• Need a formal way of documenting designs
  • Legal documents i.e patents; contracts may rely on them
• Must stand on their own – readable to any human
  • No subsequent explanation
  • No verbal assists
  • No ambiguity
• Solution: multi-view orthographic projection
  • World-wide engineering standard
  • Can easily include tolerances

• Use judgement to select most useful/informative orientation
  • Often, a projection is clearest when a significant flat surface of the object is parallel to the projection plane
• Left is a better pictorial view (it’s more 3D)
• Right is better because it’s face is parallel

“Glass-box” interpretation #

• Need an agreed way to organize different projections on the page
• Imagine projecting object onto sides of a box
• Unfold the box onto the page
• So-called “third-angle” projection
• Will not necessarily show all six projections
• Projections are aligned

Example of multi-view orthographic projection #

• Lines connecting a given point in adjacent views are always perpendicular to the “unfolding” line (of the “glass box”)

• Inclined face
  • Face in 2 views
  • Line in 3rd view

• Oblique face
  • Face in 3 views

• Example: cut from sheet material
• Unlikely to be multiple levels of relief
• Projections of edges therefore unnecessary
• Grooves or etched patterns would be labeled as such

In this case, two views not enough to describe geometry completely – we need 3

• Thick enough material that there could conceivably be multiple levels of relief: side view needed
• In this particular example, all features pass through the full thickness of the material

Would any two views be enough to describe the geometry completely?

• 3rd angle projection used in U.S.
  • glass box convention

• 1st angle projection (Europe, Japan, India)
  • top/bottom and left/right arrangement reversed

• Adopted by drafters and engineers to expedite the transfer of information
• Maximum information with the minimum drawing
• Will only cover highlights here
• Views
  • At least two views (except flat sheet)
  • Add views as required so the dimensions of the object can be defined entirely in true length measurements
  • Add views as necessary for presentation clarity
• Solid lines
  • Assumed to be intersections of planes or optical limits of cylinders
  • Tangent edges are usually not shown, or shown using phantom lines
• Hidden lines
  • Use to add information, clarity (good practice not to over-use)
  • Use views requiring the fewest hidden lines
• Center lines
  • Use to mark the centers of holes, or cylindrical surfaces ≥180º
• Circles
  • Assumed to be intersections of cylinders and orthogonal planes
• Section views
  • Used for clarification of internal geometries
  • Explained in a later lecture

Small cuts on curved surfaces

Small radii, intersections of blended planar surfaces shown as a line

Representing threads use schematic representations

Parts with odd rotational symmetry Simplify to a symmetrical view even though that is not a strictly accurate projection

Tangent and non-tangent surfaces

• A line drawn where a curved surface meets a planar surface indicates no tangency: i.e. there is an abrupt change in the angle of the surface
• No drawn line indicates tangency: i.e.surface angle is continuous/smooth