1. CGT 110 – Technical Graphics Communication
Week 9:
Tolerancing Practices

2. CGT 110 – Technical Graphics Communication
Here’s what we talked about last time…..
Week 9: Tolerancing

3. CGT 110 – Technical Graphics Communication
Dimension elements….
Week 9: Tolerancing

4. Dimensions are used to show an object’s:
CGT 110 – Technical Graphics Communication
Dimensions are used to show an object’s:
1. Overall: Width
Depth
Height
2. The actual size of features (rounds, fillets, holes, arcs, etc.)
Week 9: Tolerancing

5. CGT 110 – Technical Graphics Communication
3. And where features are located such as centers, angles, etc.
Week 9: Tolerancing

6. 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.
Week 9: Tolerancing

7. CGT 110 – Technical Graphics Communication
As a general rule…Stay off the object as much as possible.
Week 9: Tolerancing

8. CGT 110 – Technical Graphics Communication3.
Extension lines can be shared and even broken to clarify crowded dimensions.
Week 9: Tolerancing

9. CGT 110 – Technical Graphics Communication
Some features are dimensioned from their center lines.
The center line may also be used as an extension line.
Week 9: Tolerancing

10. 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.
Week 9: Tolerancing

11. CGT 110 – Technical Graphics Communication
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
Week 9: Tolerancing

12. CGT 110 – Technical Graphics Communication
Arcs are always dimensioned as a radius. Full circles are dimensioned showing their diameter value.
Week 9: Tolerancing

13. CGT 110 – Technical Graphics Communication
When dimensioning a part, always start with the inner-most dimensions and work to the outer-most values. Remember: Dimensions are used to show both the size and location of features.
Week 9: Tolerancing

14. CGT 110 – Technical Graphics Communication
Always dimension features and not lines…..and remember…. NEVER, NEVER, NEVER dimension to hidden lines!
Week 9: Tolerancing

15. Today’s Lecture - Week 9: Tolerancing
CGT 110 – Technical Graphics Communication
Today’s 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.

16. Representing Tolerance Values
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?
Week 9: Tolerancing

17. Tolerances can also be expressed as:
CGT 110 – Technical Graphics Communication
Tolerances can also be expressed as:
1. Geometric Tolerances.
“GDT”
2. Notes Referring to Specific Conditions.
3. A General Tolerance Note in the Title Block.
Example: ALL DECIMAL DIMENSIONS TO BE HELD TO ± .002”
Week 9: Tolerancing

18. Plus and Minus Dimensions
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.
Week 9: Tolerancing

19. Important Terms of Toleranced Parts
CGT 110 – Technical Graphics Communication
Important Terms of Toleranced Parts
A System is two or more mating 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”
Week 9: Tolerancing

20. Important Terms of Toleranced Parts
CGT 110 – Technical Graphics Communication
Important Terms of Toleranced Parts
Limits – the maximum and minimum sizes shown by the tolerance dimension.
The slot has limits of .502 & .498, and the mating part has limits of .495 & .497.
The large value on each part is the Upper Limit, the small value = Lower Limit.
Actual Size is the measured size of the finished part after machining.
The Actual Size of the machined part above is .501”
Week 9: Tolerancing

21. Important Terms of Toleranced Parts
CGT 110 – Technical Graphics Communication
Important Terms of Toleranced Parts
Allowance – the tightest fit
between two mating 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 it’s smallest allowable size
of .498 and the mating part is machined to its
largest allowable size of The difference
between .498 and .497, or .001, is the allowance.
Week 9: Tolerancing

22. Important Terms of Toleranced Parts
CGT 110 – Technical Graphics Communication
Important Terms of Toleranced Parts
Tolerance – the total allowable variance in a dimension;
the difference between the upper and lower limits.
The tolerance of the mating part is .002”
( = .002)
The tolerance of the slot is .004”
( = .004)
Week 9: Tolerancing

23. Important Terms of Toleranced Parts
CGT 110 – Technical Graphics Communication
Important Terms of Toleranced Parts
Maximum Material Condition (MMC)
The condition of a part when it contains
the greatest amount of material. The MMC
of an external feature, such as a shaft,
is the upper limit. The MMC of an internal
feature, such as a hole, is the lower limit.
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.
Week 9: Tolerancing

24. Important Terms of Toleranced Parts
CGT 110 – Technical Graphics Communication
Important Terms of Toleranced Parts
Piece tolerance
The difference between the upper and lower limits of a single part
(.002 on the insert in this example, .004 on the slot.).
System tolerance
The sum of all the piece tolerances.
For this example (.006)
Week 9: Tolerancing

25. Fit Types: Clearance & Interference fits between two shafts and a hole
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
Week 9: Tolerancing

26. Fit Types: Transition Fit CGT 110 – Technical Graphics Communication
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.
Week 9: Tolerancing

27. Functional Dimensioning
CGT 110 – Technical Graphics Communication
Functional Dimensioning
Functional Dimensioning begins with tolerancing the most important features.
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.
Week 9: Tolerancing

28. Tolerance Stack-up AVOID THIS!!!
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
Dimensioned
from the
left.
Dimensioned
from the
right.
AVOID THIS!!!
Week 9: Tolerancing

29. Avoiding Tolerance Stack-up CGT 110 – Technical Graphics Communication
Better still, relate the two
holes directly to each other,
not to either side of the part.
The result will be the best
tolerance possible of ±0.005.
Tolerance stack-up can
be eliminated by careful
consideration and
placement of dimensions.
(Dimension from same side).
Week 9: Tolerancing

30. Basic Hole System CGT 110 – Technical Graphics Communication
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.
Week 9: Tolerancing

31. Creating a Clearance Fit Using The Basic Hole System
Check the work by determining the piece tolerances for the shaft and the hole. To do so, first find the difference between the upper and lower limits for the hole. Subtract .500” from .503” to get .003” as a piece tolerance. This value matches the tolerance applied in Step 4. For the shaft, subtract .493” from .496 to get .003” as the piece tolerance. The value matches the tolerance applied in Step 3.
The difference between the largest hole (.503” upper limit) and the smallest shaft (.493” lower limit) equals a positive .010”. Because both the tightest and loosest fits are positive, there will always be clearance between the shaft and the hole, no matter which manufactured parts are assembled.
Using the basic hole system,
assign a value of .500” to the
smallest diameter of the hole,
which is the lower limit.
The allowance of .004” is subtracted from the diameter of the smallest hole to determine the diameter of the largest shaft, .496”, which is the upper limit.
The lower limit for the shaft is determined by subtracting the part tolerance from .496”. If the tolerance of the part is .003”, the lower limit of the shaft is .493”
Using the assigned values results in a clearance fit between the shaft and the hole. This is determined by finding the difference between the smallest hole (.500” lower limit) and the largest shaft (.496” upper limit), which is a positive .004”. As a check, this value should equal the allowance used in step 2
The system tolerance is the sum of all the piece tolerances. T o determine the system tolerances for the shaft and the hole, add the piece tolerances of .003” and .003” to get .006”
The upper limit of the hole is determined by adding the tolerance of the part to .500”. If the tolerance of the part is .003”, the upper limit of the hole is .503”
The parts are dimensioned on the drawing.

32. Creating an Interference Fit Using The Basic Hole System
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.
Week 9: Tolerancing

33. This Week’s Assignments

34. CGT 110 – Technical Graphics Communication
Week 10:
Working Drawings