Unit 7-Section View Drawings.Pdf

UNIT 7

SECTION VIEWS 7.1 Introduction A section view is an orthographic projection view drawn to reveal internal or hidden features in an object. Section views are used to supplement standard orthographic view drawings in order to completely describe an object. They improve visualization of designs, clarify multiviews and facilitate dimensioning of drawings. Hence they are an important aspect of design and documentation. Section views are created by defining an imaginary cutting plane or planes on the object so that the observer can see the internal details. Hidden lines are generally not shown in sections. Hatch lines (also called section lines) are used to indicate solid materials that are cut through. A combination of hatch lines is called a pattern. A hatch pattern has certain attributes such as orientation and line spacing. Some hatch pattern types are used to represent specific materials or group of materials. Both part and assembly sections can be created. Sometimes, auxiliary section views may be needed for clarity.

7.2 Concept of Sections In orthographic projection, the standard projection planes are top (horizontal), front (frontal), side (profile). The frontal and profile planes are vertical, while the top is horizontal. Standard drawing views are created on these planes with preferred view directions. The view direction for the frontal and profile planes is horizontal while the view direction for the horizontal plane is vertical. Fig. 7.1a shows the standard front and top views a cylinder. Fig. 7.1b shows the same cylinder in mixed (standard, section, cut isometric) views with the front view converted to a section view. Several elements associated with the concept of sections are indicated in Fig. 7.1b. These are i) cutting plane line, ii) view direction, iii) removed portion, iv) retained portion, v) hatching, and vi) section caption or label.

The cutting plane is an imaginary plane that passes through the object at a position of interest. It is represented by a line (the edge view of the section plane) in an adjacent view to the section view. In Fig. 7.1b, the cutting plane is vertical since its edge view is seen on the horizontal plane. Cutting planes can change direction within an object. The view direction is the line of sight or the direction an imaginary viewer is facing. The view direction is indicated by the arrow head and is perpendicular to the cutting plane. In Fig. 7.1b, the view direction is horizontal. The removed portion is the portion of an object that is assumed to have been removed in order to expose the interior. It is the cut- out portion of the object. The viewer is able to directly see the interior of the object when the cut-out is removed.

Top view

Section view

Front view a) Standard views b) Mixed views Fig. 7.1: Concept of sections

The retained portion is the portion of an object that is assumed to be left in front of the viewer. The hatching is the pattern of hatch lines used to indicate solid material. The section label is the name given to the section or cutting plane. A very important difference between standard and section views is the replacement of hidden lines in standard views with visible lines in section views. This is very fundamental since it indicates the feature is now visible in a section view. Comparing the standard and mixed views of Fig. 7.1 shows the clarity advantage of section views. Another advantage of section views is that the visible lines from hidden lines in standard views can be used for 1 dimensioning; hidden lines are not used for dimensioning. When parts or assemblies have complex internal features, hidden lines in standard views become confusing, sections are then indispensable.

7.3 Cutting Plane Linestyles A cutting plane is represented by a line that shows the edge view of the cutting plane. A limited number of linestyles are used to represent cutting planes. In Fig. 7.2, the common linestyles for cutting planes are shown. They are a) thick center line, b) thick phantom line and c) broken visible line. The representation in c) is used if the cutting plane line would hide important details in a drawing. Each of these lines is usually joined to two arrows at their ends. The direction of the arrow is the viewer’s line of sight. Cutting plane lines are drawn in the view adjacent to the section view and may go beyond the boundary of the adjacent view. The thickness of a cutting plane line should be more than that of normal visible line.

a) Thick centerline b) Thick phantom line c) Broken visible line

Fig. 7.2: Cutting plane linestyles

7.4 Hatch Patterns Hatch lines are thin lines and when they are laid out in a specific angle and spacing, a hatch pattern is formed. A hatch pattern is always within a closed boundary. If there is a gap in an section, hatching will not occur when using CAD systems. Spacing of hatch lines should enhance readability. Depending on the size of the drawing, it may be between 1.5 mm (0.06 in) to 6 mm (0.25 in) in relatively small drawings. Likewise, the inclination of hatch lines should be guided by clarity. The angle of inclination for hatch lines normally varies between 15 o and 75 o. Popular angles are 15 o, 30 o, 45 o, 60 o, and 75 o. The angle 45 o is the default angle in most CAD software. Hatch lines must not be drawn or placed parallel to object lines or features in a section. In Fig. 7.3, the left column views have hatch lines parallel to some object features and they are, therefore, unacceptable. The acceptable representations are shown in the right column views. The angle of inclination of the hatch lines must be different from the angles of inclination of all the features forming the boundary of a hatch pattern. Fig.7.4 shows some examples of assembly hatch patterns. When components are assembled, the hatch patterns must not be parallel to object lines or features of hatch boundary. Also, hatch lines are inclined at different angles in each component in order to distinguish them.

Fig. 7.3: Hatch pattern layout Fig. 7.4: Assembly hatch patterns

Conventionally, some hatch patterns are associated with specific materials. However, the proliferation of available materials today makes it impractical to have a unique hatch pattern for material types and grades. Thus selected material hatch patterns are in common use. In architectural drawings, some material hatch patterns are in popular use. Machine drawings use few material hatch patterns and ANSI 31 pattern for cast iron in Fig. 7.5 is the recommended today for machine drawings. This pattern may be used for all types of materials in machine drawings. Fig. 7.5 shows some material hatch pattern types.

Fig. 7.5: Material type hatch patterns

7.5 Section View Representation and Placement Proper representation of section features is very important. Every feature directly exposed to the view needs to be included as visible entities in the section view. Gaps between feature segments must not be allowed. In Fig. 7.6, two section view representations are given. The left representation is right while the right representation is wrong because of gaps between the view segments. The line features omitted in the right representation are clearly visible in the indicated section plane.

Section view position in a drawing has a definite relationship with the view direction. A section view should be placed behind the tail end of the view direction arrow. Fig. 7.7 illustrates the application of this principle for a) top section view, b) front section view, and c) right section view. Sufficient gap should be allowed between the section and the adjacent view it is derived from. This gap is very important because sufficient space must be made available for dimension placement.

a) Right b) Wrong Fig. 7.6: Section view representation

a) Top section view b) Front section view c) Right section view

Fig. 7.7: Placement of section views

7.6 Section View Types Section views may be classified in different ways. For our discussions, we shall group them into full, partial, and special section views. In full section views the full length of the principal dimension perpendicular to the view direction of the object is shown in section view. Hence the cutting plane or planes pass through the whole cross- section of the object. Partial section views do not reveal the whole sections, but show a portion of the interior. Special sections include auxiliary sections, assembly sections, and un-sectioned features.

Full Section Views Full section views provide section views along the full length of the cross-section of an object. They include straight, offset, removed, revolved, and aligned section views. Each is briefly discussed below.

Straight Sections A straight section is also called a full section by some authors. The cutting plane for a straight sections cuts right through the middle of the object so that one half of it is revealed after the second half is imagined removed. Thus a straight section is created from a single cutting plane. Straight sections are best for objects with an axis of symmetry. In multiview drawings, a section view can replace a standard view, and straight sections are commonly thus employed. Fig. 7.8 shows an example of a straight section view. Offset Sections Offset sections are similar to straight section except the cutting plane changes direction at 90 o at a time as it goes through the object. Offset section have two or more parallel cutting planes. They are used for complex part with a number of important features that do not lie on the same plane. Fig. 7.9 is an example of an offset section view with three cutting planes offset from one another. Note that in the section view, the offset cutting planes appear collinear. Multiple offset sections are possible in irregular objects.

Fig. 7.8: Straight section view Fig. 7.9: Offset section view Fig. 7.10: Removed section views 4

Removed Sections Removed sections are full section views placed at a convenient position from the adjacent view but linked with the cutting plane either by a line or view label as shown in Fig. 7.10. They are displaced from the normal view position and do not need to be of the same scale as the adjacent view they are derived from. If the scale for the removed view is different from the adjacent view, it should be indicated as a local note. It is convenient to display different removed section views along the length of an object if it has varying cross-sections in that direction; see Fig. 7.10.

Revolved Sections A revolved section is similar to a removed section except that the section view is superimposed on the cutting plane after the section has been rotated through 90 o. The axis of revolution is indicated with a centerline as shown in Fig. 7.11. This representation is attractive when space constraint is an issue. The section view scale is the same as the standard view. The section views can be placed with or without breaking the visible lines adjacent to them on the standard view. Feature lines from the standard view within the revolved view should be removed completely. Revolved section views should be drawn as seen from the view direction. Sections of bars, lever arms, spokes and other elongated objects are commonly represented in revolved section view.

Fig. 7.11: Revolved section views

Aligned Sections In aligned sections the cutting planes are not parallel but inclined at some angle. The line of intersection is usually at the center of the object. Like offset sections, the cutting planes are made to pass through features of interest as shown in Fig. 7.12. In the section view representation, one of the planes is rotated through some angle to align it with the other as indicated in Fig. 7.12a. The feature is then projected on the aligned plane. This forced alignment makes the section view looks pleasing and easier to visualize. Practical considerations or conventional rules thus override strict projection principles in aligned section views.

a) Component with arms b) Component without arms Fig. 7.12: Aligned Section views

7.6.2 Partial Section Views Partial section views provide section views of a portion of an object. They include half, broken, and detail section views. Each is briefly discussed below.

Half Section Views Half sections have two cutting planes that at 90 o allowing a quarter of the object to be imagined removed. A centerline is used to demarcate the sectioned portion from the un-sectioned portion in the section view as shown in

Fig. 7.13. Sometimes, hidden lines are shown on the un-sectioned part. Half sections are best for objects with two axes of symmetry.

Broken Section Views A broken section is a section exposed by a cut out of a portion of an object as shown in Fig. 7.14. A brake-line is used to show the boundary between the sectioned and un-sectioned portion of the drawing. A cutting plane is not shown in a broken section. Broken section is used to limit the area of interest in a object. It saves time and could substitute for full or half section.

Fig. 7.13: Half section Fig. 7.14: Broken section Fig. 7.15: Detail section view

Detail Section Views A detail section view is similar to a broken section view except it is positioned outside the standard view. Also, it is usually of enlarged scale so as to reveal greater detail around the area of interest in the object. This gives more clarity and it is often easier to place dimensions on detail section views. Fig .. 7.15 shows a detail view of a keyway.

7.6.3 Special Section Views Section views derived from non-principal views such auxiliary sections may be considered as special section views. Similarly, special application section views such as assembly sections and features or parts that are not sectioned in drawings by convention are special section views.

Auxiliary Sections Auxiliary views (full or partial) may be sectioned and standard section conventions apply to them. For example, the section view should be placed on the tail end side of the view direction arrow and there should be a visible gap between the section view and adjacent standard view. Hatch pattern should be placed with care since the auxiliary view is inclined in position and hatch lines must not be parallel to boundary line features. Fig. 7.16 shows an example of an auxiliary section view.

Fig. 7.16: Auxiliary section view Fig. 7.17: Assembly section view 6

Assembly sections Assembly sections are section views with more than one component shown in their fitted relative positions. They are very useful in checking clashes or interferences of adjacent components in a unit. Adjacent components in an assembly section are hatched at different angles to clarify drawing. Components are usually numbered and a bill of materials (BOM) or parts list is attached to the drawing. Fig. 7.18 is an example of an assembly (full) section view but without a parts list. Assembly sections may be full or half section orthographic or pictorial views of the unit or product.

Un-sectioned Features Some features are not sectioned in section views if the cutting plane is parallel to their axes. Such features are normally thin and include ribs, webs, lugs, and spokes. However, if the cutting plane is perpendicular to their axes, they may be sectioned. Fig. 7.18 shows the cutting planes are parallel to a lug and spokes. In section view, these features are un-hatched or un-sectioned because of this parallel geometric relationship. In Fig. 7.19, the cutting planes are parallel to

Fig. 7.18: Un-sectioned features

Fig. 7.19: Hatching un-sectioned features

Fig. 7.20: Un-sectioned parts 7 a rib and web in the A-A section views and the rib and wed are un-hatched or un-sectioned. The cutting planes for the B-B sections are defined perpendicular to the rib and web. In this case, the rib and web are hatched or sectioned because of the perpendicular geometric relationship. Fig. 7.20 shows a shaft, key, bolt and nut in section. By convention, these components are not hatched as indicated. Some other standard parts or components not hatched in section views include pins, dowels, fasteners, gears, bearings, and springs.

7.7 Conventional Breaks When objects are long and of constant cross-section, their length may be reduced with break lines. Break lines are effective in saving time and space. They allow the scale of a drawing to be increased. Fig. 7.21 shows examples of conventional break lines.

Fig. 7.21: Break lines for different shapes and materials

7.8 Constructing Section Views In the following discussion, two illustrative examples for constructing section views are considered. This is because, creating the different types of section view with constructional techniques follow the same basic procedure of: Step 1: Create standard view(s); Step 2: Create section plane line and features; and Step 3: Hatch section and finish the drawing. Solid Edge software was used in the following examples.

7.1 Constructing a Straight Section Fig. 7.22 shows the three step procedure applied in the construction of a straight section view.

Step1: Create standard view Using the techniques discussed in Chapter 6 for standard orthographic view creation, create one or two standard views as necessary. After creating the necessary standard views, the outline of the section view is then created as shown in Step 1 of Fig. 7.22. Center lines may be omitted at this stage, but it is recommended that they should be included.

Fig. 7.22: Constructing a regular section 8

Step 2: Create section plane line and features Choose a section plane linestyle and use it to define the section plane. Using projection lines as shown in Fig. 7.22 Step 2, project the features on the section plane from the top view to the section view. Remember that hidden lines change to visible lines in section view.

Step 3: Hatch section and finish drawing Apply the hatch lines to the section and make sure they are properly inclined. Then check and ensure that center lines are placed correctly. Check and correct any error. Add section label.

7.2 Constructing aligned Section Fig. 7.23 shows the three step procedure applied in the construction of a aligned section view.

Fig. 7.23: Constructing an aligned section Step1: Create standard view Create one or two standard views as necessary. The section view outline is then created as shown in Step 1 of Fig. 7.23.

Step 2: Create section plane line and features Using projection lines as shown in Fig. 7.23 Step 2; project key points on the features of the lower right arm to the vertical center line. The base of the slot and the tip of the arm are the necessary key points in this case. Then transfer the projected points to the center vertical line to the section view. Remember that hidden lines change to visible lines in section view.

7.9 Generating Section Views from Solids Creating section views from solid models is quite straight forward. The routine for creating sections assumes that the user has a standard view on screen. The next step is to create the cutting plane line and this is most often a step of its own. Then the user may be prompted to place the view or may terminate the step for the cutting plane and activate another button or initiate another command to place the view. Center lines are most often added to view on a separate step and the inclination of the hatch lines may have to be adjusted. Fig. 7.24 shows the three step procedure applied in the generation of an aligned section view.

Step1: Create standard view Create one or two standard views as necessary as shown in Step 1 of Fig. 7.23 using the routine for orthographic view generation.

Step 2: Create section plane line

Using the CAD routine for the cutting plane line creation, define the cutting plane as shown in Step 2 of Fig. 7.24. Some CAD package gives the direct projection for aligned section. This must be manually corrected to conform to conventions in section views.

Step 3: Hatch section and finish drawing Check the hatch lines of the section and make sure they are properly inclined. The inclination may have to be adjusted for satisfactory result. Then check and ensure that center lines and section label are placed correctly. Check and correct any error.

Section A-A

Fig. 7.24: Generating a section from solid model

7.10 SUMMARY A section view is an orthographic projection view drawn to reveal internal or hidden features in an object. Section views are used to improve visualization of designs, clarify multi-views and facilitate dimensioning of drawings. Section views are created by defining an imaginary cutting plane or planes on the object so that the observer can see the internal details. The direction of the arrows on the cutting plane line point away from the observer and the portion of the object in front of the cutting plane line is removed to reveal the internal feature or features.

In section views, hatch lines (also called section lines) are used to indicate solid materials that are cut through. Some hatch pattern types are used to represent specific materials. Hatch lines are usually drawn at angles between 15 o and 75 o but must not be parallel or perpendicular to object visible lines. Hatch lines of 15 o are most common. In section assembly drawings, coded hatch lines are used to distinguish different materials.

Section views of parts and assemblies can be created. A straight section is usually along a centerline and cuts through the object completely. A half section has a quarter of the object cut away. It is used for symmetrical objects only. A centerline is used to separate sectioned and un-sectioned areas in a half section. Revolved section is used when a feature has constant shape along its length and cannot be shown in external view. Removed section favored when a part has variable cross-sections along its length. Cutting plane lines are not shown for broken and revolved sections. Auxiliary sections are sometimes needed in design documentation. Some parts such a nut and bolts are not sectioned in engineering drawings.