04 - MECH1009Y - Dimensioning I - (LN)
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Transcript of 04 - MECH1009Y - Dimensioning I - (LN)
4
Dimensioning
(ka) | MECH1009Y | Engineering Graphics & Computer Aided Drafting — Page | 2
Objectives
Review and understand the standard dimensioning practices
for mechanical drawings.
Apply the standard dimensioning practices.
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1 Dimensioning
Geometrics:
It is the science of specifying and tolerancing the shapes and
locations of features on objects.
Once the shape of a part is defined with a set of orthographic
drawings, the size information is added in the form of dimen-
sions. Dimensioning a drawing also identifies the tolerance (or
accuracy) required for each dimension.
2 Size and Location Dimensions
A well-dimensioned part or structure communicates the size
and location requirements for each feature
Designs are dimensioned based on two criteria:
1. Basic sizes and locations of features.
2. Details for construction and for manufacturing.
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2.1 Units of measure
Most countries outside of the United States use the metric sys-
tem of measure, or the international system of units (SI), which
is based on the meter.
The common metric unit of measure on engineering drawings is
the millimetre, abbreviated as .
Angular dimensions are shown either in decimal degrees or in
degrees, minutes, and seconds. The symbol used for degrees
is °, for minutes ', and for seconds ". Where only minutes and
seconds are specified, the number of minutes or seconds is
preceded by the 0°.
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2.2 Dimensioning terminology
There are a number of terms relevant and important to dimen-
sioning practices (refer to drawing).
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Dimension - Numerical value that defines the size, shape, location, surface texture, or geometric charac-teristic of a feature. Normally, dimension text is
or high, and the space between
lines of text is or
Basic Di-mension
- A numerical value defining the theoretically ex-act size, location, profile, or orientation of a feature relative to a coordinate system.
Basic dimensions have no tolerance.
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Reference Dimension
- A numerical value enclosed in parentheses, provided for information only and not directly used in the fabrication of the part.
It is a calculated size without a tolerance used to show the intended design size of a part.
Arrows - Symbols placed at the ends of:
dimension lines to show the limits of the di-mension
leader lines (leaders)
cutting plane lines Uniform in size and style, regardless of the size of the drawing.
Arrow heads are usually about long and should be one-third as wide as they are long.
Visible gap
- There should be a visible gap of between the feature’s corners and the end of the exten-sion lines.
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Leader line
- Thin, solid line used to indicate the feature with which a dimension, note, or symbol is associ-ated.
A leader line is generally a straight line drawn at an angle that is neither horizontal nor verti-cal.
Leader lines are terminated with an arrow touching the part or detail. On the opposite end of the arrow, the leader line will have a short, horizontal shoulder ( long). Text is ex-tended from this shoulder such that the text height is centered with the shoulder line.
Adjacent leaders on a drawing should be drawn parallel to each other.
Tolerance - The amount that a particular dimension is al-lowed to vary. All dimensions (except reference dimensions) have an associated tolerance.
The tolerance is the difference between the maximum and minimum limits.
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2.3 Size and location dimensions
A size dimension might be:
(a) the overall width of a part or structure
(b) the diameter of a drilled hole
A location dimension might be the length from the edge of an
object to the centre of a feature. The basic criterion is,
“What information is necessary to manufacture or construct the object?”
The drawings below give examples of horizontal, vertical, di-
ameter and radius dimensions.
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2.4 Standard practices
The guiding principle for dimensioning a drawing is clarity.
2.4.1 Placement
2.4.2 Spacing
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2.4.3 Grouping & Staggering
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2.4.4 Extension Lines
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2.4.5 Reading Direction
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2.4.6 View Dimensioning
2.4.7 Repetitive Features
Use the symbol to dimension repetitive features.
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3 Detail Dimensioning
The drawing that follows gives examples of how to correctly
and incorrectly dimension a hole.
If it is not clear whether a hole extends completely through a
part, the word THRU can follow the numerical value.
Symbols are used for spotface, counterbored, and countersunk
holes. These symbols must always precede the diameter sym-
bol.
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3.1 Symbols for Drilling Operations
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3.2 Dimensioning a Blind Hole
The depth symbol is used to indicate the depth of a hole. The
depth symbol precedes the numerical value. When the depth of
a blind hole is specified, the depth is to the full diameter of the
hole and not to the point.
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3.3 Diameter v/s Radius
If a full circle or an arc of more than half a circle is being di-
mensioned, the diameter is specified, preceded by the diameter
symbol. If the arc is less than half a circle, the radius is speci-
fied, preceded by an R.
Concentric circles are dimensioned in the longitudinal view
whenever practical.