First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

First-angle projection is commonly used in countries other than the United States. First-angle projection places the glass box in the first quadrant of Fig 1. Views are established by projecting surfaces of the object onto the surface of the glass box. In first angle projection arrangement, the object is between line of sight and the projection plane, as shown in Fig.2.

First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

Fig.1Quadrants of spatial visualization.

First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

Fig.2 Glass box in first-angle projection.

When the glass box in the first-angle projection quadrant is unfolded, the result is the multi view arrangement shown in Figure 3. A first-angle projection drawing is identified by the first-angle projection symbol. The angle of projection symbol typically appears in the angle of projection block near the title block, Drafting Equipment, Media, and Reproduction Methods. Figure.4 shows the standard first angle projection symbol as specified by ASME Y14.3. Figure.5 shows a comparison of the same object in first- and third-angle projections.

First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

Fig.3 Views established using first-angle projection.

First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

Fig.4 First-angle projection symbol. ASME Y14.3 defines the projection symbol dimensions based on a .12 in. (3 mm) letter height. A larger symbol is usually more appropriate for use in the angle of projection block.

First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

Fig.5 First-angle and third-angle projection compared.

VIEW SELECTION

Although there are six primary views that you can select to describe an object completely, it is seldom necessary to use all six views. As a drafter, you must decide how many views are needed to represent the object properly. If you raw too many views, you make the drawing too complicated and are wasting time, which costs your employer money.

If you draw too few views, then you have not completely described the object. The manufacturing department then has to waste time trying to determine the complete description of the object, which again costs your employer money.

(i) Selecting the Front View

Usually, you should select the front view first. The front view is generally the most important view and, as per the glass box description, it is the origin of all other views. There is no exact way for everyone to select the same front view always, but there are some guidelines to follow. The front view should:

• Represent the most natural position of use.

• Provide the best shape description or most characteristic contours.

• Have the longest dimension.

• Have the fewest hidden features.

• Be the most stable and natural position.

Look at the pictorial drawing in Figure 6. Notice the front- view selection. This front-view selection violates the guidelines for best shape description and the fewest hidden features. However, the selection of any other view as the front would violate other rules, so in this case there is possibly no correct answer.

First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

Fig.6 Front-view selection.

Given the pictorial drawings in Figure.7, identify the view that you believe is the best front view for each object. The figure caption provides possible answers. More than one answer is given for some of the objects, with the first answer being the preferred choice.

First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

Fig.7 Select the best front views that correspond to the pictorial drawings at the left. You can make a first and second choice

(ii) Selecting two or Three Views

Use the same rules when selecting other views needed as you do when selecting the front view:

• Most contours.

• Longest side.

• Least hidden features.

• Best balance or position.

Given the six views of the object in Figure 8, which views would you select to describe the object completely? If your selection is the front, top, and right side, then you are correct.

First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

Fig.8 Select the necessary views to describe the object from the six principal views available.

First Angle Projection | Engineering Graphics Fundamentals | Orthographic Views | Orthographic Projection

Fig.9

Now take a closer look. Figure 9 shows the selected three views. The front view shows the best position and the longest side, the top view clearly represents the angle and the arc, and the right-side view shows the notch. Any of the other views have many more hidden features. You should always avoid hidden features if you can.




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