This weeks topic:

Orthographic Drawing With Instruments

Introduction:

Previously in the section on sketching, orthographic projection was introduced as a way to develop a design concept. Now, we will use the same techniques to turn these sketches into orthographic views drawn to scale with instruments to define the design more precisely.

Orthographic drawings are three-dimensional objects represented by separate views arranged in a standard manner that are readily understood by those involved in the design and manufacturing process.

These drawings are commonly called working drawings, or detailed drawings, when sufficient dimensions, notes, and specifications are added to enable the product to be manufactured from the drawings.

The Basics:

Draw all orthographic views with dark dense lines as if drawn with ink. Only the line widths should vary - except for guidelines and construction lines, which are drawn lightly for layout and lettering. Figure 7.4 gives examples of lines used in orthographic drawings.

As with orthographic sketching there are six possible views of any given item, and their arrangement in relation to each other is the same. Figures 7.10 & 7.11 show the rules of projection and the proper alignment of dimensions.

Selection of views:

When preparing to create a detailed drawing select the sequence of views with the fewest hidden lines. Figure 7.12A shows that the right side view is preferable to the left side view because it has fewer hidden lines

The most descriptive view is usually selected as the front view. If an object (such as a chair) has predefined views that people generally recognize as the front and top views, you should use the accepted front view as the orthographic front view.

Views by Subtraction:

Figure 7.18 illustrates how three views of a part are drawn by beginning with a block having the overall height, width, and depth of the finished view and removing volumes from it.

Laying-out Drawings:

Layout Rules:

1. Draw orthographic views in their proper positions.
2. Select the most descriptive view as the front view.
3. Select the sequence of views with the fewest hidden lines.
4. Place dimensions between the views to which they apply.
5. Use the proper line types
6. Leave adequate room between views for dimensions.
7. Draw the views necessary to describe the part.

Figures 7.20 thru 7.24 show examples of drawings done by applying these rules.

One and Two View Drawings:

Economize on time and space by using only the views necessary to depict an object. Figures 7.25 &7.27 show objects that don't require a three view drawing in order to fully describe them.

Partial Views:

Partial views of symmetrical of cylindrical parts may be used to save time and space. Omitting the rear of the circular top view in Figure 7.30 saves space without sacrificing clarity.

Conventional Practices:

The readability of an orthographic view may be improved if the rules of projection are violated. These violations of the rules are commonly called conventional practices.

Symmetrically positioned features such as ribs, webs, and circles are drawn at their true radial locations in the top view. Then imagine that the features are revolved to the centerline in the top view before being projected to the front view (see Figures 7.33 thru 7.35)

Conventional Intersections:

In orthographic projection, lines are drawn to represent the intersections between planes of an object. Wherever planes intersect, forming a sharp edge, this line of intersection is projected to its adjacent view.

Figure 7.40 shows how to draw intersections between cylinders rather than plotting more complex, orthographically correct lines of intersection.

Fillets & Rounds:

Fillets and rounds are rounded intersections between the planes of a part that are used on castings, such as the body of the part in Figure 7.44.

A fillet is an internal rounding, and a round is an external rounding on a part. The radii of fillets and rounds usually are small, about 1/4" . Fillets give added strength at inside corners, and rounds improve appearance and remove sharp edges.

A casting will only have square corners when its surface has been finished, which is the process of machining away part of the surface to a smooth finish. Indicate finished surfaces by placing a finish mark (see Figure 7.45) on all edge views of finished surfaces.

• You will now combine the methods learned in the last two sections to create orthographic drawings using drafting tools