Unit 3: Orthographic Projections - Subjective Questions

Orthographic projection is a method of representing a three-dimensional object using two-dimensional views obtained by projecting its features onto mutually perpendicular reference planes.

Main characteristics:

• The projectors are parallel to one another.
• The projectors are perpendicular to the plane of projection.
• Separate views are used to show the length, width, and height of an object.
• The commonly used views are the front view, top view, and side view.

Importance in engineering drawing:

• It provides an accurate description of the shape and size of an object.
• It allows dimensions and manufacturing information to be added clearly.
• It avoids the visual distortion generally present in perspective drawings.
• It serves as a universal means of communication among designers, engineers, and manufacturers.
• It helps in production, inspection, assembly, and maintenance of engineering components.

The principle of orthographic projection is based on viewing an object from different directions and projecting its features onto reference planes.

Basic elements:

• Object: The component whose views are required.
• Observer: The person assumed to view the object from a particular direction.
• Projectors: Imaginary straight lines drawn from points on the object to the projection plane.
• Plane of projection: An imaginary plane on which the view is formed.

In orthographic projection, the projectors are:

• Parallel to each other.
• Perpendicular to the plane of projection.

The principal planes are:

• Vertical Plane (VP): Produces the front view.
• Horizontal Plane (HP): Produces the top view.
• Profile Plane (PP): Produces the side view.

Since each view generally shows only two dimensions, multiple views are combined to describe the complete shape of the object accurately.

The principal orthographic views represent an object as seen from mutually perpendicular directions.

• Front view or elevation:

  • Obtained by looking at the object from the front.
  • Projected on the Vertical Plane.
  • Shows length and height.
  • It is normally selected to reveal the maximum number of features with minimum hidden details.

• Top view or plan:

  • Obtained by looking at the object from above.
  • Projected on the Horizontal Plane.
  • Shows length and width.

• Side view or end view:

  • Obtained by looking from the left or right side.
  • Projected on a Profile Plane.
  • Shows width and height.

The dimensions shared between views must align. Length is common to the front and top views, height is common to the front and side views, and width is common to the top and side views.

In the first-angle projection system, the object is assumed to be placed in the first quadrant. It is above the Horizontal Plane and in front of the Vertical Plane.

Principle:

• The object lies between the observer and the plane of projection.
• After obtaining the views, the Horizontal Plane and Profile Planes are unfolded into the plane of the drawing sheet.

Arrangement of views:

• The top view is placed below the front view.
• The right-side view is placed to the left of the front view.
• The left-side view is placed to the right of the front view.

Thus, each view is placed on the side opposite to the direction from which it is observed. First-angle projection is widely used in India and many countries following ISO drawing standards. Its standard projection symbol uses a truncated cone and its end view in the first-angle arrangement.

In the third-angle projection system, the object is assumed to be placed in the third quadrant. It is below the Horizontal Plane and behind the Vertical Plane in the theoretical arrangement.

Principle:

• The plane of projection lies between the observer and the object.
• The views are projected onto the planes nearest to the observer.
• The projection planes are then unfolded to form the drawing.

Arrangement of views:

• The top view is placed above the front view.
• The right-side view is placed to the right of the front view.
• The left-side view is placed to the left of the front view.

Therefore, a view is positioned on the same side from which the object is observed. Third-angle projection is widely used in the United States, Canada, and some other countries. The projection method should always be indicated by its standard symbol or drawing note.

First-angle and third-angle projection differ mainly in the relative positions of the observer, object, projection plane, and views.

Both systems represent the same object accurately. However, their view arrangements must not be mixed in one drawing. The applicable method should be identified using the standard first-angle or third-angle projection symbol.

A systematic first-angle projection procedure is as follows:

1. Study the object: Identify its overall length, width, height, holes, slots, steps, inclined faces, and curved features.
2. Select the front view: Choose the viewing direction that shows the maximum shape details and minimum hidden lines.
3. Plan the layout: Leave adequate space for the front, top, and side views, dimensions, and notes.
4. Draw the front view: Construct the overall boundary first, followed by visible internal features.
5. Project the top view: Draw light projectors downward from the front view because the top view lies below it in first-angle projection.
6. Transfer the width: Use a mitre line or equivalent construction to transfer dimensions between the top and side views.
7. Draw the right-side view: Place it to the left of the front view.
8. Add hidden details: Represent invisible edges with hidden lines.
9. Add centre lines: Mark axes of holes, cylinders, arcs, and symmetrical features.
10. Check alignment: Confirm that corresponding lengths, heights, and widths agree across all views.
11. Finish the drawing: Darken visible outlines, apply correct lineweights, erase construction lines, and add dimensions if required.

The completed drawing must use only the first-angle view arrangement throughout.

The following procedure can be used for third-angle projection:

1. Analyze the component: Determine the overall dimensions and identify visible, hidden, circular, and symmetrical features.
2. Choose the front view: Select the most descriptive position of the component.
3. Draw the front view: Begin with the main outline and then add steps, slots, holes, arcs, and other details.
4. Construct the top view: Project vertically upward from the front view because the top view is placed above it.
5. Construct the right-side view: Place it to the right of the front view.
6. Transfer depth dimensions: Use projectors and a mitre line between the top and side views.
7. Maintain alignment:
   • Front and top views must have matching lengths.
   • Front and side views must have matching heights.
   • Top and side views must have matching widths.
8. Apply line conventions: Use continuous lines for visible edges, dashed lines for hidden edges, and centre lines for axes.
9. Verify view placement: Every view must be located on the same side as the direction from which it is seen.
10. Complete the drawing: Remove unnecessary construction lines, set appropriate lineweights, and add dimensions and the third-angle projection symbol.

A correct third-angle drawing must clearly distinguish itself from first-angle projection through its arrangement and symbol.

Different line types communicate different kinds of engineering information.

• Visible or object lines: Thick continuous lines used for edges and boundaries that are directly visible in a view.
• Hidden lines: Dashed lines used for edges, holes, or surfaces that cannot be seen from the viewing direction.
• Centre lines: Thin chain lines made of alternating long and short dashes. They indicate axes, centres of circles, symmetry, and paths of rotation.
• Construction lines: Very thin continuous lines used temporarily for projection, alignment, and geometric construction. They are usually removed or placed on a non-printing layer.
• Dimension lines: Thin continuous lines terminated by arrowheads. They show the extent and direction of a dimension.
• Extension lines: Thin lines extending from the feature to the dimension line.

Correct line type and lineweight create a visual hierarchy. Visible outlines should dominate, while hidden, centre, dimension, and construction lines should remain lighter.

An AutoCAD linetype is a repeating pattern of dashes, dots, spaces, symbols, or continuous segments assigned to an object or layer.

Common linetypes:

• Continuous: Visible outlines and general geometry.
• Hidden: Invisible edges and features.
• Center: Axes, centres, and lines of symmetry.
• Phantom: Alternate positions or paths of moving parts.

Important properties:

• Layer: Controls the logical grouping of objects.
• Color: Often used to control plotted lineweight.
• Linetype: Determines the dash-dot pattern.
• Lineweight: Controls printed line thickness.
• Linetype scale: Controls the size and spacing of the linetype pattern.

Useful controls include LTSCALE for the global linetype scale, CELTSCALE for the current object scale, and PSLTSCALE for consistent display in paper-space viewports. Using ByLayer properties is recommended because it provides consistent standards and allows several objects to be modified by changing their layer settings.

MOVE command:

• Changes the location of selected objects without changing their size or orientation.
• Procedure: Start MOVE, select objects, specify a base point, and specify the destination point or displacement.
• It is useful for positioning completed views or features accurately.

COPY command:

• Creates one or more duplicates while preserving the original objects.
• Procedure: Start COPY, select objects, specify a base point, and specify one or more destination points.
• Options such as multiple copies can be used for repeated features.

ERASE command:

• Removes selected objects from the drawing.
• Procedure: Start ERASE, select unwanted objects, and confirm the selection.
• Accidental erasure can normally be reversed using UNDO or OOPS when appropriate.

For accurate operation, base points should be selected using object snaps such as Endpoint, Midpoint, Center, or Intersection.

The ROTATE command changes the angular orientation of selected objects about a specified base point.

Procedure for an exact angle:

1. Start ROTATE.
2. Select the required objects.
3. Specify the base point or centre of rotation.
4. Enter the rotation angle .

A positive angle normally rotates the object counterclockwise, while a negative angle rotates it clockwise, depending on the current angle settings.

Reference option:

• Choose the Reference option after specifying the base point.
• Enter or identify the object's current reference angle.
• Enter or identify the required new angle.
• AutoCAD rotates the object by the difference between the two angles.

The reference option is useful when the original angle is unknown or when an inclined edge must be aligned precisely with another line. The command may also create a rotated duplicate if its copy option is used.

The TRIM command removes unwanted portions of objects up to selected boundaries. It is commonly used to clean intersections and form the final profile of a 2D component.

General procedure:

1. Start TRIM.
2. Select the cutting edges or accept all suitable objects as boundaries, depending on the AutoCAD version and settings.
3. Select the portions of objects to be removed.
4. Press Enter to finish.

Useful methods and options:

• Individual segments can be selected one at a time.
• Fence selection trims every object crossed by a temporary line.
• Crossing selection trims multiple objects inside or crossing a selection window.
• The Edge option can permit trimming to an apparent or extended boundary.
• Holding the appropriate modifier key may temporarily switch between trimming and extending.

Trimming should be performed after checking that the cutting edges intersect the target geometry. Incorrect elevation, gaps, or unsuitable projection settings may prevent the command from working as expected.

The MIRROR command creates a reversed copy of selected objects about a specified axis called the mirror line. It is especially useful for symmetrical components.

Procedure:

1. Start MIRROR.
2. Select the objects to be mirrored.
3. Specify the first point of the mirror line.
4. Specify the second point of the mirror line.
5. Choose whether to erase the source objects.

Mirror line:

• It acts as the axis of reflection.
• Its location and angle determine the final position and orientation of the mirrored geometry.
• Object snaps should be used to define it accurately.

Text mirroring:

• The MIRRTEXT system variable controls whether text appears reversed.
• A value of generally keeps text readable.
• A value of mirrors the text geometry.

For a symmetric orthographic view, one half can be drawn first and mirrored about a centre line, reducing effort and improving accuracy.

The SCALE command changes the size of selected objects relative to a specified base point while preserving their proportions.

Uniform scaling procedure:

1. Start SCALE.
2. Select the objects.
3. Specify the base point.
4. Enter a scale factor.

Effect of scale factor:

• A factor greater than enlarges the object.
• A factor between and reduces the object.
• For example, a factor of doubles all linear dimensions, while a factor of halves them.

Reference scaling:

• Select the Reference option.
• Specify the current reference length.
• Specify the required new length.
• AutoCAD calculates the necessary factor automatically.

Reference scaling is useful when imported geometry has an incorrect size but contains a known dimension. Scaling drawing geometry should not be confused with setting a viewport or plotting scale. Engineering geometry is generally drawn at full size in model space.

FILLET and CHAMFER modify the corner formed by two objects, but they produce different results.

FILLET:

• Creates a rounded corner or tangent arc between two objects.
• Start FILLET, set the required radius, and select the first and second objects.
• A radius of can join or trim two objects to a sharp intersection.
• It is used for rounded corners, internal radii, and manufacturing fillets.

CHAMFER:

• Creates a straight bevel between two objects.
• Start CHAMFER, set two chamfer distances or a distance and angle, and select the two objects.
• It is used for bevelled corners, edge preparation, and removal of sharp edges.

Difference:

• A fillet produces a curved transition defined mainly by radius.
• A chamfer produces a straight transition defined by distances or distance and angle.

Both commands can trim the original objects automatically, and their multiple or polyline options can modify several corners efficiently.

The ARRAY command creates multiple copies of selected objects in an organized pattern. Associative arrays can be edited later as a single array object.

• Rectangular array:

  • Arranges copies in rows, columns, and levels.
  • Important parameters include row count, column count, spacing, and total distance.
  • Suitable for grids of holes, slots, or repeated rectangular features.

• Polar array:

  • Arranges copies around a specified centre point or axis.
  • Important parameters include item count, fill angle, and rotation of items.
  • Suitable for bolt circles, fan blades, and equally spaced radial holes.

• Path array:

  • Distributes copies along a selected line, arc, polyline, spline, or other path.
  • Items may be spaced by number or distance and may align with the path.
  • Suitable for repeated features along curved or irregular routes.

Using an array is faster and more consistent than creating each repeated feature separately with multiple COPY operations.

A suitable workflow is:

1. Set up the drawing:

   • Select the required units and limits.
   • Create separate layers for object, hidden, centre, dimension, and construction lines.
   • Assign suitable linetypes, colors, and lineweights using ByLayer.

2. Create the basic profile:

   • Draw construction lines and the centre line.
   • Construct one half of the component using lines, circles, arcs, and offsets.
   • Apply TRIM to remove excess geometry.

3. Form edge details:

   • Use FILLET for rounded corners.
   • Use CHAMFER for bevelled edges.

4. Complete symmetry:

   • Use MIRROR about the centre line.
   • Keep the source objects if both halves are required.

5. Create repeated holes:

   • Draw one accurate hole.
   • Use a rectangular or polar ARRAY, depending on the pattern.
   • Use COPY for nonuniform repetitions.

6. Position and orient features:

   • Use MOVE to place geometry.
   • Use ROTATE for inclined details.
   • Use SCALE only when a proportional size correction is required.

7. Add orthographic information:

   • Project the required front, top, and side views.
   • Put invisible details on the hidden-line layer.
   • Add centre lines through holes and symmetric features.

8. Clean and verify:

   • Use ERASE for unwanted construction objects.
   • Check view alignment, feature spacing, linetype scale, dimensions, and projection-system arrangement.

This sequence minimizes repeated work while preserving accuracy and drawing standards.

A completed 2D orthographic drawing should be checked for both geometric accuracy and drafting quality.

Geometric checks:

• Confirm that the front, top, and side views represent the same component.
• Ensure that corresponding features align through projectors.
• Verify all overall and feature dimensions.
• Check the positions and diameters of holes, arcs, slots, and repeated details.
• Ensure that fillet radii and chamfer sizes are correct.

Projection checks:

• Confirm whether first-angle or third-angle projection is required.
• Check the placement of top and side views.
• Do not mix view arrangements from the two systems.

Line checks:

• Visible edges must use continuous object lines.
• Invisible features must use hidden lines.
• Axes and symmetry must use centre lines.
• Linetype scales and lineweights must be readable at the plotting scale.

AutoCAD checks:

• Remove duplicate and unwanted objects.
• Ensure objects are on the correct layers and use ByLayer properties.
• Check that object snaps, coordinates, and repeated spacing were used accurately.
• Review the drawing at print preview and confirm that no required information is clipped.

A final visual and dimensional inspection should be completed before plotting or submission.

Common errors and remedies include:

• Incorrect view placement: A top or side view is placed according to the wrong projection system. Verify the first-angle or third-angle rules before beginning.
• Misaligned views: Corresponding edges and centres do not lie on common projectors. Use construction lines, object snaps, tracking, and a mitre line.
• Missing hidden lines: Internal features are not represented. Study the object from each viewing direction and add hidden edges on the correct layer.
• Excessive hidden lines: Unnecessary dashed lines make the view confusing. Include only geometrically valid invisible edges.
• Incorrect linetype display: Hidden or centre lines appear continuous. Load the proper linetype and adjust LTSCALE, object scale, or viewport settings.
• Wrong command base point: MOVE, COPY, ROTATE, MIRROR, or SCALE gives an inaccurate result. Select a meaningful base point using object snaps.
• TRIM failure: Objects do not intersect or are on different elevations. Check intersections, elevation, and cutting boundaries.
• Inaccurate repeated features: Manually copied items have unequal spacing. Use rectangular, polar, or path arrays.
• Distorted proportions: Geometry is scaled unintentionally. Draw at full size and use SCALE only with a verified factor or reference length.
• Poor drawing organization: Objects have inconsistent properties. Use named layers and assign color, linetype, and lineweight ByLayer.