School of Technology

TECH 13580 - Engineering Graphics I

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MAJOR CONCEPTS 

A. Categories of Multiview Projection.... Multiview is a sub-category of Orthographic
("right angle") as is Axonometric. Multiview is divided into First, Second, Third, and Fourth 
Angle Projection. U.S.A. uses Third Angle; Europe uses First Angle.   People commonly say orthographic when 
they more correctly should say multiview. Most engineering drawings throughout industry 
are orthographic (multiview). 

B. Unfolding the Glass Box which represents the Planes of Projection through which you see Views. The planes are mutually perpendicular. Horizontal Plane to see the Top View Frontal Plane to see the Front View ( best describes object's shape ) Profile Plane to see the Side View (sometimes called the END VIEW) Note: The planes are invisible and endless while the views are actual representations of an object's shape. Also note that architects refer to the top view as the "Plan View" and call the front and side views as "Elevations". 

Note: You are unable to exactly hold and view an object in orthographic positions. We see in perspective, not orthographic. Projectors are parallel in orthographic which requires that the observer be imagined to be at infinity.

C. Conventional terms of Measurement... Height, Width, & Depth Most mechanical objects are described in terms of height, width, and depth. Some objects ,such as automobiles , are described using height, width, and length. The object is three dimensional; each view is two dimensional. Front view has width and height. Top view has width and depth. Side view has height and depth.

D. Six regular orthographic views available ( top, front, right side, left side, rear, & bottom ), but all six very seldom drawn. Usually one, two, or three views will suffice. Note the "mirror" between the side views , front and rear, and top and bottom as you study .

E. Types of flat surfaces .

Normal - parallel to one plane and perpendicular to the other two planes

In a three view drawing, a normal surface will appear as a surface in one view and on edge in the other two views.

Inclined - perpendicular to one plane ( a sloping surface )

In a three view drawing, an inclined surface will appear as a foreshortened surface in two views and as an edge view in the remaining view. ( A single auxiliary view is required to reveal the normal view of an inclined surface. )

Oblique ( Skewed ) - not parallel nor perpendicular to any plane.

An oblique surface will appear as a foreshortened surface in all views. ( A double auxiliary is required to show the normal view of an oblique surface, )

F. Meaning of Lines

Edge - a line representing a surface which is perpendicular to the plane of projection ( or parallel to your line of sight )

Intersection - a line representing where an inclined surface or an oblique surface meets another surface

Surface Limit - restricted to curved surfaces; a line representing the diameter of a hole or the diameter of a cylinder. Consider that a surface limit represents how far the curved surface got from its center, not unlike the longest day (solstice) of the year concept.

A. Hidden Lines - dash lines approx 1/8" long with 1/16" gap thin and black to represent features which exist, but hidden from a particular view 

B. Centering Views - to balance placement of views on drawing Horizontal - Add width, space between views (1"min.), and depth. Subtract total from available horizontal space ( from vertical border to other vertical border; then divide by 2.

Vertical - Add height, space between views, and depth. Subtract total from available vertical space ( from horizontal border to other horizontal border ); then divide by 2.  In other words, add up what you need including the space between views which you determine, subtract from the available space, and then divide by two to discover how far to move in from the border to begin the views. 

C. Alternate Position of Side View Placement of side view next to top view  rather than front view to avoid being crowded with title block.

D. Partial Views Deliberate incomplete view to save space, time, improve scale size for better visibility, or to ignore less important features. Use the short break symbol or, in the case of a round object, a centerline to imply the missing half is identical to that drawn. 

E. Precedence of Lines This means which line has priority when several different types occupy the same location. 

First Priority - Object ( Visible ) Lines

Second Priority - Hidden Lines

Third Priority - Center Lines

F. Revolution Conventions This means that you do not always draw an object exactly as seen to minimize confusion or congestion.  Features are rotated from their center; then projected from a center line.

G. Conventional Edges Lines may be drawn to reveal features even though they are not being projected from sharp corners. 

H. Intersections & Tangencies Lines are drawn to separate intersecting surfaces.  To the contrary, surfaces which blend together by being tangent have no line at the point of tangency. 

I. Fillets, Rounds, & Run outs Fillets are rounded inside corners while rounds are rounded outside corners. Fillets and rounds strengthen an object by eliminating sharp corners which could crack, are safer, and make manufacturing easier . Run outs are curves at lines which indicate where fillets occur and toward which direction. 

J. Counter bored & Countersunk Holes Counter bores are concentric holes to allow a bolt head or nut to locate below the surface. Countersunk holes have 82 degree sloping sides to allow flathead screws to locate flush with the surface. Remember to draw the hidden lines. 

K. Plotting Curves A curve in orthographic is plotted from data provided by two views using the 45 degree miter. 

L. Deformation of Cylinders Note how cylinders being sliced generate intersection lines. 

M. Reading a Drawing via Numbering System Assign numbers or letters to given features to visualize the object and determine missing views. 

Each discipline has its own vocabulary which is commonly used and must be understood. Be responsible for understanding the following terms which are associated with orthographic projection.

Parallel

Perpendicular

Project ( in the context of projecting lines, not building a project )

Delineate