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Symmetrical parts may be drawn, half in plain view and half in section (Fig 2.23).
A local section may be drawn if half or full section is not convenient. The local break may be shown by a continuous thin free hand line (Fig. 2.24).
Successive sections may be placed separately, with designations for both cutting planes and sections (Fig. 2.25) or may be arranged below the cutting planes.
Certain draughting conventions are used to represent materials in section and machine elements in engineering drawings. As a variety of materials are used for machine components in engineering applications, it is preferable to have different conventions of section lining to differentiate between various materials. The recommended conventions in use are shown in Fig.2.26. When the drawing of a component in its true projection involves a lot of time, its convention may be used to represent the actual component. Figure 2.27 shows typical examples of conventional representaion of various machine components used in engineering drawing.
A drawing of a component, in addition to providing complete shape description, must also furnish information regarding the size description. These are provided through the distances between the surfaces, location of holes, nature of surface finsih, type of material, etc. The expression of these features on a drawing, using lines, symbols, figures and notes is called dimensioning. Dimension is a numerical value expressed in appropriate units of measurment and indicated on drawings, using lines, symbols, notes, etc., so that all features are completely defined.
1. As far as possible, dimensions should be placed outside the view. 2. Dimensions should be taken from visible outlines rather than from hidden lines. 3. Dimensioning to a centre line should be avoided except when the centre line passes through the centre of a hole. 4. Each feature should be dimensioned once only on a drawing. 5. Dimensions should be placed on the view or section that relates most clearly to the corresponding features. 6. Each drawing should use the same unit for all dimensions, but without showing the unit symbol. 7. No more dimensions than are necessary to define a part should be shown on a drawing. 8. No features of a part should be defined by more than one dimension in any one direction. The elements of dimensioning include the projection line, dimension line, leader line, dimension line termination, the origin indication and the dimension itself. The various elements of
dimensioning are shown in Figs. 2.28 and 2.29. The following are some of the principles to be adopted during execution of dimensioning:
1. Projection and dimension lines should be drawn as thin continuous lines. 2. Projection lines should extend slightly beyond the respective dimension lines. 3. Projection lines should be drawn perpendicular to the feature being dimensioned. Where necessary, they may be drawn obliquely, but parallel to each other (Fig. 2.30). However, they must be in contact with the feature. 4. Projection lines and dimension lines should not cross each other, unless it is unavoidable (Fig. 2.31). 5. A dimension line should be shown unbroken, even where the feature to which it refers, is shown broken (Fig. 2.32). 6. A centre line or the outline of a part should not be used as a dimension line, but may be used in place of projection line (Fig. 2.31).
Dimension lines should show distinct termination, in the form of arrow heads or oblique strokes or where applicable, an origin indication. Two dimension line terminations and an origin indication are shown in Fig. 2.33. In this, 1. the arrow head is drawn as short lines, having an included angle of 15°, which is closed and filled-in. 2. the oblique stroke is drawn as a short line, inclined at 45°.
3. the origin indication is drawn as a small open circle of approximately 3 mm in diameter. The size of the terminations should be proportionate to the size of the drawing on which they are used. Where space is limited, arrow head termination may be shown outside the intended limits of the dimension line that is extended for that purpose. In certain other cases, an oblique stroke or a dot may be substituted (Fig. 2.34). Where a radius is dimensioned, only one arrow head termination, with its point on the arc end of the dimension line, should be used (Fig. 2.35). However, the arrow head termination may be either on the inside or outside of the feature outline, depending upon the size of feature.
Dimensions should be shown on drawings in characters of sufficient size, to ensure complete legibility. They should be placed in such a way that they are not crossed or separated by any other line on the drawing. Dimensions should be indicated on a drawing, according to one of the following two methods. However, only one method should be used on any one drawing. METHOD1 (Aligned System) Dimensions should be placed parallel to their dimension lines and preferably near the middle, above and clear-off the dimension line (Fig. 2.36). An exception may be made where superimposed running dimensions are used (Fig. 2.44 b) Dimensions may be written so that they can be read from the bottom or from the right side of the drawing. Dimensions on oblique dimension lines should be oriented as shown in Fig. 2.37. Angular dimensions may be oriented as shown in Fig. 2.38.
METHOD2 (Uni-directional System) Dimensons should be indicated so that they can be read from the bottom of the drawing only. Non-horizontal dimension lines are interrupted, preferably near the middle, for insertion of the dimension (Fig. 2.39). Angular dimensions may be oriented as in Fig. 2.40.
Dimensions can be, (i) above the extension of the dimension line, beyond one of the terminations, where space is limited (Fig. 2.34) or (ii) at the end of a leader line, which teminates on a dimension line, that is too short to permit normal dimension placement (Fig. 2.34) or (iii) above a horizontal extension of a dimension line, where space does not allow placement at the interruption of a non-horizontal dimension line (Fig. 2.41). Values of dimensions, out of scale (except where break lines are used) should be underlined as shown in Fig. 2.41.
The following indications (symbols) are used with dimensions to reveal the shape identification and to improve drawing interpretation. The symbol should precede the dimensions (Fig. 2.42). : Diameter
S : Spherical diameter
R : Radius
SR : Spherical radius
The arrangement of dimensions on a drawing must indicate clearly the design purpose. The following are the ways of arranging the dimensions. Chains of single dimensions should be used only where the possible accumulation of tolerances does not endanger the functional requirement of the part (Fig. 2.43). In parallel dimensoning, a number of dimension lines, parallel to one another and spaced-out are used. This method is used where a number of dimensions have a common datum feature (Fig. 2.44 a).
These are simplified parallel dimensons and may be used where there are space limitations (Fig. 2.44 b).
These are the result of simultaneous use of chain and parallel dimensions (Fig. 2.45).
The sizes of the holes and their co-ordinates may be indicated directly on the drawing; or they may be conveniently presented in a tabular form, as shown in Fig. 2.46.
Diameters should be dimensioned on the most appropriate view to ensure clarity. The dimension value should be preceded by . Figure 2.47 shows the method of dimensioning diameters. The dimensioning of chords, arcs and angles should be as shown in Fig. 2.48. Where the centre of an arc falls outside the limits of the space available, the dimension line of the radius should be broken or interrupted according to whether or not it is necessary to locate the centre (Fig. 2.35). Where the size of the radius can be derived from other dimensions, it may be indicated by a radius arrow and the symbol R, without an indication of the value (Fig. 2.49). Linear spacings with equi-distant features may be dimensioned as shown in Fig. 2.50.
Chamfers may be dimensioned as shown in Fig. 2.51 and countersunks, as shown in Fig. 2.52.
Screw threads are always specified with proper designation. The nominal diameter is preceded by the letter M. The useful length of the threaded portion only should be dimenioned as shown in Fig. 2.53. While dimensioning the internal threads, the length of the drilled hole should also be dimensioned (Fig. 2.53).
Tapered features are dimensioned, either by specifying the diameters at either end and the length, or the length, one of the diameters and the taper or the taper angle (Fig. 2.54 a). A slope or flat taper is defined as the rise per unit length and is dimensioned by the ratio of the difference between the heights to its length (Fig. 2.54 b).
Notes should always be written horizontally in capital letters and begin above the leader line and may end below also. Further, notes should be brief and clear and the wording should be standard in form. The standard forms of notes and the method of indication, for typical cases is shown in Fig. 2.55. The meaning of the notes is given in Table 2.8.
Table 2.8 Meaning of notes given in Fig. 2.55 S.No.
DIA 25 DEEP 25
Drill a hole of diameter 25 mm, to a depth of 25 mm.
DIA 10 CSK DIA 15
Drill a through hole of diameter 10 mm and countersink to get 15 mm on top.
4 HOLES, DIA 12 C BORE DIA 15 DEEP 8
Dirll through hole of 12 mm, counterbore to a depth of 8 mm, with a 15 mm, the number of such holes being four.
6 HOLES, EQUISP DIA 17 C BORE FOR M 16 SOCKET HD CAP SCR
Drill a through hole of 17 and counterbore to insert a socket headed cap screw of M 16. Six holes are to be made equi-spaced on the circle.
KEYWAY, WIDE 6 DEEP 3
Cut a key way of 6 mm wide and 3 mm depth.
KEY SEAT, WIDE 10 DEEP 10
Cut a key seat of 10 mm wide and 10 mm deep to the length shown.
U/C, WIDE 6 DEEP 3
Machine an undercut of width 6 mm and dpeth 3 mm.
(a) DIAMOND KNURL 1 RAISED 30°
Make a diamond knurl with 1 mm pitch and end chamfer of 30°.
(b) M 18 × 1
Cut a metric thread of nominal diameter 18 mm and pitch 1 mm.
(a) THD RELIEF, DIA 20 WIDE 3.5
Cut a relief for thread with a diameter of 20.8 mm and width 3.5 mm.
(b) NECK, WIDE 3 DEEP 1.5
Turn an undercut of 3 mm width and 1.5 mm depth
(c) CARB AND HDN
Carburise and harden.
10. (a) CARB, HDN AND GND (b) MORSE TAPER 2
Carburise, harden and grind. Morse taper No. 1 to be obtained.
11. DIA 6 REAM FOR TAPER PIN
Drill and ream with taper reamer for a diameter of 6 mm to suit the pin specified.
12. 6 ACME THD
Cut an ACME thread of pitch 6 mm.
Standard abbreviations in draughting are recommended as notes to provide a brief and clear instructions. Table 2.9 provides the draughting abbreviations for general terms and Table 2.10 represents material abbreviations. Table 2.9 Draughting abbreviations Term
Across corners Across flats Approved Approximate Assembly Auxiliary Bearing Centimetre Centres Centre line Centre to centre Chamfered Checked Cheese head Circular pitch Circumference Continued Counterbore Countersunk Cylinder Diameter Diametral pitch Dimension Drawing Equi-spaced External Figure General Ground level Ground Hexagonal Inspection Inside diameter Internal Left hand Machine
A/C A/F APPD APPROX ASSY AUX BRG Cm CRS CL C/L CHMED CHD CH HD CP OCE CONTD C BORE CSK CYL DIA DP DIM DRG EQUI-SP EXT FIG. GNL GL GND HEX INSP ID INT LH M/C
Maunfacture Material Maximum Metre Mechanical Millimetre Minimum Nominal Not to scale Number Opposite Outside diameter Pitch circle Pitch circle diameter Quantity Radius Radius in a note Reference Required Right hand Round Screw Serial number Specification Sphere/Spherical Spot face Square Standard Symmetrical Thick Thread Through Tolerance Typical Undercut Weight
MFG MATL max. m MECH mm min. NOM NTS No. OPP OD PC PCD QTY R RAD REF REQD RH RD SCR Sl. No. SPEC SPHERE SF SQ STD SYM THK THD THRU TOL TYP U/C WT
Table 2.10 Abbreviations for materials Material
High carbon steel
High speed steel
High tensile steel
Low carbon steel
Pearlitic malleable iron
Tungston carbide steel
Violations of some of the principles of drawing are indicated in Fig. 2.56 a. The corrected version of the same as per the BIS, SP46: 1988 is given in Fig. 2.56 b and the reasons are given below: 1. Dimension should follow the shape symbol (Fig. 2.42). 2. and 3. As far as possible, features should not be used as extension lines for dimensioning. 4. Extension line should touch the feature. 5. Extension line should project beyond the dimension line. 6. Writing the dimension is not as per the aligned system. 7. Hidden lines should meet without a gap (Table 2.5 A). 8. Centre line representation is wrong. Dot should be replaced by a small dash. 9. Horizontal dimension line should not be broken to insert the value of the dimension (Figs. 2.36 to 2.49).
10. 11. 12. 13.
Dimension should be placed above the dimension line (Fig. 2.39). Radius symbol should precede the dimension (Fig. 2.42) Centre lines should cross at long dashes (Table 2.5 B). Dimension should be written by symbol (not abbreviation) followed by its value (Fig. 2.42). 14. Note with dimensions should be written in capitals. 15. Elevation is not the correct usage. 16. Usage of the term plan is obsolete in graphic language. THEORY QUESTIONS 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9
Describe the drawing sheet designations and their sizes as per ISO-A series. What is the principle involved in fixing the sizes of the drawing sheets ? What is the information generally provided by the title block and what is its maximum length ? What do you understand by the terms, (a) borders and frames, (b) centring marks, (c) metric reference graduation, (d) zoning and (e) trimming marks ? What are the scales recommended for machine drawing ? What do you understand by, (a) scale = 5:1 and (b) scale = 1:10 ? List out the standard thicknesses of lines that are used in machine drawing. What should be the ratio of thick to thin line used in machine drawing ? While finishing a drawing, what is the order of priority in the following coinciding lines: (a) centre lines (b) visible lines (c) hidden lines.
2.10 How are leader lines terminated ?
2.11 How are sizes of letters and numerals specified ? 2.12 How do you represent a sectioned surface on a drawing ? 2.13 Name the features which should not be shown hatched, when they are sectioned longitudinally. 2.14 What is the angle at which hatching lines are drawn to the axis or to the main outline of the section. 2.15 What do you understand by revolved and removed sections ? 2.16 Explain the terms, (a) half section, (b) local section and (c) successive sections. 2.17 List out the elements of a dimension line. 2.18 Give the shape identification symbols for the following: (a) diameter, (b) radius, (c) square and (d) spherical radius. 2.19 List out the various principles to be followed while dimensioning a drawing. 2.20 What are the rules to be adopted during execution of dimensioning ? 2.21 Discuss the two methods, normally followed while dimensioing a drawing. 2.22 Discuss the various ways of arranging dimensions. 2.23 Explain the following notes: (a) 4 HOLES, EQUI-SP 12 C BORE 15 DEEP 8 (b) U/C WIDE 6 DEEP 3 (c) 6 REAM FOR TAPER PIN 6 × 50
DRAWING EXERCISES 2.1 Sketch the following types of lines:
(a) centre line, (b) cutting plane line and (c) long break 2.2 Sektch the conventional representation of the following materials: (a) bronze, (b) cast iron, (c) concrete, (d) wood and (e) white metal. 2.3 Sketch the conventional representation of the following: (a) External threads, (b) internal threads, (c) splined shaft, (d) bearing, (e) square on shaft, (f) compression spring, (g) tension spring, (h) spur gear and (i) helical gear. 2.4 Sketch the various dimension line terminations and origin indication. 2.5 Sketch the method of dimensioning chamfers and countersunks. 2.6 How are, (a) screw threads and (b) tapered features, dimensioned ? 2.7 Identify (i) Functional, (ii) Non-functional and (iii) Auxiliary dimensions in Fig. 2.57.