Introduction to Engineering Drawing Part 6
Transcript of Introduction to Engineering Drawing Part 6
Introduction to Engineering Drawing Part 6- Dimensioning Basics
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Introduction to Engineering Drawing
Part 6- Dimensioning Basics
At the end of this presentation you will be able to
1. Identify the difference between Aligned and Uni-directional
dimensioning.
2. Correctly dimension a countersink and counterbore.
3. Identify an auxiliary dimension.
4. Identify a dimension not to scale.
5. Calculate a ULS and LLS around a nominal measurement value.
6. Use a baseline measurement to dimension a given component.
General
To fully understand an Engineering Drawing and convert what is drawn into
an actual job, we must be able to interpret the dimensions, abbreviations
and symbols given on a drawing.
The aim of dimensioning is to provide the reader of the drawing with the
clearest instructions relating to what is required to accurately produce the
component or overall job to the required specifications.
Dimensions should be placed on a drawing to allow all features to be clearly
seen and provide an uncluttered, aesthetically pleasing presentation.
Although there may be some variations in the conventions for drawing
dimensions from country to country, most will basically comply with the
International Standards Organization (ISO), ISO 128 Technical Drawings-
General Principles of Presentation.
The main reference text in the US is ASME Y14.5 Dimensioning and
Tolerancing and in Australia it is AS 1100.101 Technical Drawing -General
Principles. These documents should be a reference for more detail.
We will be only dealing with dimensioning of orthagonal drawings and only a
basic overview will be provided in this presentation.
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Elements of a dimension
Extension lines which are placed at either end of the dimension
Dimension line with arrows that go between and touch the extension
lines.
Dimension- figure/s that provide the numerical value of the dimension.
If the figure is not a whole number, the numbers after the decimal
point, should be expressed to two (2) decimal places unless more
exact measurements are called for. Example. 200.5 should be written
as 200.50.
General Principles of Dimensioning
Dimension and extension lines should be thin continuous Type B lines.
Dimension lines should be drawn parallel to the direction of
measurement and be placed outside the view wherever possible.
Extension lines should extend approximately 2mm past the dimension
line.
There should be a 1mm gap between the outline of the component and
the extension line.
Arrow heads are drawn heavily about 3mm long to a scale of 1:1
Arrow heads should touch the extension lines at the extremities of the
dimension.
Spacing between stacked dimensions and outline should be equal and
about 12 to 15mm apart.
Figure should be 2.5mm high at scale 1:1.
Where space is too small to place dimension between extension lines,
figures or arrow can be placed outside.
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Dimensioning using the correct technique
Dimensions are easy to read and placed outside the outline.
Dimensioning using incorrect technique
Many dimensions are inside the outline of the drawing
Holes should always be dimensioned using a diameter not a radius
Some dimensions are drawn over the centre lines.
Presentation is cluttered and does not look aesthetically pleasing.
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Methods of Dimensioning
Two (2) methods of dimensioning are in common use
1. Unidirectional
Measurements are drawn parallel to the bottom of the drawing
(Horizontal)
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2. Aligned
Dimensions are drawn parallel to the dimension line and are
readable from the bottom right hand corner of the drawing sheet.
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Techniques for Dimensioning Certain Features.
Curves and Circles
Diameters on a Side View
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Jogged Dimensions
Used on radii where the centre may be off the drawing sheet or
the radius is too big to identify the location of the centre.
Incomplete Dimension
Used where the feature cannot be completely inserted on the
drawing.
Free end terminates with a double arrow pointing in the direction
of the other end of the dimension.
Can also be used on straight lines.
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Countersunk Holes
Counter Bored Holes
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Spotfaces
Overall Dimensions
An overall dimension may be added outside the series of measurements
that add up to provide the overall length.
When an overall measurement is added, one or more of the dimensions
that make up the overall length is omitted to allow for variation during
production.
The omitted dimension is always a non-functional dimension. A
functional dimension is one that is necessary for the operation of the
component.
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Auxiliary Dimensions
Dimension enclosed in brackets
When all dimensions which add up to give an overall dimension and
none are omitted, the overall dimension can be added as an auxiliary
dimension.
Never toleranced and not binding on machine operations.
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Dimension Not to Scale
Dimension underlined with heavy line.
A typical use would be where there is a long component that requires both end details to be shown.
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Dimensioning of Flanges
Hole centres are placed around a Pitch Circle.
Pitch Circle Diameter is abbreviated to P.C.D.
Distance around the P.C.D from one hole centre to the next or horizontal/vertical centre line is called
a Radial Pitch.
Equi-Spaced Holes Unequally Spaced Holes
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Toleranced Dimensions
A tolerance is the amount of permissible variation between the Upper Limit of Size (ULS), the
biggest a dimension can be and the Lower Limit of Size (LLS), the smallest a dimension can be.
For example. If a measurement was given as 100mm (nominal) with a ULS of 101 and an LLs of
99mm, the tolerance would be ULS – LLS or 101 – 99 = 2mm.
This could also be listed as + 1.0mm on a drawing as the dimension is allowed to be 1mm bigger or
1mm smaller than the specified dimension.
Specific tolerances can be applied where necessary to any dimension on a drawing.
The tighter or smaller the tolerance, the more exact the measurement must be.
A general tolerance for linear and angular dimensions may also be supplied in the Title Block or in
the Notes section of the drawing.
Tolerance dimensions can appear in the following forms
100 + 0.50 Meaning the ULS would be 100.5 and the LLS would be 99.5
100.50 Upper limit of size (ULS) on top of the dimension line
99.50 Lower Limit of Size (LLS) below the dimension line
Note:- For further reading/viewing. It is recommended that you search You Tube for videos relating to
Geometric Dimensioning and Tolerancing. This is a very important, but complex area of
tolerancing which can be applied to standardize the manufacture and inspection of components.
It has it’s own set of symbols, methods of dimensioning and tolerancing criteria which takes a bit to
Understand. It is recommended that you gain some knowledge of this subject as you may also
encounter it on Engineering drawings. The definitive standard document for this is ASME Y14.5.
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Toleranced Dimensions using Stated Tolerance Method and Chain Dimensions
Tolerance for each dimension is placed in line with the nominal dimension.
Does not provide an Upper Limit of Size (ULS) or a Lower Limit of Size (LLS).
Chain dimensions can result in excessive accumulation of tolerance measurement. To stop this, one
unimportant, non functional measurement is omitted.
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Toleranced Dimensions using Stacked method and Chain Dimensions
Upper Limit of Size (ULS) appears above the dimension line.
Lower Limit of Size (LLS) appears under the dimension line.
Overall length is important so one unimportant nin-functional dimension is omitted.
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Tolerancing using Stacked Method and Progressive Dimensions-Baseline Measurement
Measurements are all inserted from one point on the drawing. This point is called a Datum
This method is not restricted to tolerance dimensions
To avoid accumulation of the tolerance, one unimportant non-functional dimension is omitted.
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Symbols and Abbreviations
Along with dimensions, different abbreviations and graphical symbols may
also appear on Engineering Drawings. There are a large number of
abbreviations used on Engineering Drawings and many companies will supply
a glossary of abbreviations used with the drawing sheet sets they issue.
Standard abbreviations may also be listed in Standards documents.
Special graphical symbols are used for such things including
Machining
Welding
Electrical/Electronic
Piping
Hydraulics & Pneumatics
The following pages show just a small fraction of the abbreviations and
graphical symbols you may encounter on drawings. This is by no means a
comprehensive list and they will not be explained further in this text.
The following pages are for your reference only. They are all to Australian
Standards. Please refer to your respective country’s Codes of Practice and
Standards documentation for specific information.
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Common Symbols appearing on Engineering Drawing Including
Machining
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Welding Symbols to AS1101.3
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Graphical Symbols Electrical/Electronic Components
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Graphical Symbols for Piping System Components (3D not included)
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Graphical Symbols for Hydraulic/Pneumatic Components