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book on engineering drawing for civil engineering and architecture students
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To the Students
Department of Civil Engineering Wah Engineering College,Wah Cantt.
ENGINEERING DRAWING
DR. ZAHID AHMED SIDDIQUI
DR. MUHAMMAD ASHRAF
ENGR. SHAHID AHMED SIDDIQUI
About the Author
Zahid Ahmed Siddiqui, Ph.D, is a Professor in Civil Engineering
Department of University of Engineering and Technology Lahore.
He is author of previous edition of this book published in 1992
besides various editions of four more books on the subjects titled
"Steel Structures", "LRFD Steel Design Aids", "Concrete Structures,
Part-I" and "Concrete Structures, Part-II". He has teaching,
research and practical design experience of almost 27 years. He
has vast experience of teaching to Undergraduate and Graduate
Students of Civil Engineering. He is actively involved in field
design of sizeable concrete and steel structures.
CONTENTS
Preface
Chapter 1 – Introduction
1.1 ---------- The Subject
1.2 ---------- Drawing Instruments
Chapter 2 – Use of Instruments and Lettering
2.1 ---------- Classification of Lines 2.2 ---------- Sharpening the Pencil
2.3 ---------- Horizontal and Vertical Lines on the Board 2.4 ---------- Fixing the Drawing Sheet 2.5 ---------- Border Lines and Seal 2.6 ---------- Measurements in Fractions of an Inch
2.7 ---------- Lettering on Drawing 2.8 ---------- Methods of Dimensioning 2.9 ---------- Applied Geometry 2.10 ---------- Hints for Good Linework
Chapter 3 – Orthographic Drawing
3.1 ---------- Faces of Object 3.2 ---------- Planes of Projection
3.3 ---------- Space Dimensions 3.4 ---------- Dimensions Appearing in the Views 3.5 ---------- Perspective Drawing 3.6 ---------- Orthographic Projection
3.7 ---------- Lines to Form a View 3.8 ---------- Three Orthographic Views 3.9 ---------- Classification of Surfaces 3.10 ---------- Arrangement of Views
3.11 ---------- Hidden features 3.12 ---------- Center-Lines and Precedence of Lines 3.13 ---------- Order of Drawing 3.14 ---------- Transferring Dimensions Inbetween Views
3.15 ---------- Freehand Sketching 3.16 ---------- Models 3.17 ---------- Symbols Used in Orthographic Projection
Chapter 4 – Sectional Views
4.1 ---------- Introduction to Sections
PREFACE
Drawing is one of the basic subjects of Engineering and there is no challenge to its much diversified importance and use. No single book is available in our country to explain the basic concepts and principles to the students of degree and diploma concepts and principles written in a local pattern. This book is the first effort to compensate this deficiency and the subject to the undergraduate classes for any years combined with the practical needs and requirements. Due to the financial constraints, all the figures were drawing by the principal author himself besides all the typing work which may have restricted the standard of the presentation.
The authors feel that this first edition may have many areas needing improvement both in terms of the concepts and the language. The readers are highly encouraged to point out any such improvement and to submit their suggestions which will be properly incorporated in the future editions. The authors are thankful to M/s Technical Publishers, Lahore for printing book in a presentable form.
CHAPTER NO.
INTRODUCTION
Engineering drawing, also called technical drawing and engineering graphics, is the graphical representation of shape of any physical object which may be a part of a machine, a building, a dam, or any other complicated structure. The shape of some simple objects like spheres, cubes, cylinders, etc., may be described in words and sentences but ordinary language fails for complicated objects. Even if a thick book is written to describe the shape of a building, the reader will not be able to conceive the exact shape of that building with all its minor details. It can be safely said that it is almost impossible to describe the shape of an object in words and hence only three methods are left for the purpose namely camera photographs, models and drawings.
Camera photographs can only be used to communicate the shape of existing structures but not the shape or design still in the mind of the designer. Further these photographs do not show all the inner details and actual dimensions are not communicated. Models, usually made up of wood, polystyrene, soap, plastic, etc., are perhaps the best tool for description of the shape but these are generally used for explaining finished outer shapes of the structures to non-engineering persons. Sometimes models, made smaller in size, are also used to carry out performance study of bigger structures like canals, dams and turbines. Models require much more skill and time for their creation necessitating relatively more skilled persons employed for longer periods; also more space is required in offices for their storage. Hence, cost to describe the shape with models is usually much greater and this method for shape- communication is not generally used except only in special cases. The easiest way left to describe an object is to make drawings; simply lines are drawn nr a piece of paper according to certain fixed rules. The advantages of engineering drawing are as follows:
Fig 1.1 Drawing board and T-square 1.2.2. T-Square
The T-square is composed of a long strip, called the blade, fastened rigidly at right angle to a shorter piece, called the head or stock. The upper edge of the blade and the inner edge of the head are working edges and must be straight. Transparent edge is recommended for top edge the blade since it permits the working person to see drawing underneath. T-square may be made up of wood, plywood ; or plastic but its size must be equal to the board size (called imperial size).
1.2.3. Drawing Sheet
Imperial size (30" x 22") sheet of better quality should be used. In general, paper should have sufficient grains or teeth against which lead of the pencil may work, colour of the paper should be agreeable to the eye, the sheet should provide a hard surface not easily grooved by the pencil and erasing qualities of the paper should be good. Drawing on a good quality sheet is always easier.
1.2.4. Drafting Tape (Scotch Tape)
It is a colourless sticking tape which is used to fix the sheet on the drawing board.
1.2.5. Triangles Or Set -Square
Two triangles are used in drawing. For the first triangle, angles are 45°, 45° and 90° and is called 45° triangle. The other triangle has 30°, 60° and 90° and is called 30°-60° triangle. 45°-triangle should be about 10 in. (25 cm) in size, whereas, longer side of 30°60 ° triangle should be about 14 in. (35 cm). At least one side of each triangle must be graduated in inches or centimetres depending upon the units to be used for the drawing work. These triangles are made of transparent plastic material.
Fig 1.2 45° and 30°-60° Triangle 1.2.6. Diagonal Scale
It is a 6 inches long flexible scale which is basically used for measuring very small dimensions with reasonable accuracy but, in general, it is a multipurpose scale. With decimal diagonal scale in inch units, measurements may be taken up to second decimal place.
Fig 1.3 Decimal Diagonal Scale
1.2.7. Compasses And Dividers
Compass is an instrument with the help of which we can draw circles; it consists of two arms hinged together at one end. One of the arms holds a metallic needle at the free end while the other arm may hold another needle, lead, pen, or a lengthening bar. Bow-type compass with opening and closing screw in-between the two arms is preferable because the compass may be opened exactly and further the opening is not disturbed while drawing the circle. If both arms and into needle points, the instrument is called a divider. At least two compasses,
Sandpaper pad is a small wooden strip pasted with a sandpaper on one side and a small foam on the other end. Sandpaper is used to make required shape of the lead and foam is used for final cleaning of the same.
1.2.10. Eraser
Eraser is used to rub out extra and incorrect lines. It should be of suitable grade and of good quality.
1.2.11. Erasing Shield
It is a thin metallic various plates with perforations shapes of extra completion and is used
to remove of a drawing without disturbing required lines.
1.2.12. Towel Or Brush
During drawing work, frequent cleaning of the sheet and instruments is needed. Hence, a towel or brush should always be available.
1.2.13. Triangular Scale
This instrument is triangular in shape having six edges and usually two scales are provided over each edge, one starting from the left and the other starting from the right. The scales (in F.P.S. units) available in triangular shape are generally of three types:
a) Mechanical Engineer's Scale
Fig 1.6 Sandpaper Pad
Fig. 1.7 Erasing shield
These scales are used to draw the object on the actual size, double size, half size, quarter size, or eighth size.
As usually smaller dimensions are involved in machine drawing, bigger divisions represent inches which are then subdivided in fractions.
b) Civil Engineer's Scale
This scale is graduated in the decimal system and the set of these scales is as follows: 10, 20, 30, 40 - - - - - -
On a 10-scale, inch is divided into 10 equal parts, one part is generally considered to be equal to 1 foot in survey maps. In other words, 10 ft of the actual structure will be shown by 1 inch of the line on the drawing sheet.
Mechanical Engineer's and Civil Engineer's Scales are not used for the building drawing.
c) Architectural Scale
Full, Half, Quarter, Eighth - - - - - - - or 1, 1/2, 1/4, 1/8, - - - - - - -
Architectural scale is actually an inch-foot scale and Full scale here means that one foot of the structure is represented by one inch of the drawing line; size is reduced 12 times before drawing.
Similarly half scale means that one foot of the structure is represented by half inch of the drawing line, reduction factor being 24.
These are made of plastic sheets with edges lying in irregular curves. Suitable curves may be fitted for the already plotted points and freehand curves may be changed into smooth curves.
1.2.16. Flexi-Rod
It serves nearly the same purpose as the French curves do. First the flexi-rod is shaped according to the required curvature, it then retains its shape and is used to draw smooth curves.
Fig 1.10 Irregular or French curves
Lines may be classified according to their thickness, darkness and shape:
2.1.1 Classification According to Thickness
a) Very thick lines drawn with chisel-shaped pencil Fig. 2.2 are used to show reinforcing steel bars and position of beams etc. in structural working drawings, see Fig. 2.1 (A).
b) Relatively thicker lines drawn with cone-shaped pencil Fig. 2.2, are used to show ground level in elevation of buildings, plastered surfaces of walls in plans, etc., see Fig, 2.1 (B).
c) Lines of common thickness show usual features of the object see Fig. 2.1 (C).
d) Thin lines are used for center-lines, construction lines, extension lines, and dimension lines, etc.
2.1.2 Classification According to Darkness
Line of a given thickness may be dark and bright or it may be dim depending upon grade of the pencil and the pressure applied.
a) Thick and common lines should be of greater darkness and brightness keeping their thickness in the required range. Generally try for greater brightness increases the thickness of the line.
Fig. 2.1 Types of Lines.
Lead of a pencil should always be kept sharp, only a sharp lead is capable of producing clean black lines that sparkle with clarity. For wooden pencils, lead is opened approxiately 1/2" in length and then required shape of is made over sandpaper pad. For cone-shaped lead, pencil is rotated in hand while sharpening and a very fine cone is made having at least 3/8" length and ending in a perfect wedge point. For chisel-shaped pencil, grinding of lead is done from the two opposite side and for the lead to be used in compass, grinding is done from one side.
Fig. 2.2 Lead points
For right-handers, board should always be placed in such a position so that its working edge is on the left of the working person.This working edge of the drawing board serves as a reference vertical line. All vertical lines on the drawing sheet should be parallel to the working edge and all the horizontal lines should be perpendicular to it. The head of the T-square can be slided over the working edge of the board as shown in Fig. 2.3 (A). When the working edges of the board and head of the T-square are in close contact throughout the length, the working edge of the later becomes vertical. As blade of the T-square is at right angle to the head, the working edge of the blade comes in the horizontal direction when working edge of the head is made parallel to the reference vertical edge. By sliding the T-square upwards or downwards on the board, a set of horizontal lines may be drawn. Following steps may be taken while using a T-square:
i) Only one point is marked on any vertical line on the sheet from where horizontal line is to be drawn. It is not a good practice to mark points at the two ends which are later on joined with any straight edge.
ii) While using a T-square, working edge of the board and head of T-square must be in close contact throughout the contact-length. To draw a horizontal line, hold the head of T-square with the left hand, as shown in Fig. 2.3 (A), and apply a slight but uniform lateral pressure over the T-sqaure of the board. Slide the left hand until the working edge of blade just passes through the already marked point. Chang your hold sothat fingers remain on the T-square and the thumb is placed on the board, making sure that the position of the T-square is not changed, Fig. 2.3 (B). Draw horizontal line as shown.in Fig. 2.3 (C) with the right hand.
Fig 2.3 (A) Figure 2.3 (B)
Fig 2.3 (C) iii) While sliding the T-square, the blade should be allowed to move freely. If it is touching with the right hand, note or the instruments, either the angle of the Tsquare will be disturbed or the T-square will lose its connection with the working edge of the board and the lines drawn will not be horizontal.
To draw vertical line, place any of the triangle on the T-square with the vertical edge on the left as shown in Fig. 2.4. Slide the T-square up or down until the combination is vertically in the required range. Slide the triangle towards the left or the right until working edge 04 the triangle passes through the marked point, the triangle and blade of the T-square being in full contact. Hold the combination with the left hand and draw line with the right hand.
