CGT 110

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CGT 110 Technical Graphics Communication

• Week 9
• Tolerancing Practices

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CGT 110 Technical Graphics Communication
Heres what we talked about last time..

• Week 9 Tolerancing

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CGT 110 Technical Graphics Communication
Dimension elements.
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CGT 110 Technical Graphics Communication
Dimensions are used to show an
objects 1. Overall Width Depth
Height 2. The actual size of features (rounds,
fillets, holes, arcs, etc.)
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CGT 110 Technical Graphics Communication
3. And where features are located such as
centers, angles, etc.
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CGT 110 Technical Graphics Communication
Dimensions should be stacked in a broken chain
format.
Breaking the Chain refers to leaving out one
dimension as shown above so that manufacturing
tolerances are maintained.
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CGT 110 Technical Graphics Communication
As a general ruleStay off the object as much as
possible.
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CGT 110 Technical Graphics Communication
3.
Extension lines can be shared and even broken to
clarify crowded dimensions.
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CGT 110 Technical Graphics Communication
Some features are dimensioned from their center
lines. The center line may also be used as an
extension line.
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CGT 110 Technical Graphics Communication
Leaders with dimensions are used to show negative
cylinders (holes).
The leader should always be placed to penetrate
the center of all round features.
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Features such as counterbores, countersinks and
spot faces are all dimensioned using a
leader.Note Each of these features has a
special dimensioning symbol that can be used to
show a. Diameter b. Shape c. Depth
CGT 110 Technical Graphics Communication
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Arcs are always dimensioned as a radius. Full
circles are dimensioned showing their diameter
value.
CGT 110 Technical Graphics Communication
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When dimensioning a part, always start with the
inner-most dimensions and work to the outer-most
values.RememberDimensions are used to show
both the size and location of features.
CGT 110 Technical Graphics Communication
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Always dimension features and not lines..and
remember.NEVER, NEVER, NEVER dimension to
hidden lines!
CGT 110 Technical Graphics Communication
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CGT 110 Technical Graphics Communication

• Todays Lecture - Week 9
• Tolerancing

Tolerances are used to control the variation
that exists on all manufactured parts.
Toleranced dimensions control the amount of
variation on each part of an assembly.
The amount each part is allowed to vary depends
on the function of the part and of the
assembly. For example the tolerances placed
on a swing set is not as stringent as those
placed on jet engine parts.
The more accuracy needed in the machined part
the higher the manufacturing cost.
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CGT 110 Technical Graphics Communication
Representing Tolerance Values
Tolerance is the total amount a dimension
may vary and is the difference between the
maximum and minimum limits. (A) Tolerance
.04 (B) Tolerance .006
Tolerances are represented as Direct Limits
(A) or as Tolerance Values (B).
Which part costs more to manufacture?
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CGT 110 Technical Graphics Communication
Tolerances can also be expressed as
2. Notes Referring to Specific Conditions.
3. A General Tolerance Note in the Title Block.
Example ALL DECIMAL DIMENSIONS TO BE HELD TO
.002
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CGT 110 Technical Graphics Communication
Plus and Minus Dimensions
With this approach, the basic size is
given, followed by a plus/minus sign and the
tolerance value.
Notice that a Unilateral Tolerance varies in only
one direction, while Bilateral Tolerances varies
in both directions from the basic size.
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CGT 110 Technical Graphics Communication
Important Terms of Toleranced Parts
Nominal Size is used to describe the general size
(usually in fractions). The parts above have a
nominal size of 1/2
Basic Size theoretical size used as a starting
point for the application of Tolerances. The
parts above have a basic size of .500
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CGT 110 Technical Graphics Communication
Important Terms of Toleranced Parts
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CGT 110 Technical Graphics Communication
Important Terms of Toleranced Parts
(The minimum clearance or maximum interference).
For these two parts, the allowance is
.001, meaning that the tightest fit occurs when
the slot is machined to its smallest allowable
size of .498 and the mating part is machined to
its largest allowable size of .497. The
difference between .498 and .497, or .001, is the
allowance.
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CGT 110 Technical Graphics Communication
Important Terms of Toleranced Parts
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CGT 110 Technical Graphics Communication
Important Terms of Toleranced Parts
Least Material Condition (LMC) The condition of a
part when it contains he least amount of
material possible. The LMC of an external feature
is the lower limit. The LMC of an internal
feature is the upper limit.
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CGT 110 Technical Graphics Communication
Important Terms of Toleranced Parts
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CGT 110 Technical Graphics Communication
Fit Types
Clearance Interference fits between two shafts
and a hole
Shaft A is a Clearance fit, shaft B is an
Interference fit
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CGT 110 Technical Graphics Communication
Fit Types
Transition Fit
A Clearance Fit occurs when two toleranced mating
parts will always leave a space or clearance when
assembled.
An Interference Fit occurs when two toleranced
mating parts will always interfere when assembled.
A Transition Fit occurs when two toleranced
mating parts are sometimes an interference fit
and sometimes a clearance fit when assembled.
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CGT 110 Technical Graphics Communication
Functional Dimensioning
Then, the material around the holes
is dimensioned (at a much looser tolerance).
Functional features are those that come in
contact with other parts, especially moving
parts. Holes are usually functional features.
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CGT 110 Technical Graphics Communication
Occurs when dimensions are taken from opposite
directions of separate parts to the same point of
an assembly.
Tolerance Stack-up
AVOID THIS!!!
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CGT 110 Technical Graphics Communication
Avoiding Tolerance Stack-up
Tolerance stack-up can be eliminated by
careful consideration and placement of
dimensions. (Dimension from same side).
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CGT 110 Technical Graphics Communication
Basic Hole System
The basic hole system is used to apply tolerances
to a hole and shaft assembly.
The smallest hole is assigned the basic diameter
from which the tolerance and allowance is applied.
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Creating a Clearance Fit Using The Basic Hole
System
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CGT 110 Technical Graphics Communication
Creating an Interference Fit Using The Basic Hole
System
ADD here
Follow the same sequence of steps as you did for
a Clearance Fit, except that you ADD the
allowance in Step 2, instead of subtract.
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This WeeksAssignments
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CGT 110 Technical Graphics Communication

• Week 10
• Working Drawings