Chapter One Viewing an Object

Chapter One Viewing an Object

ObjectivesUpon completion of this chapter the student will be able to:

• Define descriptive geometry.• Define the following terms: projection planes, projection lines, and line-of-

sight.• Describe the two types of orthographic projections and where they are

used.• Name the six views produced by a multiview drawing.• Define folding lines.• Explain the importance of folding lines.• Describe how measurements are transferred from view to view.• Construct an orthographic (multiview) projection using manual methods

and AutoCAD.

ObjectivesUpon completion of this chapter the student will be able

to:

• Define and list the three categories of axonometric projections.• Describe how an isometric scale is constructed.• Construct an isometric projection using manual methods and AutoCAD.• Define and describe the three categories of oblique projections.• Construct an oblique projection using manual methods and AutoCAD.• Define ground plane, station plane, vanishing point, picture plane, ground

line, and visual rays.

• Define one-point perspective, two-point perspective, and multiview perspective.

• Describe and construct a multiview perspective.

ObjectivesUpon completion of this chapter the student will be able

to:

• List and describe the three types of three-dimensional models produced using AutoCAD.

• Describe the difference between nonparametric and parametric solid models.

• Describe the difference between paper space and model space.• Know how to toggle between paper space and model space.• Use mview and vpoint to produce an orthographic and isometric

projection.

• Use mvsetup to generate orthographic and isometric projections.

Types of Two Dimensional ViewsTwo Dimensional Views can be divided into four major

classes:

• Orthographic Projections• Axonometric Projections• Oblique Projections• Perspective Projections

Orthographic Projection

Two standards used in the creation of Orthographic Projections:

• First Angle Projections• Third Angle Projections

Orthographic ProjectionKey Words and Phrases

Orthographic ProjectionMultiview DrawingPlaneHorizontal PlaneProfile PlaneFrontal PlaneProjection PlaneLine of SightObject LinesAdjacent ViewFolding LinesMiter Lines

First Angle ProjectionReceives it name because the object, is placed in the firstquadrant. In a first angle projection, the top viewis generated by projecting the object’s contours down onto thehorizontal plane. The right side view is generated by projectingthe right side of the object through the object to the profileplane. Thus resulting in an arrangement as follows:

Third Angle projectionReceives its name from the fact that the object is placed in the third quadrant. In this method the top view is projected up onto the horizontal plane. The right side view is generated by projecting the object’s contours onto the profile plane.

Constructing a multiview

Using AutoCAD to create two-dimensional multiview drawings

• Step #1– Start by using the RECTANG command to draw a 2" square. The coordinates for the location of the

square are not important at this time.

• Step #2– Now start transferring measurements to the front view by first drawing a line from the lower left-hand

corner of the object to an undetermined point below the object. Then, draw a line from the lower right-hand corner of the object to an undetermined point below the object. Thus establishing the sides of the object in the front view.

• Step #3– The bottom edge of the object is created by moving down several inches below the top view of the

part and drawing a line that crosses both lines that were extend downward.

• Step #4– The top edge of the object (in the front view) is created by using the OFFSET command. Finally, the

FILLET command with a zero radius can be used to trim off the excess length of the lines revealing a plan and front view of the object.

Axonometric Projections

Isometric Projection

Isometric Scale

Using AutoCAD to produce an Isometric ProjectionIsometric Left Isometric Right

Isometric Top

Diametric ProjectionsIn a Diametric Projection only two axis form equal angles, instead of all three like the isometric projection. These angles will be greater than 90° and less than 180°, but not to equal 120°. An angle of 120° is not used because it would produce an isometric projection. The third angle will be less than or greater than the angle chosen for the first two.

Trimetric Projections

In a Trimetric Projection none of the three-principle axis form equal angles. The result is a view that contains three different scale factors, making these types of projections difficult to generate. However, an infinite number of possible projections that can be achieved.

Oblique ProjectionThe projection plane is set parallel with the front surface of the object. However, the line of sight will be at an angle to the projection plane producing a view that reveal all three axis’s (length, width and depth)

Cavalier Oblique

Cabinet Oblique

General Oblique

Using AutoCAD to produce an oblique projection

Step #1Start by drawing the front face of the angle bracket. This is the edge that reveals the thickness of the bracket.

Step #2Next, continue working on the front view, but now concentrate on drawing the face labeled “Back Face”. This includes placing the hole in that face.

Step #3Execute the UCS command to rotate the cross hairs to a 45° angle, to generate the protruding lines and giving the object the illusion of depth. Finally, the face containing the hole, is moved to its proper location at the back of the part. Resetting the ucs in this step it is not necessary when using AutoCAD’s Object SNAP commands.

Step #4Draw a line from the upper left-hand corner of the face showing thickness, to the lower left-hand corner of the face containing the hole. Next, copy the hole from its present position to a position 0.5 inches in front of the current hole. This will give the hole the illusion of depth when finished.

Step #5 Complete the projection by adding any missing lines, and remove the excess lines by using the TRIM and ERASE commands.

Perspectives Projections

Perspective projections give the object a more natural appearance by allowing the projection lines to converge to a single point. This type of drawing is more readily used in the art world, because this illusion of lines appears more realistic.

One-point Perspective

Two-Point Perspective

Constructing a Perspective from a Multiview

Types of Three-Dimensional Views

Orthographic ProjectionIsometric ProjectionPerspective Projections

Types of three-dimensional Objects

Layout Space/Model SpaceModel Space Layout Space

Orthographic Projection / Isometric Projections Using Mview - Point

Using Mvsetup

Command: MVSETUPInitializing...Enable paper space? (No/<Yes>): YEntering Paper space. Use MVIEW to insert Model space

viewports.Regenerating drawing.Creating the default file mvsetup.dfs in the directory C:\AV2\

AVWIN\.Align/Create/Scale viewports/Options/Title block/Undo: C

Delete objects/Undo/<Create viewports>: C

Available Mview viewport layout options:

0: None 1: Single 2: Std. Engineering 3: Array of Viewports

Using Mvsetup Continued

Redisplay/<Number of entry to load>: 2Bounding area for viewports. First point:Other point:Distance between viewports in X. <0.0>:Distance between viewports in Y. <0.0>:Align/Create/Scale viewports/Options/Title block/Undo:Command:

Using Amdwgview