Unit 4: Sectional Views - Subjective Questions

A sectional view is an orthographic view that represents an object as if it has been cut by an imaginary plane and the portion between the observer and the cutting plane has been removed.

Purposes of sectional views:

• To reveal internal holes, cavities, slots, ribs, and other concealed features.
• To reduce the excessive use of hidden lines.
• To make a complicated drawing easier to read and interpret.
• To show the shape and arrangement of internal components clearly.
• To support accurate dimensioning and manufacturing.

The imaginary plane used to cut the object is called the cutting plane, and the exposed cut surfaces are identified by section or hatch lines.

The principle of a sectional view is based on imagining that an object is cut by a suitable cutting plane.

Procedure:

1. Select a cutting plane that passes through important internal features.
2. Imagine that the portion of the object between the observer and the cutting plane is removed.
3. View the remaining part in the direction indicated by the cutting-plane arrows.
4. Draw the exposed surfaces with section lines.
5. Draw all visible outlines beyond the cutting plane using continuous lines.

The cutting plane is normally represented by a thick chain line or another standard cutting-plane convention, with arrows showing the viewing direction. The cutting-plane line is identified by letters such as A–A. Only the surfaces actually cut by the plane are hatched; empty spaces, holes, and features located beyond the plane are not hatched.

Section lines, also called hatch lines, indicate the solid material cut by a cutting plane.

Standard rules include:

• Use thin, continuous, uniformly spaced parallel lines.
• Hatch lines are commonly drawn at an angle of to the principal outlines or axis.
• Select another angle, such as or , if lines become parallel to major outlines.
• Maintain uniform spacing within the same component.
• Adjacent components should be distinguished by changing the hatch angle or spacing.
• Holes, slots, and other empty spaces must not be hatched.
• Section lines should stop at visible boundaries.
• Very thin sections may be shown completely black, leaving a small gap between adjacent thin parts.
• Dimensions and notes should preferably be placed outside the hatched area; hatching may be interrupted around unavoidable text.

A full sectional view is obtained when a single cutting plane passes completely through an object, usually along its center, and the entire portion in front of the plane is imagined to be removed.

Construction steps:

1. Study the object and identify the internal features that must be shown.
2. Place the cutting plane through the important internal details.
3. Mark the cutting plane and viewing direction in the related orthographic view.
4. Project the remaining visible outlines into the sectional view.
5. Remove the outlines belonging to the imaginary front portion.
6. Draw visible features located behind the cutting plane.
7. Hatch only the solid surfaces intersected by the plane.
8. Add centerlines, dimensions, cutting-plane identification, and the title such as SECTION A–A.

A full section is particularly suitable for objects whose internal construction can be clearly exposed by one straight cutting plane.

A half sectional view combines one-half of an external view with one-half of a sectional view. It is produced by imagining two mutually perpendicular cutting planes passing through the center of a symmetrical object and removing one-quarter of the object.

Characteristics:

• One half shows the internal construction using hatching.
• The other half shows the external shape.
• A centerline normally separates the sectional and external halves.
• A visible outline is generally not drawn along the dividing centerline.
• Hidden lines are normally omitted unless they are essential for clarity.

Applications:

• Symmetrical objects such as bushes, pulleys, flanges, bearings, and cylindrical housings.
• Components for which both internal and external details must be shown in one view.

It is not normally suitable for highly asymmetrical objects because the half view may fail to represent their complete geometry.

Thus, a full section provides a complete internal view, whereas a half section balances internal and external information in a single view.

An offset sectional view is produced by using a cutting plane that bends or offsets so that it passes through several important features that do not lie in one straight line. The resulting section is drawn as if all portions of the cutting plane were in a single plane.

Construction procedure:

1. Identify the holes, slots, bosses, or cavities that need to appear in the section.
2. Draw a stepped cutting-plane line passing through the centers of these features.
3. Add arrows to indicate the viewing direction and label the plane, for example A–A.
4. Imagine the cutting-plane segments rotated or aligned into one plane.
5. Project all intersected features into one sectional view.
6. Hatch the solid surfaces cut by each segment.
7. Do not show the bends or offsets as visible lines in the final sectional view.

Advantages:

• Several non-aligned internal features can be shown in one view.
• Additional sectional views may be avoided.
• Internal relationships become easier to understand.

The cutting plane should be offset only where necessary, and its route must not create a misleading representation of the object.

In the first-angle projection system, the object is imagined to be located in the first quadrant, with the object between the observer and the plane of projection. Views are placed opposite to the direction from which they are observed.

Placement of sectional views:

• A sectional front view is placed in the normal front-view position.
• A sectional top view is placed below the front view.
• A sectional view observed from the right side is placed on the left of the front view.
• A sectional view observed from the left side is placed on the right of the front view.

The cutting-plane arrows indicate the actual viewing direction. The sectional view must remain correctly aligned with the corresponding orthographic views. Hatching, centerlines, visible outlines, and the cutting-plane designation must follow standard conventions.

In the third-angle projection system, the plane of projection is imagined to lie between the observer and the object. Therefore, each view is placed on the same side from which it is observed.

Placement of sectional views:

• A sectional top view is placed above the front view.
• A sectional view observed from the right side is placed on the right of the front view.
• A sectional view observed from the left side is placed on the left of the front view.
• A sectional bottom view is placed below the front view.

The cutting-plane line is shown in the related view, and its arrows specify the direction of observation. The sectional view is projected in alignment with the other views and is labeled appropriately, such as SECTION A–A.

Common sectional conventions:

• The arrows on the cutting-plane line determine the viewing direction.
• Only solid material intersected by the plane is hatched.
• Visible outlines beyond the plane are included.
• Section labels and projection symbols should be shown clearly.

The geometry represented by the section does not change between the systems; only the arrangement of views on the drawing sheet changes.

Engineering drawing uses special conventions to prevent sectional views from becoming misleading.

• Ribs, webs, spokes, and thin arms: When the cutting plane passes longitudinally through such a feature, it is generally not hatched. Its outline is still shown. If the plane cuts it transversely, it is hatched.
• Shafts, pins, bolts, screws, rivets, keys, and similar fasteners: These are normally not sectioned when the cutting plane passes along their longitudinal axes.
• Nuts and washers: They are commonly shown unsectioned in an assembly section when cut longitudinally.
• Holes and slots: These are empty spaces and therefore remain unhatched.
• Separate adjacent components: Their hatching should differ in angle or spacing.

These conventions improve readability and prevent a thin reinforcing feature or standard fastener from appearing thicker, solid, or structurally different from its actual form.

The AutoCAD STRETCH command changes the shape or size of selected geometry by moving selected vertices while keeping the remaining geometry connected.

Typical procedure:

1. Enter STRETCH or S.
2. Select the required vertices using a crossing window or crossing polygon.
3. Press Enter.
4. Specify a base point.
5. Specify the second point or enter the displacement.

Important points:

• Objects must cross the selection boundary for only selected vertices to be stretched.
• Objects completely enclosed by the crossing selection may move as complete objects.
• Circles generally move rather than stretch because they do not have independently stretchable endpoints.

Difference from MOVE:

• MOVE changes the position of an entire selected object without changing its size or shape.
• STRETCH relocates selected endpoints or vertices and modifies the object's dimensions or shape.

It is useful for changing lengths, shifting walls, modifying slots, and adjusting sectional profiles.

The AutoCAD EXPLODE command breaks a compound object into its individual component objects.

Examples:

• A polyline may become separate line and arc segments.
• A block may become its original component geometry.
• A hatch may be separated into hatch-line objects, depending on its type and settings.
• A region may be converted into boundary curves.

Procedure:

1. Enter EXPLODE or X.
2. Select the compound object.
3. Press Enter.

Uses:

• Editing individual parts of a block or polyline.
• Preparing separate segments for trimming, extending, or deleting.
• Accessing geometry contained in a compound object.

Limitations and precautions:

• Associativity and object intelligence may be lost.
• Exploding dimensions, hatches, or blocks can make later editing difficult.
• Some objects cannot be exploded or may require multiple explode operations.

Therefore, the command should be used only when component-level editing is necessary.

The AutoCAD OFFSET command creates a parallel or concentric copy of an existing object at a specified distance or through a specified point.

Procedure using a distance:

1. Enter OFFSET or O.
2. Enter the offset distance.
3. Select the source object.
4. Indicate the side on which the new object is required.
5. Repeat as needed and press Enter to finish.

Options and applications:

• The Through option creates the copy through a selected point.
• Erase can remove the source object after offsetting.
• It can create concentric circles and arcs.
• It is used to create wall thicknesses, flanges, slots, borders, parallel edges, and repeated sectional boundaries.

The offset distance should be entered accurately, and the correct side must be selected. Intersections produced by offsets may require subsequent use of TRIM, EXTEND, or FILLET.

The AutoCAD EXTEND command lengthens an object until it meets a selected boundary edge.

Basic procedure:

1. Enter EXTEND or EX.
2. Select the objects that will act as boundary edges, if required by the command mode.
3. Press Enter.
4. Select the ends of the objects that must be extended.
5. Press Enter to complete the command.

Uses in sectional drawing:

• Extending profile lines up to the cutting boundaries.
• Closing gaps before creating a hatch or region.
• Making centerlines reach the required limits.
• Completing the intersections of offset edges.

The object must be capable of intersecting the boundary when extended. Depending on AutoCAD settings, implied boundaries may also be used. EXTEND lengthens objects, whereas TRIM removes unwanted portions beyond a boundary.

The AutoCAD JOIN command combines compatible objects into a single object or a continuous chain.

Procedure:

1. Enter JOIN or J.
2. Select the source object and the other compatible objects.
3. Press Enter.

General conditions:

• Lines should normally be collinear and have matching or compatible endpoints.
• Arcs should have suitable centers, radii, and endpoints.
• Lines, arcs, and polylines can often be joined into a polyline if they form a continuous sequence.
• Objects should normally lie in the same plane.
• Large gaps, overlaps, or incompatible object types may prevent joining.

Applications:

• Converting a sectional profile into one continuous boundary.
• Preparing closed geometry for REGION, HATCH, or area calculation.
• Simplifying the selection and editing of related segments.

If JOIN fails, endpoint gaps should be corrected using object snaps, EXTEND, TRIM, or suitable polyline-editing tools.

A region is a two-dimensional enclosed area represented as a single AutoCAD object. It contains area properties rather than behaving merely as a collection of boundary lines.

Procedure:

1. Create a closed, coplanar boundary using lines, arcs, circles, ellipses, or polylines.
2. Enter REGION or REG.
3. Select the closed boundary objects.
4. Press Enter.

Requirements:

• The boundary must be closed.
• Objects must be coplanar.
• Invalid gaps, self-intersections, or unsuitable overlaps should be removed.

Applications:

• Calculating area, perimeter, centroid, and moments through mass-property tools.
• Performing Boolean operations such as union, subtraction, and intersection.
• Creating complex sectional areas.
• Extruding a closed 2D section into a 3D solid.

A region is particularly useful when a sectional profile must be analyzed or converted into a solid model.

The AutoCAD BREAK command removes a portion of an object or separates it into two objects at specified points.

Breaking between two points:

1. Enter BREAK or BR.
2. Select the object.
3. Specify the first break point.
4. Specify the second break point.
5. The portion between the two points is removed.

Breaking at one point:

• Specify the same location as both the first and second break points, or use the appropriate break-at-point tool.
• No visible length is removed, but the original object is divided into two separate objects.

Applications:

• Creating a gap in a line, arc, circle, or polyline.
• Interrupting hatching or outlines for notes and dimensions.
• Separating an object before modifying one portion.
• Producing openings in sectional profiles.

Accurate object snaps should be used to place the break points precisely.

The HATCH command fills an enclosed area with a pattern, solid fill, or gradient. In sectional drawings, it is used to represent material cut by the cutting plane.

Creating a hatch:

1. Ensure that the sectional boundary is closed or has only an acceptable small gap.
2. Enter HATCH or H.
3. Choose Pick Points or Select Objects.
4. Select a suitable hatch pattern, commonly ANSI31 for general section lining.
5. Set the angle, scale, origin, transparency, and associativity.
6. Preview and accept the hatch.

Using HATCHEDIT:

• Select an existing hatch or enter HATCHEDIT.
• Change its pattern, scale, angle, origin, color, transparency, or boundary association.
• Add or remove boundary areas and islands where supported.

Good practices:

• Use consistent spacing within one component.
• Apply different angles or scales to adjacent components.
• Keep holes and voids unhatched.
• Prefer associative hatching so that the pattern updates when its boundary changes.

A systematic workflow for a 2D sectional drawing is as follows:

1. Set up the drawing: Select units, limits, scale, text style, dimension style, and line types.
2. Create layers: Use separate layers for visible outlines, hidden lines, centerlines, cutting planes, hatching, dimensions, and construction lines.
3. Draw the main profile: Use accurate coordinates and object snaps to construct the external shape.
4. Create thickness and repeated geometry: Apply OFFSET to form walls, flanges, slots, and concentric features.
5. Correct intersections: Use EXTEND, TRIM, and BREAK to complete or remove profile segments.
6. Modify proportions: Use STRETCH when selected dimensions or vertices must be adjusted without redrawing the entire component.
7. Prepare closed boundaries: Use JOIN where necessary and verify that sectional areas are closed.
8. Create analytical areas: Convert suitable closed loops with REGION if area properties or Boolean operations are required.
9. Construct the section: Draw the cutting-plane line, arrows, and identification, and project the sectional view according to first-angle or third-angle conventions.
10. Apply hatching: Use HATCH for cut material and HATCHEDIT to correct pattern scale, angle, or boundaries.
11. Use EXPLODE cautiously: Explode compound objects only when individual elements require editing.
12. Finish and check: Add centerlines, dimensions, labels, and notes; verify projection alignment, line weights, hatch conventions, closed boundaries, and plotting scale.

The completed drawing should communicate both the external geometry and internal construction clearly while following standard sectional-view conventions.