Machine Drawing

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Example 1.1 Define the following:

(a) Drawing

(b) Engineering drawing

(c) Artistic drawing, and

(d) Machine drawing.

Solution.

(a) Drawing — Drawing may be defined as the representation of an abject by systematic lines. Ordinarily, the idea conveyed by the word ‘drawing’ is a pictorial view in which an object is represented as the eyes see it. A pictorial view shows only the outside appearance of an object.

(b) Engineering Drawing — Engineering drawing is a graphic language which has its own rules. It gives complete description of an object or a machine part as regards shape, size and all other internal details from which it can be constructed or manufactured.

(c) Artistic Drawing — It is the art of representation of an object by an artist as per his imagination or by keeping the object before him such as painting, advertisement board, etc.

(d) Machine Drawing — Machine drawing may be defined as the representation of a machine component or machine by lines according to certain set rules. A machine drawing generally gives all the external and internal details of the machine component from which it can be manufactured. The machining symbols, tolerances, bill of material, etc. are specified on the drawing. The-relative position of the different components and to make assembly drawing are also specified. IS: 696—1972 is the BIS Code for Machine Drawing.

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Example 1.2 Define the following:

(a) Assembly drawing

(ii) Part drawing

(c) Shop drawing

(d) Catalogue drawing

(e) Schematic drawing, and

(t) Patent drawing.

Solution.

(a) Assembly Drawin—An assembly drawing shows all the complete drawing of a given machine indicating the relative positions of various components assembled together.

(b) Part Drawing— A part drawing shows the number of views of each single part of a machine to facilitate its manufacturing. It should give all the dimensions, limits, tolerances and special finishing; if any.

(c) Shop Drawing— A shop drawing includes the part drawing, subassembly and the complete assembly of a product for manufacturing.

(d) Catalogue Drawing—A catalogue drawing shows only the outlines of an assembly drawing for illustration purpose.

(e) Schematic Drawing — A schematic drawing is the simplified illustration of a machine or system, replacing all the elements by their respective conventional representations, to understand the principle of operation.

(J) Patent Drawing — A patent drawing gives the correct and complete features of a new technology or innovation adopted for a machine or system. The drawings are pictorial in nature and self — explanatory but not useful for production purposes.

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Example 1.3 What are the standard sizes of drawing sheets? Draw the layout of a drawing sheet.

Solution. The standard sizes of drawing sheets are given in Table 1.1.

The layout of drawing sheet is shown in Fig. 1.1.

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Example 1.4 What are the various types of scales used in machine drawing ? Specify the standard scales.

Solution.

The various types of scales used in machine drawing are

1. Full scale

2. Reduced scale. and

3. Enlarged scale.

The standard scales are given in Table L2.

Solution. The main requirements for lettering are

1. Legibility 2. Uniformity 3. Ease of writing, and 4. Rapidity of execution. Single stroke letters meet these requirements and are universally used. All letters should be in capital except where lower case letters are accepted in international usage for abbreviations. Good lettering should conform to uniformity of thickness, style, scope, size and spacing.

(a) Modern Roman — Refer to Fig. 1.2.

(b) Commercial Gothic — Refer to Fig. 1.3.

(c) Single stroke vertical lettering — Refer to Fig. 1.4.

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Example 1.5 Specify the standard height of letters.

Ans. The standard height of letters as specified by BIS is given in Table 1.4.

Table 1.4 Standard height of letters.

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Example 1.6 What is the need of representing machine components conventionally? How machine components are represented conventionally?

Solution. When the complete drawing of a machine component involves a lot of time or space, it may be drawn in conventional form to represent the actual machine component. The conventional representation of various components is given in Table

Table 1.5 Conventional representation of common features.

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Example 1.7 How various materials are represented conventionally?

Solution. A variety of materials are used for making machine components. It is therefore preferable to follow different conventions of section lining for different materials as given in Table 1.6.

Table 1.6 Conventional Representation of Materials

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Example 1.8 What do you understand by dimensioning ? What are the various types of dimensions?

Solution. Every drawing besides showing the true shape of an object, must supply

its exact length, breadth, height, sizes and position of holes, grooves, etc. The procedure of presenting this information on a drawing sheet is called dimensioning. Lines, symbols, numerals and notes are used for this purpose.

There are two types of dimensions required to be shown on a drawing:

j) Size or functional dimension, and

(ii) Location or datum dimensions, shown by letters S and L respectively. The size

dimension indicates size such as length, breadth, height, diameter, etc. The

location dimension shows locations or exact position of various constructional

details within the objects.

Fig. 1.6 shows the size and location dimensions.

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Example 1.9 What are the various types of dimension lines ? Explain the various dimension lines with the help of a neat diagram.

Solution. The various types of dimension lines are:

1. Dimension line — It is a thin continuous line terminated by arrowheacs touching

the outlines, extension lines or centre lines.

2. Extension line — It is a thin line drawn outside and along the outline. There

should be a gap of about 1 mm between the extension line and the outline.

3. Leader line — A leader line or a pointer is a thin line connecting a note or a

dimension figure with the feature to which it applies. One end of the leader

terminates either in an arrowhead or a dot. The arrowhead touches the outline, while the dot is placed within the outline of the object. The other end of the leader is terminated at a horizontal line at the bottom level of the first or the last letter of the note.

4. Arrowhead — An arrowhead is placed at each end of a dimension line. Its pointed end touches an outline, an extension line or a centre line. The length of arrowhead should be about three times its maximum width. The width has to be selected depending upon the size of the drawing. The triangle of the arrow should be completely filled in.

These various types of lines are shown in Fig. 1.7.

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Example 1.10 Explain the two methods of placing the dimensions.

Solution. The two methods for placing the dimensions are:

1. Aligned system, and

2. Unidirectional system.

1. Aligned system — In the aligned system, the dimensions are placed above the dimension lines and may be read either from the bottom or from the right side of the drawing, as shown in Fig. 1.8.

2. Unidirectional system — In the unidirectional system, all dimensions are placed with respect to the bottom of the drawing, irrespective of the disposition of the dimension line. In the system, the dimension lines are broken to insert their dimensions as shown in Fig. 1.9. This system is preferred for big drawings specially when it is not convenient to read the dimension from the right side or any other direction.

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Example 1.11 Explain the following:

(a) Progressive dimensioning, and (b) Continuous dimensioning.

Solution.

1. Progressive dimensioning — In this method of dimensioning, all dimensions on the drawing are shown from a common base line, as shown in Fig. 1.10. Cumulative error is avoided in this method.

2. Continuous dimensioning — In this method, dimensions are arranged in a straight line. An overall dimension is placed outside the smaller dimension. One of the smaller dimensions is generally omitted which is least important, as shown in Fig. 1.11.

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Example 1.12 Specify the general rules for dimensioning.

Solution. The following rules should be observed while dimensioning:

1. Standard sizes of letters and figures should be used.

2. All dimensions should be specified in mm. The use of mm should be avoided by giving a general note “All dimensions are in mm”.

3. As far as possible dimensions should be placed outside the views.

4. Dimension lines should not run in the direction included in the hatched area.

5. Dimensions should be taken from visible outlines rather than from dashed lines.

6. Dimensions should be given from a base line, a centre line, an important hole, or a finished surface which may be readily established.

7. dimensions should be quoted only once in one view.

8. Overall dimensions should be placed outside the intermediate dimensions.

9. Dimensions should be placed outside a sectional view.

10. Zero should precede the decimal point when the dimension is less than unity.

11. Dimension line should not cross. Also dimension lines and extension lines should not cross.

12. When there are several dimension lines, the shorter dimension should be nearer the view.

13. Leaders should not be drawn curved or made free hand.

14. Do not use outlines for dimensions.

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Example 1.13 Illustrate the methods of dimensioning common features.

Solution. 1. Diameter —

The diameters of a cylinder should generally be given in rectangular view in preference to the view in which it occurs as circles. Never give radius of a cylinder. [Fig. 1.12 (a)]

(ii) Dimensions of diameters should be followed by symbol or D or abbreviation “DIA” only where it is not obvious that dimension represents diameter.

(iii) The circles should be dimensioned as shown in Fig. 1.12 (b).

2. Radii—

(r) The dimensions of small radii should be shown outside the outline of the object and should be followed by the letter R. An arrowhead should terminate the dimension line of radius on the outline and no arrowhead should be in the dimension line of radius touching the centre [Fig. 1.13 (a)].

(ii) The radius of a spherical surface should be dimensioned as shown in Fig. 1.13 (b).

3. Holes — Location of holes should be dimensioned where possible in the plan view of holes as shown in Fig. 1.14.

4. Bent-up parts — The method of dimensioning bent-up parts is shown in Fig.

5. Chords, arcs and angles—The chords, arcs and angles should be dimensioned as shown in Figs. 1.16 and 1.17.

6. Countersunk and counterbore—It is dimensioned by giving maximup’ diameter

and the included angle as shown in Fig. 1.18.

7. Spot face — The spot faced hole may be dimensioned as shown in fig. 1.19.

8. Chamfer—A chamfer is specified by the length and angle on chamfer as shown in Fig. 1.20.

9. Screw thread — A metric screw thread is designated by the letter M followed by the outside diameter.

10. Taper—A taper is defined as unit alteration in a specified length measured along the axis in case of a shaft and a base line or a center line in case of flat pieces as shown in Fig. 1.21.