Orthogonal Projection
Since all objects have three dimensions, Width, Depth and Height, see Figure 0400, it is near impossible to represent a 3–Dimensional object on a 2–Dimensional surface and include all of the required information for it to be manufactured or constructed. In order for an object to be communicated clearly we use a system of projection called ‘Orthogonal Projection’. This drawing method allows us to describe the 3-Dimensional form of an object by depicting each surface in a separate view. There are two styles of orthogonal projection used worldwide, ‘1st Angle Projection’ and ‘3rd Angle Projection’. In Australia, Australian Standards recommends the use of only 3rd angle projection. This chapter will discuss both 1st and 3rd to ensure that the differences between the two projection systems can be recognised.
The words ‘Orthographic’ and ‘Projection’ originate from the conjunction of a number of phrases. Ortho means ‘Straight’ or ‘at right angles (90°)’ and ‘Graphic’ means ‘written or drawn’. Similarly, projection can be linked to two Latin phrases, ‘pro’ meaning ‘Forward’ and ‘jacere’ meaning to ‘throw’. Putting all of these phrases together we get ‘thrown forward, drawn at right angles’.
The Projection Symbols
The two styles of projecting an object’s surfaces are derived from intersecting a horizontal drawing plane (HP) and a vertical drawing plane (VP) at right angles to form four right angles or what are labelled the dihedral angles.
If an object is then placed within either the 1st or 3rd of the dihedral angles, each of the object’s surfaces can be projected onto one of the horizontal or vertical drawing planes.
1ST Angle Projections
To understand how the surfaces of a 3-dimensional object are depicted using 1st angle projection, it is first important to image that the 1st angle of the dihedral angles is actually a translucent box and that each side of the box acts like a projection surface or projection plane, similar to projecting an image onto movie screen. If a 3-Dimensional form is then placed in the centre of the 1st angle dihedral box, it’s surfaces can then be projected onto each of the six sides or projection planes of the box. When using 1st angle projection the surfaces of the object are projected with the object between the viewer and the projection plane.
As shown in Figure 0400, a 3-dimensional form has been placed in the absolute centre of the 1st dihedral angle. The features of the object facing the viewer, the front view, have then been projected onto the far projection plane. This process can then be continued for each surface of the 3-dimensional form. Figure 400 shows each surface after is has been projected although the view projected onto the top of the box (the bottom view) has been left out for clarity. The dihedral box can then be unfolded to show the aligned or Orthogonal Views. Figure 400 shows the dihedral box unfolded, the front view is in the centre with the left and right hand side views on either side and the top and bottom view above and below the front view
It is important to note that these views appear in the opposite position to their name. That is, the right side view appears on the left and the left side view appears on the right. Likewise, the top view appears on the bottom, and the bottom view appears on the top.
3RD Angle Projection
Again, with 3rd projection, it is first important to image that the 3rd of the dihedral angles is once again a translucent box with each side of the box acting like a projection surface or projection plane. If a 3-Dimensional form is placed in the centre of the 3rd angle dihedral box, it’s surfaces can also be projected onto each of the six sides or projection planes of the dihedral box. However, when using 3rd angle projection the surfaces of the object are projected with the projection plane between the object and the viewer.
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***INSERT Caption Figure 400 – Projecting 3rd Angle View***
As shown in Figure 400, the 3-dimensional form has been placed in the absolute centre of the 3rd dihedral angle. With the features of the object facing the viewer, the front view, has then been projected onto the projection plane. This process can then be continued for each surface of the 3-dimensional form. Figure 400 shows each surface after is has been projected although the view projected onto the top of the box (the top view) has been left out for clarity. The dihedral box can then be unfolded to show the exterior of the box with the aligned or orthogonal views. Figure 0400 shows the dihedral box unfolded, the front view is in the centre with the left and right hand side views on either side and the top and bottom view above and below the front view. This time, the views appear in the correct positions relevant view to their name. That is, the right side view appears on the right of the front and the left side view appears on the left side of the front view. Likewise, the top view appears above the front view, and the bottom view appears underneath the front view.
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***INSERT Caption Figure 400 – 3rd Angle Projection – Folded dihedral box***
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***INSERT Caption Figure 400 – 3rd Angle Projection – Unfolded Dihedral box***
Projection Symbols
All instrumental drawings must identify the projection method used by depicting either the third angle or first angle projection symbol or writing the phrase ‘THIRD ANGLE PROJECTION’ or ‘FIRST ANGLE PROJECTION’ within the titleblock or as best fits on the drawing paper. Remember, Australian Standards recommends that only third angle projection be used.
The standard for drawing the 3rd angle projection symbol can be seen below. It should be observed that the symbol is drawn with separate centres lines for each view of the frustum (or truncated cone).
Did You Know?
Orthogonal Projection is a fantastic way to conceptualise an object or environment and communicate to designers, manufacturers, engineers or builders but almost useless to those with an untrained eye. This method of drawing provides abstract representations of a space which is great for showing how to make it but reveals nothing about how an object or environment will be experienced. If using orthogonal views on presentations make sure they are supported by perspective design drawings to help your target audience know what they’re looking at.
Drawing an orthogonal Projection
Before beginning an orthogonal projection it is essential that you have the necessary information and prepare for what has to be drawn. The following list can help you prepare and complete an orthogonal drawing part, assembly or an environment.
- Ensure drawing area is clear and all required media, materials and equipment have been prepared
- Decide on the number of views required to represent the part or environment
- Sketch a draft of the Orthogonal drawing to calculate the layout of the drawing
- Ensure all drawing equipment and hands are clean
- Prepare the drawing surface by squaring of the page
- Drawing the borderline and positioning of the views
- Construct the views
- Line-in the views
- Dimension the features
- Insert subtitles and notes
- Draw the title block, parts list and revisions tables
Number of Views
Although it is possible to project a total of 6 views from any 3-dimensional object it is rare that all 6 views are required in an orthogonal drawing. It is convention the front view is always provided and the inclusion of addition view is based on its ability to further communicate the part or environment’s features. Figure *** shows the example of 3, 2 and 1 view Orthogonal projection drawings.
Layout of the Views
When a number of views are required to communicate the form of an object or environment it is necessary to do so in a way that provides a balanced and visually attractive drawing sheet. Knowing this, before starting to draw an orthogonal instrumental drawing it is worth sketching a plan for how the drawing will be laid out.
Firstly, draw a sketch of your drawing sheet with borders and label is overall length and width less the width of the borders.
***INSERT AW0400 – Orthogonal Drawing 01***
Sketch in any additional requirements such as the title block, materials or parts lists, or revision tables. Make note of the measurements for additional requirements added to the drawing sheet.
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Sketch in boxes to represent the views required, in this case 3. Label the views and ensure the correct views are given and they appear in the correct positions.
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Mark in the overall dimensions for each of the views and the desired gaps between each view. It is important to know that the gap between the different views (for example Front – Top and Front – Side) does not need to be the same however; the gap between any 2 views should not be any less than 50mm to allow for dimensioning and view or elevation labels.
The distance between the border and views or the general drawing area can then be calculated by using the following equations. Note that if either equation results in an answer less than 50mm consider shorting the gap between the views, scaling your drawing or increasing the size of your drawing sheet, this will make sure that your drawing can be easily distinguished from the other parts of the drawing sheet.
Meas. Left & Right = ((Sheet Width – Width of Parts List or Revisions List – Borders) – (Width of Part + Gap + Depth of Part))
2
Meas. Top & Bottom = ((Sheet Height – Height of Title block - Borders) – (Height of Part + Gap + Depth of Part))
2
***INSERT AW0400 – Orthogonal Drawing 04***
Mark in all of the dimensions for centring the views on the drawing sheet sketch. Ready all the equipment to begin drawing an instrumental drawing, adhere drawing sheet down to the drawing board and then begin to construct your orthogonal drawing.
Constructing an Orthogonal Drawing
HintRemember! Before beginning any drawing, technical or freehand, it is important to ensure that the drawing area has been cleared and that your hands and all pieces of drawing equipment are clean.
Now that you have measurements mapped out you can begin to construct an orthogonal projection. Note that your drawing should be slowly constructed using a harder style of pencil such as a 2H. This will help ensure your line work is light and easily erasable.
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Begin your drawing by squaring off your drawing sheet by lightly sliding it down against the tee square and placing small pieces of masking tape on each corner to hold it in place on the drawing board or drafting machine.***
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Squaring off your page ensures that you drawing sheet is perpendicular to the edge of the drawing board. This guarantees that any line drawn with the Tee Square will be straight and parallel to the top and bottom edges of the drawing sheet. It also ensures that any lines drawn with the Tee Square and Set Squares are drawn at the correct angles.
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Using numerous pieces of equipment at one time can be tricky. The more practice you have at controlling the better you become. It can be frustrating but keep with it. The more dexterous you become using the equipment the faster you will become at completing instrumental drawings with accuracy.
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Using a Tee Square, ruler and 45° set square measure out from the bottom left hand corner and up the drawing the distance calculated in the earlier equation. Place a light dot to indicate the measurement. ***
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To indicate a datum or measuring point on your drawing sheet accurately place the tip of your pencil against the ruler, spin the pencil between your thumb and finger while apply a small amount of pressure downwards. If this is done with a sharp pencil you will have a small identifiable datum that is accurate and can be erased or hidden beneath your finished line work. Avoid doing this with dull pencils or draw large dots to indicate as these will not be accurate and can smudge if your drawing equipment passes over them.
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Use the Tee Square to draw a light line from the datum across the drawing sheet for the full measurement of the 2 views and gap.***
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It is important that construction lines are drawn as lightly as possible. Since they are only to construct the drawing, they should be drawn with the intention to be erased. If drawn to hard or dark, they cannot be erased and will cause ambiguity on the drawing.
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***INSERT Caption Figure 400
Use the Tee Square and Set Square draw a vertical construction line for the full height of the 2 views and gap. ***
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Measure and indicate with a datum (d2) the width of the front view out from the original datum. ***
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Use the Tee Square and Set Square draw a vertical construction line for the full height of the 2 views and gap from d2. ***
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Measure from d1 and indicate with a datum (d3) the height of the front view and accurately indicate the measurement.
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Use the Tee Square to draw a horizontal construction line for the full width of the 2 views and gap from d3. ***
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Use the Tee Square and Set Square to measure and mark the gap between the Front – Top views (d4) and the Front – Side views (d5). Add construction lines at the 2datum points, d4 and d5 to close the outlines for the Top and Side Views. ***
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Fill in the detail for the Front View***
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Using the Tee Square and Set Square, project the appropriate features of the Front View vertically onto the Top View. This will allow you to draw the Top View without having to measure certain features.
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Using the Tee Square, project the appropriate features of the Front View horizontally onto the Side View. This will allow you to draw the Top View without having to measure certain features.
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To complete any missing features from the Top or Side Views you will now project features between the two views. Starting with the Side View, project the appropriate features vertically to the outer perimeter of rectangle above. Using a 45° Set Square and Tee Square continue to draw the projection lines to the left hand side of the rectangle’s perimeter. Using the Tee Square alone, continue the projection lines onto the Top View.
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***INSERT Caption Figure 400
Using the previous step in reverse, project the appropriate features of the Top View down to the Side View.
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Check to ensure all features on all views are correct and are shown using the correct line styles and weights.
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Line-in the views using a HB pencil or technical drawing pens. Erase any construction or projection lines.
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Instrumental drawing is now ready for annotations, labels and dimensioning.