Computer Numerical Control
Lecture 11
Standards for CAD
Dr .Ibrahim Al-Adwan
Standards for dimensioning
v
According to the American National Standards Institute ANSI standards, the
following are the basic rules that should be observed in dimensioning
any drawing:
1.
Show enough dimensions so that the intended sizes and shapes can be
determined without calculating or assuming any distances.
2.
State each dimension clearly, so that it can be interpreted in only one
way.
3.
Show the dimensions between points, lines, or surfaces that have a
necessary and specific relation to each other or that control the
location of other components or mating parts.
4.
Select and arrange dimensions to avoid accumulations of tolerances
that may permit various interpretations and cause unsatisfactory
mating of parts and failure in use.
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Standards for dimensioning
5.
6.
7.
Show each dimension only once.
Where possible, dimension each feature in the view where
it appears in profile, and where its true shape appears.
Wherever possible, specify dimensions to make use of
readily available materials, parts, tools, and gauges. Savings
are often possible when drawings specify (a) commonly
used materials in stock sizes, (b) parts generally recognized
as commercially standard, (c) sizes that can be produced
with standard tools and inspected with standard gauges,
and (d) tolerances from accepted published standards.
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Conventional tolerancing
Since it is impossible to produce the exact dimension specified, a
tolerance is also used to show the acceptable variation in a dimension.
The higher the quality a product has, the smaller the tolerance value
specified. Tighter tolerances are translated into more careful
production procedures and more rigorous inspection. There are two
types of tolerances: bilateral tolerance and unilateral tolerance (as
shown in the following Figure). Unilateral tolerances, such as
,
specify dimensional variation from the basic size (i.e., decrease) in one
direction in relation to the basic size; for example,
1.00 00..00
05
The basic location where most dimension lines originate is the
reference location (datum). For machining, the reference location
provides the base from which all other measurements are taken. By
stating tolerance from a reference location, cumulative errors can be
eliminated.
1.00 00..00
05  0.95  1.00
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Conventional tolerancing
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Conventional tolerancing
Surface control symbols
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Conventional tolerancing
Lay symbols
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Dimensioning
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TOLERANCE
Dimensional tolerance - conventional
Geometric tolerance – modern
REASON OF HAVING TOLERANCE
• No manufacturing process is perfect.
• Nominal dimension (the "d" value) can not be achieved exactly.
• Without tolerance we lose the control and as a consequence cause
functional or assembly failure.
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TOLERANCE
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TOLERANCE STACKING
1. Check that the tolerance & dimension specifications are reasonable for
assembly.
2. Check there is no over or under specification.
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TOLERANCE STACKING
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TOLERANCE STACKING
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TOLERANCE GRAPH
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