fig 5. .

(a) Basic symbol for Surface (b) Symbol for Surface (c) Symbol for Surface
Roughness by any Production Roughness by Machining Roughness without

Process other than Machining Process Removal of Material

Fig 5 Machining Symbol

12. 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.

13. 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.

Indication of Surface Roughness

14. The surface roughness may be indicated by value in micrometer, grade number or symbol as follows:

(a) Roughness value in micrometer which is the arithmetical mean deviation from the mean line of the profile.

(b) Roughness grade numbers.

The Bureau of Indian Standards (BIS) recommends the first two types. Method of indicating the surface roughness by all the three methods are explained below.

Indication of Surface Roughness by Values in Surface Finish Symbol

15. The value of surface roughness which is the arithmetical mean deviation from the mean line of the profile, abbreviated as R_a is expressed in micrometer or microns (1 microns = 0.001 mm). Table 4 shows the recommended values of surface roughness.

Indication of Surface Roughness by Grades in Surface Finish Symbol

16. The surface roughness is also indicated by the grade number instead of their numerical values. The Bureau of Indian Standards has recommended twelve grades of surface roughness. These standard grades of surface roughness are numbered as N1, N2, N3………N12. (Ref Table 4). For Russian systems, the standard grades of surface roughness are denoted as (Ref Table 5).

Table 4 Surface roughness: Values, Grades and Symbols (British System)

Table 5 Surface Finish : (Delta) Symbol (Russian System)

Indication of Surface Roughness by Triangle Symbol

17. Although the Bureau of Indian Standards prefers the indication of surface roughness by grades, or by values, from the consideration of the requirements of the general engineering industries, it is suggested to indicate the surface roughness on drawing by symbols. The BIS recommended symbols for indicating the surface roughness are shown in Table 4. For the roughness values greater than 25 microns, the symbol ~ is used. (Refer Fig 6).

Fig 6 Indication of Surface Roughness by Triangle Symbol

Surface Finish Symbol with all Characteristics

18. Fig 7 shows a surface finish symbol with all the characteristics of surface roughness indicated in their appropriate places. Instead of the roughness values in µm, the corresponding grade numbers may be indicated.

Fig 7 Surface Finish with all Characteristics

Use of Symbols and Abbreviations

19. Symbols and abbreviations are intended for saving time and space. Some symbols and abbreviations used in Engineering Drawing are given in Table 6 for reference.

Table 6 Abbreviations and Symbols

Term	Abbreviations	Term	Abbreviations	Symbols
Across Corner	A/C	Material	MATL
Across Flat	A/F	Mechanical	MECH
Approved	APPD	Number	No.
Approximate	APPROX	Not to Scale	NTS
Assembly	ASSY	Outside Diameter	OD
British Standard Fine	BSF	Pitch Circle	PC
British Standard Witworth	BSW	Right Hand	RH
Cast Iron	CI	Rivet	RVT
Cast Steel	CS	Reference	REF
Case Hardened	CH	Screw	SCR
Centre line	CL	Sheet	SH
Chamfered	CHMED	Serial Number	Sl. No.
Countersunk	CSK	Standard	STD
Counter Bore	C’BORE	Spot face	SF
Cylinder	CYL	Specification	SPEC
Diameter	DIA	Spherical	SPHERE
Drawing	DRG	Square	SQ
Dimension	DIM	Symmetrical	SYM
Extruded	EXTD	True Position	TP
External	EXT	Traced	TCD
Figure	FIG	Unified Fine	UNF
Hydraulic	HYD	Round	RD
Hexagonal	HEX	Undercut	U/Cut
Horizontal	HORZ
Indian Standard	IS	Beam		I
Inside diameter	ID	Channel		]
Internal	INT	Number of teeth (Gear)		Z
Machine	M/C	Parallel		II
Machined	M/CD	Tee (Structural section)		T

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