Basic Engineering DrawingFST 213 Lecture Notes

Prepared by:Dr. M. O. Afolabi

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IntroductionGeometrical ConstructionPolygonsTangentsLociPictorial ProjectionsOrthographic ProjectionsFreehand Sketching

Chapter 1: Introduction•

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The end goal of an engineering drawing is to convey all the requiredinformation that will allow a manufacturer to produce a component.Drawing instruments are used to prepare neat and accurate drawings. Theaccuracy of the drawings depend on the quality of instruments used toprepare them.

Drawing instruments include drawing board, T-square, set squares, protractor,ruler, drawing sheet, compass, French curves, drawing pencils, drawing pinsand clips, etc.

Drawing board: made of strips of well softened wood generally 25mmthick. The typical standard sizes are given in Table 1.

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T-square: the name arises fromthe T shape of the instrument.It slides on the drawing board.It is used as a guide for drawinghorizontal lines.

Set squares: they come in two forms: one with 90-45-45 degree angles, theother with 30-60-90 degree angles. By combining the two types, by placingtheir hypotenuses together, 15 and 75 degree angles are obtained.Protractor: used for measuring or marking-off angles.Ruler: used to rule lines, also used to measure distance.Drawing pins/clipsFrench curvesCompass

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Drawing pencils: the quality of pencil used determines the accuracy andappearance of a drawing. The grade of a pencil lead is marked on it.

HB denotes medium grade.Increase in hardness is shown by value in front of H such as in 2H, 3H etc.Softer pencils are marked as 2B, 3B, 4B etc. 3B is softer than 2B and 4B issofter than 3B.Beginning of a drawing may be made with H or 2H, lettering anddimensioning should be done with H and HB pencils.

Drawing sheets: drawing papers used forengineering drawings are categorized according to ISO standard. The A-formats are therecommended papers for engineering drawing.

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ScalesEmployed in engineering drawing to map the size of object or design onpaper. It is very useful when certain components are drawn larger/smallerthan the full size.A scale is simply the ratio of the linear dimension appearing on thedrawing compared to the corresponding linear dimension of the real object.Scale 1:1 (read as one-to-one) means the object has been drawn to truesize; 2:1 means the object has been enlarged twice its true size; 1:2 meansthe object has been reduced to half its size.

Setting upDrawing paper should be set horizontally in respect

to the T-square and pinned to the board.a margin of 10 mm should be drawn at the edges of the paper using T-squareA title block should be drawn at the lower right hand corner of paper.

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Title block: Conveys certain specific and important information onthe drawing to the user. These include:

Name of the firm/schoolNumber of drawing (where more than one drawing areconcerned)Scale usedDimensioning unit (usually millimeters i.e. mm)Date when the drawing was finished.

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DimensioningPut outside the drawing.Dimension identical features only once.Arrowhead ~3 mm long and 1 mm wideFirst dimension should be ~12 mm from theobject.Extension lines begin 1.5 mm from theobject and extend 3 mm from the lastdimension line.Show full size dimensions regardless of thescale used in the drawing.For any feature, place the dimensions wherethe feature appears most explicitly.

Class WorkUsing the general hint on dimensioning, draw and dimension the figure in thebox (use arbitrary sizes). Lines and LetteringIn engineering drawing, different lines convey different messages. Typicaluses of lines are summarized.Letterings can be made either by freehand or lettering devices. To save time,use a guide lining device. Lines of lettering should be spaced the samedistance apart. Do not attempt to erase guide lines after lettering has beencompleted.

Chapter 2: Geometrical Constructions•

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Principles of geometric construction and its applications are essential toengineering drawing.Geometric constructions involves the construction of various types of lines:a straight line, circle, an arc of a circle or

ellipse, a circular curve, a non-circular curve, tangent or combination of these types of lines.

Bisection of a Line and Construction of Perpendicular to a Line

2. Straight Lines and Division of LinesOne method of drawing horizontal and vertical lines orperpendicular and parallel lines or inclined lines is byusing a T-square with a triangle. Another practicalmethod of constructing straight lines is by usingcompass.

3. Division of Lines

4. Angle Bisection

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Application of Angle and LineBisections

Line and angle bisections can be used for the following:Location of centre of an arcCircumscription of a circle on a triangleInscription of a circle in a triangleConstruction of polygons

The application of line and anglebisection in constructing polygons willbe fully dealt with in the next section.

Chapter 3: Polygons•

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Polygons are geometrical shapes formed by the combination of three ormore straight lines.A polygon’s name reflects the number of sides it has.Sum of interior angles of a polygon with n sides is 180 x (n-2).The lines forming a polygon intersect at points called the vertices.From three-sided to ten-sided; they are triangle, quadrilateral (square,rectangle, parallelogram), pentagon, hexagon, heptagon, octagon, nonagon,and decagon.If sides are of equal length and angles are equal, the polygon is regularotherwise it is an irregular polygon.The most commonly used in engineering drawing are regular hexagon andoctagon, since most nuts and bolt heads are of this shape.

1. Constructing Regular Hexagon

NOTE:‘Across corner (A/C)’ is a case ofinscription of the hexagon in a circle whosediameter becomes the distance across thecorners of the hexagon

NOTE:‘Across flat (A/F)’ is a case ofcircumscription of the hexagon on a circlewhose diameter becomes the distanceacross the flats of the hexagon

2. Constructing Regular Octagon

3. Constructing Any Regular Polygon

4. Determination of the Circumference of a Circle Using GraphicalMethod

Chapter 4: Tangents•

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A tangent is a straight line, which just touches the circumference of acircle or the arc of a circle without cutting through.The point where the line touches the circumference is known as the pointof tangency.For example: if a disc stands on its edge on a flat surface, it will touch thesurface at one point. This point is known as the point of tangency and thestraight line which represents the flat plane is known as a

tangent.A line drawn from the point of tangency to the

centre of the disc is called a normal, and the tangent makes an angle of 90 degrees with the normal.

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There are three major principles of tangency which are applied inengineering drawing: joining an arc with a straight line; drawing tangents tocircles; and joining of circles.

Joining an arc with straight line and angles

2. Drawing of Tangents toCircles

3. Joining of Circles

Chapter 5: Loci

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Locus is a set of points, the positions of which satisfy a set of algebraicconditions. When a point moves according to a given law, its path is said tobe a locus. For example: a point moving so that it is always at a constantdistance from a second fixed point traces out a circle as its locus. Equallymany important geometrical curves may be considered as loci. Theseinclude ellipse, parabola and hyperbola.

EllipseIts path is traced by a point which

moves in order that the summation of its distances from two invariable points is a constant.

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ParabolaThe locus of a point which moves so that itsdistances from a fixed point (the focus) and afixed straight line (the directrix), are alwaysequal.Has a property that if a source of light orsound is placed at the focus, rays reflectedfrom the parabola form a parallel beam.This property is used in hand torches, carheadlamps and some electric fires

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HyperbolaThe locus of a point which moves so that the ratio of its distances from the focus and directrix isconstant and greater than unity.

• A curve generated by a point on the circumference of the circle as the circle rolls along a straight linewithout slipping. The moving circle is called the ‘Generating circle’ and the straight line is called the‘Directing line’ or the ‘Base line’. The point on the Generating circle which generated the curve is calledthe ‘Generating point’.

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InvolutesA curve that is traced by a point on a taut cord unwinding from a circle or regular polygon, which iscalled a base.Has a property that makes it important to the gear industry: if the teeth of two mating gears have theshape of an involute, their relative rates of rotation are constant while the teeth are engaged.

4. Cycloid

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HelixThe locus of a point which moves round thecircumference of a right cylinder, at the same timemoving axially, the ratio of

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the two movements being constant.The axial movement during one revolution is called thelead.Occurs in practice as screw threads, springs, worm gears,propeller blades and cylinder cams.

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Applications of Loci to Points and MechanismsIt is often necessary to study the parts of oscillating, reciprocating, or rotating mechanisms; from aknowledge of displacement and time, information regarding velocity and acceleration can be obtained.A general method of finding the locus of a point on a mechanism is to construct a number of positionsof such mechanism during one cycle of its operation and plot all the locus of the points identified at thevarious positions of the mechanism.For example: consider the slider mechanism OXY, the rotating rigid member does not change in lengthand the end of the link X describes an arc or a circle while the end Y describes a straight line. The locus of a point, say P, on the slider mechanism can

be plotted given that OX = 25 mm at 60 degree to the horizontal, OY = 80 mm and XP = 50 mm.

Chapter 6: Pictorial Projections•

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Pictorial projections are drawings in which the object is drawn in threedimensions (3-D), i.e. three sides of the object appear in one drawing.A three-dimensional drawing shows the length, width and height of theobject simultaneously.Two standards are currently used for pictorial projections: diametricprojection and isometric projection.

Diametric ProjectionIn diametric projection, all dimensions along two axes are drawn to true size.The dimensions along the third axis are halved. This projection is preferredwhen one view of the object is to beemphasized than the other two views(i.e. the one view is of more interestthan the other two).

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Isometric ProjectionAll dimensions along all three axes are drawn to true size.Preferred when the three views are of equal importance for accurate presentation ofthe object, this is the more important in engineering drawing.In isometric drawing, the object’s vertical lines are drawn vertically, and the horizontallines in the width and depth planes are shown at 30 degrees to the horizontal.

A more detailed representation and explanation of the steps are nowpresented in respect of Figure 6.3.

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(b)Figure 6.8: Isometric circle is drawn using the

appropriate method

Take home Exercises

Chapter 7: Orthographic Projections•

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Any engineering drawing should show everything: a completeunderstanding of the object should be possible from the drawing. If theisometric drawing can show all details and all dimensions on one drawing,it is ideal. One can pack a great deal of information into an isometricdrawing.However, if the object had a hole for instance on the back side, it would notbe visible using a single isometric drawing.In order to get a more complete view of the object, an orthographicprojection may be used.A minimum of two orthographic views is required to show the threedimensions of any object and therefore to describe its shape completely.Some features of the object that do not directly appear on viewing theobject from any direction (known as hidden details) are shown on thedrawing as dotted lines.

1. First and Third Angle ProjectionConsider the block shown in Figure 7.2. The five arrows point to different surfaces of theblock and five views will result. The arrows themselves are positioned square to thesurfaces, that is at 90 degrees to the surfaces and they are also at 90 degrees (ormultiples of 90) to each other.

NOTE:The student is expected to commit the above rules tomemory. It is customary to state the projection used onorthographic drawings to remove all doubt, or use thedistinguishing symbol which is shown on the arrangementin Figure 7.3

NOTE:The difference between the distinguishing symbol usedhere and the one used in First angle projection.

2. Orthographic projection drawingprocedure

Generally, in orthographic projection, do not complete oneview on a drawing before starting the next, but rather workon all views together.

Chapter 8: Freehand Sketching•

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A freehand sketch is a drawing in which all proportions and lengths are judged by eyes and all lines are drawn without the use of drawing instruments.

Sketches may be schematic or instructional and produced to convey ideas between engineering personnel.

Take home ExercisesCopy the following objects:

Disclaimer•

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This material which is an excerpt from the ‘Engineering Drawing Manual ofthe Mechanical Engineering Department of The Federal University ofTechnology, Akure (FUTA) is strictly meant as a lecture note for students ofthe FST 213 course.It is meant for the purpose of illustrative teaching only and should not becommercialized.