Sectional Views of Solids: Sectional Plane Parallel to HP

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1. Need and Purpose of Sectional Views

When an engineering component has complex internal contours, chambers, stepped bores, or ribs, standard orthographic projections produce a confusing web of overlapping dashed (hidden) lines.

A Sectional View is obtained by imagining that a portion of the object in front of an imaginary cutting plane has been sliced and removed, thereby exposing the internal geometry as solid, visible outlines.

       UN-SECTIONED VIEW                         SECTIONED VIEW
   (Confusing hidden lines)                  (Clear internal profile)
        +------------+                           +------------+
        |  :      :  |                           |////////////|
        |  :      :  |     ========>             |    +--+    |
        |  :      :  | (Cutting Plane S-S)       |    |  |    |
        |  :......:  |                           |////+--+////|
        +------------+                           +------------+

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2. Cutting Plane (Section Plane) Conventions (IS 10714 / BIS SP 46: 2003)

1. Representation: A cutting plane is drawn as a Chain thin line with thick ends and directional arrows (Line Type H).
2. Directional Arrows: Point in the direction of the observer's line of sight (away from the portion assumed to be removed).
3. Identification: Capital letters placed at the arrows (e.g., $A\text{---}A$ or $S\text{---}S$).

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3. Cutting Plane Parallel to the Horizontal Plane (HP)

When the cutting plane is parallel to HP, it is automatically perpendicular to the Vertical Plane (VP).

Behavioral Rules of the Views:

• In Front View (on VP): The cutting plane appears as a single horizontal line parallel to $XY$ (edge view) at the specified height above the base.
• In Top View (on HP): The cut surface is projected directly onto the HP, forming the Sectional Top View.
• Golden Rule: Because the cutting plane is parallel to the Horizontal Plane, the Sectional Top View displays the TRUE SHAPE OF THE SECTION directly, without requiring an auxiliary plane!

                FRONT VIEW (Elevation)
                       Apex (o')
                         /\
                        /  \
     Cutting Plane S --+----+-- S (Parallel to XY)
                      /      \
                  ---+--------+--- XY (Base on HP)
                     a'       b'
                     
                SECTIONAL TOP VIEW (Plan)
                     +--------+
                     |////////|  <-- True Shape of Section (Hatched at 45 deg)
                     |////////|
                     +--------+

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4. Sectional Profiles for Standard Solids Cut Parallel to HP

Solid	Orientation	Sectional Shape in Top View (True Shape)
Cylinder	Axis vertical (resting on HP)	Circle of diameter equal to cylinder diameter $D$.
Right Cone	Axis vertical	Circle of smaller diameter $d < D$ ($d$ measured from FV at cutting height).
Square Prism	Axis vertical	Square identical in size to the base.
Square Pyramid	Axis vertical	Square geometrically similar to the base, but smaller in size (concentric with the base in TV).
Hexagonal Prism	Axis vertical	Hexagon identical in size and orientation to the base.
Hexagonal Pyramid	Axis vertical	Hexagon geometrically similar to base, scaled down proportionally.

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5. Hatching (Section Lining) Conventions (BIS SP 46: 2003)

Hatching lines are used to highlight the solid material that was physically severed by the cutting plane.

• Line Type: Continuous thin line (Type B, $0.25\text{–}0.35\text{ mm}$).
• Angle: Strictly $45^\circ$ to the horizontal or to the main centerline/outline of the section.
• Spacing: Uniformly spaced between $1.5\text{ mm}$ and $3\text{ mm}$ depending on the size of the cross-section.
• Crucial Restrictions:
  1. Hatching lines must never cross visible outlines.
  2. Hollow cavities, holes, and air spaces must never be hatched.
  3. Standard hardware components (shafts, bolts, nuts, rivets, keys, pins, balls/rollers of bearings) and thin features (webs, ribs, spokes) are never sectioned longitudinally.

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6. Summary of Key Exam Points

1. True Shape Condition: The Sectional Top View is the True Shape if and only if the cutting plane is parallel to HP.
2. Cutting Plane Representation in FV: $S\text{---}S$ is a horizontal chain-dot line with thick ends, located at the specified distance above the base line $XY$.
3. Cut Edges in FV: The portion of the solid above the cutting plane is shown with thin phantom lines or erased, while the retained lower portion is drawn in bold outlines.
4. Hatching Angle: $45^\circ$ to the reference baseline; drawn with thin, sharp $2\text{H}$ pencil.