[Funk Game Loop by Kevin MacLeod plays] [Liu Jin Xuan]: Hello, I’m Jin Xuan. Creating a technical drawing is more than creating a single view. While the single top view drawing can show many of this object’s features, it cannot show everything that we need to know. In engineering a drawing that expresses the details and information about a three-dimensional object is most commonly done using orthographic projections. In this lesson we will learn about orthographic projections and how to draw them. Projection is a technique used to represent a three-dimensional object on a two-dimensional drawing surface. The top view that was drawn in the previous lesson is simply a projection of an object’s top surface onto a piece of paper. The paper represents what we call the projection plane. Understanding projection is important for making and interpreting two-dimensional drawings that represent three-dimensional objects or structures in multiple views. An orthographic projection is where the plane of projection is parallel to the surface of the object. Features on the surface of the object are projected through lines called projection lines. These projection lines are parallel to one another and are perpendicular or at a right angle to the plane of projection. The outline of the object on the projection plane represents how the object appears to someone observing it. There are six principal perpendicular directions from which any object can be viewed. For example, the top view that we have already drawn represents one of these principal views. The five other views of our object are the front view, the left view, the right view, the bottom view, and the rear view. The top view is also called a plan view, which is a very common term used with architectural and civil engineering drawings. The term elevation is also commonly used for architectural and civil engineering drawings for any view that shows the height or vertical dimension of a building or engineering structure. Understanding how the six orthographic views are projected and arranged on paper can be done by thinking about our object as being inside of a box. Each of the six sides of the box represents the six planes of projection onto which the six principal views are drawn. As we look around the box we can see how this projection would appear from the outside. The outlines of the object and its features are simply projected outward and perpendicular to the projection planes. This projection system is the one used in the United States and is called third angle projection. By unfolding the box in front of the object we can then see how the third angle projection produces the six principal views of an orthographic projection on the outside of the box. Another projection system that is used most elsewhere in the world is called first angle projection. In first angle projection the outlines of the object and its features are projected through the object and perpendicular to the projection plane on the other side. This is very similar to the idea of a shadow being cast. Unfolding the box behind the object allows us to see how first angle projection produces the six principal views of an orthographic projection on the inside of the box. Although both projection systems are important, we will focus on third angle projection in this lesson. The unfolded box showing the six principal views from the third angle orthographic projection also presents how the different views are arranged when drawing on paper and how they relate to one another. This is important for making sure that the geometry of each view is accurately drawn. Each object has three principal dimensions that are shared between the views. These are the width, the depth, and the height. In orthographic projection these dimensions must be scaled the same in any view that they are presented. Width is presented in the front, top, bottom and the rear views. Depth is presented in the top, bottom, right, and the left views, and height is presented in the front, rear, right, and left views. Transferring dimensions from one view to another can be done using simple methods. First, it is important that the datum lines in the different adjacent views are aligned. Then, the transfer of width and height dimensions between views can be done by projecting these dimensions directly from one view into another. Transferring a depth dimension cannot be done directly because as we have unfolded our box, we do not have any views presenting depth that are adjacent to one another. One method for transferring a depth dimension is by measurement. When transferring the depth by measurement, the datum line in each view is considered as an edge of a common plane and the depth is simply measured from this common datum. A second method for transferring the depth dimension is by projection through an auxiliary line. This auxiliary line is drawn at 45 degrees from the two views you are projecting between and positioned to pass through the perpendicular intersection of the common datum lines extended from each view. A depth dimension is then transferred from one view with one projection line that is drawn parallel to its datum line until it intersects the auxiliary line. A second projection line is then drawn from the auxiliary line perpendicular to the first projection line and into the second view. This projection line is parallel to the second view’s datum line and it will be located at the proper depth. When drawing orthographic projection views, we also need to be able to locate the same point in different views. This can be done by measuring the location of the point directly or it can be done by projecting the point between views. Projecting points between views follows the same method as transferring dimensions with the exception that the point must be accurately located. For example, if we have drawn the top and front views of an object and want to locate a point in the right view, we can do this by transferring the point’s depth and height into the right view to find where these dimensions intersect. The height dimension is transferred by direct projection from the front view into the right view, and the depth dimension is transferred by projecting from the top view into the right view through the auxiliary line. The point is now located at the intersection of these two transferred dimensions. A line is also projected from one view into another by projecting its two end points. Each orthographic projection represents one side of the six principal view directions and only certain object features will be visible in any given view. When a view is drawn, the difference between visible features and hidden features must be clear. The outlines of visible features are drawn with thick, dark lines. The outlines of hidden features are drawn with dashed lines. When a hidden feature’s outline is directly behind a visible feature’s outline, it is not drawn. [Funk Game Loop by Kevin MacLeod plays]