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Engineering Drawing

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DRAWING, ENGINEERING, the general term for the drawing used in the industrial world by engineers and designers, mechanical, architectural, structural, etc., as the formula in which is expressed and recorded the ideas and information necessary for the building of machines and structures. It is distinguished from drawing as a fine art in that it is not pictorial representation but a complete graphical language in which exact and positive infor mation is given regarding every detail of the structure or machine to be built. Since it describes the object as it actually is to be and does not show it in pictorial form as it would appear to the eye it can be read and understood only by one trained in its use. When this language is written exactly and accurately it is done with the aid of mathematical instruments and is called mechanical drawing. When done freehand it is known as technical sketching. As it cannot be read aloud like a written language it must be inter preted by forming a mental image of the subject represented, and the engineer in reading a drawing that would appear to be only a complicated mass of lines has as clear a picture of the structure standing in space as if it were actually before him. Apart from its practical utility, the value of teaching drawing in the schools is in the training of this constructive imagination, the perceptive ability to think in three dimensions, to visualize quickly and accu rately, to build up a clear mental image, a training useful not only to professional designers but to all who may be interested in technical industries.

The basis of engineering drawing is orthographic projection, which may be defined as the method of representing the exact shape of an object by two or more views on planes at right angles to each other, by dropping perpendiculars from the object to the planes. There are two systems in use; the first and older is first angle projection, in which the object is assumed to be placed in the first quadrant of the four dihedral angles formed by the intersection of two reference planes called the co-ordinate planes or planes of projection,fig. I (cf. DESCRIPTIVE GEOMETRY), and its points projected to these planes, the horizontal plane (H) then revolved to coincide with the vertical plane (V), the two being represented by the plane of the drawing paper. A third or profile plane (P) perpendicular to the H and V planes is used for a third view if necessary. Fig. 2 shows an object in the first angle and fig. 3 the resulting arrangement of views when the planes are opened. This is the system in present use in Great Britain and other European countries except the Netherlands. It was used in the United States until about 1890 when the industrial works began to change to the newer system of third angle jection which in a few years entirely replaced the former method. It is significant that this movement originated in the shops instead of in the colleges, after experiments with workmen demonstrated that they could read third angle drawings much more easily than those made in the first angle.

Third angle, or, as it is called in Europe, American projection, assumes the object to be placed in the third quadrant of the co-ordinate planes, and the observer to be looking through the planes at the object, as shown in fig. 4. These planes when opened into one plane give an arrangement of views as in fig. 5. Thus the object may be thought of as surrounded by a glass box with its sides hinged to each other (fig. 6), the object projected to these sides and the box opened up into one plane. In both sys tems the projection on the front plane is known as the front view, elevation or vertical projection, that on the top plane the top view, plan or horizontal projec tion, that on the side plane as the side view or end view, side or end elevation or profile projection. For a simple object two views are often sufficient, others may require three or more. Sometimes the left side view can be used to better advantage than the right side. In some cases the bot tom view, and more rarely the back view will be required. Fig. 7 shows the box as it opens and indicates the positions of these different views.

It is a growing practice in the United States to teach elementary projection drawing without reference to the planes of projec tion, by explaining that the problem is to represent a solid, with three dimensions, on a flat sheet of paper having only two dimen sions, in such a way as to tell its exact shape, and that this is done by drawing a system of "views" of the object as seen from different positions and arranging these views in a definite man ner, each view showing two of the three dimensions. Taking, for example, the block shown in pic torial form in fig. 8, if the ob server imagine himself as in a position directly in front (theoreti cally at an infinite distance, practically at a reasonable seeing dis tance but imagining the rays of light from each point to his eye as parallel) its front view would appear as in fig. 9a. This view tells the length and height but not the width of the block nor the depth of the notch. Then let the observer change his posi tion so as to look down from directly above the block. He will see the top view (fig. 9b), giving the length and width, and the shape of the notch. It is neces sary to have another view from the side in this case to show the shape of the triangular part. Fig.

9c is the right side view. These three views arranged in their natural position with the top view directly above the front view and the right side view to the right of the front view, completely describe the shape of the block.

Note that in the top and side views the front of the block always faces toward the front view.

The argument for this teaching method is that the student visualizes the object itself without being confused in trying to visualize the projections. Its success is indicated in that some engineering schools are now teaching the whole subject of de scriptive geometry without using the reference planes.

Reading a Drawing.

A line on a drawing always indicates either an intersection of two surfaces, as in the projection of a prism, or a contour, as in the projection of a cylinder (fig. 1o), a visible edge being represented by a full line and an invisible one by a "dotted" line, i.e., a line made up of short dashes. One can not read a drawing by looking at one view. Each line on the view (except a contour line) denotes an abrupt change in direc tion, but the corresponding part of another view must be con sulted to tell what the change is.

For example, a circle on a front view might mean either a hole or a projecting boss. A glance at the side view or top view will tell immediately which it is. In reading a drawing one should first gain a general idea of the shape of the object by a rapid survey of all the views given, then should select for more careful study the view that best shows the char acteristic shape, and by referring back and forth to the adjacent views see what each line represents. In looking at any view one should always imagine that it is the object itself, not a flat pro jection of it, that is seen, and in glancing from one view to an other the reader should imagine himself as moving around the object and looking at it from the direction the view was taken.

Auxiliary Views.

A surface is shown in its true shape when projected on a plane parallel to it. In the majority of cases an object may be placed with its principal faces parallel to the three reference planes and be fully described by the regular views.

Sometimes however the object may have one or more inclined faces whose true shape it is desirable or necessary to show, espe cially if irregular in outline. This is done by making an auxiliary view looking straight against the surface, that is, imagining a projection on an extra or auxiliary plane parallel to the inclined surface, therefore perpendicular to the same reference plane to which the inclined surface is perpendicular, and revolving it into the plane of the paper. There are three kinds of auxiliary views, first auxiliary elevations (fig. I 1), made on planes which are perpendicular to the hori zontal plane but at an angle with the vertical plane, or in other words the kind of views that would be seen if one walked around the object starting from the position at which the front view is seen. Thus an auxiliary elevation would have the same height as the front view. The second kind, called sometimes left and right auxiliary views, are used much more frequently. They are made on planes perpendicular to V but inclined to H. Fig. 12 is an example, showing that the width of the auxiliary view is the same as the width of the top view. Third, front and rear auxiliary views, on planes perpendicu lar to the profile plane but inclined to H and V, in which the width of the auxiliary view is the same as the width of the front view (fig. 13). Often an auxiliary view will save making one or more of the regular views and at the same time show the shape or construction of the object to better advantage. They are used extensively in the drawing of machine parts and usually are only partial views showing the inclined surface alone. In fig. 14 a front view, partial top view and two partial auxiliary views, describe the shape of the piece in the simplest way.

Engineering Drawing

Sectional Views.

When an object is solid or the interior simple the invisible parts can be represented satisfactorily by dotted lines, but if there is much interior detail, especially if the object is made in more than one piece, the dotted lines become confusing and hard to read. In such cases a view is made "in section," as if for that particular view a part of the object were supposed to be cut away and removed, exposing the interior.

This view is known as a sectional view or simply a section. If the object is symmetrical the cutting plane is usually passed through one of the main axes and the front half imagined as re moved. The exposed cut surface of the material is indicated by "section-lining" or "cross-hatching" with uniformly spaced fine lines. It must be understood clearly that in thus removing the front portion in order to show the sectional view this portion is not removed from the other views. Fig. 15 shows in pictorial form a casting intersected by a cutting plane and its appearance when the front half is removed ; fig. i6 shows the two views of the casting, the front view in section. The edge of the cutting plane is indicated by the line symbol of a dash and two dots, with reference letters and arrows showing the di rection in which the view is taken. The cutting plane need not be in a single con tinuous plane but may be offset in any part of its length to go through some detail. Shafts, bolts, nuts, keys, rods, rivets and the like whose axes occur in the plane of the section are left in full and not sectioned. Adjacent pieces are section-lined in op posite directions, and are often brought out more clearly by varying the pitch, using closer spacing for smaller pieces. The same piece in different views or in different parts of the same view should always be section-lined identically in direction and spacing. A common and econom ical way of showing an object which is symmetrical about a centre line is by making what is called a half section, drawing one side in section and the other in full. In such a view dotted lines are unnecessary. Revolved sec tions, made by passing a cutting plane through some detail such as a rib or the arm of a wheel and turning it in place are often used (fig. 17). Detail sections are for the same purpose but in stead of being drawn on the view they are set off to some adjacent place on the paper. The cutting plane, with reference letters, should always be indicated. Phantom sections are exterior views with the interior construction brought out by dotted cross-hatching.

A working drawing is a drawing that gives all the information necessary for the complete construction of the object represented. It includes (I) The full representation of the shape of every part of the object (orthographic projection). (2) The size of every part, in figures (dimensioning). (3) Explanatory notes giving specifications as to materials, finish, etc. (4) A descriptive title. Of ten as in architectural and structural drawing the notes of ma terials and workmanship are too extensive to be lettered on the drawings so are made up separately in typewritten or printed form and are called the specifications, hence the term "drawings and specifications." Working drawings are divided into two gen eral classes, assembly drawings and detail drawings. An assembly drawing is as its name implies, a drawing of the machine or structure put together, showing the relative positions of the dif ferent parts (fig. 18). Its particu lar use is in the erection of the structure. It may give the all over dimensions and distances from centre to centre or part to part of the different pieces, show ing their relation to each other, usually indicating the different parts by "piece numbers," often enclosed in circles. It frequently includes a "bill of materials," a tabulated statement of all the parts used, including stock parts such as bolts, screws, cotters, etc. Classified under the general term of assembly drawings would be other forms, as the design drawing, the preliminary layout, full size if possible, on which the scheming, inventing and designing are worked out accurately after freehand sketches and calcula tions have determined the general idea. From it the detail draw ings of each piece are made. Sometimes the finished assembly drawing is traced from the design drawing, more often it is re drawn, perhaps to smaller scale to fit a standard sheet, using the detail drawings to work from, thereby checking their correct ness. An outline assembly is used to show the appearance of the machine, sometimes for catalogue or other illustrative purposes.

Piping, wiring and oiling dia grams are also forms of assembly drawings. An assembly working drawing showing fully the dimen sions and construction of each piece as well as their relative posi tions, so that no separate detail drawings are needed, may be made for a simple machine. A unit assembly drawing is a drawing of a related group of parts, in a complicated machine or structure.

A detail drawing is a complete description of each separate piece, giving its shape, size, material and finish, what shop opera tions are necessary, what limits of accuracy are demanded and how many of each are wanted (fig. 19). Sometimes smaller parts of the same material or character are grouped together, as forg ings on one sheet, special bolts and screws on another, etc., but in large production the accepted practice in a set of drawings is to have each piece, no matter how small, on a separate sheet.

In commercial drafting, accu racy and speed are the two re quirements. The drafting room is an expensive department. There are therefore many conventional methods or idioms and abbrevia tions of the language, with which the draftsman must be familiar. There are also allowable violations of the strict principles of projection when added clearness may be gained. One of the time saving conventions is in the representation of screw threads. The helical curves are never drawn except on screws of very large diameter, but are conventionalized into straight lines, and on screws less than perhaps an inch in diameter the thread contours are omitted, the threaded portion of a shaft being represented by one of a number of conventional symbols, of which three are shown in fig. 20, A being the commonest. As another example : in making working drawings of gears and toothed wheels the teeth are not drawn but are represented by drawing the pitch cir cle, addendum and root circles. On detail drawings for cast gears the full-size outline of one tooth is added and for cut gears the blank is drawn with notes and dimensions giving full information.

On patent drawings, however, all the teeth on a gear must be shown.

Fig. 21 is an illustration of the violation of theory, in which the true projection of the sectional view is not as good an explanation of the piece as the preferred form in the second view. When a cutting plane passes through a rib (fig. 22), a true section, A is heavy and misleading. The usual method is to omit the section lines from the rib, B, as if the cutting plane were just in front of it. Another method sometimes used is to section the rib as at C. There have been a number of different codes of symbols proposed and published for the indication of different metals and materials in section, but there is no established uni versal standard. At the present time, how ever, all the countries where drawings are made have either officially adopted each its own standard set of rules and symbols for all the conventions used in drawing, as threads, finish, dimensioning, materials, etc., or are working on such standards through the Government or the engineering societies.

Scales.

In representing objects which are larger than can be drawn to their natural or "full size" on the paper, it is necessary to reduce the dimensions on the drawing proportionally, and for this purpose the so-called architect's scale of proportional feet and inches is used. The first reduction is to what is commonly called "half-size," or correctly speaking "to the scale of 6"--=-1'. This scale reduction is used on working drawings even if the object be only slightly larger than could be drawn in full size, and is erally worked with the full size scale by halving the dimensions. If this is too large for the paper the drawing is made to the scale of 3"= I', commonly called quarter-size. This is the first scale of the usual commercial set. Others are i 2"; I"; a" ; 2"; and " to the foot. Drawings to odd proportions as 1', etc., are not used except in rare cases when it is desired to make it difficult or impossible for a workman to measure them with an ordinary rule. The scale 4"= i' is a usual one for ordinary house plans and is often called by architects "quarter-inch-scale," meaning not quarter size but that one quarter inch on the drawing represents one foot on the building. For plotting and map drawing the civil engineer's scale of decimal parts, 10, 20, 30, 40, 5o, 6o, 8o, Too to the inch, is used but this scale should never be used for machine or structural work. Drawings in the metric system are not made to half-size or quarter-size. The first regular scale smaller than full size is one fif th size, then one-tenth size, although sometimes the scale of i to 21 is used. The unit of measurement is the millimetre and figures are all understood to be millimetres, without any indicating mark. Dimensioning.—After the correct representation of the object by its projections, that is, telling the shape, the entire value of the drawing as a working drawing lies in the dimensions, i.e., telling the size. Successful dimensioning requires not only a knowledge of the principles and conventions but an acquaintance with the shop processes which enter into the construction. A dimension line is usually made as a fine full line terminated by carefully made arrow-heads which indicate exactly the points to which the dimension is taken. Some use a dash line and some a red line for dimension lines. On machine drawings a space for the figures is left in the dimension line ; in structural and much architectural practice the figure is placed above a continuous dimension line. Extension or witness lines not touching the out line, indicate the distance measured when the dimension is placed outside the figure.

In dimensioning there are some conventional practices which have come to represent good form to such an extent as to have the force of rules: I. Dimensions on horizontal and inclined dimension lines should read from left to right ; those on vertical lines from bottom to top; i.e., so as to be read from the right hand side of the sheet.

2. Preferably keep dimensions outside the view unless added clear ness, simplicity and ease of reading will result from placing them inside. They should for appearance's sake be kept off the cut surfaces of sections. When necessary to be placed there the section-lining is omitted around the numbers.

3. Feet and inches are designated thus, 5'-3". When a dimension is in even feet it is indicated thus 5'–o".

4. Fractions are always made with horizontal division lines.

5. Dimensions should generally be placed between views.

6. Do not repeat dimensions unless there is a special reason for it.

7. Do not crowd dimensions.

8. In general give dimensions from or about centre lines. Never locate holes or other machine operations from the edge of unfinished castings.

9. Never give dimensions to the edge of a circular part but always from centre to centre.

Io. If it is practicable to locate a point by dimensioning from two centre lines do not give an angular dimension.

II. Never use a centre line as a dimension line.

12. Never use a line of the drawing as a dimension line.

13. Do not allow a dimension line to cross an extension line unless unavoidable.

14. The diameter of the "bolt circle" of holes in circular flanges is given, with the number and size of holes.

15. Give the diameter of a circle, not the radius.

16. Give the radius of an arc, marking it

R or Rad.

17. Never place a dimension so that it is crossed by a line.

Fits and Tolerances.

With the demand for interchange ability and quantity production the exact size in decimals is speci fied for "essential dimensions" with the amount of "tolerance" over and under which will be allowed by the inspector, since it is not possible to work to an absolutely accurate dimension. These limits are set by the engineering department and placed on the drawing, and the shop follows orders explicitly. In fitting one piece with another, as a shaft and hub, the diameters in decimals with allowed tolerances are given for each, superseding the older practice of leaving the amount of allowance for different kinds of fits to the machine shop. Much experience in manufacturing is needed as well as a study of the particular mechanism involved before the draughtsman is able to know just the accuracy necessary and to specify proper tolerance. When unnecessarily small toler ances are set the cost of manufacture is greatly increased. The general tolerance is often stated in a note near the title.

Checking.

Bef ore being sent to the shop a working drawing is carefully checked for errors and omissions. A first check of the pencil drawing is made by the chief designer, who knows the price at which the machine is to be made and checks the design and its mechanism for soundness and economy, sees if existing patterns for any parts can be used, checks for correct representation, e.g., adequate lubrication. He sees that every piece is correctly de scribed, checks all dimensions by scaling and computation, checks for tolerances, checks for finishes, checks for specifications of material, looks for interferences and clearances, sees that small details are standard and stock sizes where possible, checks the title and bill of material.

Working drawings are always duplicated for shop use by some printing process, and the original is not allowed to be taken out of the office. The great majority are blueprinted. Photostat prints, and reproductions made by various forms of gelatine, stencil and lithographic processes are also used. Drawings are usually made in pencil on cream or buff detail paper and traced, either for econ omy on tracing paper, or on tracing cloth, a transparentized cotton fabric which gives a better print and is much more durable.

One-plane Projection.

Orthographic projection with its two or more views describes an object completely, but requires an effort of the geometrical imagination to visualize its appearance. On the other hand, a picture of the object showing it as it would appear to the eye can be made by perspective drawing, but is not useful as a working drawing as its lines cannot be meas ured directly. To obtain the pictorial effect of perspective draw ing with the possibility of measuring the principal lines several kinds of one plane projection or conventional picture methods have been devised. With the combined advantages are some seri ous disadvantages which limit their useful ness. They are distorted until the appear ance is often unpleasant, only certain lines can be measured, the execution requires more time, and it is difficult to add many figured dimensions, but with all this, a knowledge of these methods and facility in their use is of great value to the draughts man. Mechanical or structural details not clear in orthographic projection may be drawn pictorially or illustrated by supple mentary pictorial views. Technical illus trations, patent office drawings, layouts, piping and wiring dia grams, preliminary free-hand sketches, etc., can all be done advantageously in one-plane projection. Aside from perspective drawing there are two general divisions of pictorial projection, axonolnetric projection with its divisions into isometric, dimetric and trimetric, and oblique projection with several variations.

Axonometric projection, theoretically, is simply a form of ortho graphic projection in which only one plane is used, so placed with relation to the object that a rectangular solid projected on it would show three faces. Usually the object is considered as turned from its natural position and the vertical plane taken as the plane of projection. Imagine a vertical plane with a cube behind (or in front) of it, having one face parallel to the plane. Its projection will be a square. Rotate the cube about its vertical axis through any angle less than 9o°, the projection will now show two faces, foreshortened. From this position tilt the cube forward any amount and three faces will show on the projection. There are thus an infinite number of axonometric positions, only a few of which are ever used as a basis for drawing. The simplest of these is the "isometric" (equal meas ure) position, where the three faces are foreshortened equally, as would occur if the cube were rotated about the verti cal axis through then tilted for ward until the edge OC (fig. 23) is f ore shortened equally with OA and OB thus making the body diagonal from 0 perpendicular to the plane of projection. (This makes the top face slope 35°-16' approx.) The three lines of the front corner, OA, OB, OC, make equal angles with each other and are called the isometric axes. Since parallel lines have their projections parallel, the other edges of the cube will be respectively parallel to these axes. Any line parallel to an isometric axis is called an isometric line. The planes of the faces of the cube and all planes parallel to them are called isomet ric planes. It will thus be noticed that any line or plane which in its regular orthographic projection is perpendicular to either of the reference planes, will be an iso metric line or plane. In this iso metric projection the lines have been foreshortened to approxi mately M- of their length and to measure them would require a special scale. In all practical use of the isometric system this fore shortened scale is not used but the full scale lengths are laid off on the axes. This gives a figure slightly larger but of exactly the same shape and is called isometric drawing. As the effect of in creased size is usually of no consequence and the advantage of measuring the lines with standard scales is of such great conven ience, isometric drawing is used almost exclusively instead of iso metric projection. In making an isometric drawing the axes are first drawn, apart, drawing one vertical and the other two with the 3o° triangle. On these three lines are measured the length, breadth and thickness of the object. Lines not parallel to one of the isometric axes are called non-isometric lines. The one important rule is, measurements can be made only on isometric lines. Since a non-isometric line does not appear in its true length its extremities must be located by isometric co-ordinates. A circle on any isometric plane will ap pear as an ellipse, and is usually drawn as a four-centred approxi mation with the construction of fig. 24. It is sometimes desirable to show the lower face of an object, by tilting it back instead of forward, and drawing it on reversed axes. Fig. 25 shows a sketch on reversed axes. Isometric drawings are from their pictorial nature usually outside views but sometimes an isometric section or half-section can be employed to good advantage. The cutting planes are taken as isometric planes. Fig. 26 is a half-section, made by outlining the figure, then cutting out the front quarter.

The reference cube can be turned into any number of positions where two edges would be equally foreshortened and the third to a different length, and any one of these positions might be taken as a basis for a system of dimetric drawing. A simple dimetric posi tion is one with the ratios i : i :1. In this position the tangents of the angles of the axes are s and $ making the angles approximately 7 and 41 degrees. Fig. 27 is a drawing in this system. Trimetric drawing, with three unequal axes, has little if any practical value.

Oblique Projection is a one-plane method in which the project ing lines are parallel but make an angle other than 9o° with the picture plane. Suppose the reference cube to be set with one face parallel to the picture plane and the projectors to make an angle of 45° with the plane, in any direction.

The face parallel to the picture plane would be projected in its true size and the edges perpendicular to the plane would be pro jected in their true length. This system with 45° projectors is sometimes called cavalier projection. It is similar to iso metric drawing in having three axes repre senting three mutually perpendicular lines, upon which measurements can be made. Two of the axes are always at right angles to each other, being in a plane parallel to the picture plane. The third or cross axis may be at any angle, 3o° or 45° being generally used. Any face parallel to the picture plane will evidently be pro jected without distortion, an advantage over isometric of particular value in the representation of objects with circular or irregular out line, thus objects should always be placed with their character istic contour parallel to the picture plane (fig. 28). Oblique drawing always gives the distorted effect of excessive thick ness. A variation called cabinet drawing devised to overcome this effect is an oblique projection, with the projectors assumed at such an angle that all measurements in the direction of the cross axis are reduced one-half (fig. 29), which makes easy measure ment but the effect is often too thin. Other ratios such as 3 or may be used with more pleas ing effect. The cross axes may be at any angle, but are usually made either 3o° or 45°. A special system of oblique projection c a 11e d clinographic projection, used in drawing mineral crystals in crystallography, is based on the axes of a cube first revolved about a vertical axis through an angle whose tangent is 1, then projected obliquely to the vertical plane with the eye (at an infinite distance) elevated through an angle whose tangent is .

Execution.—As drawing instruments are used for all accurate work, the first requirement in making a drawing is the ability to use them with facility and in good form. The drawing table, with softwood top or carrying a softwood drawing board, should be set so that the light comes from the left, the paper held in place with thumb tacks, and a hard pencil selected, sharpened to a long sharp point. A T-square, 45° and 30°-60° triangles, com passes, dividers, scale, pencil eraser and sandpaper pad should be at hand. Horizon tal lines are drawn with the T-square guided by the left edge of the drawing board, and vertical lines are drawn with the triangle set against the T-square, always with the per pendicular edge nearest the head of the square and toward the light (fig. 3o). These lines are always drawn up from the bottom to top, consequently their location points should be made at the bottom. With the triangles against the T-square, lines at and 6o° may be drawn, and the two triangles may be used in combination for angles of 15° and 75°, directly (fig. 31) . Thus any multiple of 15° may be drawn and a circle may be divided with the 45° triangle into 4 or 8 parts, with the 6o° triangle into 6 or r 2 parts and with both into 24 parts. The dividers, used for transferring distances, etc., are manipulated with one hand, and opened by pinching at the chamfer with the thumb and second finger. This puts them in correct position with the thumb and forefinger on the outside of the legs and the second and third fingers on the inside (fig. 32). The compasses are manipulated in the same way, adjusting to the radius marked on the paper, then raising the hand to the handle and drawing the circle (clockwise) in one sweep by rolling the handle with the thumb and forefinger, inclining the compasses slightly in the direction of the line (fig. 33). In making a working diagram the order of pencilling should be somewhat as follows : first, make a preliminary freehand layout sketch, estimating the position and space required for each view ; ond, decide the scale to be used ; third, draw the centre lines for each view and block in the views with the principal outlines ; fourth, finish the projections, rying them on together ; fifth draw all dimension lines, then put in the dimensions; sixth, lay out the title; seventh, check the drawing as carefully as possible.

After being made in pencil, drawings, except when inked on the paper, as patent drawings or display drawings, are traced for blueprinting or other method of reproduction. When intended to be used perhaps only once, as tool-room drawings, architectural details, etc., they are very com monly traced in firm pencil lines on tracing paper. Production drawings and works of permanent value are traced in ink on tracing cloth. The ruling pen is always used with drawing ink and guid ing edge, either T-square or tri angle. To fill it touch the quill filler of the ink bottle between the nibs, being careful not to get any ink on the outside of the blades. The pen is held as in fig. 34 and the width of line adjusted by turning the screw. The pen is held against the straight-edge with the blades parallel to it and in a plane perpendicular to the paper. If the ink refuses to flow it is because it has dried and clogged in the point of the pen. If pinching the blades slightly or touching the pen on the finger does not start it, it should be wiped out and fresh ink supplied. Faulty lines may occur by pressing the pen too hard against the T-square, by sloping the pen away from the T-square, or by having it too close to the blade, when the ink will run un der ; by having ink on the outside of the blade ; by not keeping the blades parallel to the line, or by letting the triangle slip into the wet line. Visible outlines should be strong full lines, invisible lines much lighter lines made with short dashes. Centre lines of long dash and dot, and dimension lines are made either of the same weight as invisible lines or still finer. Inking should be done in a systematic order, first, all visible circles, beginning with the smallest; second, full lines, horizontal, vertical, inclined; third, dotted circles and lines; fourth, centre lines; fifth, extension and dimension lines; sixth, arrow heads and dimensions; seventh, section lines; eighth, notes and title; ninth, border; tenth, check the tracing.

In lettering a working drawing the two requirements are legibility and speed. Lettering is not mechanical drawing and the use of "geometrical" letters, "block" letters, etc., made of straight lines and ruled with T-square and triangles is not approved in good practice. The "commercial gothic" or "sans serif" letter made freehand in a single stroke, either vertical or inclined is the style almost universally used.

Technical Sketching.

From its use in connection with art the word sketch suggests a free or incomplete or careless render ing of some idea. This is not its meaning in engineering drawing. A sketch is a working drawing made freehand, a quick expression of graphic language, but complete in its information. In all me chanical thinking in invention, all preliminary designing, all ex planation and instruction of the designer to the draftsman, sketching is the mode of expres sion. It represents the mastery of the language, gained only of ter proficiency in mechanical exe cution. Sketches of machine parts would be made in orthographic projection, explanatory of illus trative sketches either in ortho graphic or in one of the pictorial methods. Design sketches are often made on co-ordinate paper. The memory for form may be strengthened and the capacity for "stored observation" greatly increased by systematic practice in sketching from memory, studying a drawing or casting with close concentration until every detail is stored for future visualization, then making an accurate sketch without further reference to the original and comparing it when finished. See also ENGI

view, lines, plane, projection, fig, object and line