Notes on technical sketching and free hand lettering for engineering students

Preface

The modern engineer must know how to make drawings in order to know how to read them. Very Uttle of his time is spent at the drafting board, however. His drawings consist chiefly of sketches showing his ideas in more or less detail which are turned over to subordinates to be worked out and put in the conventional form.

In the solution of his engineering problems, he is obliged to think in three dimensions and a preliminary sketch stands in the place of a model which he can work over and examine in order to clarify and fix his ideas. Such sketches must often be made in great profusion, and dexterity with the pencil is frequently an inspiration to the brain.

One of the primary functions of a course in drawing is to cultivate and extend the faculty of thinking in space. To accomplish this successfully, the draftsman must learn how to express his ideas quickly and accurately. It is the purpose of this book to suggest how this may be done.

Worcester, Aug. 23, 1907. Alton L. Smith.

Chapter I Description Of Methods Of Representation

1. Probably the best way to describe a material thing is to make a picture of it, and a written language composed of hieroglyphs would prove satisfactory if it had to express only what we sense through the eyes. To depict odours, sounds or emotions would severely tax such a language as reference to the ancient Egyptian monuments will prove.

The written language of modern engineering construction has to deal chiefly with shape and size of material things. It is a picture language and its superiority over the language in common use will be quickly recognized, if one attempts to read a written description of a modern machine without the aid of an illustrative drawing.

A perusal of legal documents will show how difficult it is to express an idea or fact concisely and with exactness. In the modern sciences, extended terminologies permit this. There is, of course, a special vocabulary of technical terms used by engineers and shop workmen, and it would be possible to write a specification describing each part of a machine so it could be built, but to write and to read such a description would be a tedious and a costly process involving many chances for error. There was a time when construction was carried on in shops by oral directions from the foreman, or the workman made a part of a machine to suit his own notions, very much as some repair work is now done. If such methods had prevailed, the general use of our numerous modern contrivances would have been deferred to the remote future.

2. An engineering drawing must describe the machine or structure completely, exactly and concisely that it may insure economy of time for the maker and the reader. Most drawings used in engineering work are made mechanically with instruments, because most of them can be thus made more economically. There are, however, many drawings which an engineer or draftsman has to make, where it would be very impracticable to make them with instruments. Such are the innumerable preliminary sketches used in designing, the incidental sketches made for illustrative or explanatory purposes, sketches of parts of existing machines and sketches

S for work of which no record is preserved such as repair jobs. It is also true that some desirable forms of representation which can be made well and quickly free-hand, become expensive when drawn with instruments.

3. The problem in illustrative drawing is to produce a representation of an object having three dimensions on a flat surface having only two dimensions. The difficulty lies in properly representing the third dimension. The different methods for accomplishing this, their underlying principles and their adaptation for mechanical and free hand treatment will be considered.

Nearly all simple objects can be represented without ambiguity by a single outline drawing. A stick of wood looked at endwise tells nothing regarding its length. It might be a block or a long beam. By looking at it from some other point of view its true proportions are indicated. In the case of a sphere the outline from any point of view is a circle. There are three ways of completing this representation. If we draw a circle and write "sphere " on it, the size and shape are defined. If we draw two circles and indicate that these are views from two different points, the object is defined. If we draw a circle and shade its surface to represent the light and shade effect on the original, the object is defined. Each method has its advantages dependent on the use to which the drawing is put.

4. If a die is held close to the face, but far enough from the eyes to be seen distinctly, it will appear as in Fig. i, A. On closing the right eye it appears as at B and on closing the left eye, as at C. The fact is,

^ S Q we get a separate image of the object with each eye and if both eyes are open the

two images are mei'ged more or less into one. To test this, set up a card about 10 inches high edgewise between B and C and look at the figures from the top edge, the card serving as a partition to shield B from the right eye and C from the left. These two images will always occur when both eyes are used, but the difference between the two is not noticeable, except when the distance of the object from the eye is small as compared with the greatest horizontal dimension of the object. A drawing like A would not be a satisfactory representation of the die, but either B or C would be satisfactory. We therefore derive the conclusion that to make a satisfactory representation of an object, it should be drawn as seen with one eye. It is also true that if this dz'awing is to produce the same effect on the eye that the original object did, it should be looked at with one eye, the drawing being held at the same distance from the eye as when made.

Fig. 1.

Stereoscopic photographs made in pairs and viewed in the stereoscope give an increased reaUty to the third chmension. A similar effect is produced when a single pictm'e is viewed with one eye through a conical^tube, or thi'ough the closed hand.

Actual DfiAw/Nq ON Pictu/^e: Plane:

Fig. 2.

5. Referring to Fig. 2, A, we have a cube ABCD-H resting on the right end of the top of a table OPQ. Let the eye be placed at E and interpose a transparent piece of glass KLMN between it and the cube. The cube is visible to the eye, because light is reflected from its faces and as these faces have different degrees of illumination, their bounding edges appear conspicuously as lines. We may consider that the light reflected from any point, B, on the cube to the eye passes along a straight line, BE, through the glass at some point b. If we mark this point, b, on the glass, it will shut off our view of the corner, B, of the cube which is in line with it. In the same way, we may mark the other points on the glass where we see the other corners of the cube. These points are now connected forming lines which appear to coincide with the edges of the cube. That is, line ab shuts off edge, AB, be shuts off BC and in the same way, the others. The cube could now be removed and the figure abcd-h would produce the same effect on the eye that the edges of the cube did. It stands in place of, or represents the cube. Such a drawing is called a Linear Perspective. It is designated Linear because it represents lines, but not the light and shade nor the color efl'ect.

Fig. 2, B, is the actual drawing, as it is on the glass plane. The transparent plane is called the picture plane. The line ES from the eye to the center of the object is called the line of sight.

6. If in Fig. 2, A, the picture plane is revolved about the line X-Y as an axis, until it is perpendicular to the line ES, the perspective drawing would change to that shown in Fig. 3, A. Here the upright edges are not quite vertical and produce a false impression regarding the object. If they are made vertical, as in Fig. 3, B, the drawing will be an Artists' Perspective of the cube.

7. Referring again to Fig. 2, A, suppose the eye to be moved along the line SE so that it is much further away from the object. Then the angles which the light rays make with each other at E would become much less. If E were removed along SE to a very great distance, then the angle between the light rays would reduce practically to zero and the light rays would become practically parallel. The drawing on the picture plane would be called an Oblique Projection. It is so called, because if we projected, or threw on the picture plane, each point of the object by a series of parallel lines oblique to the picture plane, we should get the same result.

8. If the cube in Fig. 2, A, were placed further to the left with its front face parallel to the picture plane the perspective drawing of it would be like Fig. 4, A. If an oblique projection were now made by removing

Linear

PLRSPECnyE

Artists' Perspective

Fig. 3.

the eye to a great distance, the drawing would be Uke Fig. 4, B. Such a drawing is called a Cabinet Projection.

Its peculiar features are that one face of the object is shown in its true size and shape, while lines perpendicular to this face appear inclined at 45° and of one-half their true length.

Fig, 4.

a 6 y

OfiTHOQfiAPHIC

Projection d c h

Fig.

9. Referring again to Fig. 2, A, suppose the eye be removed to a great distance from the object along a line RE which is perpendicular to the picture plane. The light rays from the object to E would then become practically parallel to RE and therefore perpendicular to the picture plane.

The drawing on the picture plane would then become like Fig. 5 and it would be called an Orthographic Projection, because if the object were projected on the picture plane by lines perpendicular to that plane, we should get the same result. The term projection is always understood to mean orthographic projection unless otherwise stated. It is thus apparent that a projection drawing is merely a perspective drawing in which the eye is placed at a great distance from the object.

10. In Fig. 6, A, the cube, ABCD-H, is elevated slightly from the table and tm-ned so all its edges are obliciue to the plane of projection. It is also placed so its upright edges are all parallel to a side plane, not shown, but which is perpendicular to the table top and the plane of projection. If a projection of the cube is now made on the vertical plane, its actual shape will be like Fig. 6, B. If this drawing be compared with Fig. 2, B, a marked similarity is noticed, although there are also important differences. A projection made in this way is the basis of an Axometric Drawing.

11. If the cube in Fig. 6, A, had been placed so that the three edges meeting at a corner, as for instance B, were equallj^ inclined to the plane of projection, then the resulting drawing would have been like Fig. 6, C. This is called an Isometric Projection. Its peculiar features are that the three edges meeting at B are 120° apart and eciual in length. Any line of the drawing, as for instance, be, is shorter than the edge of the cube it represents.

12. If a drawing were made of the cube, which was exactly Uke Fig. 6, C, in shape, but in which the Hnes be, ab, bg etc. were each equal to the true length of the edge of the cube then we should have an Isometric Drawing. An isometric drawing is exactly like an isometric projection, but larger.

Fig. 6.

13. Solids have three principal dimensions; length, breadth or width and thickness or height. These terms are applied in various ways, depending on whether the object is large or small, movable or fixed and other characteristics. The essential thing to remember is that these three dimensions are perpendicular, each to the others. It might be difficult to agree on the length, breadth and thickness of so irregular a form as a potato, but three measurements could be arbitrarily assumed, which would have the essential feature of such dimensions, namely, mutual perpendicularity. In the case of most artificial forms, however, there is little difficulty in selecting these principal dimension lines or reference axes of measurement. Generally they will be partly or entirely determined by the physical peculiarities of the object. The rectangular block and the circular cylinder are the predominant artificial forms. In the former, the edges, and in the latter the axis of S3aTimetry and two perpendicular diameters would be selected. In the case of the sphere, three perpendicular diameters would be chosen If a drawing is to be useful as a guide in construction, it must satisfy the following conditions.

First, it must give an idea pictorially of the shape of the object.

Second, it must be of such a nature that all necessary dimensions and specifications can be appended. Third, when completed with all dimensions and specifications, the whole must be capable of being read with a minimum amount of study.

14. To permit satisfactory application of dimensions, the object must be placed so its projections show the lines of the object in their true length. To accomplish this, the object must be placed so two of its principal dimensions are parallel to the plane of projection. The result of this is to lose the third dimension, so that nearly always two or more projections of the object are required.

In Fig. 7, the object to be represented is a triangular pyramid JKL-0. It is placed inside a glass box ABCDEFGH, the back side of which is lacking. A working drawing for such an object should give the size and exact shape of the base, the length of the altitude and the location of the vertex relative to the base. The pyramid is therefore placed with its base parallel to the top face of the box and this location brings the altitude parallel to the front face of the box. One side of the base, JK, is parallel to the face BCFG. The pyramid is now projected onto each of the five faces. The joints of the box along edges AE, BF, CG and DH are then broken. Keeping the front face, ABCD, stationary, swing the top, bottom and two side faces about their hinge lines AB, CD, BC and AD, until they come into the same plane with the front face, as shown. II

Third Angle Projection _

Fig. 7.

15- The projection figures on the four revolved faces are now grouped about the central projection on the front face and certain features of their relations should be noted.

Considering the central projection, or front view, the principal one, it is seen that the view obtained from above the object, that is the top view, is placed above the front view; the view of the right side is placed at the right ; the bottom view below and the left view at the left of the front view. This is a logical arrangement and it is called third angle arrangement, or Third Angle Projection.

This is the arrangement of views used in nine-tenths of the drafting rooms in the United States. The other arrangement most used is known as first angle projection. With this method of grouping, the top view is placed below the front view, the bottom view above, the right view at the left and the left view at the right. It is entirely illogical, renders a drawing more difficult to construct and to read and has advantages in only a few instances. First angle projection is used for shop drawings in Great Britain, on the continent and by the other tenth of draftsmen in the United States.

i6. It should be noted next, that any point of the object, as the vertex 0, will have its projections in the front, left and right views, that is 0^, 0* and 0'^ on the same horizontal line. Also any point, as 0, will have its projections in the front top, and bottom views, that is, 0^ , 0^ and 0^ on the same vertical line. This relation between the views is a very important one, and it facilitates greatly the making and reading of the projections.

17. Inspection of the projections shows that two views, the front and top would suffice in this case to represent the object, and accommodate all necessary dimensions. Thus the top view shows the exact size and shape of the base and the location of the vertex, while the front view gives the exact altitude. Though two views are really necessary here, for some objects, one view would suffice. On the other hand, for some very irregular machine parts, five views supplemented by auxiliary sections, dotted lines and specifications are none too many, to make them intelligible.

i8. The names used in Fig. 7 for the different projections are those commonly employed. Others are also in use for architectural drawings and Descriptive Geometry. They are given in the following table.

Common Name

Front View Top View Right View Left View Bottom View

Architectural Drawings

Front Elevation

Plan

Right Elevation

Left Elevation

Plan

Descriptive Geometry

Vertical Projection Horizontal Projection Right Profile Projection Left Profile Projection Aux. Horizontal Projection

Chapter II Working Drawings

19. A working drawing is one used by a workman in actually making the machine or structure which it represents.

20. While many of the methods of representation described in the preceding chapter might be used for working drawings, the one last described is usually employed. Though somewhat deficient pictorially it has the following advantages. The process of making the projections is easily explained and generally understood. The drawings are composed principally of horizontal and vertical straight lines and circles, all of which are easily made with ordinary instruments. The large number of views available makes it possible to avoid the confusion of lines and figures which occurs when one view only is used.

21. There are two kinds of working drawings. A detail drawing shows each piece by itself with complete dimensions and specifications for its construction as shown in Fig. 10. An assembly drawing shows all the parts of a machine or structure assembled, or put together : Or it may show a group only of parts put together. A drawing of an engine would illustrate the first, while a drawing of the connecting rod of an engine would illustrate the second. An assembly drawing may be used in a pictorial way, merely, to give a general idea of the

H machine, in which case, much of hidden detail is not indicated and only the principal dimensions are given. Such a drawing may be used for assembl ng or for erecting the machine and then everything is shown in greater or less detail, but with only a few dimensions. An assembly drawing may be used as a shop drawing for actual construction, and then complete dimensions are given for every detail. It is obvious that only the simplest machines, tools or structures could be thus drawn. A shaft hanger or an arbor press would be illustrations. Such a drawing has an advantage over a detail drawing, in that there is less chance of error, both in making the drawing and in making the parts. In the case of the draftsman, the drawing helps to check the dimensions and in the case of the workman, he sees how the parts fit together. If a machine is made on the interchangeable system, this last feature is of no particular value.

22. Scale. Drawings should be made large enough so they can be easily and accurately read when covered with dimensions. For convenience in filing, most drafting departments have adopted standard sheet sizes, which are particularly adapted to their special line of work.

It is often desirable to place on one sheet all the parts of a machine constituting a natural group; for instance, all the parts of the tailstock of a lathe; or all the forgings; or all the castings. These conditions, therefore, will usually determine the scale. A bridge is drawn to a greatly reduced scale, the general run of machine parts are made full size, while instruments or machines with exceedingly small parts, like those of a watch, should be drawn larger than full size.

23. The scales in common use are as follows, 12 inches, 6 inches, 3 inches, 1^ inches to one foot for ordinary details of machinery; 1 inch, f inch, ^ inch, | inch, I inch and |- inch to one foot for larger structural work. A drawing made to a scale of 1^ inches to one foot is one in which 1 J inches on the drawing represents one foot in the object; that is, the drawing is I of full size. Full size is a very desirable scale, especially for the designer, because it conveys an exact idea of the size of the part and there is also less liability of errors in dimensioning.

SELECTION OF VIEWS

24. Select those views and the least number of views that will completely and clearly represent the object. Do not use two views, if one will suffice. If more than one view is necessary, one of them should be

IS the front view, as this makes it possible to project from points in one to corresponding points in the other projection by horizontal or vertical projecting lines. In Fig. 7 the object might have been placed so as to make a front view of what is now the right view. A corresponding change in the positions of the other views would have been necessary. A judicious use of dotted lines or of sections will often permit a reduction in the number of views, as is explained in Sections 29 and 30. The pipe fittings in Fig. 14, the Latch Handle in Fig. 8, A and the Spur Gear in Fig. 13 illustrate this. On the Lag Screw in Fig. 11, by specifying Sq. Head, an additional view is avoided. Also in the Anchor Bolt for concrete in Fig. 11, by giving the letter d after the |" dimension, a round bar is indicated thus making one view sufficient.

25. While limitations of space or the clearness of the drawing may sometimes decide otherwise, yet the desirable and the customary front view is that view which shows the object most characteristically and in a natural position. For a building, it would be the facade; for a bridge, the longtitudinal view; for a pulley, the view showing the radiating arms; for a machine, the view a workman gets as he stands at work before it. Some objects have no characteristic view, others have several and a moving part like the crank on an engine may have many natural positions. These are exceptional.

26. It is allowable to have one view showing the object with a part removed as in the Cylinder Cap, Fig. 10, while another view may show it entire or with a different part removed as in the Worm Gearing, Fig. 13. To condense a drawing, it is often desirable to break out a portion as in the Pulley, Fig. 10. Under Broken Ends in Fig. 8 is indicated how to break rods, pipes, structural steel etc., so as to suggest the shape of the cross-section.

An auxiliary view is sometimes needed which cannot be properly grouped with the other views. Its location or relation to the others must be very definitely specified by projecting lines or otherwise.

27. When two pieces are exactly alike except in some minor detail or dimension, it is often possible to make one drawing serve for both, as in the Hoist Arm Yoke, Fig. 10.

28. Threaded Parts are so numerous that to save the draftsman's time they have been conventionalized. The same is true regarding Riveting in Structural Work, and the Fittings in Pipe Systems. These are represented in Fig. 11 and Fig. 14.

USE OF DOTTED LINES.

29. Some draftsmen show all hidden edges, but this is plainly a mistake, for in many instances it produces only a confusion of lines and obscures the meaning of the drawing. Hidden edges should be shown, only when they contribute to the clearness of the drawing or give it a more finished appearance. Thus in Bevel Gears, Fig. 13, the dotted lines complete the representation partly shown by the half section. See also Section 83 for the proper way of making a dotted line. Figures 10, ir, 13 and 14 illustrate the use of dotted lines.

USE OF SECTIONS.

30. A slice or section is used to show the contour of an irregular shape or of a shape not clearly shown by dotted lines. The section of the puhey arm, Fig. 10, and of the Latch Handle, Fig. 8, A, are illustrations. The section may be specified in any- one of the three ways shown in Fig. 8, A. A section of this kind shows nothing more than the figure cut from the object by the sectioning plane.

31. A sectional view is one which shows not only the cut surfaces, but everj^hing back of them also. The chief use of a sectional view is to explain the internal construction of the object. In Fig. 8, B, is an illustration of this. Note that the center is not cut, as there is nothing inside of it to be explained. Sectioning planes may be taken in any way to facilitate the explanation of the object, but they are usually taken parallel to some one of the planes of projection. Unless the location of the cutting plane is perfectly obvious, it should be indicated by its projection on the plane to which it is perpendicular, where it will be shown as a line, marked as in Fig. 8, A.

32. Sometimes when it is not desirable to remove the part of the object in front of the sectioning plane, a dotted section may be used as in Fig. 14, V.

33. Section lining or cress hatching may be at any angle, so long as it is not parallel to the bounding lines of the surfaces sectioned. The angles generally used are 30°, 45° and 60°, which taken both ways give six directions. If two different pieces come together in the same section, as in Fig. 8, B, a different angle should be used for each piece; but for different parts of the same piece, even though disconnected, the same sectioning

Cast Iron

Use of Sections

Babbitt

Y^TPTTA

Brick Wood

Rubble.

Concrete

:0 o-o

V<i'

.0:0'= r>i

Sand

EAF!TH

Water

Broken Ends.

B£AM k

CHANNE.L

irf

Pipe

Wood

N

Angle

Tee r^ Max. Space Width

^^ FOf< A QIVEN WIDTH v-\ Section AT MN

Z Bah

A Sectional i//£i/v

Cross Lm/nq or o/rrsReA/T pa^ts

OF AREA.

^v

^r

3£CT/o/v LiNiNq McjT A/or

Fig. 8.

will be used. Width of spacing is determined by the smallest sectioned part of a given piece and large areas may have wider spacing than small ones. Fig. 8, C, shows satisfactory spacing for different areas.

34. The kinds of material used in construction have multiplied to such an extent in recent years, that it is no longer feasible to have a distinctive symbolical section lining for each. In Fig. 8 are shown some of the kinds which are in use chiefly in a pictorial way. On a working drawing these are seldom used, plain sectioning and definite printed specification of the material being the custom.

35. Section lining is one of the most tedious parts of a draftsman's work. To secure uniform spacing on large areas, a special instrument is often used. For ordinary work, spacing by the eye is sufficiently accurate. Make the first few spaces carefully and then look back to them frequently for a gage on the other spaces, rather than at the last spaces made. For free-hand sectioning, the slant from the upper right to the lower left is the one to be preferred for a right handed draftsman.

USE OF SHADE LINES.

36. If an object is illuminated by direct light it will cast a shadow. The outline of this shadow is composed of the shadows of certain edges or lines of the object. A line which is said to cast shadow is one which separates a lighted surface from one that is in shade. In a projection drawing, these lines are made twice as heavy as the other lines and are called shade lines. The object of using shade lines on a drawing is for the pictorial effect only. They impart an appearance of solidity. Shade lines are now seldom used on working drawings, however.

37. Light is assumed to be coming down in parallel rays over the left shoulder of the observer as he stands looking at the object which is supposed to be built out solid on its projection. The slant of the rays is such that their projections on the planes of projections have an inclination of 45°. To select the lines that cast or form the shadow, the pencil may be set up as a light ray, as shown in Fig. 9 P, and applied to the projection.