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Sketching Fundamentals

Learning Outcome When you complete this module you will be able to:

Make basic engineering sketches of plant equipment.

Learning Objectives Here is what you will be able to do when you complete each objective:

1. Set up a sketch using center lines. 2. Recognize standard view of an object. 3. Apply simple techniques for drawing parallel lines, circles, and ellipses. 4. Apply and recognize cross hatching methods in sectional drawings.

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INTRODUCTION A technical sketch may be a freehand sketch or one carried out with the use of some simple drawing instruments. Its purpose may be to describe a machine part to the person responsible for making the part or it may be to illustrate an idea without making an actual drawing. Whatever its particular purpose at the time, it is always a valuable aid to effective communication between engineers. To be able to sketch clearly and well is one of the most useful abilities that an engineer can possess. TECHNICAL SKETCHING Suppose for instance that in the operation of a plant, a special repair part is required, which must be made to certain measurements. By means of a sketch, all the necessary information regarding shape and size that is required for the making of the part can be conveyed in a few lines. Without a sketch it may be a difficult matter to explain just what is wanted verbally or in writing, and there will be a greater possibility of error. Sketching may be done freehand, and some hints on this are included in this module. Generally, however, it is just as quick to use a few simple drawing instruments and these make a much better job. Two triangles (60° and 45°), a scale, compasses, a pencil and eraser, are all that is required and their cost is very little. The proper way to make a sketch can be summed up in a few simple instructions.

1. Make sketches large. It is easier to draw on a large scale and the details can be shown more clearly.

2. Begin with the center lines, and make all necessary measurements from

these center lines.

3. Make the sketch in proper proportion so that it looks like the object it is supposed to represent.

4. Lines that are needed only for construction purposes, such as center lines,

should be drawn lightly and heavier lines used to outline the object being drawn.

5. Print the name of the article drawn beneath the sketch and put the names

of the principal parts on the sketch. Do not confuse a sketch with unnecessary detail. In answering a question show no more than is asked for, but show that to a large scale.

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Beginning a sketch by drawing the foundation or center lines is illustrated by the very simple example shown in Fig. 1. This sketch of a knuckle joint is accompanied by three preliminary steps or stages which illustrate how easily a freehand sketch of the right size and proportions can be made when the first half-dozen lines are placed where they belong. In making this particular sketch, first draw the horizontal center line and then four horizontal parallel lines spaced so that the inner part of the joint is somewhat wider than the outer sections of the forked member. Next draw a vertical center line for the pin. The sketch is now as represented in the upper left hand corner of the illustration. For the second stage, draw lines spaced to suit the pin diameter. Since these lines, excepting at the top, represent a part of the pin that is concealed, they may be drawn in lightly, and dotted in afterwards. Draw vertical lines to locate the end of the fork. The third and fourth stages are then achieved by adding the lines as shown. The resulting sketch shows good proportioning and has all prominent parts clearly drawn and ready for dimensioning.

Figure 1

Sketch of a Knuckle Joint

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Dimensioning The sketch so far only shows a shape, to be a useful working tool it must be given size. Fig. 2 shows an example of dimensioning on a simple sketch. Note that the points at which the measurements are to be taken are brought out as “extension lines”. The dimension line then extends the full distance between these extension lines, with a break for the dimension number. In all sketching, remember this: First draw the most important lines. These are the ones which establish the main proportions of the sketch and enable this proportioning to be done in a most simple and direct manner. The practical application of this fundamental principle in sketching will be illustrated by the different examples of sketching to follow.

Figure 2 Typical Dimensions on a Working Drawing

Sketches Requiring One View Only The sketch, in Fig. 3(a) shows an American Standard flat-head cap-screw and the sketch in Fig. 3(b) shows an American Standard button head machine bolt. One view of such simple parts is sufficient. An end view to show that the head is round is unnecessary.

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Figure 3

Examples of Sketches Requiring One View Only Sketches Requiring Two or More Views The bracket shown by the sketch in Fig. 4 requires three views to represent its shape. The plan view shows the circular form of the pad or raised part on the top of the bracket, and the end view at the right shows the shape of the lower part and the width of the central stiffening rib as well as the location of the bolt holes. If any one of the three views is removed, it will be evident that the sketch is incomplete.

Figure 4 Sketch of a Bracket Requiring Three Views to Show its Form

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Standard Views Fig. 5 serves to illustrate the standard views used in engineering drawing practice. Six views are shown but there are rarely more than three used. The most commonly used three are the Front Elevation, the Plan (or top view) and one Side Elevation.

Figure 5 Six Views of a House

Fig. 6 gives further illustration of the six standard views. Note that the front elevation is usually used as the key view, the remaining views are projected from this one.

Figure 6 Six Views

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In each case the view named shows the object as it appears when viewed from the stated direction. For example, the Right Side View shows the appearance of the object when viewed from the right side and so on. BASIC SKETCHING TECHNIQUES A few simple techniques that can be used to make sketching easier and to improve the quality of your drawings will be discussed in the following sections. Dividing a Line or a Space into Equal Parts Lay a scale or rule across the space such that the required number of divisions can be counted on the scale between the boundary lines. In the example shown this is 10 - 1/4" divisions. Mark these divisions on the paper and draw horizontal lines through these points, the space is then divided into 10 equal parts.

Figure 7 Dividing a Space into Equal Parts

A similar method may be used to divide a line into equal parts when drawing gear teeth for example as in Fig. 8.

Figure 8

Dividing a Line into Equal Parts

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Sketching Circles Small circles and arcs can be easily sketched in one or two strokes, as for the circular portions of letters, without any preliminary blocking in. One method of sketching a larger circle, Fig. 9 (a), is to first sketch lightly the enclosing square and mark the mid-points of the sides, then draw light arcs tangent to the sides of the square, and finally darken in the circle. Another method, Fig. 9 (b), is to sketch the two center lines, add 45° radial lines, then sketch light arcs across the lines at the estimated radius distance from the center, and finally sketch the required circle heavily.

Figure 9

Sketching Small Circles Another method, particularly for large circles, Fig. 10(a), is to mark the estimated radius on the edge of a card or scrap of paper, set off from the center as many points as desired, and then sketch the final heavy circle through these points. Fig. 10 (b), (c), and (d) show further suggested methods of sketching circles without the use of drawing instruments.

Figure 10

Sketching Large Circles

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Sketching Ellipses If a circle is viewed obliquely, it appears as an ellipse. With a little practice, you can learn to sketch small ellipses with a free arm movement, Fig. 11(a). Hold the pencil naturally, rest the weight on the upper part of the forearm, and move the pencil rapidly above the paper in the elliptical path desired; then lower the pencil so as to describe the several light overlapping ellipses, as shown at I. Another method, (b), is to sketch lightly the enclosing rectangle, I, mark the midpoints of the sides, and sketch light tangent arcs, as shown. Then, II, complete the ellipse lightly.

Figure 11 Sketching an Ellipse

Finally, the method shown in Fig. 12 is a practical style using a pencil and string.

Figure 12 Ellipse Using Pencil and String

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Measurement of Angles In the construction of many figures, diagrams, or illustrations of machinery parts, it becomes necessary to measure the angle between two intersecting lines. Fig. 13 illustrates the meaning of the generation of an angle by a rotating arm. Imagine a folding rule hinged as shown; if one section is held horizontally and the other rotated in an anticlockwise direction a circle will be traced out by the outer end of the rotating section, the hinge being at the center of the circle. When the folding ruler is closed, the angle between the two parts is zero. As the ruler is opened, the angle between the two parts increases.

Figure 13 Angular Parts of a Circle

One complete rotation measures out 360° or 90° in each of four-quarter turns. 1. Typical Angle Measurements

Fig. 14 illustrates typical angle measurements and Fig. 15 shows the instrument used to measure angles, the protractor, and the method used to measure an angle of 75° with this instrument.

Figure 14 Typical Angle Measurements

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A protractor is used for measuring and for laying out angles. To measure an angle with a protractor set the center on the vertex of the two lines forming the angle and measure off as shown in Fig. 15. (The vertex is the point of intersection of two lines.)

Figure 15

A Protractor 2. Uses of Triangles

Triangles may be used to draw lines set at certain angles to each other. The two triangles in use are the 45° triangle and the 60°, 30° triangle.

Fig. 16 shows these triangles set upon a horizontal line (which may be drawn by a T-square) and the angles which they make with that line.

Note: the definition of angles measured to the horizontal and to the vertical.

Figure 16 Angles Drawn Using Triangles

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Figure 16c, 16d

Angles Drawn Using Triangles Drawing Parallel Lines One of the main uses of triangles is in the drawing of lines parallel to, or at right angles to each other. Assuming the triangles are held by the fingers of the left hand, in the position shown in Fig. 17, and a horizontal line AB is drawn along the top edge of the upper triangle, then by holding the lower triangle firmly in the same position and sliding the other triangle upward or downward, lines may be drawn perpendicular to or at right angles to AB. Thus by sliding the upper triangle to the position shown by the dotted lines in the sketch, CD may be drawn parallel to AB and line DE at right angles to AB.

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Figure 17 Drawing Parallel Lines

Sectioning In order to give all the necessary details on a sketch, the object is sometimes shown in section or as it would appear if cut in halves, through the center line and the half next to the observer removed. When this is done, all the parts that are supposed to be cut through are cross hatched or section lined. Different metals can be indicated by the method of cross hatching used.

Figure 18

Section Lining

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Fig. 19 shows the American Standard symbols for section lining to indicate various materials while Fig. 18 illustrates the method of section lining parts that are adjacent to each other. Section lining generally should be drawn at an angle of 45°. Two adjacent parts should be sectioned in opposite directions. A third part, adjacent to both should be sectioned at 30° or 60°.

Figure 19

American Standard Symbols Fig. 20 shows a sectioned sketch of a gate valve. Note that the internal details of the valve are illustrated by correct sectioning, and the cross hatching used indicates that the material is brass. Fig. 21 shows the conventional method of indicating breaks in a long rod or tube or shaft where it is not convenient to draw the whole length, or where it is required to indicate the shape to be used.

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Figure 20 Section Lining in a Valve

Figure 21 Indicating Breaks

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Notes: