Dimensions provide the sizes and locations needed to fabricate and assemble parts. Tolerances ensure accurate fits by specifying the permitted error in dimensions. There are two main methods for dimensioning drawings: conventional dimensioning lists overall sizes, while baseline dimensioning uses reference lines and three-place decimals. Scale size is always in direct proportion to the actual object being dimensioned.

1. Blueprint Reading for
Welders
8th Edition
Unit 4
Dimensions

2. Objectives
• Define common dimensioning
terms
• Identify common types of
dimensions
• Discuss the importance of
tolerances

3. Objectives (cont’d.)
• Identify the following elements of a
tolerance
– Minimum
– Maximum
• Understand the importance of a
drawings scale

4. Objectives (cont’d.)
• Understand the following
dimensioning methods:
– Conventional dimensioning
– Baseline dimensioning

5. Purpose of Dimensions
• Dimensions serve two purposes:
– Give the sizes needed to fabricate the
part
– Indicate locations where components
of the part should be:
• Placed
• Assembled
• Machined
• Welded

6. Purpose of Dimensions
(cont’d.)
• Both U.S. Customary and metric
units used on dual-dimensioned
drawings

7. Linear and Angular
Dimensions
• U.S. Customary linear dimensions
given as whole numbers, fractions,
decimals
– Preferred practice is to show
decimals
• Fractional dimensions for weld
drawings

8. Linear and Angular
Dimensions (cont’d.)
• Decimal dimensions for machining
operations
• Current practice uses unidirectional
dimensioning system
• Decimal fraction dimensions used
when precision sizes required

11. Linear and Angular
Dimensions (cont’d.)
• Angular dimensions when line is at
an angle to a horizontal, vertical,
angular line
– Included angle shown in degrees
• Angular dimensions sometimes
shown in degrees and minutes
– Decimal degrees preferred

13. Linear and Angular
Dimensions (cont’d.)
• Bevel: sloping edge that extends
full or partial length of the edge
• Sharp edge formed commonly
called a feather edge
• Root face area along the edge
often called a land
• Chamfer often identified and
treated as a bevel

15. Linear and Angular
Dimensions (cont’d.)
• Bevel features dimensioned with a
note and a leader
– Amount of bevel given as a linear and
a degree dimension
• Bevels may be dimensioned using
extension and dimension lines
– Sizes given as two linear dimensions
or as one linear and one angular
dimension

17. Radius and Arc Dimensions
• Several methods for dimensioning
a radius, arc, or curve:
– Angular dimension and radius
– Two linear dimensions and radius
– Radius and centerlines
• Arcs with multiple radii
– Dimensions shown are locations for
radii and size of radii

19. Drilled Hole Dimensions
• Shown by leader and note
• Applied to the view that shows the
shape of the hole
• Note gives size of drill, number of
holes to be drilled, depth of hole
• Holes with no depth dimension
drilled all the way through
– May also be indicated by adding the
word “thru” to drill size dimension

21. Countersunk and
Counterbored Holes and
Spotface Dimensions
• Abbreviation DP or symbol for
depth applied to counterbored hole
dimension
• Depth of a spotfaced hole not given
• Depth of countersunk hole not
given

22. Countersunk and
Counterbored Holes and
Spotface Dimensions
(cont’d.)
• Major diameter of countersink
provided
• Dimensions for flame cut holes
followed by the words “flame cut” or
“FC”

27. Tolerance Dimensions
• Figure given as a plus or minus
quantity that allows for a variation
in the dimension
• Specifies the permitted amount of
error
• Any or all dimensions on a print
can be given tolerances
• Tolerances follow the dimension to
which it refers

28. Tolerance Dimensions
(cont’d.)
• May be given as a note or
specification
• Ensure accuracy and proper fit of
parts
• Tolerances standardized for many
parts
– Found in tolerance tables

29. Tolerance Dimensions
(cont’d.)
• If no tolerances given, assume:
– Tolerance of 1/64 for fraction
dimensions
– Tolerance of .010” for decimal
fractions shown to two decimal places
– Tolerance of .005” for decimal
fractions shown to three decimal
places

30. Scale Sizes
• Dimensions may be full, enlarged
or reduced scale
– Reduced scale size more commonly
used on prints
– Enlarged scale size often used for
detail drawings describing small
components

31. Scale Sizes (cont’d.)
• Scale size always in direct
proportion to actual size
• Listed in the form of an equation

32. Thread Dimensions
• Standard thread symbols are used
on prints to represent threaded
parts
• Threads dimensioned using a
leader followed by the thread
specification
• Left-handed threads denoted with
LH following class of fit
specification

33. Thread Dimensions
• Common V-thread forms:
– National Coarse (NC) or National
Fine (NF)
– Unified National Coarse (UNC) or
Unified National Fine (UNF)

38. Dimensioning Methods
• Conventional dimensioning
– Overall dimensions of length, width,
height given
– Reference dimension given by adding
or subtracting other dimensions on
the drawing
• Facilitates reading the drawing
– If overall sizes not given, dimensions
added to find overall sizes
– Decimal dimensioning preferred

40. Dimensioning Methods
(cont’d.)
• Baseline dimensioning
– All dimensions originate from
baselines
• Dimensions normally expressed as
three-place decimals
– Baselines: object lines that usually
represent machined surfaces
• May also originate from centerlines

41. Dimensioning Methods
(cont’d.)
• Baseline dimensioning (cont’d.)
– Calculations not required to find a
dimension
– Commonly used to dimension
precision parts

43. Other Terms Commonly
Used in Dimensioning
• Basic size: base size of a part to
which a tolerance value is applied
• Actual size: size of part when
measured
• Nominal size: dimension that
represents an approximate size

45. Other Terms Commonly
Used in Dimensioning
(cont’d.)
• Limits of size: dimensions that
indicate upper and lower limits
• Call out: identification of each of
the parts by means of numbers

46. Dual Dimensioning
• Application of both U.S. Customary
and metric units
• Permits the manufacture of parts in
either unit of measurement

47. Summary
• Two purposes for dimensions
– Give the sizes needed to fabricate the
part
– Indicate the locations where
components of the part should be
placed
• Tolerances ensure fit of parts

48. Summary (cont’d.)
• Scale size in direct proportion to
size of object
• Two methods for dimensioning
– Convention dimensioning
– Baseline dimensioning