EDT 300/ENGR 306 1

Geometry For Technical Drawing Geometry For Technical Drawing Chapter 4Chapter 4

Sacramento City CollegeEDT 300/ENGR 306

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ObjectivesObjectivesIdentify and describe geometric shapesand constructions used by drafters.

Construct various geometric shapesusing

Points.Lines.Planes.

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ObjectivesObjectivesApply geometric construction as a problem-solving tool in technical drawing.

Use geometry to reduce or enlarge a drawing or to change the proportions of a drawing.

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VocabularyVocabularyBisectChordCircumscribedDiameterEntitiesGeometric constructionsGeometryHypotenuseInscribed

IntersectObject snapsOgee curvesParallelPerpendicularPolygonRegular polygonRight angleTangent

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GeometryGeometryGeometry is the study of the size and shape of things.

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GeometryGeometryGeometry has always been important.

It was used in ancient times formeasuring land and making right-angle corners for buildings and other kinds of construction.

Egyptian “rope stretchers” used rope with marks or knots at 12 equal spaces.

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GeometryGeometryThe rope was divided into 3, 4, and 5 space sections.

A right angle was constructed by stretching the rope and driving pegs into the ground at the 3, 4, 5 space marks.

Refer to figure in text.

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The Use of the 3The Use of the 3--44--5 Triangle5 TriangleThe use of the 3-4-5 triangle for making right angles was proved by Pythagorasin the sixth century B.C.

The proof is the Pythagorean Theorem.A2 + B2 = C2

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The Use of the 3The Use of the 3--44--5 Triangle5 TriangleThe Pythagorean Theorem works for triangles that have the same proportions as 3-4-5.

6-8-109-12-15Etc.

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Geometric Construction Geometric Construction Geometric Constructions are:

Drawings made of individual lines and points drawing in proper relationship to one another.

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Geometric Construction Geometric Construction Geometric constructions are used by

Drafters.Surveyors.Engineers.Architects.Scientists.Mathematicians.Designers.

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Geometric ConstructionsGeometric ConstructionsEveryone, in all technical fields, needs to know the geometric constructionsexplained in this chapter.

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Solving Problems Using GeometrySolving Problems Using Geometry

Geometric constructions are often usedby drafters to help solve design problems.

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Solving Problems Using GeometrySolving Problems Using GeometryA designer may be designing a complicated mechanical assembly that has moving parts.

The designer must determine whether the design allows enough clearance for the parts to move freely.

The drafter may need to construct accurate circles and various tangents, parallel and perpendicular lines to measure clearances.

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Designing Within ParametersDesigning Within ParametersA designer may be given very specific design parameters (guidelines).

Example:A part or assembly may have to fit within a specific area.The principles of geometric construction are used to meet the specification.

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Accuracy Is FundamentalAccuracy Is FundamentalRemember, accuracy is a fundamental of drafting.

Sloppy drafting may give the designer the wrong answer, which may lead to a part being manufactured incorrectly.

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Accuracy Is FundamentalAccuracy Is FundamentalSloppy drafting may result in:

Lack of clarity and people may misread the information.Change orders.Parts not ordered correctly.

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Geometric ShapesGeometric Shapes

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Geometric ShapesGeometric Shapes--TrianglesTrianglesEquilateral Triangle

all sides equal.Isosceles Triangle.

two sides have equal length.3-4-5 Right Angle Triangle

right angle in triangle.Scalene Triangle

no equal sides or angles.

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Geometric ShapesGeometric Shapes--Circles/ArcsCircles/ArcsSemi-Circle and Circle Components

ChordDiameterRadiusSegmentSectorQuadrant

Concentric circlesEccentric circles

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Geometric ShapesGeometric Shapes--Four SidesFour SidesSquares.Rectangles.Rhombus.Rhomboid.Trapezoid.Trapezium.

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Geometric ShapesGeometric Shapes--MultisidedMultisidedPentagon.Hexagon.Heptagon.Octagon.Nonagon.Decagon.Dodecagon

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Geometric ShapesGeometric Shapes--3D3DCube.Right Rectangle.Right Triangle Prism.Right Triangular Prism.Right Cylinder.Right Cone.

Frustum of a coneSphere.

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Laying Out Your DrawingLaying Out Your Drawing

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Laying Out Your DrawingLaying Out Your Drawing1. Determine the width of the drawing space available.

2. Determine the height of the drawing space available.

3. Determine how wide the object to be drawn is.

4. Determine how high the object to be drawn is.

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Laying Out Your DrawingLaying Out Your Drawing5. Find the center of your drawing area.

6. Divide the object width by 2.

7. Measure from the center point out to the left and right this same distance

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Laying Out Your DrawingLaying Out Your Drawing8. Divide the object height by 2.

9. Measure from the center point out to the top and bottom this same distance.

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