Introduction to Engineering Drawing

Introduction
Drawing is the Graphical means of expression of technical details without the barrier of a language. Engineering Drawing is the Universal Language for Engineers

Communication in engineering is necessary for effectively transferring one’s ideas to others.  While communicating,  we use our memory to remember objects, sense organs to perceive objects and mind to imagine objects. Our perception are coloured or modified by our past experiences.

We see things around us, perceive the objects and identify them by their names. Later, when we hear these names we can remember these items easily and imagine various features like the shape, size, color, functions  etc,. e.g. if I say that a particular object is having the shape of a cricket bat, it is easy for a high school student to imagine the shape of the object since he has seen a cricket bat and has perceived the object. If I say that a particular object looks like the Dendrite formed during solidification of a metal, then it is difficult for the same student to imagine, since he has not seen a dendrite. His question will “how do a dendrite look like?” when we say that dendrite means tree-like structure,  his imagination can go to different types of trees, but still he may not have a clear picture of the dendrite. If we show him a picture of a dendrite, he can very easily perceive that object.

One picture/drawing is equivalent to several sentences. It is not easy for anyone to make another person understand somebody’s face just by explaining the features. Even if several sentences are used to explain the features of the face, words it would be difficult for the listener to perceive  the image of the face.  However, if you show a sketch or a photograph of the person, all these sentences can be saved. i.e., We grasp information easily if it is illustrated with diagrams, sketches, pictures, etc.

Engineering drawing
Drawings help us in developing our thoughts and ideas in to a final product. 
Drawings are also necessary for engineering industries since they are required and are being used at various stages of development of an engineering product.  Engineering drawing is completely different from artistic drawing, which are used to express aesthetic, philosophical, and abstract ideas. In an industry, these drawings help both the technical as well as commercial staffs at various stages like:

·        conceptual stage

·        design stage

·        modification stage

·        prototype development stage

·        process and production planning

·        production

·        inspection

·        marketing

·        Servicing and maintenance,  etc.

What information should be available in an engineering drawing?
A  perfect engineering drawing should have the following information:

·        Shape of an object

·        Exact Sizes and tolerances of various parts of the object

·        The finish of the product

·        The details of materials

·        The company’s name

·        Catalogue no of the product

·        Date on which the drawing was made

·        The person who made the drawing

Drawings are the road maps which show how to manufacture products and structures. No industrial level construction/manufacturing of any (man-made) engineering objects is possible without engineering drawing.

What will you learn from this course?
In this course you will learn how to communicate technical information by:
Visualization – the ability to mentally understand visual information.
Graphics theory – geometry and projection techniques used for preparation of drawings.
Use of standards – set of rules for preparation of technical drawings.
Use of conventions – commonly accepted practices in technical drawings.
Tools – devices used to create technical drawings and models.
Applications – the various uses for technical drawings.
Developing an engineering drawing can be either by manual drawing or by computer graphics

Computer has a major impact on the methods used to design and create technical drawings. The tools are Computer aided design and drafting (CADD).Design and drafting on computer are cheap and less time consuming. Then why we go for manual drawing?

Computer cannot replace the drafting board and equipment as a learning tool. In schools, students are not allowed to use calculators up to calss 12. During this period if they use calculators from class 1, they will not improve their mathematical skills. After calss 12, i.e., once they have learned the basics of mathematics, they are allowed to use calculators and computers. Hence before use of the drafting software, their fundamentals regarding drawing should be clear.  If basic fundamentals are clear, better use can be made of the power of the software. To be an expert in technical drawing, this first course on Engineering (manual) drawing is the first step.

DRAWING INSTRUMENTS AND ACCESSORIES
The following set of instruments are required for ensuring perfection in manual drawing:

1.    Drawing board

Drawing board is made of soft wooden platens.  Almost perfect planning of the working surface of the drawing board is to be ensured. A  strip of hard ebony edge is fitted up in a groove on the shorter edge of the board and perfectly lined to provide the guide for the T-square.  The standard sizes of the drawing board is shown in Table 1.1 below. D2 size of drawing board is normally recommended for the First year Engineering students.

2.    Drawing Sheet

Drawing sheet is the medium on which drawings are prepared by means of pencils or pen.  Drawing sheets are available in standard sizes as shown in Table 1.2. A standard A0 size sheet is the one with an area of 1 m-2 and having dimensions of 1189 x 841. Each higher number sheet (A1, A2, A3, etc. in order) is half the size of the immediately lower  numbered sheet.  For drawing practice for first year engineering students A2 size is the preferred drawing sheet. The recommended  sizes obtained for various drawing sheets are shown in figure 1

 recommended  sizes obtained for various drawing sheets

3.    Mini-drafter

This is a device used to draw parallel or inclined lines very effectively with ease. This is mounted on the top left corner of the drawing board  by means of a clamping mechanism which is an integral part of the device. Figure 2 shows the photograph of a typical college level mini drafter.An L-shaped scale which is graduated in millimeters acts as the working edge of the mini-drafter. The L-Shaped scale also has a degree scale for angle measurement. The working edge can be moved to any desired location on the drawing board. 

Procedure for clamping the mini-drafter 
Set the protractor head with reference mark indexing zero degree, then fix the clamp of the  mini-drafter at the top left corner either along the top horizontal edge of the board or along  the left vertical edge of the board. With the drawing sheet placed underneath the scales of the mini-drafter, fix the drawing sheet to the drawing board with the scales of the mini- drafter aligned either with the vertical or the horizontal borderlines of the drawing sheet.

4.    Set squares

Set squares are a set of 45° set square and 30°-60° set-square, as shown in figure 3.They are used in conjunction with each other and with T-square to draw parallel, inclined and perpendicular lines. They are made of transparent acrylic. Each is having beveled edges with engraved mm or inch marking. The 45° set square generally has a protractor where as the 30°-60° set-square includes French curves.

5.    Compasses

These are used to draw arcs or circles. Generally two sizes of compasses: one large compass and the other a small spring bow compass  are commonly found. Each compass consist of a needle point and a pencil point. For drawing very large radius arcs, the pencil point leg can be removed from the knee joint and a lengthening bar can be inserted to increase the radius of the arc. Figure 4 shows the photograph of a compass.Figure 4 shows the photograph of a compass.

6.    Divider

Dividers are used to transfer lengths to the drawings either from scales or from the drawing itself. Similar to the compasses, two sizes of dividers are used in technical drawings. One large divider and the other small spring bow divider.

7.    Pencils / lead sticks/ pencil sharpener / eraser/etc:

The primary tool used in technical drawings is the pencil or lead sticks. Generally for technical drawings, the three grades of pencil used is HB, H and 2H . For different purposes, different grades of pencils are used. Pencil sharpener is is used to mend the pencils. Eraser  is used to erase the unnecessary part of the pencil drawing.

8.    French curve/Flexible curve

French curve is free form template make of acrylic and is used to draw a smooth curve passing through a  number of points. The outer profile of the French curve is adjusted such the smooth curve passes through more than three points and a curve passing through these lines are drawn. The next part of the curve is then drawn by using the next three points in addition to the last two points of the previous curve.A typical French curve is shown in figure 6.
A flexible curve is consists of a flexible, generally made of metallic wire coated with a thick rubber material. This can be bend in to any shape so that its working edge can be matched with a number of points and a smooth curve can be .

Layout of drawing sheets

Any engineering drawing has to follow a standard format. The drawing sheet consist of drawing space, title block and sufficient margins. After fixing the drawing sheet on the drawing board, margins should be drawn.   The layout should facilitate quick reading of important particulars. Drawings are prepared at various locations and shared and quick references should be located easily.
A typical drawing sheet is shown in figure 4 and consist of the following:

a.    Borders – space left all around in between the trimmed edges of the sheet. A minimum of 10 mm

b.    Filling margin – 20 mm minimum on left hand side with border included. This is provided for taking perforations. 

c.     Grid reference system – For all sizes of drawing sheets for easy location of drawing within the frame. The length and the width of the frames are divided into even number of divisions. Number of divisions for a particular sheet depends on complexity of the drawing. The length of the grids  lies between 25 mm to 75mm depending on the Drawing sheet size. The grids along vertical edges are named by capital letters where as grids along the horizontal edges are by numerals. Numbering and lettering start from the corner of the sheet opposite to the title box and are repeated on the opposite sides. The numbers and letters are written upright. Repetition of letters or numbers like AA, BB, etc. are practiced in case they exceed that of the alphabets.

f.     d.Title box – An important feature which is a must in every drawing sheet. The title box is drawn at the bottom right hand corner of every drawing sheet and provides technical and administrative details regarding the drawing/component. Though there are various dimensions for the title box, for Engineering students it is advisable to use a title box of size 170 mm x 65 mm.

g.    The title box is divided in to two zones: (a) part identification zone and (b) additional information zone. In the part identification zone, information like  the component identification number , name of the part, the legal owner of the drawing (i.e. the name of firm/component/etc will be highlighted where as in the additional information zone, technical information like symbols indicating the system of projection, scale of drawing,  method of indicating surface texture, geometric tolerances, etc. will be highlighted.