CBSE Book Class 11 Engineering Graphics Lines Angles

LINES, ANGLES, LETTERS, DIMENSIONING AND RECTILINER FIGURES

1.1 INTRODUCTION

You are welcome to the subject of Engineering Graphics. Though it is a new subject for us still we have studied a bit in our previous classes. Let us recall the simple geometrical constructions where we have drawn them without much caring about the thickness of lines etc. and written the steps of construction also. At this level of study we shall study geometrical constructions again but here we shall use proper lines to construct them and there is no need of writing steps of construction. We have studied systeme International& units (S.I.Units).

These units are used throughout the world. Similarly, many languages are spoken in the world and different dialectsare used for expression. We can understand few of them but not all of them. It will not be a surprising fact if we say that there is a language which is understood by a class of persons, known as ‘Engineers’ who speak the same dialects and understand each other very well. This is the universal language of Engineers known as ‘Engineering Graphics’. In Engineering Graphics we use different lines,symbols and conventions.

Each line, symbol and convention has a definite purpose and sense to convey. Thus, we can say thatEngineering Graphics is the language of Engineers. It is a graphic representation of thinking, planning and a language of every technical person who uses to communicate his ideas clearly to other technical persons. In the age of automation, Engineering Graphics has greatly developed. Without the fundamental knowledge of Graphics a student cannot succeed in Industry. There are many uses of Engineering Graphics in practical field, especially in modern industry.

These Engineering Graphics are widely used in Mechanical, Electrical, Electronics, Automobiles, Marine, Aeronautical, Chemical, Computer and instrumentation industry etc. In Civil Engineering and Architecture, Graphics are used to draw top view and front view of buildings and structures. 

In this chapter we shall learn about the basics of this universal language of Engineers. In Engineering Graphics we make extensiveuse of various types of lines, symbols and conventions (standard symbols). We shall learn about the standard letters and Numerals. These letters and Numerals are very necessary for writing notes, dimensions and the information needed on Graphics. Lastly, weshall undertake some simple constructions on rectilinear figures such as angles, triangles, quadrilaterals and polygons. We will also learn about dimensioning as it is an important aspect of our subject ‘Engineering Graphics’. These methods dimensioning allows a standard shape and size of the product to be.

ACTIVITY

1. Measure the size of your books, note books and files etc. Draw a rough sketch of these in your sketch book and write their sizes.

2. Collect different types of cartons of consumer goods such as : Medicine cartons, Tea cartons, Chocolate cartons, Toothpaste Cartons etc. Identify their shapes and note them in the sketch book.

3. Note down the different shapes of your rooms in your house, Measure their length, width and height. And write these measurements in your sketch book.

4. Note different types of rectilinear figures in your study room such as note books, geometry box, pencil set, square etc.

1.2.1 LINES

First of all we shall study about the quality and use of different lines, symbols and conventions which we shall use at this stage. We shall come to know about different pencils used for this purpose.

These lines are black (Bold), medium and thin. In Engineering graphics we use the term ‘Thick’, ‘Extra wide’ and ‘wide’ for these lines. The General ratio of these lines is 4 : 2 : 1. For drawing these lines with pencil, we use ‘HB’, ‘2H’ and ‘4H’. These pencils are used withequal pressure of hand. When we are confident and habitual with the quality of these lines, we can use a single pencil of 0.3 mm or 0.5 mm and get the same result by applying different pressure on the pencil.

It will be interesting to not that all lines should be sharp, distinct, and uniform in thickness and according to the type of line to be used.

Now let us learn more about these lines.

1. Continuous thick line : It is used as a boundary line of the drawing sheet and title block lines at the bottom of the drawing sheet. For showing the end of threaded portion and visible out lines of the object, so that the form of the object is at once clear. It is also known as object line.

2. Thin continuous line : This line is used to show projection lines, dimension lines, extension lines etc. These are drawn continous thin.

3. Given Line : It is drawn with medium thickness to show the given lines and angles.

(a) Construction lines : These lines are drawn for constructing drawings. These are continuous thin lines.

(b) Extension line : Extension lines are projected from the out lines at right angle to the boundary line to be dimensioned. These lines extend at least 3 mm beyond the dimension line.

(c) Dimension line : This line is drawn thin and at a distance of 6 to 8 mm away from the boundary line and parallel to it. Dimension line is terminated by the arrow heads on both sides touching projection line, extension line or centre line.

(d) Leader line : Leader line is an inclined line followed by a horizontal line. Numerals or notes are written on the horizontal portion of the leader line.

4. Hidden Line : This line is used where outline is not visible at viewing surface. This line is represented by thick short dashes evenly spaced. This line is a broken line, composed of short strokes of equal lengths (approximately 2 to 3 mm) and spaced at equal distances (1 mm), must be used.

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