Projections Methods Used in Machine Drawing and CAD
written by: Asheesh
• edited by: KennethSleight
• updated: 8/29/2011
Projection is a method used to represent a 3-D object on a plane surface without including the mathematical calculations. It actually makes the study of design of an object or structure easier.
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Graphical projection has an important role in machine drawing and design. Various types of projection can be used to understand a 3-D structure. Here we discuss the important types of projections, including their advantages and disadvantages.
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When you look at a machine, you see that the machine has planes based on parallel lines. For example, a lathe machine has a rectangular base, which is a combination of parallel lines. If parallel lines are drawn to represent the parallel lines actually present on the machine, we call it a parallel projection. This type of projection is widely used by draftsman and architects to make blueprints and schematics.
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Orthographic projection is the most common parallel projection due to its simplicity. It is represented using three views; front view, side view and top view. In graphical terms, you need to draw the object in XY plane, YZ plane, and ZX plane, separately.
To draw the front view you just imagine how the object looks at front. Don’t think about slope of the object, just draw whatever it looks. Similarly, the top and side view of the object can be drawn (refer the image).
Through this projection, accurate measurement can be obtained because all the views are drawn using same scale. However, it doesn’t provide a realistic projection of a 3-D model; besides, it requires multiple views to feel the object design.
The orthographic projection can be drawn two ways, first angle projection and third angle projection. Both of them have differences only in the positioning of various views. In first angle projection, the front view is placed onto first quadrant; top and side view are placed onto the forth and second quadrants, respectively. In the third angle projection, the third quadrant is used for front view; whereas top and side view are placed onto the second and forth quadrants.
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Axonometric projection is a type of orthographic projection. Drawing the object in this method is somewhat complex because it requires only one image to draw the 3-D structure of the object onto a plane paper. Suppose you are using a projector and an object is placed in front of projector lines. Now, you can see the image of 3-D object onto a 2-D plane just behind the object. This projection is nothing but an axonometric projection.
Axonometric projection is classified into three categories depending upon orientation of the object.
The first and most convenient type is isometric. In this type of projection, angles between the three axes are equal. As the diagram says, if we project a cube onto a 2-D surface, you see all the three sides AB, AC, and AD are equal.
The second type is diametric projection, in which only two angles between the axes are equal. You can see the diagram in which only two sides AB and AC are equal.
The third is trimetric projection that can be drawn using three axes having different angles between them. It’s the most common type of axonometric projection and the object can be placed anywhere with respect to the observer.
All these methods are used for furniture and structural design. Axonometric projection is good for rectangular structures rather than curved objects.
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Oblique projection is comparatively easier than axonometric projection. It requires only one image and traditional equipments to draw any object. In this method, first you need to draw the front view or side view and then draw rest of the object with respect to it.
Draw the front or side view of the object just like in orthographic projection.
The measurements drawn backwards must be in the scale equal to half of the actual measurement.
The lines drawn backwards should have the angle in between 300 to 450; however it is convenient if you use 450 angles.
Oblique projection is further divided into two types based on scaling.
The first one is cavalier projection, in which the length in X and Z axis are drawn in scale 1:1 and there is no required scaling for the length of the Y-axis. It is very easy to draw and often used for drawing an object when you can use only your hands such as drawing a cube on the blackboard. The most common use of this type of projection is in military fortification.
The other type is cabinet projection. Unlike cavalier projection, this method uses the 2:1scale to draw lengths in X and Z direction, respectively. Besides, lengths in Y-axis must also be scaled properly. This projection is very useful in furniture industries.
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Perspective projection doesn’t use parallel lines to project an object, instead it is a projection along the lines emerging from a single point. Due to this, the nearer part appears bigger than distant part. This is much like the working of our eyes in respect to depth perception. For example, when we see a railway line, it appears converging towards a single point called the vanishing point. View of the object feels more realistic using this projection.
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You can use any type of projection for drawing an object onto plane surface. However, it requires good imagination power so as the object would become more realistic. The scaling must be done properly to avoid distortion. Always use clips to place the drawing sheet rigidly on the desk. Furthermore, use clean equipment to draw a neat and clean image.