1. Basic Dimensioning with AutoCAD1

2. Learning Objectives
Be able to understand the basic rules of dimensioning
Apply dimensions to objects in accordance with engineering standards
Define the following items: Dimension line, Extension line, Reference dimension, and Leader
Be able to recognize the following machined holes: Spotface, Counterbore, Countersink, & Depth. 2

3. Basic Dimensioning
Dimensions allow a part to be manufactured – called a working drawing
Dimensions are ALWAYS in real world units
Dimensions should be applied in a concise, coherent manner (clear and concise) 14

4. Dimension Terminology:

5. What’s wrong with this?

6. Dimensioning Rules
Avoid placing any dimensions on the part (inside the view) unless there is no other option.
Avoid dimensioning to hidden features
Always place the dimension where the characteristic shape is shown in the most descriptive view. (This means don’t place a dimension on object lines making a “T joint”.)
Always dimension holes in their circular view by stating the diameter of drilled holes. Specify the hole depth of special features such as countersinking with a note following the dimension.
Dimension rounded corners and arc features as radii where they appear in their rounded views.
If the same value is repeated many times, then use a general note for the features.
Dimension cylindrical objects as diameters in their rectangular view.
Always place the first row of dimensions a minimum distance of 3 text heights away from the edge of the part. Additional stacks can be a minimum of 2 text heights away from each other.
Keep dimensions between the views whenever possible
Extension lines may cross each other and over other lines on the part, but dimension lines should never be crossed.
The overall dimension should always be given. It should be placed outside of smaller dimensions and be the furthest dimension from the part. 16

7. Dimensioning Rules continued…
Do not duplicate dimensions and avoid using unnecessary or superfluous dimensions
When all of the dimensions are expressed in inches, do not use inch mark (“) or the abbreviation for inches (in.)
For drawings dimensioned in inches, values less that one inch should not be preceded with a zero.
For metric drawings, omit the use of the millimeter (mm) notation following the numeral, as millimeters are the default units.
The origin for baseline or ordinate dimensions used as a datum should be extended from a finished edge of the part.
Conserve space and time by using abbreviations and standardized symbols whenever possible.
Reference dimensions should be placed in parentheses or should include the abbreviation “REF”.
Basic sizes (tolerance) should be placed inside a rectangular box
Extend leaders from the first of last word in a note. Point them toward the center of circular features that they are specifying.
Place dimensions among the various views to avoid crowding. Stagger horizontal dimensions to avoid contact or crowding of values. 16

8. Avoid placing dimensions on the part (inside of the view).

9. Avoid dimensioning to hidden features

10. Always place dimensions where the characteristic shape is shown in the most descriptive view

11. Always dimension holes in their circular view with the Ø
Always dimension holes in their circular view with the Ø. Specify special features (hole types) with a note.
Also: Remember to locate hole position with ordinate dimensions to hole centers

12. Dimension rounded corners and arc features as radii where they appear in their rounded view.

13. If the same value is repeated many times, then use a general note for the feature. (ALL FILLETS AND ROUNDS ARE .125R)

14. Dimension cylinders in their rectangular view with a diameter symbol…

15. Place the first row of dimensions 3 text heights (3/8 ”or 10 mm) away from the edge of the part.
Additional stacks of dimensions can be a minimum of two text heights (1/4 ”or 6 mm) away from each other

16. Keep dimensions between views whenever possible .

17. Extension lines may cross each other and over other lines on the part, but dimension lines should never be crossed. (Hint no “arrow-headed” lines can cross “arrow-headed” lines)

18. The overall dimension should always be given
The overall dimension should always be given. It should be placed outside of smaller dimensions and be the farthest from the part.

19. Do not duplicate dimensions and avoid using unnecessary or superfluous dimensions

20. When all of the dimensions are expressed as inches
When all of the dimensions are expressed as inches. DO NOT use inch marks ('') or the abbreviation (.in)

21. For drawings dimensioned in inches, values less than 1 inch should not be proceeded with a zero.

22. For metric dimensions less than 1 mm DO place a zero in front of the decimal point.

23. For metric drawings, omit the use of the millimeter (mm) notation following the numeral, as millimeters are the default units.

24. The origin for baseline or ordinate dimensions used as a datum should be extended from a finished edge  of the part.

25. Conserve space and time by using abbreviations and standardized symbols whenever possible.

26. Reference dimensions should be placed in parenthesis or should include the abbreviation “REF”. Basic sizes (tolerance) should be placed inside of a rectangular box

27. Extend leaders from the first or last word in a note
Extend leaders from the first or last word in a note. Point them toward (but not touch) the center of the circular features that they are specifying.

28. Place dimensions among the various views to avoid crowding
Place dimensions among the various views to avoid crowding. Stagger horizontal dimensions to avoid contact or crowding of the values

29. Angular features: Dimension angular features with either:
X,Y coordinate location or Vertex location & degrees

30. Dimension Styles
Aligned dimensions align with individual dimension lines.
Unidirectional dimensions are read from the lower left corner of the sheet

31. Dimensioning in AutoCAD
AutoCAD does semi-automatic dimensioning
You select the features which should be dimensioned and the location for each dimension
AutoCAD measures the distance in question and applies the appropriate dimension complete with arrowheads, extension lines, and dimension lines

32. The Dimensioning Toolbar
The dimensioning commands can be easily accessed by:
Right Click on an existing toolbar and activate the dimensioning toolbar;
Or, From the “pull down” menu
View
Toolbars

33. Linear Dimensions
Linear dimensions are use to show basic dimensions of parts
They can be orientated either in a horizontal or vertical (or even aligned) fashion
AutoCAD will automatically measure the length specified and insert all the parts of the dimension.

34. Leaders Leaders are used to apply thread notes and other annotations
Leaders begin at the arrow and continue from their elbow to their annotation
The annotation can be any text string you want to type

35. Radius and Diameter
By selecting a circular feature (arc or circle) with the Radius or Diameter option AutoCAD will measure the size and place the dimension at a user selected point
Using the Center option will place automatic centerlines on arcs or circles

36. AutoCAD Dimension Variables:

37. AutoCAD Dimension Variables:
Default
Description
dimaso on
All parts of the dimension are one entity.
dimscale
1.00
Changes sizes of text arrows.
dimasz
.18
Sets arrowhead length.
dimexe
Extension beyond dimension line.
dimtad
off
Places text above the dimension line.
dimtxt
Sets height of text.
dimtix
Forces text inside of extension lines.
dimdec ??
Sets number of decimal places of dimension
dimcen
Sets center tick mark size

38. Special Characters in AutoCAD:
%%c will show as the Diameter symbol: Ø
%%d will show as the Degree symbol: 
%%p will show as the Plus-minus symbol: 
%%u will start/stop text underlined
%%o will start/stop text overline

39. Spotfaced holes: No depth required
Spotfaced holes: No depth required. Cleans an uneven surface deep enough to allow full contact with bolt or washer.

40. Counterbore holes Require both a diameter and a depth dimension.

41. Countersink holes:
Used with flat head screws and bolts. They become flush when tightened. Used extensively in the aerospace industry. The 82 angle is common for cutting tools and screw threads

42. Chamfers:
Break away sharp edges and help guide the part into a mating assembly. May be specified with one distance and one angle, or with two distances.

43. Necks: Necks are used for “O” ring seals, parts turned on lathes,
and designed failure points

44. Knurls:
Knurling is a process of rolling patterns onto cylindrical objects to make them easier to grip.

45. Dimensioning in small places

46. Repetitive features A combination of
dimensioning & notes is used. A number in parenthesis indicates how many are required. The word TYPICAL (or TYP) indicates that all shown are the same.

47. Dimensioning Errors:
What is wrong with:

48. Dimensioning Errors:
What is wrong with:

49. Dimensioning Errors:
What is wrong with: