Report on Dimensional tolerance

Contents
1 Origin

2 Dimensioning and tolerancing philosophy

3 Symbols

3.1 Datums and datum references

4 Data exchange

5 Documents and standards

5.1 ISO TC 10 Technical product documentation

5.2 ISO/TC 213 Dimensional and geometrical product specifications and verification

5.3 ASME standards

5.4 GD&T standards for data exchange and integration

6 See also

7 References

8 Further reading

9 External links

DEFINATION: Geometric dimensioning and tolerancing (GD&T) is a system for defining

and communicating engineering tolerances. It uses a symbolic language on engineering drawings and
computer-generated three-dimensional solid models that explicitly describe nominal geometry and its
allowable variation. It tells the manufacturing staff and machines what degree of accuracy and
precision is needed on each controlled feature of the part. GD&T is used to define the nominal
(theoretically perfect) geometry of parts and assemblies, to define the allowable variation in form and
possible size of individual features, and to define the allowable variation between features.

Dimensioning specifications define the nominal, as-modeled or as-intended geometry. One example is
a basic dimension.

Tolerancing specifications define the allowable variation for the form and possibly the size of
individual features, and the allowable variation in orientation and location between features. Two
examples are linear dimensions and feature control frames using a datum reference (both shown
above).

There are several standards available worldwide that describe the symbols and define the rules used in
GD&T. One such standard is American Society of Mechanical Engineers (ASME) Y14.5. This article
is based on that standard, but other standards, such as those from the International Organization for
Standardization (ISO), may vary slightly. The Y14.5 standard has the advantage of providing a fairly
complete set of standards for GD&T in one document. The ISO standards, in comparison, typically
only address a single topic at a time. There are separate standards that provide the details for each of
the major symbols and topics below (e.g. position, flatness, profile, etc.).

The dimensioning and tolerancing are mainly used in engineering design,

mechanical drawing & assembly and product inspection. In this course module
a fundamental introduction of the concepts, standards, principles and rules for
dimensioning and tolerancing in engineering drawings are discussed and
various examples of how to symbol and assign dimensions and tolerance are
also illustrated in details.

THERE ARE TWO TYPEES OF DIMENSIONS

Size Dimensions: Size dimensions are placed directly on a feature to identify a

specific size or may be connected to a feature in the form of a note.

Location Dimensions: The relationship of features of an object is defined with

location dimensions.

ANSI (American National Standard Institute) The ANSI standard document for
dimensioning is titled Dimensioning and Tolerancing ANSI Y14.5M. The
following fundamental rules for dimensioning are adopted from ANSI Y14.5M.

i. Each dimension shall have a tolerance, except for those identified

as reference, maximum, minimum, or stock dimensions
ii. Dimensions for size, form, and location of features shall be complete
to the extent that there is full understanding of the characteristics of each
feature.
iii. Each necessary dimension of an end product shall be shown. No more
than those necessary for complete definition shall be given.
iv. Dimensions shall be selected and arranged to suit the function and
mating relationship of a part and shall not be subject to more than one
interpretation.
v. The drawing should define a part without specifying manufacturing
methods.
Dimension Line Spacing: Dimension lines should be placed at a uniform
distance from the object, and all succeeding dimension lines should be equally
spaced.

a) Avoid crossing extension lines over dimension lines

b) Avoid dimensioning over or through the object) Avoid dimensioning to

hidden featured) Avoid unnecessarily long extension lines) Avoid using any
line of the object as an extension line) Avoid dimensions between views)
Group adjacent dimensions) Dimension to views that provide the best shape
description.

Tolerances define the manufacturing limits for dimensions. All dimensions

have tolerances either written directly on the drawing as part of the dimension
or implied by a predefined set of standard tolerances that apply to any
dimension that does not have a stated tolerance:
 Specified Dimensions: A specified dimension, also known as nominal size, is
that part of the dimension from which the limits are calculated. For example: 5
is the specified dimension of 5 ± 0.2

Dimensional Tolerance: Dimensional Tolerance is defined as the permissible or

acceptable variation in the dimensions (height, width, depth, diameter, angles)
of a part. Tolerances become important only when a part is to be assembled or
mated with another part.

Standard Fits

Calculating tolerances between holes and shafts that fit together is so common
in engineering design that a group of standard values and notations has been
established. There are three possible types of fits between a shaft and a hole: :

Clearance Fit: Clearance Fits -A clearance fit is a condition when, due to the
limits of dimensions, there will always be a clearance between mating parts.

Transition Fit:
Interference Fit: Interference Fits -An interference fit is the condition that exist
when, due to the limits of the dimensions, mating parts must be pressed
together.

TYPES OF TOLERENCING

Bilateral Tolerance -A bilateral tolerance is allowed to vary in two directions

from the specified dimension as shown.
Unilateral Tolerance -A unilateral tolerance varies in only one direction from
the specified dimension as shown
DATUM FEATURE SYMBOL-A symbolic means of indicating a datum
feature consists of a capital letter enclosed in a square frame and a leader line
extending from the frame to concerned feature, terminating with a triangle. The
triangle is filled or not fillet.

MAXIMUM MATERIAL CONDITION

The symbol for maximum material condition is a capital “M” in a circle,

Maximum material condition may abbreviated MMC. The term maximum
material condition is used to describe the maximum condition of a feature of
size. For example, a hole is a feature of size that is permitted to vary in size
within the limits of a plus/minus tolerance. For holes or any internal feature,
MMC is the smallest size for that feature. In other words, the max. material
remains in the piece the hole was put in.

LEAST MATERIAL CONDITION

The symbol is a capital “L” in a circle,…. The term least material condition
(LMC), is used to describe the opposite condition as maximum material
condition. Least material condition also applies to external features of size.
TOLERANCE OF PROFILE

Profile

Definition: Profile tolerancing is a method of specifying control of deviation

from the desired profile along the surface of a feature.

Tolerance: Profile tolerance may be specified either as a surface or line profile.

The tolerance provides a uniform zone along a desired true profile of the part.
The surface of controlled feature must lie within this zone.

APPLICATIONS: Current and future potential applications for three-dimensional (3D) fibre
reinforced polymer composites made by the textile processes of weaving, braiding, stitching and
knitting are reviewed. 3D textile composites have a vast range of properties that are superior to
traditional 2D laminates, however to date these properties have not been exploited for many
applications. The scientific, technical and economic issues impeding the more widespread use of 3D
textile composites are identified. Structures that have been made to demonstrate the possible uses of
3D composites are described, and these include applications in aircraft, marine craft, automobiles,
civil infrastructure and medical prosthesis.

Introduction to Surface Finish:

Surface finish, also known as surface texture or surface topography, is the nature of a surface
as defined by the three characteristics of lay, surface roughness, and waviness.

1.It comprises the small, local deviations of a surface from the perfectly flat ideal (a true
plane).
Surface texture is one of the important factors that control friction and transfer layer
formation during sliding. Considerable efforts have been made to study the influence of
surface texture on friction and wear during sliding conditions. Surface textures can be
isotropic or anisotropic. Sometimes, stick-slip friction phenomena can be observed during
sliding, depending on surface texture.

Each manufacturing process (such as the many kinds of machining) produces a surface
texture. The process is usually optimized to ensure that the resulting texture is usable. If
necessary, an additional process will be added to modify the initial texture. The latter process
may be grinding (abrasive cutting), polishing, lapping, abrasive blasting, honing, electrical
discharge machining (EDM), milling, lithography, industrial etching/chemical milling, laser
texturing, or other processes.

Surface Finish Symbols:

The surface texture of a component often affects its performance. Therefore, one has to
specify the surface finish that is required for acceptable performance. Figure 11.12 illustrates
the surface texture features, and how the finish mark is used for communicating the desired
finish.

The different terms used in describing the surface finish can be explained as follows: Surface
Texture is the variation in the surface in the form of roughness, waviness, lay, and flaws
.Roughness refers to the finest of the irregularities in the surface. They are caused by the
process(es) used to smooth the surface.

Figure 11.12 Surface texture characteristics

Roughness Height is the average deviation from the mean plane of the surface (micrometer’s , or
micro inches ).

i. Roughness Width is the width between successive peaks and valleys of the roughness.
 ii. Roughness width cutoff is the largest spacing of irregularities including average roughness
height.

iii. Waviness is the widely spaced variation (millimeter’s, or inches) exceeding the roughness
width cutoff. It is assumed that the roughness is superimposed on a surface that is wavy in
nature.

iv. Waviness height is the crest to trough height variation of the waves.

v. Waviness width is the wave length i.e. distance from crest to crest or from trough to trough.

vi. Lay is the orientation of the surface pattern. This is determined by the manufacturing
processes used.

vii. Flaws are defects, or irregularities, that occur more or less at random over the surface.
These defects can be such things as cracks, blow holes, ridges, scratches etc.

viii. Contact Area is the surface that will make contact with a mating surface.

Figure 11.13 Surface control symbols

i. the Perpendicular symbol shown in figure 11.12 indicating that the lay direction is
perpendicular to the line to which the symbol is applied;

ii. . the = symbol which indicates that the lay direction is parallel to the line;
 iii. the X symbol indicates that the Lay is angular in two directions relative to the line
representing the surface;

iv. the M symbol indicates a multidirectional Lay;

v. the C symbol indicates that the lay is approximately circular relative to the centre of
the surface;

vi. the R symbol indicates that the lay is approximately radial relative to the centre of
the surface;

vii. the P symbol indicates that the Lay is particulate, non-directional, or protuberant.

Welding Symbols:
Figure 11.14A Symbols for different butt weld notches
 Figure 11.14B Symbols indicating fillet welds

Electrical Engineering Symbols:

Figure 11.14C Intermittent fillet welds.