Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Orthographic Projection
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Orthographic Projection
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Subject: Technical Drawings
Class: Senior Secondary 2
Term: 1st Term
Week: 10
Theme: Building And Engineering Design And Drawing
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Explain or thographic projection and its principles. State types of or thographic projections Draw symbols of first and third angle or thographic projection. Draw properly or thographic views of objects. Draw is ometric projection from given or thographic views.
seen from the top, using the projected lines and the object's width. In First Angle, the Plan View is directly below the Front View. Add hidden lines for any features not visible from the top.
6. Project to Draw End View (EV): From the rightmost edges of the Front View, project light horizontal construction lines to the right. From the Plan View, project light horizontal construction lines to the right, ending at a 45-degree mitre line drawn from the intersection of the FV and PV projection areas. From the 45-degree mitre line, project vertical construction lines upwards to intersect the horizontal lines from the Front View. Measure the Width (40mm) from the right of the Front View, using the projected lines from the mitre line. Draw the outline of the L-bracket as seen from the left end (for First Angle, Left End View goes to the right of the Front View). The End View will show the Width (40mm) and Height (50mm). Add hidden lines for any features not visible from the end.
7. Darken Visible Lines: Once all views are complete and checked, darken all visible outline lines with a darker pencil (e.g., HB or 2B). Hidden lines (short dashes) and centre lines (long-short-long dashes) should be drawn with a lighter pencil (e.g., H or 2H).
8. Add Projection Symbol: Draw the first angle projection symbol in a designated area (e.g., bottom right corner) of the drawing sheet.
Example for Third Angle Projection: The procedure is similar, but the arrangement of views differs: Plan View above Front View, Left End View to the left of Front View. The projection lines would extend upwards for the Plan View and to the left for the Left End View. 2.
6. Drawing Isometric Projection from Given Orthographic Views (PO5) Isometric projection is a type of pictorial drawing where all three dimensions (length, width, height) are shown in a single view, making the object appear 3D. It is characterized by having its three main axes (representing length, width, and height) at 120 degrees to each other, with two axes at 30 degrees to the horizontal and one vertical. General Procedure (
Example: Isometric of an L-shaped Bracket from Orthographic Views)
Given: Orthographic views (Front, Plan, End) of an L-shaped bracket with dimensions: L=60mm, W=40mm, H=50mm.
1. Establish Isometric Axes: Draw a vertical line. From the bottom of this line, draw two lines at 30 degrees to the horizontal, one to the left and one to the right. These are the isometric axes (representing height, length, and width).
2. Determine Overall Dimensions: From the given orthographic views, identify the maximum length (60mm), width (40mm), and height (50mm) of the object.
3. Construct an Isometric "Crate" (Bounding Box): Along one 30-degree axis (e.g., to the right for length), measure 60mm. Along the other 30-degree axis (e.g., to the left for width), measure 40mm. Along the vertical axis (for height), measure 50mm. Complete the isometric box (a rectangular prism) using parallel lines for each axis. This box encloses the entire object.
4. Block in Main Features: Refer to the orthographic views. For the L-bracket, the Front View shows the overall 'L' shape. Start by sketching this 'L' shape on the front face of your isometric box, using the dimensions from the Front View. For the horizontal base (20mm thick), measure 20mm up from the bottom along the height axis at the front, and draw a line parallel to the length axis. For the vertical arm (20mm thick), measure 20mm from the right along the length axis at the bottom, and draw a line parallel to the height axis.
5. Add Depth (Width): From the corners and edges of the 'L' shape blocked in the front, draw lines parallel to the width axis (the 30-degree axis not used for length) for a distance of 40mm. Connect the ends of these lines to complete the 3D form. For the L-bracket, this means extending the top horizontal surface backwards by 40mm, and the vertical surface backwards by 40mm.
6. Add Details and Refinements: smaller end on the right, as if viewing the object from the left and projecting it onto a plane to its right. The arrangement of circles within the symbol reinforces the "plane behind object" concept.
Drawing Representation: ``` (Large circle) --- (Small circle) | | / \ / \ / \ / \ / \ / \ ----------- ----------- (Larger end) (Smaller end) ``` Detailed explanation: The symbol is drawn by projecting the end view of the truncated cone. If you look at the cone from the left, you see a small circle on the left and a larger circle behind it. In First Angle, the Left End View is projected to the Right of the Front View. So, if the cone is viewed from the left, its projection will be to the right, showing the smaller circle on the left and the larger circle on the right (as if the viewer sees the smaller circle first, and the projection of the larger end is further away).
Third Angle Projection Symbol: Shows the smaller end of the cone on the left and the larger end on the right, as if viewing the object from the left and projecting it onto a plane in front of it. The arrangement of circles within the symbol reinforces the "plane in front of object" concept.
Drawing Representation: ``` (Small circle) --- (Large circle) | | / \ / \ / \ / \ / \ / \ ----------- ----------- (Smaller end) (Larger end) ``` Detailed explanation: In Third Angle, the Left End View is projected to the Left of the Front View. If the cone is viewed from the left, the plane is in front of the object. So, the projection on the left plane shows the smaller circle on the left and the larger circle on the right (as if the viewer sees the smaller circle first, and the projection is on the plane directly in front of that view). 2.
5. Drawing Orthographic Views of Objects (PO4) This involves converting a 3D pictorial view of an object into its 2D orthographic projections. General Procedure (
Example: First Angle Projection of an L-shaped Bracket)
Object: An L-shaped bracket with overall dimensions: Length (L) = 60mm, Width (W) = 40mm, Height (H) = 50mm. The vertical arm is 20mm thick, and the horizontal base is 20mm thick.
1. Study the Object and Select Front View: Analyze the pictorial view to understand its features, dimensions, and hidden parts. Select the most informative view as the Front View (usually the view that shows the most features and the overall shape). For the L-bracket, viewing it from the front along its longest dimension (60mm) is ideal.
2. Determine Projection Method: For this example, use First Angle Projection.
3. Prepare Drawing Sheet: Draw faint horizontal and vertical reference lines (X-Y and X1-Y1 axes). These lines separate the drawing planes. Ensure adequate spacing between views for clarity and dimensioning.
4. Draw Front View (FV): Draw the outline of the L-bracket as seen from the front. The Front View will show the Length (60mm) and Height (50mm). Include all visible edges. Add hidden lines (short dashed lines) for features not visible from the front (e.g., if there were a hole passing through the back of the L-bracket).
5. Project to Draw Plan View (PV): From the top edges of the Front View, project light vertical construction lines downwards. Measure the Width (40mm) below the Front View, starting from a suitable gap (e.g., 20mm). Draw the outline of the L-bracket as seen from the top, using the projected lines and the object's width. In First Angle, the Plan View is directly below the Front View. Add hidden lines for any features not visible from the top.
6. Project to Draw End View (EV): From the rightmost edges of the Front View, project light horizontal construction lines to the right. From the Plan View, project light horizontal construction lines to the right, ending at a 45-degree mitre line drawn from the intersection of the FV and PV projection areas. * 2.
1. Definition and Principles of Orthographic Projection Orthographic projection is a method of representing a three-dimensional (3D) object in two-dimensions (2D) by showing multiple views of the object as seen from different angles. It achieves this by projecting the object onto mutually perpendicular planes. The purpose is to provide an unambiguous and accurate representation of an object's shape, size, and features, which is essential for manufacturing and construction.
Principles:
1. Parallel Projectors: All lines of projection (projectors) from the object to the projection plane are parallel to each other.
2. Perpendicular Projectors: The projectors are perpendicular to the projection plane. This means that each view shows only two dimensions of the object (e.g., length and height, width and height, or length and width), eliminating depth in that particular view.
3. Multiple Views: Typically, at least three standard views (Front View, Plan View, End View) are drawn to fully describe the object. These views are arranged in a specific, systematic manner relative to each other.
4. True Shape and Size: Features parallel to the projection plane are shown in their true shape and size.
5. Relationship between Views: Corresponding features in different views are aligned, meaning that a length in the Front View must correspond to the same length in the Plan View, and a height in the Front View must correspond to the same height in the End View. 2.
2. Projection Planes Orthographic projection uses three main mutually perpendicular planes: Vertical Plane (VP): The plane onto which the Front View (or Elevation) is projected.
Horizontal Plane (HP): The plane onto which the Plan View (or Top View) is projected. Auxiliary Vertical Plane (AVP) / End View Plane (EVP): The plane onto which the End View (or Side View) is projected. These planes intersect to form quadrants. The standard projection methods place the object in either the first or third quadrant. 2.
3. Types of Orthographic Projections There are two primary types of orthographic projection, differing in the arrangement of the object, observer, and projection plane: 2.3.
1. First Angle Projection Principle: The object is imagined to be in the first quadrant, positioned between the observer and the projection plane.
View Arrangement: The views are projected onto the plane behind the object. The Plan View is placed below the Front View. The Left End View is placed to the right of the Front View. The Right End View is placed to the left of the Front View.
Usage: Predominantly used in Nigeria (following British/European standards), India, and other parts of Asia.
Mnemonic: "Observer - Object - Plane" (OOP). 2.3.
2. Third Angle Projection Principle: The object is imagined to be in the third quadrant, positioned behind the projection plane, with the projection plane between the observer and the object.
View Arrangement: The views are projected onto the plane in front of the object. The Plan View is placed above the Front View. The Left End View is placed to the left of the Front View. The Right End View is placed to the right of the Front View.
Usage: Predominantly used in the United States, Canada, and some Asian countries.
Mnemonic: "Observer - Plane - Object" (OPO). 2.
4. Drawing Symbols of First and Third Angle Orthographic Projection To indicate the projection method used on a drawing, a standard symbol is placed in the title block. The symbol is a truncated cone (frustum of a cone) viewed from its end.
First Angle Projection Symbol: Shows the larger end of the cone on the left and the smaller end on the right, as if viewing the object from the left and projecting it onto a plane to its right. The arrangement of circles within the symbol reinforces the "plane behind object" concept.
Drawing Representation: ``` (Large circle) --- (Small circle) | | / \ / \ / \ / \ / \ / \ ----------- ----------- (Larger end) (Smaller end) ``` Detailed explanation: The symbol is drawn by projecting the end view of the truncated cone. If you look at the cone from the left, you see a right along the length axis at the bottom, and draw a line parallel to the height axis.
5. Add Depth (Width): From the corners and edges of the 'L' shape blocked in the front, draw lines parallel to the width axis (the 30-degree axis not used for length) for a distance of 40mm. Connect the ends of these lines to complete the 3D form. For the L-bracket, this means extending the top horizontal surface backwards by 40mm, and the vertical surface backwards by 40mm.
6. Add Details and Refinements: Add any specific cut-outs, holes, or chamfers indicated in the orthographic views. Remember that circles in isometric projection appear as ellipses. Use the four-centre method or a template for ellipses. * Ensure all lines that should be parallel in the object are drawn parallel to the isometric axes.
7. Darken Visible Lines: Erase all construction lines and lightly drawn sketch lines. Darken only the visible lines of the object with a darker pencil.
8. Check for Accuracy: Verify that the isometric view accurately represents all features shown in the orthographic views.
Architectural and Building Construction (Nigeria): Orthographic projection is fundamental to creating blueprints for houses, schools, hospitals, and commercial buildings across Nigeria. Architects produce floor plans (plan view), elevations (front/side views), and sections (cut-away views) using orthographic principles. Understanding these drawings is critical for masons, carpenters, electricians, and plumbers working on Nigerian construction sites, ensuring accurate interpretation of design specifications for structures like a typical Nigerian bungalow or a multi-storey office block in Lagos. Mechanical Engineering and Fabrication (Nigeria): In Nigerian industries, from vehicle assembly plants to small-scale metal fabrication workshops (e.g., fabricating gates, window frames, agricultural tool parts in Aba or Nnewi), orthographic drawings are used to design and manufacture components. Engineers and technicians rely on these drawings to specify dimensions, tolerances, and material types for parts of machines, vehicles, or industrial equipment. This ensures that manufactured components fit together precisely. Product Design and Manufacturing (Nigeria): For local product designers creating anything from furniture (e.g., a modern take on a 'talking drum' inspired stool) to casings for locally assembled electronics or household appliances, orthographic projection is used to communicate design intent to manufacturers. It allows for clear representation of external forms and internal mechanisms, critical for mass production and quality control in Nigerian manufacturing hubs.