1.DRAWING INSTRUMENTS

AND SHEET LAYOUT

Drawing is an art of representing objects or forms on a surface chiefly by means of lines, using any of a wide variety of tools and techniques. It generally involves making  marks on a surface by moving graphite pencils, ink pen, wax colour pencils, crayons,  charcoals, pastels, and markers on a plane surface such as paper, canvas etc.Engineering drawing is a type of drawing used to fully and clearly convey graphically  the ideas and information necessary for engineered items. They are usually created in  accordance with standard conventions for layout, nomenclature, interpretation,  appearance, size, etc. The purpose of engineering drawing is to provide exact  geometrical configuration for the construction or analysis of machines, structures, or  systems. Today the mechanics of the drawing task has been largely automated, and  greatly accelerated, through the use of CAD systems. This chapter deals with the  introduction and basic techniques associated with the use of drawing instruments and  accessories commonly used in preparing engineering drawings and also the layout of  the drawing sheet standard.

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2.LINES, LETTERING AND DIMENSIONING

Engineering drawing is supposed to give complete information about the shape and size of the objects like machine parts, buildings etc. The shape of the object is conveyed through the appearance of the drawing while the size description is expressed in the form of figured dimensions and notes. The Bureau of Indian Standards has recommended various types of lines, letters and dimensions to be used. This chapter introduces the standard practice suggested by Bureau of Indian Standards for various types of lines to specify shape, size of letters for writing notes, dimensions to convey the size and their correct way of implementation.

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3.GEOMETRICAL CONSTRUCTIONS

The engineers should be familiar with the principles of plane and solid geometry. A thorough knowledge of these principles is a prerequisite to solve engineering graphics problems. Plane figures such as circle, triangle, and different polygons frequently constitute a part of various objects for preparing engineering drawings. This chapter presents some of the important methods of geometrical constructions based on the principles of plane geometry studied earlier.

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4. SCALES

It is always convenient to represent objects to their actual size in drawings, if their size permits. eg. A 200 mm diameter plain disc should be represented by a circle of 200 mm diameter on the drawing sheet. This gives complete information of the object. When drawings are prepared equal to the actual size of the object, the scale is said to be full size scale and the drawings are said to be full size drawings. However, it is not always possible to make drawings of all objects, such as large machines, buildings, town plans, etc. to their actual size. When the objects are of very large sizes, the actual dimensions of the object have to be reduced on some regular proportion to make their drawings on the sheet. eg. A rectangular plot of size 25m X 10m can be represented by a rectangle of 250 mm X 100 mm. The scale selected in the present case is 1 mm = 0.10 m. In other words 1 mm on the drawing represents 0.10 m  l ength of the object. When the drawings are prepared smaller than the actual size of the object, the scale is said to be reducing scale and the drawings are said to reduce sized drawings. Similarly very small objects, such as gear mechanism of a wristwatch, components of an electronics instrument, atoms configuration, etc., are shown by drawing them larger then their actual size. When the drawings are prepared larger than the actual size, the scale is said to be an enlarging scale and the drawings are said to enlarge sized drawings.

This is being illustrated by drawing of a bottle

 

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5.CONIC SECTIONS

In engineering practice we come across a number of objects containing plane curves such as ellipse, parabola, hyperbola, etc. The curve, which is obtained by cutting a right circular cone with the help of a plane in different positions relative to the axis, is called a conic section. This chapter deals with a few common methods of construction of the conic sections and the field of their application.

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6. ENGINEERING CURVES

Roulettes are curves generated by the rolling contact of one curve or line on another curve or line. There are infinite varieties of roulettes. The most common types of roulettes used in engineering applications are cycloidal curves, involutes and spirals

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7. ORTHOGRAPHIC PROJECTIONS 

Projection is defined as an image or drawing of an object made on a plane. All drawings used in the field of engineering are based on the principles of projection. That is why engineering drawings are capable to precisely convey the external as well as internal features of objects in terms of their shape and size. Projections can be classified on the basis of line of sight and the position of plane on which the drawing is made.

 

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8. PROJECTIONS OF POINTS

A point is defined as a geometrical element that has no dimensions. In engineering drawing / graphics the point is represented as a dot. This chapter deals with the projections of points.

LOCATION OF A POINT

We know that the reference planes divide the space in four quadrants. A point lying in the space may be situated in the following positions with respect to principle planes of  projections.

1. Point situated above the HP and in front of the VP.

2. Point situated above the HP and behind the VP.

3. Point situated below the HP and behind the VP.

4. Point situated below the HP and in front of the VP.

5. Point situated on the HP and in front of the VP.

6. Point situated above the HP and on the VP.

7. Point situated on the HP and behind the VP.

8. Point situated below the HP and on the VP.

9. Point situated on the HP and VP both

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9. PROJECTIONS OF STRAIGHT LINES

A straight line is defined as the locus of a point which moves linearly. A straight line is the shortest distance between two points. The projections of straight lines are drawn by joining the respective projections of its end points. We have used the word ‘line’ for straight lines for the sake of simplicity. The actual length of the line is known as true length and is denoted by TL.

ORIENTATIONS OF STRAIGHT LINES

The possible orientations of straight lines with respect to the principal planes are as following.

1. Line parallel to both HP and VP.

2. Line perpendicular to HP (and parallel to VP).

3. Line perpendicular to VP (and parallel to HP).

4. Line inclined to HP and parallel to VP.

5. Line inclined to VP and parallel to HP.

6. Line situated in HP.

7. Line situated in VP.

8. Line situated in both HP and VP (i.e. on the reference line).

9. Line inclined to both the reference planes.

a. Line inclined to both HP and VP such that θ + Ø ≠ 90º.

b. Line inclined to both HP and VP such that θ + Ø = 90º.

Let us first consider the projections of straight lines situated in the first quadrant, is inclined to one of the reference planes. Projections of a straight line lying in the first quadrant will have its front view above the xy line (reference line) and the top view below xy line. Concept of projections of points and orthographic projections is required to understand the projections of straight lines.

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10. PROJECTIONS OF PLANES

In this chapter we deal with two dimensional objects called planes. Planes are having length, breadth and negligible thickness (i.e. thickness equivalent to a line). Only  those solids are considered in the chapter whose shape can be defined geometrically  and are regular in nature. Some of them are shown

 

ORIENTATIONS OF PLANES

The possible orientations of the surface of a plane with respect to the principal planes are given below:

1. Surface of plane is parallel to HP (and perpendicular to VP).

2. Surface of plane is parallel to VP (and perpendicular to HP).

3. Surface of plane is perpendicular to both HP and VP (i.e. parallel to profile plane).

4. Surface of plane is inclined to HP and perpendicular to VP.

5. Surface of plane is inclined to VP and perpendicular to HP.

6. Surface of plane is inclined to both HP and VP.

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11. PROJECTIONS OF SOLIDS

This chapter deals with the orthographic projections of three dimensional objects called solids. However, only those solids are considered, the shape of which can be defined geometrically and are regular in nature. The basic concepts of orthographic projections discussed in earlier chapters shall also apply here.

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12.SECTIONS OF SOLIDS

It is observed that the orthographic views of a solid may contain a number of dotted lines. These lines indicate the presence of hidden details which may lie behind or somewhere in the middle of the object. The interpretation of the object’s shape becomes difficult with increasing number of such lines. As a remedy, it becomes obligatory to draw sectional views for a better and easier interpretation of the internal details. The present chapter describes the methods of obtaining sectional views and other related drawing. The object considered to be cut by a plane called a section or a cutting plane.The portion of the object, which falls between the cutting plane and the observer, is assumed to be removed. Thus the exposed internal details become visible. The projections of the remaining object are termed as sectional views.

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13.DEVELOPMENT OF SURFACES

 In engineering practice, a large number of objects like funnel, bucket, hopper, chimney, duct of air conditioner, boiler shell, storage tank and tray etc. are made of metal sheets. The fabrication of these objects can be planned in an economic way if the accurate shape and size of metal sheet is known. This chapter deals with proper layout planning of the surface of the object on a single plane called the development of surfaces.

CLASSIFICATION OF SURFACES

Surfaces of various geometrical objects may be classified as:

1. Plane surfaces: Surfaces of prism, pyramids, cube and polyhedra are plane surfaces.

2. Singly curved surfaces: Surfaces of object like cylinder and cone are singly curved surfaces.

3. Doubly curved surfaces: Surfaces of spheres, paraboloid, ellipsoid, hyperboloid are doubly curved surfaces

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14.INTERSECTION OF SURFACES

When a solid penetrates into another solid, it is known as interpenetration of solids. Due to such interpenetration their lateral surfaces intersect to produce closed loop which may either be made of straight lines or curves. These loops are known as lines or curves of intersection.
Since two plane surfaces intersect in a straight line the intersection of prism with prism or pyramid with pyramid or prism with pyramid results in a polygon. Similarly if any one or both of the two solids have curved surface, it will result in a closed curve. In both
the cases, the term “curve of intersection” is frequently used. It is important to note that
the points lying on the curve of intersection are always common to the surfaces of both the solids.

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15.ISOMETRIC PROJECTIONS

Isometric projection is a type of single view projection in which a pictorial view is obtained by keeping the object in such a way that all the three mutually perpendicular geometrical axes are equally inclined to the plane of projection. The projectors follow the rules of multi-view projections i.e. they are parallel to each other and perpendicular to the plane of projection. In multi-view orthographic projections, each view provides information of two axes (length & breadth or length & height or breadth & height). For a complete understanding, there is always a need of more than one view of the object. These views can only be correctly interpreted and visualized by those persons who have a good knowledge of principles used for these projections. Whereas in isometric projection, a single view is drawn in such a manner that it gives an overall view of the object at the first sight. Thus, it is necessary to draw a pictorial view of one kind or the other so as to enable a common man to understand.

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16.OBLIQUE PROJECTIONS

Oblique projection is defined as a pictorial projection in which projectors are parallel to each other and inclined to the plane of projection at any angle other than right angle.

In orthographic projections (both multiview and axonometric) the projectors are parallel to each other and perpendicular to the plane of projection. Whereas in oblique projection the projectors, although parallel to each other, are oblique to the plane of projection. See Fig. 16.1. It may be seen that the face of an object parallel to the plane of projection will have the same appearance in both multi-view and oblique projections. To take this advantage, it is customary to have one of the faces of the object parallel to the plane of projection. This is the chief advantage of oblique projection over other forms of pictorial drawings.

 

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17.PERSPECTIVE PROJECTIONS

Perspective projection is a three dimensional representation of an object on a plane as it is perceived by the human eye from a particular point. It is a geometric method of obtaining images which are similar to the photographs taken by a camera.

The major difference between parallel projection, be it orthographic oblique or isometric, and perspective projection lies in the fact that in the later case the point of sight is at a finite distance from the object. The projectors from the object therefore converge to the point of sight instead of being parallel to each other as in the former types of projection. Such drawing is also known as scenographic projection or central projection.

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18. COMPUTER AIDED DESIGN(CAD)

The drafting work can be automated and accelerated through the use of Computer Aided Design (CAD) systems. It may be applied for a wide variety of products in the field of automotive, electronics, aerospace, naval, architecture, civil and other disciplines of engineering. CAD systems were originally used for automated drafting only, but now they also include three-dimensional modeling and computer-simulated operations of the models. Sometimes CAD is translated as “computer-assisted drafting”, “computer-aided drafting”, or a similar phrase. Related acronyms are CADD, which stands for “computer aided design and drafting”; CAID, for Computer-aided Industrial Design; and CAAD, for “computer-aided architectural design”. All these terms are essentially synonymous, but there are some subtle differences in meaning and application

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