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

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