RTMNU ENGINEERING GRAPHICS-I

Unit 1

Question banks

1.     What is the Role of Engineering Drawing?

The ability to read drawing is the most important requirement of all technical people in any profession. As compared to verbal or written description, this method is brief and clearer. Some of the applications are: building drawing for civil engineers, machine drawing for mechanical engineers, circuit diagrams for electrical and electronics engineers, computer graphics for one and all.

The subject in general is designed to impart the following skills.

1. Ability to read and prepare engineering drawings.

2. Ability to make free - hand sketching of objects.

3. Power to imagine, analyze and communicate, and

4. Capacity to understand other subjects.

2.    Explain the Use of various drawing instruments?

Drawing paper is made up of variety of qualities and manufactured in sheets or rolls. White drawing papers that will not turn yellow with age or exposure are used for finished drawings, maps, charts, and drawings for photographic reproduction. For pencil layouts and working drawings buff detail papers are preferred as they are easier on the eyes compared to white papers.

Tracing paper

Tracing papers are thin papers, natural or transparent, on which drawings are traced, in pencil or ink, and from which blueprints or similar contact prints can be made. In most drafting rooms original drawings are penciled on tracing papers, and blueprints are made directly from these drawings, a practice increasingly successful because of improvements both in papers and in printing.

Thumbtacks

The best thumbtacks are made with thin heads and steel points screwed into them. Cheaper ones are made by stamping. Use tacks with tapering pins of small diameter and avoid flat-headed (often colored) map pins, as the heads are too thick and the pins rather large.

Pencils

The basic instrument is the graphite lead pencil, made in various hardness’s. Each manufacturer has special methods of processing design to make the lead strong and yet give a smooth clear line.

In the left there is an ordinary pencil, with the lead set in wood and the other is a semi-automatic pencil with thinner leads. Both are fine but have the disadvantage that in use, the wood must be cut away to expose the lead (a time-consuming job), and the pencil becomes shorter and shorter until the last portion of it is to be discarded. Where as semi automatic pencils, with a chuck to clamp and hold the lead, are more convenient, has a plastic handle and changeable tip (for indicating the grade of lead).

Drawing pencils are graded by numbers and letters from 6B which is very soft and black, to 5B, 4B, 3B, 2B, B, and HB to F, the medium grade; then H, 2H, 3H, 4H, 5H, 6H, 7H, and 8H to 9H, the hardest. The soft (B) grades are used primarily for sketching and rendered drawings and the hard (H) grades for instrument drawings.

Pencil Pointer

After the wood of the ordinary pencil is cut away with a pocketknife or mechanical sharpener, the lead must be formed to a long, conic point.

A pencil pointer is a tool for sharpening a pencil’s writing point by shaving away its worn surface.

Erasers

The Ruby pencil eraser, large size with beveled ends, is the standard. This eraser not only removes pencil lines effectively but is better for ink, as it removes ink without seriously damaging the surface of paper or cloth.

Art gum or a soft-rubber eraser is useful for cleaning paper and cloth of finger marks and smears that spoil the appearance of the completed drawing.

Penholders and Pens

The penholder should have a grip of medium size, small enough to enter the mouth of a drawing-inkbottle easily yet not as small as to cramp the fingers while in use. A size slightly larger than the diameter of a pencil is good.

Triangles

Triangles, are made of transparent hard or other plastic material. Through internal strains they sometimes lose their accuracy. Triangles should be kept flat to prevent warping. For ordinary work, a 6- or 8-in. 45° and a 10-in. 30-60° are good sizes.

The T squares

The fixed-head T square, is used for all ordinary work. It should be of hardwood, and the blade should be perfectly straight. The transparent-edged blade is much the best. A draftsman will have several fixed-head squares of different lengths and will find an adjustable-head square of occasional use.

Curves:

Curve rulers, called "irregular curves" or "French curves," are used to draw curved lines other than circular arcs. The patterns for these curves are laid out in parts of ellipse and spirals or other mathematical curves in various combinations. For the student, one ellipse curve of the general shape or one spiral, either a logarithmic spiral is a useful small curve.

The Case Instruments:

We have so far, except for curves considered only the instruments needed for drawing straight lines. A major portion of any drawing is likely to be circles and circle arcs, and the so called “case” instruments are used for these.

Divider

It is used for laying off or transferring measurements.

Next is the large compass with lengthening bar and pen attachment. The three “bow” instruments are for smaller work. They are almost always made without the conversion feature.

The ruling pen is used for inking straight lines.

Protractor is used to measure angles up to 180°.

3.    What is the Guide lines used for lettering?

Lettering

Graphic representation of the shape of a part, machine, or structure gives one aspect of the information needed for its construction. To this must be added, to complete the description, figured dimensions, notes on material and finish, and a descriptive title—all lettered, freehand, in a style that is perfectly legible, uniform, and capable of rapid execution. As far as the appearance of a drawing is concerned, the lettering is the most important part. But the usefulness of a drawing, too, can be ruined by lettering done ignorantly or carelessly, because illegible figures are apt to cause mistakes in the work.

4.    Explain single stroke lettering ?

Single stroke lettering:

By far the greatest amount of lettering on drawings is done in a rapid single-stroke letter, either vertical or inclined, and every engineer must have absolute command of these styles.

The term "single-stroke," or "one-stroke," does not mean that the entire letter is made without lifting the pencil or pen but that the width of the stroke of the pencil or pen is the width of the stem of the letter.

5.    Draw an example Layout of Drawing Sheets?

Engineering drawings are prepared on standard size drawing sheets. The correct shape and size of the object can be visualized from the understanding of not only its views but also from the various types of lines used, dimensions, notes, scale etc. For uniformity, the drawings must be drawn as per certain standard practice.

Drawing sheet layout

The layout of a drawing sheet used on the shop floor is shown in Fig.1. The layout suggested to students is shown in Fig.2.

Figure 1 General features of a drawing sheet

                                  Figure 2  Layout of sheet for a class work

6.    Explain the Sizes of Drawing Sheets?

The standard drawing sheet sizes are arrived at on the basic Principal of x: y = 1: and xy = 1 where x and yare the sides of the sheet. For example, A0, having a surface area of 1 Sq. m; x = 841 mm and y = 1189 mm. The successive sizes are obtained by either by halving along the length or doubling the width, the area being in the ratio 1: 2. Designation of sizes is given in Fig.3 and their sizes are given in Table 1. For class work use of A2 size drawing sheet is preferred.

Table 1

Figure 3

7.    Explain Title block for the drawing sheet with a neat sketch?

The title block should lie within the drawing space at the bottom right hand comer of the sheet. The title block can have a maximum length of 170 mm providing the following information.

1. Title of the drawing.

2. Drawing number.

3. Scale.

4. Symbol denoting the method of projection.

5. Name of the firm, and

6. Initials of staff who have designed, checked and approved.

The title block used on shop floor and one suggested for student’s class work are shown inFig.4& Fig. 5.

Figure 4 Shop floor drawing sheet layout

Figure 5 Students class work

8.    Different types of lines used in drawing practice?

Just as in English textbook the correct words are used for making correct sentences; in Engineering Graphics, the details of various objects are drawn by different types of lines. Each line has definite meaning and sense to convey.

IS 10714 (Pint 20): 2001 (General principles of presentation on technical drawings) and SP 46:2003specify the following types of lines and their applications:

1. Visible Outlines, Visible Edges: (Continuous wide lines) the lines drawn to represent the visible outlines/ visible edges / surface boundary lines of objects should be outstanding in appearance.

2. Dimension Lines: (Continuous narrow Lines) Dimension Lines are drawn to mark dimension.

3. Extension Lines: (Continuous narrow Lines) There are extended slightly beyond the respective dimension lines.

4. Construction Lines: (Continuous narrow Lines) Construction Lines are drawn for constructing drawings and should not be erased after completion of the drawing.

5. Hatching / Section Lines: (Continuous Narrow Lines)

Hatching Lines are drawn for the sectioned portion of an object. These are drawn inclined at an angle of 45° to the axis or to the main outline of the section.

6. Guide Lines: (Continuous Narrow Lines)

Guide Lines are drawn for lettering and should not be erased after lettering.

7. Break Lines: (Continuous Narrow Freehand Lines)

Wavy continuous narrow line drawn freehand is used to represent break of an object.

8. Break Lines: (Continuous Narrow Lines with Zigzags)

Straight continuous arrow line with zigzags is used to represent break of an object.

9. Dashed Narrow Lines: (Dashed Narrow Lines)

Hidden edges / Hidden outlines of objects are shown by dashed lines of short dashes of equal lengths of about 3 mm, spaced at equal distances of about 1 mm. The points of intersection of these lines with the outlines / another hidden line should be clearly shown.

10. Center Lines: (Long-Dashed Dotted Narrow Lines)

Center Lines are drawn at the center of the drawings symmetrical about an axis or both the axes. These are extended by a short distance beyond the outline of the drawing.

11. Cutting Plane Lines:

Cutting Plane Line is drawn to show the location of a cutting plane. It is long-dashed dotted narrow line, made wide at the ends, bends and change of direction. The direction of viewings shown by means of arrows resting on the cutting plane line.

12. Border Lines

Border Lines are continuous wide lines of minimum thickness 0.7 mm.

9.    Explain Principles of Dimensioning?

Some of the basic principles of dimensioning are given below.

1. All dimensional information necessary to describe a component clearly and completely shall be written directly on a drawing.

2. Each feature shall be dimensioned once only on a drawing, i.e., dimension marked in one view need not be repeated in another view.

3. Dimension should be placed on the view where the shape is best seen (Fig.7)

4. As far as possible, dimensions should be expressed in one unit only preferably in millimeters, without showing the unit symbol (mm).

5. As far as possible dimensions should be placed outside the view (Fig. 8).

6. Dimensions should be taken from visible outlines rather than from hidden lines (Fig. 9).

Figure 7

Figure 8

Figure 9

7. No gap should be left between the feature and the start of the extension line (Fig.10).

Figure 10 Marking of Extension lines

8. Crossing of center lines should be done by a long dash and not a short dash (Fig.11).

Figure 11 Crossing of Centre lines

10.                       Basics of Orthographic Projections

i)                   Basic principles of orthographic projection

If straight lines are drawn from various points on the contour of an object to meet a plane, the object is said to be projected on that plane. The figure formed by joining, in correct sequence, the points at which these lines meet the plane, is called the projection of the object. The lines from the object to the plane are called projectors.

Methods of Projection:

Following four methods of projection are commonly used,

1) Orthographic projection.

2) Isometric projection.

3) Oblique projection.

4) Perspective projection.

In the above methods 2, 3 and 4 represent the object by a pictorial view as eyes see it. In these methods of projection, a three-dimensional object is represented on a projection plane by one view only, while in the orthographic projection an object is represented by two or three views on the mutual perpendicular projection planes. Each projection view represents two dimensions of an object. For the complete description of the three-dimensional object at least two or three views are required.

11.                       What is Orthographic projections with a help of a neat sketch?

Let us suppose that a transparent plane has been set up between an object and the station point of an observer's eye . The intersection of this plane with the rays formed by lines of sight from the eye to all points of the object would give a picture that is practically the same as the image formed in the eye of the observer. This is perspective projection.

Perspective projection. The rays of the projection converge at the station point from which the object is observed.

If the observer would then walk backward from the station point until he reached a theoretically infinite distance, the rays formed by lines of sight from his eye to the object would grow longer and finally become infinite in length, parallel to each other, and perpendicular to the picture plane. The image so formed on the picture plane is what is known as "orthographic projection." See Fig.

2.

Orthographic projection.

Basically, orthographic projections could be defined as any single projection made by dropping perpendiculars to a plane. However, it has been accepted through long usage to mean the combination of two or more such views, hence the following definition has been put forward: Orthographic projection is the method of representing the exact shape of an object by dropping perpendiculars from two or more sides of the object to planes, generally at right angles to each other; collectively, the views on these planes describe the object completely. (The term "orthogonal" is sometimes used for this system of drawing.)

12.                       What are the different types of Orthographic views?

The rays from the picture plane to infinity may be discarded and the picture, or "view," thought of as being found by extending perpendiculars to the plane from all points of the object, This picture, or projection on a frontal plane, shows the shape of the object when viewed from the front, but it does not tell the shape or distance from front to rear. Accordingly, more than one projection are required to describe the object.

           The frontal plane of projection. This produces the front view of the object.

In addition to the frontal plane, imagine another transparent plane placed horizontally above the object The projection on this plane, found by extending perpendiculars to it from the object, will give the appearance of the object as if viewed from directly above and will show the distance from front to rear.

The frontal and horizontal planes of projections. Projection on the horizontal plane produces the top view of the object.

If this horizontal plane is now rotated into coincidence with the frontal plane, the two views of the object will be in the same plane, as if on a sheet of paper.

The horizontal plane rotated into the same plane as the frontal plane.

Now imagine a third plane, perpendicular to the first two. This plane is called a "profile plane," and a third view can be projected on it. This view shows the shape of the object when viewed from the side and the distance from bottom to top and front to rear.

The three planes of projection: frontal, horizontal and profile. Each is perpendicular to other two.

The horizontal and profile planes are shown rotated into the same plane as the frontal plane (again thought of as the plane of the drawing paper). Thus, related in the same plane, they give correctly the three-dimensional shape of the object.

The horizontal and profile planes rotated into the same plane as the frontal plane. This makes it possible to draw three views of the object.

In orthographic projection the picture planes are called "planes of projection"; and the perpendiculars, "projecting lines" or "projectors."

13. A point A is 20 mm above HP and 30 mm in front of VP. Draw its projections?

Solution steps:

1) Draw reference line XY.

2) Mark a point a´ at a distance of 20 mm above XY.

3) Through this point draw a perpendicular line to XY and mark the top view a at a distance of 30 mm below XY

14. A point D is 20 mm below HP and 30 mm in front of VP. Draw its projections.

Solution steps:

1) Draw reference line XY.

2) Mark a point d´ at a distance of 20 mm below XY.

3) Through this point draw a perpendicular line to XY and mark the top view d at a distance of 30 mm above XY.

15. Draw the projections of the following points on the same ground line, keeping the distance between projectors equal to 25 mm.

Point A, 20 mm above HP, 25 mm behind VP;

Point B, 25 mm below HP, 20 mm behind VP;

Point C, 20 mm below HP, 30 mm in front of VP;

Point D, 20 mm above HP, 25 mm in front of VP;

Point E, on HP, 25 mm behind VP;

Point F, on VP, 30 mm above HP;

16. Draw projections of a 80 mm long line PQ. Its end P is 10 mm above HP and 10 mm in front of VP. The line is parallel to VP and inclined to HP at 30°.

Solution steps:

1) Draw the plan and elevations of the end point P.

2) Draw plan PQ of the line at an angle of 30° to XY.

3) Draw the projector of Q.

4) From the elevation of end point P draw a line parallel to XY meeting projector of Q at Q´.

5) P´Q´ is the elevation and PQ is the plan of the line.

17. A straight line AB of 40 mm length has one of its ends A, at 10 mm from the HP and 15 mm from the VP. Draw the projections of the line if it is parallel to the VP and inclined at 30° to the HP. Assume the line to be located in each of the four quadrants by turns.