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Practice MCQ

Unit 4 - Notes

MEC136

Unit 4: Sectional Views

1. Introduction to Sectional Views

In engineering drawing, internal details of an object are represented by invisible (hidden) lines. When internal features are complex, the resulting drawing can become cluttered with numerous hidden lines, making it difficult to visualize and interpret the object's geometry.

Definition: A sectional view is a drawing generated by imagining the object being cut by a plane to reveal the interior construction.

Primary Objectives:

  • To clarify internal features.
  • To reduce the number of hidden lines.
  • To facilitate dimensioning of internal parts.
  • To indicate the cross-sectional shape and materials.

2. Principle of Sectioning

The process of creating a sectional view involves three main concepts:

A. The Cutting Plane

This is an imaginary plane that cuts through the object.

  • Representation: It is represented by a Cutting Plane Line (a thick line with two dashes or one long and two short dashes).
  • Direction: Arrows at the ends of the cutting plane line indicate the direction of sight (the direction the observer is looking).

B. The Section

The portion of the object between the cutting plane and the observer is assumed to be removed. The remaining portion is projected onto the projection plane.

C. Section Lines (Hatching)

Surfaces that are cut by the cutting plane are indicated by section lines (also known as cross-hatching).

  • Standard Pattern: Thin, continuous lines.
  • Angle: Typically drawn at 45 degrees to the main outline of the drawing.
  • Spacing: Uniformly spaced (usually 2mm to 4mm apart depending on drawing size).
  • Material: Different patterns represent different materials (e.g., cast iron, steel, concrete). The universal symbol for general mechanical drawings is the ANSI31 pattern (Cast Iron).

3. Types of Sectional Views

A. Full Section

The cutting plane passes entirely through the object in a straight line.

  • Usage: Used when the internal details are consistent across the entire length of the object.
  • Result: The front half of the object is imagined to be removed.
  • Note: Hidden lines are generally omitted in sectional views unless necessary for clarity.

B. Half Section

The cutting plane cuts only halfway through the object, removing one-quarter of it.

  • Usage: Primarily used for symmetrical objects (e.g., cylinders, pipes).
  • Result: The resulting view shows one half of the drawing in section (internal details) and the other half as an external view.
  • Separation: The sectioned half and un-sectioned half are separated by a Center Line (not a solid line).
  • Advantage: Shows both internal and external features in a single view.

C. Offset Section

The cutting plane is bent or "stepped" at 90-degree angles to pass through features that are not in a straight line.

  • Usage: Used when important internal features are situated at different depths or positions.
  • Representation: The "steps" of the cutting plane are not shown in the sectioned view; the view is drawn as if the cut surfaces were on the same plane.

4. Sectional Views in Projection Systems

The placement of the sectional view depends on the projection system used (First Angle vs. Third Angle).

First Angle Projection

  • Logic: The object is placed between the observer and the projection plane.
  • Placement: If the cutting plane arrows point Left, the sectional view is placed on the Right. If looking down (Top view arrows point up), the section view (Front) is drawn below.
  • Rule: The view is projected onto the plane behind the object.

Third Angle Projection

  • Logic: The projection plane is placed between the observer and the object.
  • Placement: If the cutting plane arrows point Left, the sectional view is placed on the Left.
  • Rule: The view is projected onto the plane between the observer and the object. (Standard in US and AutoCAD defaults).

5. AutoCAD Commands for Sectioning

To create professional sectional views in AutoCAD, mastery of specific modification and drawing commands is required.

A. Modify Commands

1. OFFSET (O)

Creates concentric circles, parallel lines, and parallel curves.

  • Usage: Essential for creating thickness in walls, pipes, or placing construction lines.
  • Workflow: Type O Specify distance Select object Click side to offset.

2. EXPLODE (X)

Breaks a compound object (like a Block, Polyline, or Hatch) into its individual component objects.

  • Usage: Modifying a rectangle into four separate lines to trim only one side.

3. STRETCH (S)

Stretches objects crossed by a selection window or polygon.

  • Critical Rule: You must use a Crossing Window (Right-to-Left selection, Green window) to select the ends of the objects to be stretched.
  • Usage: Lengthening a shaft or pipe without redrawing it.

4. EXTEND (EX)

Extends objects to meet the edges of other objects.

  • Workflow: Type EX Select boundary edge (or press Enter to select all) Click lines to extend.
  • Pro Tip: While in the TRIM command, holding Shift switches to EXTEND.

5. JOIN (J)

Combines similar objects to form a single, unbroken object.

  • Requirement: Objects must be collinear (lines) or share endpoints without gaps.
  • Usage: Turning a series of lines into a polyline before creating a Region or Hatch.

6. BREAK (BR)

Creates a gap between two specified points on an object or splits an object into two.

  • Usage: Removing a segment of a line where a feature intersects, or creating space for text.

B. Drawing/Modeling Commands

7. REGION (REG)

Converts objects that enclose an area into a 2D region object.

  • Properties: Regions act as planar surfaces. They have physical properties (Centroid, Moment of Inertia).
  • Usage: Essential for 3D modeling (Extrude) and helpful for calculating area in complex cross-sections.

C. Sectioning Commands

8. HATCH (H)

Fills an enclosed area or selected objects with a hatch pattern, solid fill, or gradient.

  • Workflow:
    1. Type H.
    2. Pick Points: Click inside a closed area (boundaries are auto-detected).
    3. Select Pattern: Choose ANSI31 for general mechanical sectioning.
    4. Properties: Adjust Scale (to space lines out) and Angle (if the part is rotated).
    5. Associative: Ensure "Associative" is checked so the hatch updates if the boundary geometry changes.

9. HATCHEDIT (HE)

Modifies an existing hatch block.

  • Usage: Changing the scale, angle, or pattern of section lines after they have been drawn.
  • Workflow: Type HE Select the hatch Modify properties in the dialog box.

6. Hands-on Practice: Creating a Sectional View in AutoCAD

Exercise Workflow: Sectioning a Mechanical Bracket

Objective: Draw the Front View and a Full Sectional Side View of a bracket with a central hole.

Step 1: Create Base Geometry

  1. Set Units (UN) to Millimeters.
  2. Use LINE and CIRCLE to draw the Top View and Front View outlines.
  3. Use OFFSET to create the thickness of the bracket walls.

Step 2: Define the Cutting Plane

  1. In the Top View, draw a line passing through the center of the object.
  2. Change the Linetype of this line to "Phantom" or "Dashed" (Load via LINETYPE command).
  3. Add Arrows (using LEADER or Polyline) to indicate viewing direction.

Step 3: Project the Section View

  1. Use RAY or XLINE to project the edges from the Top/Front view to the location of the Side View.
  2. Draw the outline of the Side View.

Step 4: Clean Up Geometry

  1. Use TRIM to remove internal lines that would be removed by the cut.
  2. Use JOIN to ensure the boundary where hatching will occur is closed.
  3. Change visible edges cut by the plane to the "Visible" layer (Thick, continuous).

Step 5: Applying the Section

  1. Command: HATCH
  2. Pattern: ANSI31
  3. Pick Points: Click inside the solid walls of the bracket. Do not hatch the hole/void.
  4. Adjust Scale: If lines are too dense, increase scale (e.g., 1.0 to 5.0).
  5. Close Hatch Creation.

Step 6: Finalization

  1. Add Center Lines to holes.
  2. Dimension the drawing.
  3. Add Text: "SECTION A-A".

Practice Checklist

  • [ ] Can you successfully join four lines into a single rectangle using JOIN?
  • [ ] Can you use STRETCH to elongate a drawing without breaking the corners?
  • [ ] Can you use HATCH and modify its density using HATCHEDIT?
  • [ ] Can you identify the difference between a Rib (which is usually NOT hatched even if cut) and a Wall?

Common AutoCAD Hatching Errors & Solutions

  • "Valid hatch boundary not found": There is a gap in your geometry. Zoom in to corners and use FILLET (Radius 0) or EXTEND/TRIM to close gaps.
  • Hatch too dense (looks solid): The scale is too small. Use HATCHEDIT to increase the scale.
  • Hatch lines broken: The origin point of the hatch might be far away. Use "Set Origin" in the Hatch ribbon.