Transcript Slide 1

CGT 110 – Technical Graphics Communication
Week 9:
Tolerancing Practices
CGT 110 – Technical Graphics Communication
Here’s what we talked about
last time…..
Week 9: Tolerancing
CGT 110 – Technical Graphics Communication
Dimension elements….
Week 9: Tolerancing
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
CGT 110 – Technical Graphics Communication
3.
And where features are located such
as centers, angles, etc.
Week 9: Tolerancing
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
CGT 110 – Technical Graphics Communication
As a general rule…Stay off the object as much as possible.
Week 9: Tolerancing
CGT 110 – Technical Graphics Communication
3.
Extension lines can
be shared and even
broken to clarify
crowded dimensions.
Week 9: Tolerancing
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
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
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
CGT 110 – Technical Graphics Communication
Arcs are always
dimensioned as a
radius. Full circles
are dimensioned
showing their
diameter value.
Week 9: Tolerancing
CGT 110 – Technical Graphics Communication
When dimensioning a part,
always start with the innermost dimensions and work
to the outer-most values.
Remember:
Dimensions are used to
show both the size and
location of features.
Week 9: Tolerancing
CGT 110 – Technical Graphics Communication
Always dimension features and not
lines…..and remember….
NEVER, NEVER, NEVER dimension
to hidden lines!
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.
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
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
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
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
CGT 110 – Technical Graphics Communication
Important Terms of Toleranced Parts
Actual Size is the measured size of the finished part after machining.
The Actual Size of the machined part above is .501”
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.
Week 9: Tolerancing
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 .497. The difference
between .498 and .497, or .001, is the allowance.
Week 9: Tolerancing
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”
(.497 - .495 = .002)
The tolerance of the slot is .004”
(.502 - .498 = .004)
Week 9: Tolerancing
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
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
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
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.
Week 9: Tolerancing
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
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.
AVOID THIS!!!
Dimensioned
from the
right.
Week 9: Tolerancing
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).
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.
Week 9: Tolerancing
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.
Week 9: Tolerancing
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
The difference between the hole. Subtract .500” from
largest hole (.503” upper limit)
.503” to get .003” as a piece
and the smallest shaft (.493” tolerance. This value matches
Using the assigned values
lower limit)
equals
a positive
the tolerance
applied
Step lower limit for the shaft is
Using
the basic
hole
system,The
allowance
of .004”in
results
in a clearance
fitis The
The system tolerance is the
.010”.
Because
both
the
4. For the shaft,
subtract
assign a value of .500” to thesubtracted
from
the
diameter
determined
bypiece
subtracting the
between the shaft and the sum of all the
tightest
and
loosest
fits
are
.493”
.496hole
to get
as tolerance from .496”. If
smallest diameter of the hole,of
the from
smallest
to .003”
part
hole.
This
is
determined
by
tolerances.
T o determine the
positive,
there
will limit.
always bedetermine
the piece tolerance.
Theofvalue
which
is the
lower
the
diameter
the
the
tolerance
of thefor
part
finding the difference between
system tolerances
theis
clearance between the shaft largest
matches
the tolerance
applied
shaft,
.496”,
which
is.003”,
the the
lower
limit
of the
the
the smallest
hole
(.500”
lower
shaft and
hole,
add
and the hole, no matter whichthe
in Step
3. limit.
upper
.493”
limit)
and the
largest shaft shaft
pieceistolerances
of .003” and
manufactured parts are
(.496” upper limit), which is.003”
a
to get .006”
assembled.
parts.004”.
are dimensioned
The upper limit of the hole isThe
positive
As a check,on
determined by adding the the
thisdrawing.
value should equal the
tolerance of the part to .500”.allowance used in step 2
If the tolerance of the part is
.003”, the upper limit of the
hole is .503”
CGT 110 – Technical Graphics Communication
Creating an Interference Fit Using The Basic Hole System
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
This Week’s
Assignments
CGT 110 – Technical Graphics Communication
Week 10:
Working Drawings