Sectional Views and Auxiliary Views With easy examples.

A sectional view shows what you would see if you sliced an object with an imaginary cutting plane and removed the front half. The cut surfaces are filled with thin diagonal section lines, called hatching, so the reader sees which parts are solid and which are hollow.

An auxiliary view is projected onto a plane parallel to a slanted surface, so the surface appears in its true shape and size. Together, sections and auxiliary views reveal interiors and true shapes that standard views hide.

Key Concepts

• Cutting plane line: a thick line with arrows showing the cut location and viewing direction.
• Section lining (hatching): thin parallel lines drawn at 45 degrees on every surface the cutting plane passes through. Adjacent parts are hatched in opposite directions so they can be told apart.
• Full section: the cutting plane passes completely through the object. Half section: one quarter is removed, showing the inside on one half and the outside on the other; used for symmetrical parts.
• Offset section: the cutting plane bends to pass through several features in one view. Removed sections: show the cross-section of a spoke or rib, drawn moved aside.
• Auxiliary plane: an extra projection plane parallel to an inclined face, showing it without foreshortening.

How It Works

1. Study the object and decide which interior features need to be shown, then choose the cutting plane that passes through them.
2. Draw the cutting plane line with arrows pointing toward the half you keep, and label it with a letter such as A-A.
3. Discard the nearer half and draw the rest, hatching every cut surface at 45 degrees.
4. Leave ribs, webs and fasteners unhatched when cut lengthwise, as per convention.
5. For an auxiliary view, draw a plane parallel to the inclined surface and transfer each point perpendicular to it, keeping the same depths as the top view.

Worked Example

A steel plate is 100 mm long, 60 mm wide and 20 mm thick, with a through hole of diameter 20 mm drilled in its centre. A full section is taken along the length through the hole axis.

1. The cutting plane passes through the 100 mm × 20 mm rectangle of the plate, so that rectangle is hatched at 45 degrees.
2. The hole removes a 20 mm strip from the hatched area; its inner walls are drawn as solid lines but not hatched — showing the hole goes all the way through.
3. An inclined edge of true length 65 mm at 30 degrees appears foreshortened in front view: 65 × cos 30° = 56.29 mm. An auxiliary view parallel to the edge shows the full 65 mm the machinist needs.

Common Misconceptions

• "Everything the plane touches gets hatched." Ribs, webs, spokes and fasteners are left unhatched when cut lengthwise — hatching them would falsely suggest solid blocks.
• "The arrows show where the cut goes." The arrows show the direction of sight, not the cutting direction. You look along the arrows at the half that remains.
• "An auxiliary view is just a rotated normal view." It is a fresh projection onto a new plane, with depths transferred point by point — rotating the front view would give wrong, foreshortened shapes.

Real-World Applications

• Machine assembly drawings: sections show how shafts, bearings and housings fit together inside a gearbox.
• Buildings: sections reveal floor heights, staircases and wall construction.
• Casting and mould design: sections expose wall thickness and hollow cores that decide whether a casting will fill correctly.

Sectional views expose interiors by hatching cut faces; auxiliary views reveal true shapes of slanted faces. Together they show exactly how a part is built, inside and out.