chapter 8 tolerancing - 2010
TRANSCRIPT
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Chapter 8Tolerancing
TopicsExercises
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Tolerancing: TopicsSummary8.1) Tolerancing and Interchangeability8.2) Tolerancing Standards8.3) Tolerance Types8.4) General Definitions8.5) Inch Tolerances8.6) Metric Tolerances8.7) Selecting Tolerances8.8) Tolerance Accumulation8.9) Formatting Tolerances
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Tolerancing: ExercisesExercise 8-1: Inch tolerance definitionsExercise 8-2: Types of fitExercise 8-3: Determining fit typeExercise 8-4: Limits and fitsExercise 8-5: Milling jack assembly tolerancesExercise 8-6: Millimeter tolerance definitionsExercise 8-7: Metric fit designationExercise 8-8: SystemsExercise 8-9: Metric limits and fitsExercise 8-10: Tolerance accumulationExercise 8-11: Over dimensioning
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Tolerancing
Summary
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Summary What will we learn in Chapter 8?
→ We will learn about tolerancing and how important this technique is to mass production.
Key points→ If a feature’s size is toleranced, it is allowed
to vary within a range of values or limits.→ Tolerancing enables an engineer to design
interchangeable or replacement parts.
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Tolerancing
8.1) Tolerancing for Interchangeability
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Tolerancing / Interchangeability Tolerancing is dimensioning for
interchangeability.
What is interchangeability?
An interchangeable part is simply a mass produced part (a replacement part).
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Tolerancing / Interchangeability How is a feature on an interchangeable
part dimensioned?
→ The feature is not dimensioned using a single value, but a range of values.
1.00 →1.005
.994
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Tolerancing / Interchangeability A tolerance is the amount of size
variation permitted.→ You can choose a tolerance that specifies a
large or small variation.
1.005.994
Tolerance = 1.005 - .994 = .011
Size limits =
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Tolerancing / Interchangeability Why do we want a part’s size to be
controlled by two limits?
It is necessary because it is impossible to manufacture parts without some variation.
The stated limits are a form of quality control.
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Tolerancing / Interchangeability Choosing a tolerance for your design.
→ Specify a tolerance with whatever degree of accuracy that is required for the design to work properly.
→ Choose a tolerance that is not unnecessarily accurate or excessively inaccurate.
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Tolerancing / Interchangeability Choosing the correct tolerance for a
particular application depends on:
→ the design intent (end use) of the part→ cost→ how it is manufactured→ experience
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Tolerancing
8.2) Tolerancing Standards
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Tolerancing Standards Standards are needed to;
→ make it possible to manufacture parts at different times and in different places that still assemble properly.
→ establish dimensional limits for parts that are to be interchangeable.
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Tolerancing Standards The two most common standards
agencies are;
→ American National Standards Institute (ANSI) / (ASME)
→ International Standards Organization (ISO).
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Tolerancing
8.3) Tolerance Types
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Tolerance Types The tolerancing methods presented
are:→ Limit dimensions → Plus or minus tolerances → Page or block tolerances
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1. Limit Dimensions Limits are the maximum and minimum
size that a part can obtain and still pass inspection.
→ For example, the diameter of a shaft might be specified as follows.
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1. Limit Dimension Order The high limit is placed above the low
limit. When both limits are placed on one line, the low limit precedes the high limit.
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2. Plus or Minus Tolerances Plus or minus tolerances give a basic
size and the variation that can occur around that basic size.
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3. Page or Block Tolerances A page tolerance is actually a general
note that applies to all dimensions not covered by some other tolerancing type.
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Tolerancing
8.4) Shaft-Hole Assembly
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Shaft-Hole Assembly Used to illustrate concepts and definitions. Both the shaft and the hole are allowed to
vary between a maximum and minimum diameter.
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Tolerancing
8.5) Inch Tolerances
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Inch Tolerances Definitions Limits: The limits are the maximum and
minimum size that the part is allowed to be.
Basic Size: The basic size is the size from which the limits are calculated. → It is common for both the hole and the shaft
and is usually the closest fraction.
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Inch Tolerances Definitions Tolerance: The tolerance is the total
amount a specific dimension is permitted to vary.
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Exercise 8-2
Inch tolerance definitions
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Exercise 8-2 Fill in the
following table.
Skip to next part of the exercise
Shaft HoleLimitsBasic SizeTolerance
.47 - .51 .49 - .50
.5 or 1/2
.04 .01
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Inch Tolerances Definitions Maximum Material Condition (MMC):
The MMC is the size of the part when it consists of the most material.
Least Material Condition (LMC): The LMC is the size of the part when it consists of the least material.
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Exercise 8-2 Fill in the
following table.
Skip to next part of the exercise
Shaft HoleMMCLMC
.51 .49
.47 .50
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Inch Tolerances Definitions Maximum Clearance: The maximum
amount of space that can exist between the hole and the shaft.
→ Max. Clearance = LMChole – LMCshaft
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Inch Tolerances Definitions Minimum Clearance (Allowance): The
minimum amount of space that can exist between the hole and the shaft.
→ Min. Clearance = MMChole – MMCshaft
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Exercise 8-2 Fill in the
following table.
Max. ClearanceMin. Clearance
.50 - .47 = .03
.49 - .51 = -.02
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Exercise 8-2 What does
a negative clearance mean?
Max. ClearanceMin. Clearance
.50 - .47 = .03
.49 - .51 = -.02
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Types of Fits There are four major types of fits.
→ Clearance Fit→ Interference Fit→ Transition Fit→ Line Fit
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Types of Fits What is a clearance fit?
There is always a space.
Min. Clearance > 0
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Types of Fits What is an interference fit?
There is never a space.
Max. Clearance 0
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Types of Fits What is a transition fit?
Depending on the sizes of the shaft and hole there could be a space or no space.
Max. Clearance > 0Min. Clearance < 0
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Types of Fits What is a line fit?
There is a space or a contact (hole dia = shaft dia)
Max. Clearance > 0Min. Clearance = 0
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Exercise 8-3
Types of fits
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Exercise 8-3 From everyday life, list some examples of
clearance and interference fits. Fit ExampleClearance
Interference
Lock and KeyDoor and Door frameCoin and Coin slot
Pin in a bicycle chainHinge pin
Wooden peg and hammer toy
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Exercise 8-4
Determining fit type
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Exercise 8-4 Determine the basic size and type of fit
given the limits for the shaft and hole.
Shaft Limits Hole Limits Basic Size
Type of fit
1.498 - 1.500 1.503 - 1.505.751 - .755 .747 - .750.373 - .378 .371 - .375.247 - .250 .250 - .255
1.5 Clearance.75 Interference.375 Transition.25 Line
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ANSI Standard Limits and Fits The following fit types and classes are in
accordance with the ANSI B4.1-1967 (R1994) standard.
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ANSI Standard Limits and Fits RC: Running or Sliding Clearance fit.
→ Intended to provide running performance with suitable lubrication. • See table 8-2 for a more detailed description.
→ RC9 (loosest) – RC1 (tightest)
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ANSI Standard Limits and Fits Locational fits (LC, LT, LN).
→ Locational fits are intended to determine only the location of the mating parts. • See table 8-3 for a more detailed description.
• LC = Locational clearance fits • LT = Locational transition fits • LN = Locational interference fits
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ANSI Standard Limits and Fits FN: Force Fits.
→ Force fits provide a constant bore pressure throughout the range of sizes. • See table 8-4 for a more detailed description.
→ FN1 – FN5 (tightest)
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Exercise 8-5
Limits and fits
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Exercise 8-5 Given a basic size of .50 inches and a fit
of RC8, calculate the limits for both the hole and the shaft. → Use the ANSI limits and fit tables given in
Appendix A.
Page A-2
Basic size = .5Fit = RC8
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Exercise 8-5 Given a basic size of .50 inches and a fit
of RC8, calculate the limits for both the hole and the shaft.
→ Standard Limits Hole = +2.8 0→ Standard Limits Shaft = -3.5 -5.1
These are the values that we add/subtract from the basic size to obtain the limits.
What are the units?See page A-1.
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Exercise 8-5 Given a basic size of .50 inches and a fit
of RC8, calculate the limits for both the hole and the shaft.
→ Hole Limts = .50 - 0 = .5000 .50 + .0028 = .5028
→ Shaft Limits = .50 - .0035 = .4965 .50 - .0051 = .4949
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Exercise 8-6
Milling Jack assembly tolerances
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Exercise 8-6 Consider the Milling
Jack assembly shown. → Notice that there are
many parts that fit into or around other parts.
→ Each of these parts is toleranced to ensure proper fit and function.
The V-Anvil fits into the Sliding Screw with a RC4 fit. The basic size is .375 (3/8). Determine the limits for both parts.
The V-Anvil fits into the Sliding Screw with a RC4 fit. The basic size is .375 (3/8). What are the limits?
.3750 - .3759
.3739 - .3745
The Sliding Screw fits into the Base with a RC5 fit. The basic size is .625 (5/8). Determine the limits for both parts.
The Sliding Screw fits into the Base with a RC5 fit. The basic size is .625 (5/8). What are the limits?
.6231 - .6238
.625 - .626
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Tolerancing
8.6) Metric Tolerances
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Metric Tolerances Definitions Limits, Basic Size, Tolerance, MMC and
LMC have the same definition as in the inch tolerance section.
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Exercise 8-7
Millimeter tolerance definitions
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Exercise 8-7 Fill in the
following table.
Skip to next part of the exercise
Shaft HoleLimitsBasic SizeTolerance
2.1 – 2.2 1.8 – 2.020.1 0.2
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Metric Tolerances Definitions Upper deviation: The upper deviation is
the difference between the basic size and the permitted maximum size of the part.
→ UD = |basic size – Dmax|
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Metric Tolerances Definitions Lower deviation: The lower deviation is
the difference between the basic size and the minimum permitted size of the part.
→ LD = |basic size – Dmin|
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Metric Tolerances Definitions Fundamental deviation: The
fundamental deviation is the closest deviation to the basic size. → The fundamental deviation is the smaller of
the UD and the LD. → A letter in the fit specification represents the
fundamental deviation.
Ex: Metric Fit = H11/c11
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Exercise 8-7 Fill in the
following table.
Shaft HoleUDLDFD
0.2 00.1 0.20.1 0
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Exercise 8-7 Fill in the
following table.
Type of fit Interference
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Metric Tolerances Definitions International tolerance grade number
(IT#): The IT#’s are a set of tolerances that vary according to the basic size and provide the same relative level of accuracy within a given grade. → The number in the fit specification
represents the IT#. → A smaller number provides a smaller
tolerance.
Ex: Metric Fit = H11/c11
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Metric Tolerances Definitions Tolerance zone: The fundamental
deviation in combination with the IT# defines the tolerance zone. → The IT# establishes the magnitude of the
tolerance zone or the amount that the dimension can vary.
→ The fundamental deviation establishes the position of the tolerance zone with respect to the basic size.
Ex: Metric Fit = H11/c11
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ANSI Standard Limits and Fits The following fit types are in accordance
with the ANSI B4.2-1978 (R1994) standard.
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Available Metric Fits Hole Basis Shaft Basis FitH11/c11 C11/h11 Loose runningH9/d9 D9/h9 Free runningH8/f7 F8/h7 Close runningH7/g6 G7/h6 SlidingH7/h6 H7/h6 Locational clearanceH7/k6 or H7/n6
K7/h6 or N7/h6
Locational transition
H7/p6 P7/h6 Locational interferenceH7/s6 S7/h6 Medium driveH7/u6 U7/h6 Force
The difference between Hole and Shaft Basis Fits will be discussed in an upcoming section.
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Tolerance Designation A Metric fit is specified by stating the
fundamental deviation and the IT#. Remember!
→ IT# = the amount that the dimension can vary (tolerance zone size).
→ Fundamental deviation (letter) = establishes the position of the tolerance zone with respect to the basic size. • Hole = upper case• Shaft = lower case
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Tolerance DesignationFits are specified by using the:
→fundamental deviation (letter) →IT# (International Tolerance Grade #).
When specifying the fit:→The hole = upper case letter →The shaft = lower case letter
Ex: Metric Fit = H11/c11
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Exercise 8-8
Metric fit designation
Fill in the appropriate name for the fit component.
Basic size
Fundamental Deviation IT#
Hole Tolerance ZoneShaft Tolerance Zone
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Basic Hole / Basic Shaft Systems Metric limits and fits are divided into two
different systems; the basic hole system and the basic shaft system.
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Basic Hole / Basic Shaft Systems Basic hole system: The basic hole
system is used when you want the basic size to be attached to the hole dimension.
→ For example, if you want to tolerance a shaft based on a hole produced by a standard drill, reamer, broach, or another standard tool.
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Basic Hole / Basic Shaft Systems Basic shaft system: The basic shaft
system is used when you want the basic size to be attached to the shaft dimension.
→ For example, if you want to tolerance a hole based on the size of a purchased a standard drill rod.
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Exercise 8-9
Systems
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Exercise 8-9 Identify the type of fit and the system used
to determine the limits of the following shaft and hole pairs
Shaft Hole Type of Fit System9.972 - 9.987 10.000 - 10.022
60.002 - 60.021 60.000 - 60.030
39.984 - 40.000 39.924 - 39.949
Clearance Hole
Transition Hole
Interference Shaft
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Exercise 8-10
Metric limits and fits
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Exercise 8-10 Find the limits, tolerance, type of fit, and
type of system for a n30 H11/c11 fit. → Use the tolerance tables given in Appendix
A.
Page A-8
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Exercise 8-10 Find the limits, tolerance, type of fit, and
type of system for a n30 H11/c11 fit.
Shaft HoleLimits 29.760 - 29.890 30.000 - 30.130ToleranceSystemFit
0.13 0.13HoleClearance – Loose Running
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Exercise 8-10 Find the limits, tolerance, type of fit, and
type of system for a n30 P7/h6 fit. → Use the tolerance tables given in Appendix
A.
Page A-11
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Exercise 8-10 Find the limits, tolerance, type of fit, and
type of system for a n30 P7/h6 fit.
Shaft HoleLimits 29.987 - 30.000 29.965 – 29.986ToleranceSystemFit
0.013 0.021ShaftLocational Interference
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Tolerancing
8.7) Selecting Tolerances
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Selecting Tolerances Tolerances will govern the method of
manufacturing. → When the tolerances are reduced, the cost
of manufacturing rises very rapidly.
→ Specify as generous a tolerance as possible without interfering with the function of the part.
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Selecting Tolerances Choosing the most appropriate tolerance
depends on many factors such as; → length of engagement, → bearing load, → speed, → lubrication, → temperature, → humidity, → and material.
Experience also plays a significant role.
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Machining and IT Grades
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Tolerancing
8.8) Tolerance Accumulation
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Tolerance Accumulation The tolerance between two features of a
part depends on the number of controlling dimensions.
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Tolerance Accumulation The distance could be controlled by a
single dimension or multiple dimensions.
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Tolerance Accumulation The maximum variation between two
features is equal to the sum of the tolerances placed on the controlling dimensions.
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Tolerance Accumulation As the number of controlling dimensions
increases, the tolerance accumulation increases.
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Tolerance Accumulation Remember, even if the dimension does
not have a stated tolerance, it has an implied tolerance.
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Exercise 8-11
Tolerance Accumulation
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Exercise 8-11 What is the tolerance accumulation for
the distance between surface A and B for the following three dimensioning methods?
3070 .
2070 .
40.1
109.9
69.8
1070 .
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Exercise 8-11 If the accuracy of the distance between
surface A and B is important, which dimensioning method should be used?
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Exercise 8-12
Over dimensioning
Assuming that the diameter dimensions are correct, explain why this object is dimensioned incorrectly.
2.98 – 3.00
1. The decimal places don’t match.Formatting tolerances will be discussed next.
2. The dimensions are inconsistent.
1.98 + .99 = 2.972.01+1.00 = 3.01
This part is over dimensioned.
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Tolerancing
8.9) Formatting Tolerances
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Formatting Metric Tolerances Tolerances from standardized fit tables are
listed on drawings as;
The person reading the print has to have access to the standard fit tables.
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Formatting Metric Tolerances Unilateral tolerances
→ A single zero without a plus or minus sign.
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Formatting Metric Tolerances Bilateral tolerances
→ Both the plus and minus values have the same number of decimal places.
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Formatting Metric Tolerances Limit dimensions
→ Both values should have the same number of decimal places.
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Formatting Metric Tolerances Using Basic dimensions with the tolerance
→ The number of decimal places in the basic dimension does not have to match the number of decimal places in the tolerance.
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Formatting Inch Tolerances Unilateral and Bilateral tolerances
→ The basic dimension and the plus and minus values should have the same number of decimal places.
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Formatting Inch Tolerances Limit dimensions
→ Both values should have the same number of decimal places.
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Formatting Inch Tolerances Using Basic dimensions with the tolerance
→ The number of decimal places in the basic dimension should match the number of decimal places in the tolerance.
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Formatting Angular Tolerances Angular tolerances
→ Both the angle and the plus and minus values have the same number of decimal places.
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Tolerancing
The End