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.
Fig.1Quadrants of spatial visualization.
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.
Fig.3 Views established using ﬁrst-angle 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.
Fig.5 First-angle and third-angle projection compared.
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.
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.
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.
Fig.8 Select the necessary views to describe the object from the six principal views available.
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.