Chapter 5: Section and Sectional Views – Machine Drawing with AutoCAD

Chapter 5

Section and Sectional Views

Chapter Outline

The primary objective of an engineering drawing is to communicate the drawing information clearly to the user. From our basic engineering drawing knowledge, we can represent the details of an object fairly well through an orthographic projection drawing, indicating the dimension, shape, and size of the object. The invisible features of the object are conventionally depicted by means of hidden lines in their projected views. Sometimes, the internal features of an object are so complicated that an ordinary orthographic representation would result in a confusing bunch of overlapping hidden lines making it very difficult to interpret.

 

FIG. 5.1   This object is being cut into sections by a cutting plane

 

For a better understanding of the component(s), it is customary to assume the object, or assembly, as being cut through or made into sections by an imaginary plane known as the cutting plane as depicted in Fig. 5.1. It is further assumed that the front portion of the object between the cutting plane and the observer is removed (Fig. 5.2). The orthographic view thus obtained is called a ‘sectional view’ or view in section as shown in Fig. 5.3. In order to represent the internal material in the sectional view, hatching (section lines) is used. In Fig. 5.3a, the projection view of the object shown in Fig. 5.1 is illustrated whereas in Fig. 5.3b, the front sectional view of the object is presented. The other view (top view) are drawn in the normal fashion, without any change, except the inclusion of the cutting plane line indicating the location of the imaginary cutting plane.

 

FIG. 5.2   Section

 

A comparison between Fig. 5.3a and Fig. 5.3b clearly highlights the fact that a sectional view helps to visualise the internal details of an object which is otherwise not visible from outside. If only one view of the projection drawing of an object is shown in a section, then the other views are not at all affected. The section is shown by means of hatch or section lines. In general, hidden/dashed lines for internal details are omitted from the sectional view unless they are absolutely necessary for further clarification of the internal shape and size of the object.

 

FIG. 5.3a   Projection views

 

FIG. 5.3b   Sectional front view

 

Fig. 5.4a shows the same object cut by a vertical plane, with the orientation changed to obtain the sectional side view of the object. The latter is represented in Fig. 5.4b.

 

FIG. 5.4   Object cut by a vertical plane and its sectional view

CUTTING PLANE LINES

Cutting planes are indicated by long, thin chain lines, thickened at the ends and bends where there is change of direction. The direction of viewing is indicated by arrows resting on the cutting plane line as indicated in Fig. 5.3. A cutting plane is conventionally designated by capital letters.

FULL SECTION

When an imaginary cutting plane passes through the entire object showing the whole object in section, the resultant view is known as a full section. In Fig. 5.3b, the vertical cutting plane divides the object into two equal parts along the center line and the front view is projected in full section.

 

FIG. 5.5   An object in full section

 

Fig. 5.5 shows the full sectional view of another slightly complicated object in which a horizontal cutting plane is passed through the object resulting in a sectional top view. When an object is sectioned crosswise, it is called cross-section. Such a cross-section is frequently used to show the shape of a shaft, axle, beam, box beam, and so on.

HALF SECTION

If an object has an axis of symmetry, then the half sectional view is sufficient to represent the internal appearance of the object. In such situations, the object is assumed to be cut by two cutting planes at right angles to each other. Hence one quarter of the object between the two planes is assumed to be sliced off, resulting in a half section.

Fig. 5.6a indicates the location of two cutting planes at right angles with respect to the object. If the solid object is actually cut out in half section, it will look like as shown in Fig. 5.6b. The corresponding orthogonal projection views are illustrated in Fig. 5.7.

 

FIG. 5.6a   Placement of cutting plane for Half Section

 

FIG. 5.6b   Removal of a quarter part

 

FIG. 5.7   Half section

 

FIG. 5.8   Local section

SPECIAL SECTION

Apart from the usual sections, some sections, for example, revolved section, local (partial or skin) section, removed section, and so on are also shown to reveal the internal composition of an object.

LOCAL OR PARTIAL SECTION

Sometimes it is essential to show the sectional view of a small portion of a drawing. In such a situation, a local or partial (broken) section is drawn as shown in Fig. 5.8.

REMOVED SECTION

A section is obtained by passing a cutting plane at right angles to the axis of the object to display the cross-section of an object. When the sectional view is shown within the object, shown in Fig. 5.9a, it is known as a revolved section or cross-section of the object. In some cases, the section is drawn outside the object (Fig. 5.9b)—and it is termed removed section.

 

FIG. 5.9a   Revolved section by solid fill

 

FIG. 5.9b   Removed section

OFFSET SECTION

Even for a full section drawing, it may so happen that the cutting plane is not one single plane. Rather, it is a combination of more than one plane, as can be seen in Fig. 5.10a. This is referred to as an offset section. Fig. 5.10b shows the same object when the front part is removed. The sectional view of the object is shown in Fig. 5.11.

 

FIG. 5.10a   Position of cutting plane

 

FIG. 5.10b   Object with front portion removed

 

OMISSION OF SECTION

According to drawing convention, ribs, web, rolling elements (ball, roller) of bearing shaft, rod nuts, bolts, revets, keys, screws, studs, pins, gear teeth and so on which do not have any special internal features, are, in general, not drawn in section particularly when they are seen in profile. However, a cross-section or the broken out sections may be shown when required. An example of a web not being sectioned is shown in Fig. 5.12 and 5.13.

 

FIG. 5.12   Web not sectioned

 

FIG. 5.13

HATCHING OR SECTION LINES

The simplest form of hatching is usually adequate to show areas of sections and may be based upon continuous lines at a convenient angle, preferably 45°, to the principal outlines. Separate areas of a section of the same component are hatched in an identical manner whereas the hatching of adjacent components is carried out with different directions or spacing. The spacing between the hatching lines should be dependent on the size of the hatched areas. For the manual mode of drawing, extra precaution should be taken so that the thickness and spacing of the hatch lines appear uniform. In case of an AutoCAD drawing, the software takes care of this and users do not have to bother about it.

Section lines should never cross the outlines of a drawing. They may be interrupted for dimensioning only when it is not possible to place dimensions outside. Very thin sections (cross-sectional view of gasket, angles, and so on) may be shown entirely black—Fig. 5.9a. Some of the conventional section lines are depicted in Table 5.1.

 

Table 5.1   Types of Section Lines

No. Types of Section Lines Materials
1. Cast iron, steel, copper and its alloys, aluminium and its alloys. Section lines are inclined at 45° and uniformly spaced
2. Lead, zinc, tin, white metal. Section lines cross each other and are inclined at 45° in the opposite directions
3. Concrete
4. Liquids, such as water, oil, petrol, kerosene, and so on
5. Brick work
6. Brass
SECTIONAL VIEW AND AutoCAD

From the above discussion, it is clear that in order to obtain a sectional view, the object is first cut up into parts by passing an imaginary cutting plane through it and the projection view of the cut object is shown on a 2-D drawing sheet. AutoCAD has the facility to develop 3-D models of objects. It provides the option to slice the object along any plane as required and obtain sectional views of the object in a 3-D environment. In fact, all 3-D objects shown in this chapter are drawn using AutoCAD solid modelling features. They give the feeling of the actual object on the computer monitor. We shall discuss the sectional views of 3-D objects later in 3-D modelling. At the moment we are confining our attention to the Hatching facility of AutoCAD for displaying 2-D orthographic sectional views.

 

Bhatch Command in AutoCAD

The Bhatch or Boundary Hatch command creates a solid fill of a selected area of drawing to show sections of different materials, hidden areas, and so on in Mechanical Drafting. We shall first discuss the different facilities provided by the Bhatch command. Subsequently, we will highlight their uses. The command may be invoked using any one of the following methods.

When you click the Hatch command's toolbar icon, it opens up the Boundary Hatch dialog box shown in Fig. 5.14.

 

FIG. 5.14   The Boundary Hatch dialog box

 

Note that at the top left corner there are two options—Quick and Advanced to choose from. For our purpose, we shall concentrate on the Quick option, the default setup in AutoCAD. The following steps will incorporate hatching in the drawing.

Step 1    Define hatch pattern   First you have to define your hatch pattern by choosing one of the following three options shown in the Type box of the Bhatch dialog box.

Predefined      This will enable you to choose from AutoCAD's hatch patterns.

User defined      This will enable you to define your own hatch pattern by specifying the angle and spacing, using the current line type.

Custom      Here you will be able to choose a pattern that you have already created yourself.

Generally, we choose the Predefined option and move on to the exact pattern of our choice. When you click the pattern box's drop down arrow, a palette rolls down as shown in Fig. 5.15, with a list of patterns to choose from. AutoCAD provides many options of hatch patterns.

 

FIG. 5.15   Hatch Pattern Palette

 

The mostly used option type is the Other Predefined, though there are two other types—ISO and ANSI option to choose from as well. In the palette are different image tiles to show you the sample of the hatch patterns. Click the image tile you like and the Swatch box (situated below the pattern box) will show you how the chosen hatch will look. Click OK to confirm your selection and go back to the Bhatch dialog box. For very thin sections, select the solid image tile.

Step 2   Set the Angle and Scale of hatching patterns   You need to set up the Angle of the hatch lines and the scale. Internal spacing between the lines varies on the scale and size of your drawing. You may choose the angle from the rolled down angle list of the Angle box. Be careful about the ANSI 131 types as they are, by default, set at an angle.

NOTE

It happens sometimes that when you select an internal point, AutoCAD fails to calculate the exact area and a small warning appears on the screen that no boundary has been detected. In such cases check if the area is closed. If you are sure that the area is already closed, choose the object selection mode and select the elements of geometry defining the area of hatching.

 

Similarly, select the scale, that is, hatch density, of your hatch pattern from the scale box's drop down list. A scale of l(the default) creates the hatch as defined. A scale of 0.5 will compress the spacing to one half of its original spacing. Similarly, if you increase the scale, the inter space of the pattern will increase. For hatching sections of two adjacent objects of the same material, you can change the scale of the same pattern to get the desired effect.

Step 3   Define the boundary of the hatching area   This is the most critical part of the entire hatch command. In a mechanical drawing, the part to be hatched is often very complex and you have to define the exact boundary of your hatching; by picking the right point. To do this, there are two methods in AutoCAD.

Pick points   You can click the Pick points button at the top right of the dialog box which will take you back to the drawing screen. Now pick inside the area of your hatching. The area must be properly closed before you do this. Actually AutoCAD needs to do some calculation to determine the area of hatching; so it displays the following when it goes back to the drawing:

Select internal point: Selecting everything
Selecting everything visible
Analysing the selected data
Select internal point: You are to pick the internal points here.

When you click the point, the entire defining boundary turns into a dotted line to show your selection.

Select object   The hatch boundary can also be defined by the Select Object option reached by clicking the Select Object button. Choose the objects individually or use any of the object selection methods.

 

FIG. 5.16   Hatching with an island

 

Sometimes it is necessary to exclude an island (enclosed area within a hatch boundary) in the hatching area. For example, a rectangular area situated within a circle need not be hatched as shown in Fig. 5.16. Use the Pick Point option to select a point inside the circle and remove the rectangle by the Remove Island option.

Step 4   Preview and execution   After the selection, press Enter to return to the dialog box and click the preview button at the bottom left to see the result of your selection. If you find it correct, click OK. If not, then right click to go back to the dialog box and redo the selection. While hatching, you can right click the mouse to get all the options as discussed.

Advance mode   In the advance option (Fig. 5.17), there are different buttons, the advantages of using which are explained below.

 

FIG. 5.17   Advance mode of Boundary Hatch command

 

Island detection style   AutoCAD has three options to operate with Island selection. The most important of these is the use of Island selection for the text to be left without hatch within a hatched boundary. The options are:

Normal If you select this, AutoCAD will hatch alternating areas as shown in the sample tile above this option's radio button. Fig. 5.18 illustrates the normal option.

Outer This will hatch only the outer area.

Ignore   This will ignore the internal island and will hatch everything.

 

FIG. 5.18   Normal option

 

When you do not want to hatch the text (or any other bounded area) inside the hatching area (Fig. 5.19), follow the steps given below.

  1. Select the Normal boundary style from the Advance tab or choose the Normal Island Detection mode by right clicking the mouse.
  2. See that the Flood radio button of the Island detection method of the Advance option tab is on.
  3. Select the hatch boundary. Also select the text (object) separately. AutoCAD will leave the text as unhatched. The result is shown in Fig. 5.19.

 

FIG. 5.19   The text left as unhatched (normal option)

NOTE

Selecting the Island

When you pick points, you need not select an island because AutoCAD detects it by default. Once you pick points, the Remove Islands button becomes active. Now you can select those islands which you do not want to consider, similar to Ignore options.

 

Associative hatching   This facility provides the option of hatching your objects associatively, that is, if you change the geometry of the defining boundary of an already hatched object, the hatching will change to fit to the new boundary. You can do this by putting on the Associative radio button placed inside the composition box of the Boundary Hatch dialog box shown in Fig. 5.17.

You can edit already hatched areas by choosing the Modify Hatch option to get the Hatch Edit box. The options available are the same as above.

Before concluding this chapter, we present below an example of a civil drawing (Fig. 5.20) to show how the Hatch command can be used effectively to indicate different materials in a sectional view.

 

FIG. 5.20   The use of hatching in a civil drawing

ILLUSTRATIVE PROBLEMS

In the previous sections, we have discussed the concept of sectional views and their use to illustrate the internal features of an object.

Let us take a few simple objects and try to develop their sectional views so that our understanding of the generation of sectional views is clear. Fig. 5.21 shows a pictorial view of an object with circular and rectangular holes. The orthographic views (front and top) are also presented in the same figure. Fig. 5.22 presents a pictorial view of the same object with the front half portion removed. The darker shade distinguishes the material part of the object from the void part. Now the drawing of the sectional front view becomes very simple. It is shown in the figure along with the top view which does not alter at all. It may be pointed out that the shape of the holes cannot be identified from the sectional view alone. We need the help of the front view as well. By drawing convention, the cutting plane line is indicated only in the top view.

Let us consider a slightly modified object with a keyway (groove) cut at the side of the circular hole (Fig. 5.23), the remaining part of the object being similar to that in Fig. 5.21. The corresponding cut section is shown in Fig. 5.24. The hatch (section) lines are given only on the surface area through which the cutting plane passes and not in the groove and holes. Thus we obtain the top view and the sectional front view (Fig. 5.25). A comparison with the previous front view indicates that an additional vertical line, AB, representing the keyway is visible in this case and hence drawn with a continuous line.

 

FIG. 5.21   Object with circular and square holes

 

FIG. 5.22   Object with front portion removed

 

FIG. 5.23   Object with keyway

 

FIG. 5.24   Cut section of the object with keyway

 

FIG. 5.25   Sectional view of the object with keyway

Example of Drawing Sectional View Using AutoCAD

Let us see, stepwise, how a sectional view is generated in AutoCAD from a given orthographic view of an object. Here we shall generate a sectional view of the object shown in Fig. 5.22 through the cutting plane.

Step 1   There is no change in the top view. The front view has to be modified to accommodate the hatch lines at the proper places.

 

FIG. 5.26   Conversion of lines for sectional view

 

First of all, change the hidden lines of the orthographic view into continuous lines. At the same time, use the Trim and Erase commands to remove some of the hidden and continuous lines as marked to obtain the necessary border area for drawing hatch (section) lines (Fig. 5.26). The trimming edge may be selected suitably. You may take help of layers for conversion of linetypes.

Step 2   When you get the required outline border areas for drawing section lines (shown in Fig. 5.27 by thicker lines), we revoke the Bhatch command and select the necessary type of section lines and set the proper angle (135°) and scale. Indicate the position of the section lines (or hatching) by picking, points inside the zones to be hatched. Refer to our discussion on hatching earlier in this chapter in case you need help. You will notice that when you pick the rectangular zones for hatching, the outline of those particular zones will become dotted (Fig. 5.28) to inform you that AutoCAD has selected those zones for implementing the selected type of hatching lines.

 

FIG. 5.27   Areas to be hatched

 

FIG. 5.28   Areas selected by AutoCAD for hatching

 

Step 3   Once the areas have been selected correctly, you may ask for a preview in the Bhatch dialog box and check whether the right inclination of angle and scaling (density) of hatch lines are achieved or not. If everything is in order, click OK. Fig. 5.29 illustrates the final view of the hatched sectional drawing. Note that the material being the same, all the hatch lines will have same orientation and density.

 

FIG. 5.29   The final view of the drawing

EXERCISE
  1. Pictorial views of various objects are given in Fig. 5.30 to 5.45. In each case, the viewing direction for the front view is specified by means of arrows. Draw the sectional front view, one of the side views, and the top view of these objects, considering the sectioning plane as passing through the center of the objects. Any dimensions missing may be assumed proportionately.
  2. Pictorial views of various objects are given in Figs. 5.46 to 5.61. In each case, the viewing direction for the front view is specified by means of arrows. Draw the sectional side view, front view, and top view of these objects, considering the sectioning plane passing through the center of the objects. Missing dimensions may be assumed proportionately.

 

FIG. 5.30

 

FIG. 5.31

 

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FIG. 5.45

 

FIG. 5.46

 

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FIG. 5.61