(a) Obiect
(b) Kotated TOP
LEFT SIDE FRONT RIGHT SIDE REAR BOTTOM
(c) FIGURE 2-7. Orthographic projection.
principal view shows the complete control wheel, while the detail view is an enlarged drawing of a portion of the control wheel.
Sectional Views A section or sectional view is obtained by cutting away part of an object to show the shape and construction at the cutting plane. The part or parts cut away are shown by the use of section (cross-hatching) lines.
Sectional views are used when the interior construction or hidden features of an object cannot he shown clearly by exterior views. For example, figure 2-11, a sectional view of a coaxial cable connector, shows the internal construction of the connector. This is known as a full section. Other types of sections are described in the following paragraphs.
Half Sections In a half section, the cutting plane extends only halfway across the object, leaving the other half of the object as an exterior view. Half sections are 40
.032 THICK FIGURE 2-9. Symmetrical object with exterior half view.
FIGURE 2-8. One-view drawing.
59R 1000 32 OSR (TYP) 28- .56 B
DETAIL B FIGURE 2-10. Detail view.
PLUG BODY WASHER - GASKET — CLAMP NUT • FIGURE 2-11. Sectional view of a cable connector.
used to advantage with symmetrical objects to show both the interior and exterior.
Figure 2-12 is a half-sectional view of a quick disconnect used in aircraft fluid systems.
Revolved Sections A revolved section drawn directly on the exterior view shows the shape of the cross section of a part, such as the spoke of a wheel. An example of a revolved section is shown in figure 2-13.
Removed Sections Removed sections illustrate particular parts of an object. They are drawn like revolved sections, except that they are placed at one side and, to bring out pertinent details, are often drawn to a larger scale than the view on which they are indicated.
Figure 2-14 is an illustration of removed sections. Section A-A shows the cross-sectional shape of the object at cutting plane line A-A. Section B-B shows the cross-sectional shape at cutting plane line B-B. These sectional views are drawn to the same scale as the principal view; however, as already mentioned, they are often drawn to a larger scale to bring out pertinent details.
THE MEANING OF LINES Every drawing is composed of lines. Lines mark the boundaries, edges, and intersection of surfaces.
Lines are used to show dimensions and hidden surfaces, and to indicate centers. Obviously, if the same kind of line is used to show all of these things, a drawing becomes a meaningless collection of lines. For this reason, various kinds of standardized lines are used on aircraft drawings. These are illustrated in figure 2-15, and their correct uses are shown in figure 2-16.
Most drawings use three widths, or intensities, of lines: thin, medium, or thick. These lines may vary somewhat on different drawings, but there will always be a noticeable difference between a thin and a thick line, with the width of the medium line somewhere between the two.
QUICK
DISCONNECT COUPLING FIGURE 2-12. Half section.
SEC. A B-B FIGURE 2-13. Revolved sections.
Center Lines Center lines are made up of alternate long and short dashes. They indicate the center of an object or part of an object. Where center lines cross, the short dashes intersect symmetrically. In the case of very small circles, the center lines may be shown unbroken.
Dimension Lines A dimension line is a light solid line, broken at the midpoint for insertion of measurement indications, and having opposite pointing arrowheads at each end to show origin and termination of a measurement. They are generally parallel to the line for which the dimension is given, and are usually placed outside the outline of the object and between views if more than one view is shown.
All dimensions and lettering are placed so that they will read from left to right. The dimension of an angle is indicated by placing the degree of the angle in its arc. The dimensions of circular parts are always given in terms of the diameter of the circle and are usually marked with the letter D or the abbreviation DIA following the dimension.
The dimension of an are is given in terms of its radius and is marked with the letter R following the dimension. Parallel dimensions are placed so that the longest dimension is farthest from the outline and the shortest dimension is closest to FIGURE 2-14. Removed sections.
the outline of the object. On a drawing showing several views, the dimensions will be placed upon each view to show its details to the best advantage.
In dimensioning distances between holes in an object, dimensions are usually given from center to center rather than from outside to outside of the holes. When a number of holes of various sizes are shown, the desired diameters are given on a leader followed by notes indicating the machining operations for each hole. If a part is to have three holes of equal size, equally spaced, this information is given. For precision work, sizes are given in decimals. Diameters and depths are given for counterbored holes. For countersunk holes the angle of countersinking and the diameters are given. Study the examples shown in figure 2-17.
The dimensions given for fits signify the amount of clearance allowable between moving parts. A positive allowance is indicated for a part that is to slide or revolve upon another part. A negative allowance is one given for a force fit. Whenever possible, the tolerance and allowances for desired fits conform to those set up in the American Standard for Tolerances, Allowances, and Gages for Metal Fits. The classes of fits specified in the standard may be indicated on assembly drawings.
CENTER LINE THIN DIMENSION → THIN
LEADER THIN
BREAK (LONG) THIN BREAK (SHORT) • THICK
PHANTOM THIN
SECTIONING THIN
HIDDEN MEDIUM
STITCH LINE MEDIUM
OUTLINE OR THICK
VISIBLE LINE
DATUM LINE THICK
CUTTING PLANE EXTRA THICK
VIEMING PLANE EXTRA THICK
CUTTING PLANE FOR
COMPLEX OR OFFSET EXTRA THICK VIEW'S FIGURE 2-15. The meaning of lines.
- Extension line Dimension line - Phantom line Leader line
•Center line Sectioning fine -Break line
SECTION AA Outline Hidden line Cutting plane line FIGURE 2-16. Correct uses of lines.
Hidden Lines Hidden lines indicate invisible edges or contours. Hidden lines consist of short dashes evenly spaced and are frequently referred to as dash lines.
Outline or Visible Lines The outline or visible line is used for all lines 3/16 DRILL .3125 DRILL on the drawing representing visible lines on the 3 HOLES .3217 REAM object.
EQUALLY SPACED 3/16 DRILL 80° C'Sk Stifch Lines 7/16 C' BORE 1/4 DRILL TO 5/16 DIA Stitch lines indicate stitching or sewing lines 1/8 DEEP and consist of a series of evenly spaced dashes.
2 HOLES 80°_ Cutting Plane and Viewing Plane Lines Cutting plane lines indicate the plane in which a sectional view of the object is taken. In figure 2-16, plane line A-A indicates the plane in which section A-A is taken.
.2560 DRI 1/4 DRILL 3/8 C'BORE Viewing plane lines indicate the plane from 1/8 DEEP, 3 HOLES which a surface is viewed.
FIGURE 2-17. Dimensioning holes.
Leader Lines Leaders are solid lines with one arrowhead and indicate a part or portion to which a note, number, or other reference applies.
Break Lines Break lines indicate that a portion of the object is not shown on the drawing. Short breaks are made by solid, freehand lines. For long breaks, solid ruled lines with zigzags are used. Shafts.
rods, tubes, and other such parts which have a portion of their length broken out, have the ends of the break drawn as indicated in figure 2-16.
Phantom Lines Phantom lines indicate the alternate position of parts of the object or the relative position of a missing part. Phantom lines are composed of one long and two short evenly spaced dashes.
Sectioning Lines Sectioning lines indicate the exposed surfaces of an object in sectional view. They are generally thin, full lines, but may vary with the kind of material shown in section.
READING DRAWINGS A drawing cannot be read all at once any more than a whole page of print can be read at a glance. Both must be read a line at a time. To read a drawing effectively, follow a systematic procedure.
Upon opening a drawing, read the drawing number and the description of the article. Next, check the model affected, the latest change letter, and the next assembly listed. Having determined that the drawing is the correct one, proceed to read the illustration(s).
In reading a multiview drawing, first get a general idea of the shape of the object by scanning all the views; then select one view for a more careful study. By referring back and forth to the adjacent view, it will be possible to determine what each line represents.
Each line on a view represents a change in the direction of a surface, but another view must be consulted to determine what the change is. For example, a circle on one view may mean either a hole or a protruding boss, as in the top view of the object in figure 2-18. Looking at the top view, we see two circles; however, the other view must be consulted to determine what each circle represents. A glance at the other view tells us that the smaller circle represents a hole, and the larger 45
ance of 0.01, the drawing would show the total dimensions as: 199. A print tolerance (usually found in the title block) is a general tolerance that can be applied to parts where the dimensions are noncritical. Where a tolerance is not shown on a dimension line, the print tolerance applies.
To complete the reading of a drawing, read the general notes and the contents of the material block, check and find the various changes incorporated, and read the special information given in or near views and sections.
FIGERE 2-18. Reading Views.
circle represents a protruding boss. In the same way, the top view must be consulted to determine the shape of the hole and the protruding boss.
It can be seen from this example that one cannot read a print by looking at a single view, chen more than one view is given. Two views will not always describe an object, and when three views are given. all three must be consulted to be sure the shape has been read correctly.
After determining the shape of an object, determine its size. Information on dimensions and tolerances is given so that certain design requirements may be met. Dimensions are indicated by figures either with or without the inch mark. If no inch mark is used, the dimension is in inches.
It is customary to give part dimensions and an overall dimension that gives the greatest length of the part. If the overall dimension is missing, it can be determined by adding the separate part dimensions.
Drawings may be dimensioned in decimals or fractions. This is especially true in reference to tolerances. Many firms, instead of using plus and minus signs for tolerances, give the complete dimension for both tolerances. for cxample. if a dimension is 2 inches with a plus or minus toler-
DIAGRAMS A diagram may be defined as a graphic representation of an assembly or system, indicating the various parts and expressing the methods or principles of operation.
There are manv types of diagrams; however, those with which the aviation mechanic will be concerned during the performance of his job may be grouped into two classes or types— installation diagrams and schematic diagrams.
Instellation Diagrams Figure 2-19 is an example of an installation diagram. This is a magram of the gust lock systems of an aircraft. It identifes each of the components in the systems and shows their location in the aircraft. Each letter (A, B, C, etc.) on the principal view refers to a detail view located elsewhere on the diagram. Bach detail view is an enlarged drawing of a portion of a system. The numbers on the various views are referred to as call outs, and serve to identify each component.
installation diagrams are used extensively in aircraft maintenance and repair manuals and are invaluable in identifying and locating components and understanding the operation of varimis systens.
Schematic Dicgram Schematic diagrams do not indicate the location of individual components in the aircraft, but do locate components with respect to each other within the system. Figure 2-20 illustrates a schematic diagram of an aircraft hydraulic system.
The hydraulic pressure gage is not necessarily located above the landing gear selector valve in the aircraft; however, it is connected to the pressure line that lead: to the selector valve.
Schematic diagrams of this type are used mainly in table shooting. Note that rach line is coded 46
1. Gust lock handle 7. Elevator latch
2. Right aileron latch 8. Turnbuckles
3. Retracting springs (3) 9. Seals
4. Spring bungee 10. Left aileron
5. Push Rod 11. Tie cable
6. Rudder latch 12. Lock cable
13. Unlock cable FIGURE 2-19. Example of installation diagram (gust lock system).
RH ENGINE PUMP
SYSTEM PRESSURE
ENGINE PUMP SUCTION
IDLING CIRCUIT PUMP PRESSURE
RETURN FLOW
HAND PUMP PRESSURE
HAND PUMP SUCTION hate CHECK VALVE ACCUM. AIR GAGES GEN. SYSTEM ACCUMULATOR BRAKE ACCUMULATOR •
TO BRAKE SYSTEM
RESTRICTOR
THERMAL RELIEF VALVE
FROM EMERGENCY SEL. VALVE (BRAKE)
UNLOADING AND RELIEF VALVE
SNUBBER AND VENT ENG
VL FLAPS RESERVOIR SEL. VAL.
NORMAL LAND. CEAF SEL. VALVE EMER.
SELECTOR VALVE
XXXXXXX
LH ENGINE PUMP FIGURE 2-20. Aircraft hydraulic system schematic.
for ease of reading and tracing the flow. Each component is identified by name, and its location within the system can be ascertained by noting the lines that lead into and out of the unit.
In tracing the flow of fluid through the system, it can be seen that the engine-driven pumps receive a supply of fluid from the reservoir. One-way check valves are installed in both left and right pump pressure lines so that failure of one pump will not render the pressure from the other pump ineffective. Fluid flows to the relief side of the unloading and relief valve, and through the check valve, which will hold pressure built up beyond this point. Pressure is then directed through all lines leading to each selector valve, where it is checked if no units are being operated.
Pressure builds up in the line routed to the control port of the unloading valve and begins to 48
BLOCK IN ADD DETAIL
DARKEN VIEWS ADD DIMENSIONS ¼" →T 9-13-67 WEDGE .WOK FIGURE 2-21. Steps in sketching.
charge the system accumulator. Pressure to charge the brake accumulator is routed through a check valve incorporated in the thermal relief valve; this prevents the pressure from returning to the general system.
Although the general system accumulator starts charging at the same time, it will not charge as fast, because the fluid passes through a restrictor valve. The general system pressure will bleed into the brake system whenever the brake pressure drops below system pressure.
As soon as the pressure reaches the relief valve setting, the valve will open slightly. General system pressure increases until it reaches the value established as the system operating pressure. At this point, through the line leading to the control part of the unloading valve, the pressure will force the unloading and relief valve completely open. The pressure trapped in the system by the one-way check valve holds the value open to create an idling circuit, which prevails until some unit of the hydraulic system is operated.
Schematic diagrams, like installation diagrams, are used extensively in aircraft manuals.
DRAWING SKETCHES A sketch is a simple, rough drawing that is made rapidly and without much detail. Sketches may take many forms-from a simple pictorial presentation to a multiview orthographic projection.
A sketch is frequently drawn for use in manufacturing a replacement part. Such a sketch must provide all necessary information to those persons who must manufacture the part.
A mechanic need not be an accomplished artist.
However, in many situations, he will need to prepare a drawing to present an idea for a new design, a modification, or a repair method. The medium of sketching is an excellent way of accomplishing this.
The rules and conventional practices for making mechanical drawings are followed to the extent that all views needed to portray an object accurately are shown in their proper relationship. It is also necessary to observe the rules for correct line use (figures 2-15 and 2-16) and dimensioning.
To make a sketch, first determine what views are necessary to portray the object; then block in the views, using light construction lines. Next, complete the details, darken the object outline, and sketch extension and dimension lines. Complete the drawing by adding notes, dimensions, title, date and, when necessary, the sketcher's name. The steps in making a sketch of an object are illustrated in figure 2-21.
where on the drawing. In this case, the more easily drawn symbol for cast iron is used for the sectioning, and the material specification is listed in the bill of materials or indicated in a note.
CAST IRON MAGNESIUM, ALUMINUM AND Figure 2-22 illustrates a few standard material ALUMINUM ALLOTS symbols.
Shape Symbols Symbols can be used to excellent advantage, when it is desired to show the shape of an object.
STEEL RUBBER, PLASTIC ELECTRICAL INSULATION Typical shape symbols used on aircraft drawings are shown in figure 2-23. Shape symbols are usually shown on a drawing as a revolved or removed section.
Electrical Symbols BRASS, BRONZE, AND COPPER BABBITT, LEAD, ZINC AND ALLOYS Electrical syn bols (figure 2-24) represent various electrical de ices rather than an actual drawing of the units. Having learned what the various symbols indicate, it becomes relatively simple to look at an electrical diagram and determine what each unit is, what function it serves, and how it is WOOD ACROSS GRAIN WOOD WITH GRAIN connected in the system.
CARE OF DRAFTING INSTRUMENTS
CORK, FELT, FABRIC
ASBESTOS, LEATHER
AND FIBRE FIGURE 2-22. Material symbols.
The degree to which a sketch is complete will depend on its intended use. Obviously, a sketch not be dimensioned. If a part is to be manunecessary construction details.
DRAWING SYMBOLS The drawings for a component are composed largely of symbols and conventions representing its shape and material. Symbols are the shorthand of drawing. They graphically portray the characteristics of a component, with a minimum amount of drawing.
Material Symbois Section-line symbols show the kind of material from which the part is to be constructed. The material may not be indicated symbolically when Good drawing instruments are expensive precision tools. Reasonable care given to them during their use and storage will prolong their service life.
T-squares, triangles, and scales should not be used, or placed, where their surfaces or edges may be damaged. Use a drawing board only for its intended purpose, and not in a manner that will mar the working surface.
Compasses, dividers, and pens will provide better results with less annoyance, if they are correctly shaped and sharpened, and they are not damaged by careless handling.
Store drawing instruments in a place where they are not likely to be damaged by contact with other tools or equipment. Protect compass and divider points by inserting them into a piece of soft rubber or similar material. Never store ink pens without first cleaning and drying them thoroughly.
MICROFILM The practice of recording drawings, parts catalogs, and maintenance and overhaul manuals on microfilms was introduced in recent years. Microfilm is regular 16-mm. or 35-mm. film. Since 35-mm. film is larger, it provides a'better reproduction of drawings. Depending on the size of 50
SQUARE SECTION SQUARE SECTION (METAL) (WOOD)
ROUND SECTION ROUND SECTION (SOLID) (TUBULAR)
ANGLE SECTION CHANNEL SECTION (METAL) (METAL)
I BEAM SQUARE SECTION (METAL) (TABULAR) FIGURE 2-23. Shape symbols.
the drawing to be reproduced, a varying number of drawings can be photographed on one reel of 35-mm. film. To view or read drawings or manuals on a reel of film, you need either a portable 35-mm.
film projector or a microfilm reader or viewer.
The advantage of microfilm is that several reels, which represent perhaps hundreds of drawings, require only a small amount of storage space.
Too, a person working on an aircraft may need to refer to a specific dimension. He can place the reel of microfilm in a projector, locate the drawing or desired information, and read the dimension.
If he has to study a detail of the drawing, or work with the drawing for a long period of time, an enlarged photographic reproduction can be made, using the microfilm as a negative.
Microfilm of drawings has many other uses and advantages. However, microfilm is not intended to replace the need for original drawings, especially where the originals are modified and kept current over a long period of time.
When drawings are filmed on continuous reels, corrections can be made by cutting out superseded drawings and splicing in the revised ones.
When these corrections become numerous, the procedure becomes impractical and is discarded in favor of again filming all the related drawings.
A method that allows corrections to be made easily is to photograph the drawings and then cut up the film into individual slides. This has one disadvantage; it requires considerable time to convert the film into slides, insert them into transparent protective envelopes, and arrange them in sequence so that desired drawings can be located quickly.
A 70-mm. microfilm has become available very recently. With it, larger size drawings can be reproduced as individual frames or slides, and these can be inserted in regular paper envelopes and kept in an ordinary file. When held to the light, this large microfilm can be read with the naked eye.