RGPV

ENGINEERING GRAPHICS AND DESIGN

UNIT 1

1. Explain Principles of Engineering drawing and their significance ?

To record information on paper (or another surface), instruments and equipment are required. Even for drawings made freehand, pencils, erasers, and sometimes coordinate paper or other special items are used. The lines made on drawings are straight or curved (including circles and arcs). They are made with drawing instruments, which are the necessary tools for laying down lines on a drawing in an accurate and efficient manner. To position the lines, a measuring device, a scale, is needed.

The various instruments will be described in detail later, but the opening of this chapter will serve as an introduction. To draw straight lines, the T square, with its straight blade and perpendicular head, or a triangle is used to support the stroke of the pencil. To draw circles, a compass is needed. In addition to the compass, the draftsman needs dividers for spacing distances and a small bow compass for drawing small circles. To draw curved lines other than circles, a French curve is required. A scale is used for making measurements.

2.    What are 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.

1.     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.

2.     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.

3.     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. Whereas 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.

4.     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.

5.     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.

6.     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.

7.     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.

8.     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.

9.     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.

10. 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.

11. 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.    Explain 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.

Guide lines:

Always draw light guide lines for both tops and bottoms of letters, using a sharp pencil. Figure 1 shows a method of laying off several equally spaced lines. Draw the first base line; then set the bow spacers to the distance wanted between base lines and step off the required number of base lines. Above the last line mark the desired height of the letters.

Lettering in Pencil:

Good technique is as essential in lettering as in drawing. The quality of the lettering is important whether it appears on finished work to be reproduced by one of the printing processes or as part of a pencil drawing to be inked. In the first case, the penciling must be clean, firm, and opaque; in the second, it may be lighter.

5. Define Scales and its necessity?

 A scale is defined as the ratio of the linear dimensions of the object as represented in a drawing to the actual dimensions of the same.

Necessity

• Drawings drawn with the same size as the objects are called full sized drawing.
• It is not convenient, always, to draw drawings of the object to its actual size. e.g. Buildings, Heavy machines, Bridges, Watches, Electronic devices etc.
• Hence scales are used to prepare drawing at

      Full size

      Reduced size

      Enlarged size

      Types of Scale

6.    Explain Engineers Scale and Graphical Scale?

1. Engineers Scale :

The relation between the dimension on the drawing and the actual dimension of the object is mentioned numerically (like 10 mm = 15 m).

      Mechanical Engineers Scale:

These are divided and numbered in such a way that fractions of inches represent inches. The most common ranges are 1/8, 1/4, 1/2, and 1 in. To the inch. These scales are known as the size scales because the reduced size also represents the ratio of size, as for example one-eighth size.

Mechanical engineer's scales are almost always "full divided"; that is, the smallest divisions run throughout the entire length.

They are generally graduated with the marked divisions numbered from right to left, as well as from left to right, as shown in Figure Mechanical engineer's scales are used mostly for drawings of machine parts and small structures where the drawing size is more than one-eighth the size of the actual object.

      Civil Engineers Scale:

These are divided into decimals with divisions ranging from 10, 20, 30, 40, 50, 60, and 80 to the inch. Such a scale is usually full divided and is sometimes numbered both from left to right and right to leave. Civil engineer's scales are most used for plotting and drawing maps, although they are convenient for any work where divisions of the inch in tenths is required.

2.     Graphical Scale:

Scale is drawn on the drawing itself. This takes care of the shrinkage of the engineer’s scale when the drawing becomes old.

7.    What are the types Types of Graphical Scale?

1. Plain Scale
2. Diagonal Scale
3. Vernier Scale
4. Comparative scale

8.    Define plain scale in engineering graphics?

Plain Scale

• A plain scale consists of a line divided into suitable number of equal units. The first unit is subdivided into smaller parts.
• The zero should be placed at the end of the 1st main unit.
• From the zero mark, the units should be numbered to the right and the sub-divisions to the left.
• The units and the subdivisions should be labeled clearly.

9.    Define  diagonal scale in engineering scale?

Diagonal Scale

• Through Diagonal scale, measurements can be up to second decimal (e.g. 4.35).
• Diagonal scales are used to measure distances in a unit and its immediate two subdivisions; e.g. Dm, cm & mm, or yard, foot & inch.
• Diagonal scale can measure more accurately than the plain scale.

10.                       Define vernier scale in engineering graphics?

Vernier Scales

• Similar to Diagonal scale, Vernier scale is used for measuring up to second decimal.
• A Vernier scale consists of

(i)                a primary scale and

(ii)              a vernier.

• The primary scale is a plain scale fully divided in to minor divisions.
• The graduations on the vernier are derived from those on the Least count (LC) is the minimum distance that can be measured.