2. •Geometric Dimensioning &Tolerancing (GDE&T) Is a symbolic
language for researching, refining, and encoding the function
of each feature of a part.
•It consists of concepts, tools, rules, and processes,which are
described in various industrial standards, and are set forth
in production drawings in abbreviated form.
3. 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
• 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.
4. It is almost impossible (and sometimes uneconomical) to maintain
the strict degree of accuracy due to
inevitable inaccuracy of manufacturing methods.
• Due to interchangeability! mass production.
• 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.
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.
5. 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 generaI 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.
7. •Nominal Size: It is the designation used for general identification
.For example: 7/8 inch shaft, 25 mm shaft etc.
•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 =
8. •Deviation: is the difference between the basic size and the hole or shaft
•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.
•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.
9. 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.
In clearance fit an internal member fits in an external member (as a
shaft in a hole) and always leaves a space or clearance between the
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.ooo7”
10. 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':
12. •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.
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
13. Imagine the control of dimensions of this part shown here.
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
14. 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.
e.g. minimum size hole, or a maximum size shaft
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.
15. A constant boundary generated by the collective effects of a size
features specified MMC & the geometric tolerance for that material
It is used for analyzing mating parts, To find gauge dimensions &
to check extreme conditions.
Virtual Condition for external feature
16. Virtual Condition for internal feature
A datum feature simulator is the manufacturing or inspection
equipment contacting the datum feature of the part.( Surface
The simulated datum can be point,Axis or plane established from
the actual surface of the datum feature locator.
17. Datum Targets
Datum targets are specific portions of a surface, line or point that
may be used for datum referencing. Sometimes due to the
configuration of a part, its function in assembly or its rough or
warped surfaces, it becomes desirable to use only a portion of the
surface as a datum. The portion may be designated as a point or
points, a line or lines, or an area or areas.
18. The datum target symbol consists of a circle cut in to two halves.
The top tier contains the target area size that can be placed either
internally or externally as shown. The lower tier contains a datum
identifying letter with a target number.
The symbol is placed outside the part outline with a radical
(leader) line directed to the target. The use of solid radial line
indicates that the datum target is on the rear surface. The use of
a dashed radial line indicates that
the datum target is on the far (hidden) surface.
19. Form tolerances
A two dimensional tolerance zone defined by two parallel planes
within which the entire surface must lie. Basically all the surface
elements are constrained to lie within two parallel planes,
separated by the tolerance zone
A condition where an element of a surface or an axis is a
One of the surface elements is constrained to lie within two
parallel surface planes separated by the tolerance
20. Form tolerances
All of the points on a cylindrical surface are constrained to lie
within two circles. It is a 2-D surface form control.
It is an extension to circularity that specifies the tolerance
along the cylinder. It is a 3-D form control which controls
roundness (circularity), straightness and taper.
21. Profile tolerances
Profile of a Line
The amount of deviation that is allowed for a surface to float
within a certain dimensional range while maintaining the shape
or form of each line elements that makes up that surface.
Profile of a Surface
It is the amount of deviation that is
allowed for a surface.
24. Location tolerances
A zone within which the center, axis, or center plane of a feature
of size is permitted to vary from its true (theoretically exact)
A cylindrical tolerance zone whose .axis coincides with the
datum axis and within which all cross sectional axes of the
feature being controlled must lie
25. Locational tolerances
Symmetry is that condition where the median points of all
opposed or correspondingly located elements of two or more
feature surfaces are congruent with the axis or center plane of
26. Runout tolerances
A composite tolerance used to control the relationship of one or
more features of a part to a datum axis during a full 360 degree
rotation about the datum axis.
All surface elements across the entire surface of the part must
be within the runout tolerance.