Technical Illustration has at its foundation a knowledge base of geometric shapes and their description. There is a vast vocabulary that needs to be fundamentally known by the "Draftsman".Along with a vocabulary is the ability of an individual to see the views of a model and also be able to see the model from the views. This is a skill that develops with time but it is necessary to be able to visualize from 2 dimensions to 3 dimensions; going from xy to xyz.

Connecting Your Learning

As you work your way through this lesson, you should reflect on the following questions:

How to sketch to determine a viewpoint?
Knowing where a hidden line is?
When a model line has taken precedence over a hidden line?
An awareness of the use of auxiliary views!

The State of Arizona has assembled Drafting standards to make sure that Instruction is thorough and well thought out by the instructor. By clicking on the hyperlink below will upload to your browser window the entirety of these standards. The standards that are addressed in this lesson are below the hyperlink.

1.0 USE BOARD DRAFTING EQUIPMENT, INSTRUMENTS AND MEDIA.

• 1.1 Analyze Drawing surfaces for Drafting
• 1.6 Demonstrate Sheet Setup skills

2.0 DEMONSTRATE SKETCHING, LETTERING, AND LINE SKILLS.

• 2.1 Demonstrate freehand sketching skills for drafting applications.
• 2.2 Demonstrate lettering skills.
• 2.3 Demonstrate line work.

6.0 DEMONSTRATE GEOMETRIC CONSTRUCTIONS

• 6.1 Measure Geometric Constructions.
• 6.2 Develop Geometric Constructions.

8.0 APPLY BOARD DRAFTING SKILLS TO COMPUTER-AIDED DESIGN/DRAFTING (CADD)

• 8.1 Demonstrate computer skills for a CADD system.
• 8.2 Create dimensional CADD entities.
• 8.3 Create 3 dimensional CADD drawings.

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The State of Arizona has also assembled Math standards and many of these apply directly to the vocational subject area of Drafting. Drafting is a very precise discipline and the use of mathematics is integral to design and manufacturing. Below are the math standards that are involved in this lesson

1M-P1. Compare and contrast the real number system and its various subsystems with
regard to their structural characteristics

• PO 2. Compare subsets of the real number system with regard to their properties
  (commutative, associative, distributive, identity, inverse and closure properties)

1M-P2. Construct, interpret and demonstrate meaning for real numbers and absolute

• PO 3. Solve real-world distance problems using absolute value
• PO 4. Determine, among the solutions to a real-world problem, which, if any, is reasonable
• PO 2. Define absolute value as the distance from the origin

4M-P1. Interpret and draw three-dimensional objects

• PO 1. Sketch prisms, pyramids, cones, cylinders and spheres
• PO 2. Classify prisms, pyramids, cones, cylinders and spheres by base shape and lateral surface shape.
• PO 3. Recognize the three-dimensional figure represented by a two-dimensional drawing.
• PO 5. Make a model of a three-dimensional figure from a two-dimensional drawing and make a two-dimensional representation of a three-dimensional object (models and representations include scale drawings, perspective drawings, blueprints or computer simulations)
  

4M-P4. Deduce properties of, and relationships between, figures from given
assumptions

• PO 1. Find similarities and differences among geometric shapes and designs using a given attribute (e.g., height, area, perimeter, diagonals and angle measurements)
• PO 2. Identify arcs, chords, tangents and secants of a circle
• PO 3. State valid conclusions using given geometric definitions, postulates and theorems
• PO 4. Represent pi as the ratio of circumference to diameter
  

1. Know the dimension rules.
2. Be apply the rules of dimensioning to a drawing.
3. Interpret how a rule may apply in certain situations in a drawing and in other drawings the same rule does not apply.
4. Add the appropriate geometric symbol with text that is added to a drawing surface for edification of components parts and how they will be used in assembly or manufacturing.

 

Instruction

 

In this Lesson you will be provided allot of details and drawings regarding the rules of dimensioning and of course the vocabulary that goes with the rules.The following is the list of pages that will be dealt with in this lesson regarding dimensioning.

• The Size versus the Location of dimensions
• Dimensioning systems
• Geometric Symbols
• Standard Terminology
• Dimensioning Rules

Standard Dimensional rules

1. Dimensions should be given between features which have functional relationships.
2. Dimensions should be given that control the relationship of mating parts.
3. Dimensions and extension lines should not cross.
4. Dimension lines should not cross each other.
5. Cylindrical features should be located by the centerlines.
6. Cylindrical feat rues should be located in the circular view.
7. Extension lines from dimensions and centerlines should not extend between two views. (auxiliary view ex.)
8. Notes should be lettered horizontally o the drawing sheet.
9. A cylindrical feature should be dimensioned with its length and diameter in he rectangular view.
10. Dimensions's should be given to finished surfaces if at all possible.
11. Dimensions should be kept off the views of the object, if possible, to promote drawing clarity.
12. Dimensions applying to adjacent views should be placed between the views.
13. Dimensional figures should be .125 (3mm) tall.
14. A diameter dimension should be preceded by the Theta symbol.
15. A radius dimension should be preceded by the R symbol.
16. Each dimension may appear only one time on a drawing the exception being a reference dimension.
17. If possible, dimensions should be given so that the production personnel need not calculate any dimensions.
18. Do not scale drawings for production proposes.
19. A dimension on a drawing that is not to scale must be Underlined or denoted either NTS or NOT TO SCALE.
20. The first dimension on a view should be place at least .375" away from the view with subsequent dimensions .250" apart.
21. The abbreviation TYP (typical) may be used for non-critical repeated features such as fillets's or rounds.
22. Dimensions should be given on the view where the shape of the feature is shown.
23. No line of the drawing may be used for, or coincide with, a dimension line.
24. The shoulder of a leader must start at either the beginning or the end of a note with the shoulder mid height of the lettering.
25. Utilize only those dimensions that are necessary to produce the part.
26. Dimensioning to hidden lines is to be avoided.
27. Stagger dimensional figures to avoid crowding and poor drawing legibility.
28. If dimensional figures must appear in a sectioned area, a clear space should be provided.
29. A local not is applied directly to a view of the drawing and supplies manufacturing information.
30. A general not applies to the entire drawing.
31. Longer dimensions should be placed outside shorter dimensions to avoid crossing dimension and extension lines.
32. It is permissible for extension lines to cross extension lines.
33. A complete chain of dimensions is to be avoided.
34. Finish marks may be omitted if the part is finished all over and a general not or title block is used.
35. Circles should be dimensioned by giving a diameter dimension and arcs by a radius dimension.
36. Notes for machining operations should begin in the order they are to be performed.
37. All dimensions have a tolerance except those identified as reference, max, min or are commercial stock.

Here is an example of Dimensioning

Here are some Doo's and Do Not's

Lesson 14a: Using your CADD software draw the useful orthographic views of the following model and dimension following all the rules you have learned in this lesson, and there are allot of them. Take you time and course be accurate

Lesson 14b: Below is another model. A little bit different in character than the preceding model but you will need a half scale offset sectional in the upper right hand corner to illustrate the details of this model in its entirety. Your basic model will be orthographic and it will drawn in layout-c. Remember this assignment is not just about models but about dimensioning them according to the rules outlined above which correspond to ASME 14.5 standards.

I will explain this further in the lesson 14 media disc. Please insert the media disk and listen to lesson 14.

*Note: The Lessons are best viewed with real player and you can download the latest version by clicking on the words real player.

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Summary and Assessment