Reference Guide
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CCTSPLV332052016
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1
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manual or
Contents What
is GD& T7
Machining N~cessity
1
Flowchart
of
1
Dimensional Tolerance
2
Tolerance Dimensioning
2
Deviation
5
Fits between Mating System
of
Geometric
Parts
5
7
Fits Dimensioning
and Tolerance (GD&T) System-ASME
Terms and Definitions Maximum Material
Condition
Y14.5M-1994
9 11
(MMC)
13
GD&T Rules
15
Datums
16
GD& T Symbols and Modifiers
20
;.
,--;,f'
.
II
GD&T
Geometric Dimensioning & Tolerancing What is Geometric Dimensioning
& Tolerancing?
Geometric Dimensioning EtTolerancing (GDEtT)is a symbolic language for researching, refining, and encoding the function of each feature of a part. In addition to enabling unambiguous decoding to communicate design intent to manufacturing and quality assurance, GDEtTenables scientific tolerance stack-up analysis. and is therefore in a position to absolutely guarantee the assemble ability of in-tolerance mating parts. It consists of concepts, tools, rules, and processes,which are described in various military, national and ISO standards, and are set forth in this document in abbreviated form. Y AXIS OF DATUM REFERENCE
BASIC DIMENSION
FRAME
(A,B,C)
...
X(A,B,C)
Z(A,B,C)
-
ill}--L-----4---~--+-------------~-~I~~~1~0.-2~IA~I~B®~S1 018±0.3
(C)
027±0.3
1-$-1¢0.5@IAIB®lcl 1_L'¢0'5®'A'
DATUM FEATURE
1
A
FEATURE FRAME
1
T
(B)
LEADER
EXTENSION
LINE
CONTROL
Fig 1.1: A GDftT Encoded Drawing
Machining Flowchart Let us consider the steps involved in creating a mechanical device to solve a given problem. •
The first step is conceptual development! product design (the design stage).
•
Draft !detail the plans for each part (the drawing stage)
•
Then the individual parts are machined.
•
Next we layout an assembly plan, finally the device is assembled.
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GD&T
Machining
~
(comPletion) Fig 1.2: Machining flowchart
Necessity of Dimensional Tolerance •
It is almost impossible (and sometimes uneconomical) to maintain the strict degree of accuracy due to inevitable inaccuracy of manufacturing methods.
•
Due to interchangeability!
•
It is impossible for an operator to make perfect settings. In setting up machine .., i.e. in adjusting the tool and work piece on the machine, some errors are likely to creep in.
mass production.
To accommodate this, it is normal to display measurements with a plus or minus (+/-l tolerance which allows for some margin of above errors. Usually, the dimensional tolerance is decided at the design stage and a Machinist must take care to apply the required dimensional tolerance and to ensure that discrepancies are not introduced as a result of poor workmanship of measuring techniques. The tolerance is a compromise between accuracy required for proper functioning and the ability to economically produce this accuracy.
Tolerance Dimensioning Tolerance is the total amount that a specific dimension is permitted to vary. It is the difference between the maximum and the minimum limits for the dimension. Tolerance may be specified in 3 places: •
Directly on (with) the specified dimension
•
In a genera I note
•
In title block (tolerance block)
For example a dimension given as 1.625 ± .002 means that the manufactured part may be 1.627 or 1.623, or anywhere between these limit dimensions.
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GD&T Expressing Tolerance 1.00 ± .05
III
•
~:g~III 1.00 ~:g~ 1.00 ~:gg III • 1.00
III
Equal Bilateral Tolerance
II
Bilateral Tolerance
II
Unilateral Plus Tolerance Unilateral Minus Tolerance
1.05 .98
III
II
Plus Limits, 2 Lines
.98 1.05
III
II
Minus Limits, 2 Lines
1.05 - .98
III
II
Plus Limits, 1 Lines
.98 -1.05
III
II
Minus Limits, 1 Lines
Unilateral
Bilateral
Fig 1.3: Expressing i'o/erancing
Tolerance definition - Key terms Nominal Size: It is the designation used for general identification and is usually expressed in common fractions. For Ex. In the previous figure, the nominal size of both hole and shaft, which is 11/4", would be 1.25" in a decimal system of dimensioning. Basic Size or Basic dimension: It is the theoretical size from which limits of size are derived by the application of allowances and tolerances. Actual Size: is the measured size of the finished part. Limits: The two extreme permissible sizes between which the actual size lines are called limits. Max Limit: It is defined as the maximum permissible size for a given basic size. In fig. the max limit for the basic size of Dia30 is = Dia30 + 0.035 = Dia30.035mm. Min Limit: It is defined as the minimum permissible size for a given basic size. In fig. the min limit for the basic size of Dia30 is = Dia30 - 0.215 = Dia29.785mm. Tolerance: It is defined as the amount of variation permitted to a basic size. The difference between the max and min limits of a basic size are called tolerance. In fig. the tolerance is = Dia30.035 - Dia29.785 = 0.2Smm. --=--====--=--=--====
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GD&T
Deviation: is the difference between the basic size and the hole or shaft size. Upper Deviation: is the difference between the basic size and the permitted maximum size of the part. Lower Deviation: is the difference between the basic size and the minimum permitted size of the part. Actual Deviation: size.
It is the algebraic difference b/w the actual measured size and the corresponding basic
Zero Line: Since the deviations are measured from the basic size, to indicate the deviations graphically, the basic shaft, the min shaft, the actual shaft and the max shaft are aligned at the bottom and a straight line, called zero line is drawn through the top generator of the basic shaft as shown in fig. This is called zero Line because the deviations at the basic size will be zero. When the zero line is drawn horizontally, deviations above this line will be positive and below it will be negative. Tolerance zone: The zone bounded by the upper and lower limits of the basic size. Fundamental Deviation: It is that one of the two deviations which is conventionally chosen to define the position of the tolerance zone in relation to the zero line. Grades of tolerance: In a standardized system of limits and fits, group of tolerance are considered as corresponding to the same level of accuracy for all basic sizes.
~-- Basic Zero Line --
Shaft
.......
~-- Basic Hole International tolerance grade
Fundamental deviation
cl- ..
Min. size ------t~1
Max. size ---.-' Fig 1.4: Tolerance definition
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GD&T Deviation It is defined as the algebraic difference between a size 8: corresponding basic size.
ZERO LINE
Deviation
SHAFT
Diameter (upper limit)
(lower limit)
Basic Size
-cei»
Fig 1.5: Deviation
Upper deviation: It is the difference of dimension between the maximum possible size of the component and its basic size. i.e. it is designated by ESfor hole 8: es for the shaft. It is a positive quantity when the maximum limit of size is greater than the basic size and negative quantity when the maximum limit of size is less than basic size. Lower deviation: Similarly, it is the difference of dimension between the minimum possible size of the component and its nominal size. i.e, It is designated by EI for hole 8: ei for the shaft. It is a positive quantity when the minimum limit of size is greater than the basic size and negative quantity when the minimum limit of size is less tha n basic size. Fundamental deviation: It defines the location of the tolerance zone with respect to the nominal size. For that matter, either of the deviations may be considered. Minimum Clearance: in a clearance fit, it refers to the difference between minimum size of the hole 8: the maximum size of the shaft. Maximum Clearance: in a clearance I transition fit, it refers to the difference between maximum size of the hole 8: the minimum size of the shaft. Minimum Interference: in a Interference fit, it refers to the difference between maximum size of the hole 8: the minimum size of the shaft. Maximum Interference: in a Interference I transition fit, it refers to the difference between minimum size of the hole 8: the maximum size of the shaft.
Fits between Mating Parts Fit is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts.
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GD&T
There are three types of fits between parts:
o
Clearance Fit In clearance fit an internal member fits in an external member (as a shaft in a hole) and always leaves a space or clearance betwr _._L'_ - - - L_
Fig 1.6: Clearance fit
e
Interference
Fit
In interference fit the internal member is larger than the external member such that there is always an actual interference of material. The smallest shaft is 1.2513" and the la rgest hole is 1.2506", so that there is an actual interference of metal amounting to at least o.ooor Under maximum material conditions the interference would be 0.0019". This interference is the allowance, and in an interference fit it is always negative.
(0)
e
iNTERFERENCE
FIT
Transition Fit Transition fit result in either a clearance or interference condition. In the figure below, the smallest shaft 1.2503" will fit in the largest hole 1.2506", with 0.003" to spare. But the largest shaft, 1.2509" will have to be forced into the smallest hole, 1.2500" with an interference of metal of 0.009':
dl.2r:-td t .__ __ ~
""'1.2503
(b)
CADDO
CENTRE
TRANSITION
FIT
~ .. ~
1
II
GD&T H11/c11
Loose Running:
H9/d9
Free Running: For large temperature variations, high running speeds, or heavy journal pressures.
Ha/f7
Close Running: For accurate location and moderate speeds and journal pressures.
H7/g6
Sliding: Fit not intended to run freely, but to turn and move freely, and to locate accurately.
H7/h6
Locational Clearance: Fit provides snug fit for locating stationary parts; but can be freely assembled and disassembled.
H7/k6
Locational Transition: Fit for accurate location, a compromise between clearance and interference.
H7/n6
Locational Transition: Fit for more accurate location where greater interference is permissible.
H7/p6
Locationallnterference: Fit for parts requiring rigidity and alignment with prime accuracy of location, but without special bore pressure requirements.
H7/s6
Medium Drive: Fit for ordinary steel parts or shrink fits on light sections, the tightest fit usable with cast iron.
H7/u6
For wide commercial tolerances on external members.
Force: Fit suitable for parts which can be highly stressed or for shrink fits where the heavy pressing forces required are impractical.
System of Fits Two types of systems used to obtain various types of fits:
e
Hole Basis System In this system the different types of fits are obtained by associating shafts of varying limit dimensions with a single hole, whose lower deviation is zero. When the lower deviation of the hole is zero, the minimum limit of the hole is equal to its basic size, which is taken as the base for computing all other limit dimensions.
j_ ¢.498_c=:2_ .495~-
¢.500 .502
-r-
(0)
BASIC
HOLE.
FIT
Fig 1.9: Hole Basis System
CADD®
CENTRE
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II
GD&T
In the above figure
o
•
The minimum size of the hole 0.500" is taken as the basic size.
•
An allowance of 0.002" is decided on and subtracted from the basic hole size, making the maximum shaft as 0.498"
•
Tolerances of 0.002" and 0.003" respectively are applied to the hole and shaft to obtain the maximum hole of 0.502" and the minimum shaft of 0.495':
Shaft Basis System In this system the different types of fits are obtained by associating holes of varying limit dimensions with a single shaft, whose upper deviation is zero. When the upper deviation of the shaft is zero, the maximum limit of the shaft is equal to its basic size, which is taken as the base for computing all other limit dimensions.
_L_/~ n(.502 )U.505
j_ n(.500~_ )U.499~
~/~
'(b)
BASIC SHAFT
FIT
Fig 1.10: Shaft Basis System
•
The maximum size of the shaft 0.500" is taken as the basic size.
•
An allowance of 0.002" is decided on and added to the basic shaft size, making the minimum hole as 0.502':
•
Tolerances of 0.003" and 0.001" respectively are applied to the hole and shaft to obtain the maximum hole of 0.505" and the minimum shaft of 0.499':
IT Grade IT Grade refers to the International Tolerance Grade of an industrial process defined in ISO286 implements 20 IT tolerance. This grade identifies what tolerances a given process can produce for a given dimension. Field of use of individual tolerances of the system ISO: ITOl to IT6 - For production of gauges and measuring instruments IT5 to IT12 - For fits in precision and general engineering ITll to 1T16- For production of semi-products 1T16to IT18 - For structures ITll to 1T18- For specification of limit deviations of non-tolerated dimensions
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GD&T IT Grade
Fig 1.11: Machining
process associated
witt) tolerance
grades
Geometric Dimensioning and Tolerance (GD&T)System-ASME Y14.5M-1994 GD&T is an international language that is used on engineering drawings to accurately describe a part. With this the designer can properly apply geometric tolerance, they must carefully consider the fit and function of each feature of any part. This language consists of well defined set of symbols, rules, definition & conventions. GD&T encourage a dimensional philosophy called "FUNCTIONAL DIMENSIONING": functional dimensioning that defines a part based on how it functions in the final product. Consider the following example: Consider a table. Given table Height, we assume all 4 legs will be cut to length the same time.
Ii
I
20±1
_j Fig 1.12: Tuble with dimensions
===========
applied
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GD&T Datum axis
Datum paints
measurement
Datum planes Origino( measurement
Fig 7.17: Datums
Some common datum feature simulators are surface plates, angle plates, chucks, mandrels, and machine tables. Feature Control Frame: Imagine the control of dimensions of this part shown here. The size and the location of the feature (cylindrical hole) is specified with basic dimension. We can now add an allowed deviation also to the feature
1--1.000--
00.500±0.010
1--1.000--
00.500±0.010
Fig 1.18: Part with dimensions
Here the tolerance must be shown as applying to the feature being controlled. Like this each controlled feature (hole, shaft, slot, surface, etc) associated with the basic dimension is given a feature control frame to show a tolerance. The tolerance that appears in the feature control frame is the allowed deviation from the perfect size or location shown by dimensions. Feature control frame has the following: A geometric characteristic symbol A tolerance zone descriptor A tolerance of location A material condition symbol Primary, secondary, and tertiary datums For our example, the component, shown with details of feature control frame would appear like this.
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m
GD&T
1-$-1 ¢O.030@ I A I B lei
Fig 1.19: Feature control
frame
Material Condition: To overcome shortcomings in symbols, modifiers can be added to change their meanings. They can be either Maximum material condition or least material condition. -..........____ Maximum Material Condition is the condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size. This means that the tolerance is at the extreme that would result if too little material was cut off, and the maximum material remains.
MMCSymbol Least Material Condition is the condition in which a feature of size contains the least amount of material everywhere within the stated limits of size. This means that the tolerance is at the extreme that would result if too much material was cut off, and the minimum material remains.
LMC Symbol
Maximum Material Condition (MMC) MMC is that condition of a part or feature which contains the maximum amount of material, e.g. minimum size hole, or a maximum size shaft The maximum material principle takes into account the mutual dependence of tolerances of size, form, orientation and/or location and permits additional tolerance as the considered feature departs from its maximum material condkion. Assembly clearance is increased if the actual sizes of the mating features are finished away from their MMC, and if any errors of form or position are less than that called for by any geometrical control.
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GD&T
Its application is restricted to those features whose size is specified by tolerance dimensions incorporating an axis or median plane. It can never be applied to a plane, surface, or line on a surface. The characteristics to which the maximum material condition concept cannot be applied are as follows: flatness, roundness, cylindricity, profile of a line, profile of a surface, run-out.
VL tc~I"'o
dition
A constant boundary generated by the collective effects of a size features specified MMC/LMC 8: the geometric tolerance for that material condition. Or
Constant value outer locus 8: constant value inner locus values are derived
o 0.1 Positional
zone atMMC
030.1 MMC size of feature - 0 0.1 Positional zone at MMC 030 Virtual condition (Inner boundary)
VIRTUAL CONDITION BOUNDARY
Pin in Plate 1
Boundary Hole in Plate 2
Virtual condition for hole >= Virtual condition for pin Fig 7.20: virtuot condition
~
•.
".tf
1
The M M C modifier applied to the position tolerance implies, that a virtual condition is defined for the features and the calculations are done with the M M C limit of size.
GD&T
o
Virtual Condition for external feature
.' . ..••
Q
O~
Virtual Condition: Pin MMC Pin
~
0 26.5
26.9 milleters
+ 0 0.4
'"
o
Pin Virtual Condition
o
The virtual condition of the pin shown here Is thus an envelop of diameter
26.9
Virtual Condition for internal feature
Virtual Condition: Hole MMC Hole '" --...;_----Condition Hole Virtual
'+~-""!
