BASIC ENGINEERING DRAWING. Prepared By: Syed Basharat Ali. Basic Engineering Drawing Contents. Ortho Graphic Projection Lines Sectioning Terminology Abbreviations Conventional Representation Of Common Features Pictorial Drawing Dimensioning Limits And Fits Threads
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BASIC ENGINEERING DRAWING Prepared By: Syed Basharat Ali
Basic Engineering Drawing Contents • Ortho Graphic Projection • Lines • Sectioning • Terminology • Abbreviations • Conventional Representation Of Common Features • Pictorial Drawing • Dimensioning • Limits And Fits • Threads • Assembly Drawing
Ortho Graphic Projection In the engineering industry communication between the drawing office and the work shop is achieved mainly by means of engineering drawings. The principal method used to prepare these drawings is known as Ortho Graphic Projection. Basically, Orthographic Projection is the representation of a three dimensional component on a flat surface (the drawing sheet) in two dimensional form. At least two orthographic views, therefore, are required to indicate fully the shape and size of a component. If the component is a complicated one then usually more then two views are shown to aid understanding. In this country two methods of a Orthographic Projection are used. One is known as First Angle Orthographic Projection (often referred to as English Projection), the other as Third Angle Orthographic Projection (American Projection). Both methods of representation are illustrated and explained in this section
First Angle Orthographic Projection The pictorial drawing opposite indicates the shape of the component with a single view. An orthographic drawing indicates the Shape of a component by using a number of views each looking at a different face of the Component. At least two views are necessary to fully represent the component. Usually , however, three views are shown in order to clarify internal And external detail. • A Front View (F) • A Plan View (P) • A side View (L&R)
Front View • The front view or front elevation represents what is seen when looking at the front of the component in the direction of arrow F.
Plan View • A plan view represent what is seen when looking at the top of the component in the direction of arrow P.
Side View View looking in direction of arrow L. View looking in direction of arrow R. Left- Hand Side View (L) Right- Hand Side View (R) • The side view or side elevation represents what is seen when looking at the side of the component in the direction of either arrow R or arrow L. These arrows are at 90° to both arrow F and arrow P.
In First Angle Ortho Graphic Projection The Front View is Above the Top view. The Right-hand side view is on the Left-hand side of the front view. The left hand side view is on the Right-hand side of the front view.
Third Angle Orthographic Projection When representing a three dimensional component in Third Angle Orthographic Projection, the basic views are exactly the same as those shown when using First Angle Orthographic Projection . The difference between First Angle and Third Angle is in the positioning of the views relative to each other. In Third Angle Orthographic Projection the individual views are placed on the drawing sheet in projection with each other as shown:
Point For Third AngleOrthographic Projection • The plan is always projected ABOVE the front view. • The right-hand side view is shown on the RIGHT-Handside of the front view. • The left-hand side view is shown on the LEFT-Handside of the front view.
A Comparison Of First And Third Angle Ortho Graphic Projection • The plan is BLOW the front view. • The Right-hand side view is on the Left-hand side of the front view. • The left hand side view is on the • Right-hand side of the front view. • The plan is ABOVE the front view. • The right-hand side view is on the Right-hand side of the front view. • The left-hand side view is on the Left-hand side of the front view.
Sectioning Drawings of the outside of sample components are often sufficient to convey all the information necessary to make the component. More complicated components, however may require sectional views to clarify internals details. A sectional view is obtained when one imagines the component to be cut through a chosen section plane often on a center line. If the vee-block is cut on section plane C-C as shown the resulting sectional view projected from the plan replaces the usual front view of the block. Sectional Front View looking on cutting plane C-C End View
Sectional views are drawn only when it is necessary to explain the construction of a complex object or assembly. Some of the examples used in the next few slides have been chosen to illustrate the rules of sectioning although in practice, as in the case of the vee-block drawn above a sectional view may not have been necessary. • The draftsman has to decide how a component or assembly should be sectioned in order to provide the fullest possible information. The recommendations of BS 308 enable him to do this in a way that is understood by all engineers. • Rules Of Sectioning • A sectioned object is shown by lines drawn preferably at 45°. • Thin lines touch the outline. Size of sectioned part determines line • spacing preferably not less than 4 mm. • If two adjacent parts are sectioned , the section lines are drawn • in opposite directions. Lines are staggered where the parts are in • contact. • Where more then two parts of an assembly are to be sectioned, • the lines cannot all be opposite. Sectional lines are closer together • on the third area usually the smallest
The sectional view of a symmetrical object is obtained when the section plane • cuts through the obvious centre line. Hatching may be omitted if the meaning is • clear without it. • If an object is NOT symmetrical the section plane chosen should be clearly • stated.
Sectioning Exceptions There are a number of a features and parts which are not normally sectioned even though they may lie in the section plane. A good way to accept these exceptions to be general rule is to imagine how complicated the drawing would look if they were sectioned. They are sectioned, however, when they lie across the section plane.
Staggered Section Planes Each Part of the section plane is swung to the vertical before projecting to the sectional End view. By using the convention the draftsman avoids using too many auxiliary views. A staggered section plane should only be used when there is a resulting gain in clarity. Section C-C Revolved Section A-A Realigned Section D-D Revolved
TERMINOLOGY Communication between the drawing office and the work shop is mainly achieved via the engineering drawing orthographic or pictorial. In order to reduce drafting time a number standard parts are abbreviated. Before this engineers “shorthand” can be correctly it is necessary to understand the terms used to describe features of engineering components. This terminology is common to both drawing office and workshop and is often used when discussing the various manufacturing and machining processes used in engineering. Many different types of holes may be seen on engineering drawings. The more common ones, associated with drilling, reaming and tapping. The name and where appropriate the application of each is indicated.
A drilled hole or, if grater accuracy is required, a reamed hole. A ‘blind’ tapped hole i.e. a threaded hole which passes only a part way through the plate. A countersunk hole provides a mating seat for a countersunk head screw or rivet. A counter bore provides a housing for the heads of cap screw, bolts, etc. A spot face a much shallower circular recess. Provides a machined seat for nuts, bolt heads, washers, etc.
Abbreviations Many terms and expressions in engineering need to be written on drawings so frequently as to justify the use of abbreviations which help to reduce drafting time and costs. A selection of the more commonly used ones are stated and clarified in the following table.
Conventional Representation of Common Features Screw Threads There are many components commonly used in engineering which are complicated to draw to full. In order to save drawing time, these parts are shown in a simplified, conventional form. SubjectConvention The screw thread is represented by two parallel lines. The distance between these lines is approximately equal to the depth of thread. The inside line is THIN and the circle is broken
Springs A spring is designated by stating the diameter of the wire, the coil diameter (inside or outside), the form of the spring ends, the total number of the coils and its free length. in the case of compression spring, the pitch of the coil may be deduced from its free length and number of coil.
Shaft Details it is frequently necessary to fix a component to one end of a shaft or spindle so that a torque may be transmitted. Subject Convention Square on the end of a long Shaft Side View Splined Shaft
Knurling Knurling is a common method of providing a roughened to aid tightening or slackening of a screw by hand. This is formed by pressing special rollers against the surface of the component whist it revolves in lathe. Subject Convention Diamond Knurl on a machine screw head Straight Knurl on a circuit terminal
Long Components There are occasions when bars, shafts, spindles or tubes may be too long to be drawn to a reasonable scale. In such cases the elevation may be interrupted . Subject Convention Rectangular Bar Circular Shaft “OR” Spindle Hollow Shaft “OR” Tube
Multiple Holes When a large number of holes of equal diameter are equally spaced around a diameter or a line, only one hole need be drawn in full with the reminder marked with a short center line. That circle is called the pitch circle diameter or PCD
Gears Before gears be drawn a great deal of background knowledge about their nomenclature and construction must be acquired. Subject Convention Side view of gear wheel is in section Spur Gear Worm And Wheel
A good example of a how a complex component maybe drawn relatively simply is the bevel gear. The assembly shown blow is of a pair of gear of equal size, the direction of motion being changed through an angle 90°. In the arrangement he gears are often referred to mitre wheel. The gares ma be of differing sizes of course and the angle between the shaft may be other then 90°. In this letter case, the side view of the gear assembly would have to show one gear as then ellipse.
Pictorial Drawing A component may be represented graphically in various ways. An Orthographic Drawing, for example, requiring a minimum of two views to fully communicate the size and the shape of a component, is used in engineering mainly to convey manufacturing instruction from the designer to the craftsman. On the other hand a well executed Pictorial Drawing adequately representing all but the most complicated components using one view only, is used mainly as an aid to visualization of the shape of a component rather then for communication detailed instruction for manufacture. A pictorial drawing, generally, is a quickly produced approximately scaled representation of a component a “picture” rather then an accurately scaled line drawing. There are many different types of pictorial representation. Two of the most commonly used ones are known as Isometric Drawing and Oblique Drawing.
Isometric All receding lines are drawn at 30°
Oblique An oblique pictorial drawing presents with the component with one of its faces as a true shape. This shape is drawn on the front face of the oblique box as shown below. The longest face is usually drawn on the front of the oblique box with receding lines between ½ and ¾ full size.
Methods Of Construction OfOblique Drawing • There are many variations in angle, length of receding lines, and directions from which a component may be viewed in order to produce an oblique drawing as can be seen by examples on the previous slide. Different oblique drawings of the same component may each provide the details required. • The receding lines may be drawn at any angle to the horizontal but an • angle of 30, 45, 60 is proffered as lines can be drawn with set squares. • Receding lines may be any proportion of heir true length. A good • pictorial representation is obtained if lengths from ½ to ¾ actual length is used.
Dimensioning A number of the basic rules of dimensioning can be explained by reference to the above drawing of a thin plate. The sides marked A and B are known as DATUM faces. They are used as reference edges from which dimensions are drawn. Datum's may or may not be machined. Even if they are not machined it is good practice to choose reference edges in order to simplify the layout of dimensions. Dimension Line: Thin full lines placed outside the component where possible and spaced well away from the out lines. The longer dimension lines are placed outside shorter ones. Projection Lines: Thin full lines which extend from the view to provide a boundary for the dimension line. Drawn at 90° to the out line. Arrowheads: Drawn with sharp strokes which must touch the extension lines. A Leader line is a thin full line which is drawn from a note, a dimension or, in this case, a “balloon” and terminates in an arrowhead or a dot. Relatively small gap. Relatively short tail.
Crossing extensions lines usually a break to ensure clarity. Dimension placed above the dimension line. This is preferred to the alternative method of placing the dimension in a gap in the line. Avoid using both methods on the same drawing if possible. Dimension placed so that it may be read from bottom or right hand side of the drawing sheet.
Arrangement Of Dimensions Dimensions should be placed so that they may be read from either the bottom or right-hand side of a drawing, for example: Various methods of dimensioning narrow spaces or width are shown above.
Dimensioning Circles The way a circle is dimensioned the dimension always refers to the diameter and NOT the radius. A circle is never dimensioned on a center line. The conventional symbol for diameter is ∅. The leader line must be drawn in line with the center of the circle. In the example it is preferable to dimension the side view even tough the cylindrical shape is not apparent. Dimension in this view, however, must always be preceded by symbol ∅.
Dimensioning Radii A radius should be dimensioned by a dimension line which passes through, or is in line with, the center of the arc. The dimension line should have one arrowhead which should be placed at the point of contact with the arc. The abbreviation R should always precede the dimension.
Dimensioning Angles Angles are Expressed in: Degrees e.g. 90° Degrees and minutes e.g. 27 ° 30’ The placing of the angular dimension depends on the position of the angle in relation to the bottom and/or the right-hand side of the drawing sheet and the size of the angle.
Dimensioning Chamfers 45° chamfer should be specified by one of the methods below:
Location Dimensions The features can be located from a machined surface or center line. Such s surface or line is known as DATUM. Examples on pervious slides have been shown components and features may be dimensioned when size is the main consideration. Spigot located from two reference edges ( R). Both holes located from two reference edges ( R). Both holes located from two reference edges ( R) then hole B related to hole A.
The simple bearing bracket casting on the left shows both size and location dimensions. Reference surface are marked with machining symbol:- This is placed so that it may be read from the right of the sheet. It is preferable to place the symbol on the appropriate projection line rather than as show on the left. No symbol is required where the machine is specified i.e. in the case of the drilled holes, the reamed holes and the spot-face. The location dimension are those show with letter L and size dimensions by a letter S. Some of the size dimensions are less accurate then other e.g. the thickness of the rib is fixed during the casting process whistle the 11 mm diameter holes is accurately reamed. The 20 mm diameter hole located by the dimension from the machined base to the center line of the hole.
Threads A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a fastener.
Pipe Threads G Series • These are parallel pipe threads having thread • angle of 55° and are used where pressure-tight • joints are not made on threads. • Taper Pipe Threads Whitworth Form • These threads have a taper of 1 in 16 and a • thread angle of 55° and are used where • pressure tight joints are made on threads.
American Pipe Threads • These threads have a taper of 1 in 16 and a • thread angle of 60°. The types of threads • include NPT, NPTF & ANPT. • ACME Threads • Acme screw threads are mainly used for • the purpose of producing traversing • motions on machines, tools etc. The multi-start threads are used to provide • fast relative traversing motion.
Stub ACME • The Stub Acme screw threads are generally • confined to those unusual applications like • transmission of power and motion where a • coarse pitch thread of shallow depth is • required due to mechanical or metallurgical • considerations. • Trapezoidal Thread • Trapezoidal threads are used for • transmission of power and motion and are • nearly similar to ACME threads, but are • made to metric dimensions and standards. • The most commonly used class of threads • are 7e for external threads & 7H for internal • threads.