Dimensining

Dimesioning Explained

How to annotate simple drawings for
use in constructing an object

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: Spotfaced, Counterbored, Countersunk,
& Counterdrilled.

3

Basic Dimensioning

■ Dimensions allow a part to be manufactured

■ Dimensions are ALWAYS in real world units

■ Dimensions should be applied in a concise,

coherent manner

4

Dimension Terminology:

5

Extension lines

Extension lines
should be offset
from the part by
one half the text
height and should
extend one text
height beyond the
dimension line

6

Dimension Spacing

The first row of
dimensions must be
a minimum of 3 text
heights away from
the part.
Any dimensions
beyond it must be a
minimum of 2 text
heights apart

7

Arrowheads:
Should be 3 times longer than they are wide.

8

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 furtherest
dimension from the part.

See Essentials of Engineering Design Graphics Chapter 4

9

■

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.

■

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 (to be
toleranced) should be
placed inside a rectangular
box

Dimensioning Rules

See Essentials of Engineering Design Graphics Chapter 4

10

■

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.

■

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.

Dimensioning Rules Cont

See Essentials of Engineering Design Graphics Chapter 4

11

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

See Essentials of Engineering Design Graphics Chapter 4

12

2. Avoid dimensioning to hidden features

There is one
exception:……
When the hidden
line is a finished
(√) surface

There is one exception:……
When the hidden line is a finished (√) surface

13

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

See Essentials of Engineering Design Graphics Chapter 4

14

4. 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

See Essentials of Engineering Design Graphics Chapter 4

15

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

See Essentials of Engineering Design Graphics Chapter 4

16

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

See Essentials of Engineering Design Graphics Chapter 4

17

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

See Essentials of Engineering Design Graphics Chapter 4

18

8. 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

See Essentials of Engineering Design Graphics Chapter 4

19

9. Keep dimensions between views whenever
possible .

See Essentials of Engineering Design Graphics Chapter 4

20

10. 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)

See Essentials of Engineering Design Graphics Chapter 4

21

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

See Essentials of Engineering Design Graphics Chapter 4

22

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

See Essentials of Engineering Design Graphics Chapter 4

23

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

See Essentials of Engineering Design Graphics Chapter 4

24

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

See Essentials of Engineering Design Graphics Chapter 4

25

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

See Essentials of Engineering Design Graphics Chapter 4

26

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

See Essentials of Engineering Design Graphics Chapter 4

27

17. The origin for baseline or ordinate dimensions
used as a datum should be extended from a finished
edge ☑

of the part.

See Essentials of Engineering Design Graphics Chapter 4

28

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

See Essentials of Engineering Design Graphics Chapter 4

29

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

See Essentials of Engineering Design Graphics Chapter 4

30

20. 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.

See Essentials of Engineering Design Graphics Chapter 4

31

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

See Essentials of Engineering Design Graphics Chapter 4

32

Angular features:
Dimension angular features with either:

X,Y coordinate location or Vertex location & degrees

See Essentials of Engineering Design Graphics Chapter 4

33

Dimension Styles

Unidirectional dimensions
are read from the lower left
corner of the sheet

Aligned dimensions align
with individual dimension
lines.

See Essentials of Engineering Design Graphics Chapter 4

34

A short review on Dimension “elements”
•What is…

•an Extension line ?
•an Extension line gap ?
•an Extension line extension ?
•a Dimension line ?
•a Dimension numeral ?
•a leader ?

•When are…

•Decimals used ? Not used ?
•Unit designators (in. or mm) listed on dimension

lines ?

•What size…

•Is an Arrowhead ?

Dimensioning

In AutoCAD

36

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

37

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

38

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.

39

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

40

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

41

AutoCAD Dimension Variables:

See Essentials of AutoCAD by Whiteacre appendix 6

42

AutoCAD Dimension Variables:

Sets center tick mark size

??

dimcen

Sets number of decimal places of dimension

??

dimdec

Forces text inside of extension lines.

off

dimtix

Sets height of text.

.18

dimtxt

Places text above the dimension line.

off

dimtad

Extension beyond dimension line.

.18

dimexe

Sets arrowhead length.

.18

dimasz

Changes sizes of text arrows.

1.00

dimscale

All parts of the dimension are one entity.

on

dimaso

Description

Default

Variable

See Essentials of AutoCAD appendix 6

43

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

44

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

45

Counterbore holes
Require both a diameter and a depth dimension.

46

Counterdrilled holes:
Require both a diameter and a depth dimension. (measured to
the shoulder).

47

Countersunk 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

48

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.

49

Necks:

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

50

Knurls:

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

51

Dimensioning in small places

52

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.

53

SI symbol

When dimensions on
the drawing are all
stated in millimeters,
place an SI symbol
near the title block.

54

SI symbol:
1st vs 3rd angle projection.

Metric Units and
3rd angle projection

Metric Units and
1st angle projection

55

Finish marks:

Rough surfaces that
need to be machined to
provide a smooth,
finished surface are
indicated with finish
marks. The vertex of
the √ needs to touch
the edge view of the
surface.

56

Individual Assignment:

■ AutoCAD

❑ Plate 51………………..…….…all

❑ Plate 52……………………C & D

❑ Plate 53 …………….……..B & D

❑ Plate 54 ……………….…..A & D

■ Due: Beginning of class 12.2

Preview:

■ Tolerances:………………. Plate 57-63

❑ You will need your “paper copy” of the Vinson plate

book for this assignment

57

Dimensioning Errors:

■ What is wrong with:

58

Dimensioning Errors:

■ What is wrong with:

59

Dimensioning Errors:

■ What is wrong with:

60

Dimensioning Errors:

■ What is wrong with:

61

Learning Objectives

■ Apply dimensions to objects using

AutoCAD

■ Be able to write correct notes for

machined holes