Sacramento City College EDT 300/ENGR 306 · 2013-04-02 · EDT 300/ENGR 306 - Chapter 6 Dimension...

EDT 300/ENGR 306 - Chapter 6 1

Dimensioning and NotationDimensioning and Notation

Chapter 6Chapter 6

Sacramento City CollegeEDT 300/ENGR 306

2 EDT 300/ENGR 306 - Chapter 6

ObjectivesObjectives

Apply measurements, notes and symbols to orthographic views on a technical drawingUse ANSI standards for dimensioning and notesDifferentiate between size dimensions and location dimensions.Dimension a technical drawing using either the SI or US unitsDetermine appropriate sizes for precision fits between matching parts


ObjectivesObjectives

Specify geometric tolerances using symbols and notesDesignate appropriate surface textures


VocabularyVocabularyAligned systemAssembly drawingBasic hole systemsBasic shaft systemsBilateral tolerancesDatumsDetail DrawingDimension lineDimensioningDual-dimensioning system

Extension linesFinish markGeometric dimensioning and tolerancingLeaderOutline viewToleranceUnidirectional systemUnilateral tolerances


Goal of ChapterGoal of Chapter

This chapter discusses how to show the sizeof the objects you draw.

Dimensioning is Showing an object’s size


DimensioningDimensioning

Dimensioning - is the process whereby size and location data

Are shown for a technical drawing.

Notation - is the process whereby needed information not covered by dimensions is placed on technical drawings.



ANSI Y14.5M is the most widely accepted dimensioning standard.

This standard applies in most instances where interchangeability of parts is a major consideration.Automobile parts - good example.


DimensionsDimensions

Three dimensioning formats are used in the US.

1. Decimal-inch dimensioning.Example: 1.375”

2. Fractional inches.Example: 1-3/8”

3. Metric SI units.Example: 1.375 mm



In the US system, measurements are given in:

1. Feet and Inches.40’- 6 1/2”

2. Inches and Fractions.6-1/2”

3. Decimal Divisions of Inches.6.5”Most commonly used.



Decimal divisions are Most commonly used throughout industry.Used exclusively in ANSI Y14.5 M (the drafting standard on dimensioning).


Dimensioning SystemsDimensioning Systems

Metric units are often used for engineeringdrawings.

Civil engineering drawings may be dimensioned in meters.

Architectural drawings may use both metersand millimeters.


Lines and Symbols for DimensioningLines and Symbols for Dimensioning

Dimensioning is standardized.

Professional and trade associations.Engineering societies.Certain industries.

have agreed upon the symbols so people who use the drawings can recognize their meaning.



Organizations that publish dimensioning standards:

1. The American National Standards Institute (ANSI).2. The Society of Automotive Engineers. (SAE)

Automotive.3. The Military Standards. (MIL-Specs)

Military.4. The International Standards Organization (ISO).

Parts for international market.


Orienting DimensionsOrienting Dimensions

Dimension Formats:The aligned system.The unidirectional system.

Refer to page 167, 168.


Orienting DimensionsOrienting Dimensions


Placing DimensionsPlacing Dimensions

The aligned systemDimensions are place in line with the dimension lines. Once was the only system in use.See Figure 6-13.

Avoid placing dimensions in the shaded areas shown in Figure 6-14.



The Unidirectional SystemAll dimensions read from the bottom of the sheet no matter where they appear.Think “horizontal text”.See Figure 6-15. The Unidirectional System, has replaced the Aligned System in most industries.ANSI Y14.5M uses the unidirectional system exclusively.



Notes and dimensions with leaders ALWAYS read from the bottom of the drawing.



Notes and symbolsShow

FinishMaterials, and other information needed to make a part

Are components of dimensioning.


Working DrawingWorking Drawing

A working drawing is used to build the part or building.includes

Shape description, MeasurementsNotes and Symbols.


Working DrawingWorking DrawingDimension precision

Show only as precise as necessary to allow the manufacturer to create the part.Example: Framing dimensions

40’-5.000330 inchesIs meaningless.TOO MUCH precision is shown.

Example: Auto engine partsPiston – diameter 5.2Cylinder – diameter 5.21Not ENOUGH precision is shown.Is meaningless.


Working DrawingWorking Drawing

Dimensions are given in FeetOnes TenthsHundredthsThousandthsTen-thousandths of an inch.


Complete Working DrawingComplete Working Drawing



Lines and symbols show where the dimensions apply.

Measurements are shown on the drawings.The drawing is NOT scaled or measured !

Cannot measure a drawing accurately enough for many interchangeable parts that must fit closely together.

How would you measure 5.03456” on a drawing?



Add size information to the drawing using a system of

LinesSymbolsNumerical values.



Lines and symbols are used on drawings to show where the dimensions apply.

See Figure 6-2.


Figure 6Figure 6--22


Dimension ComponentsDimension Components



1. Dimension LinesIs a thin lineShows where a measurement begins and where it ends.Show the size of an angle.



1. Dimension LinesShow a break in it for dimension numbers.Be drawn about 0.38” from the drawing to allow for the dimension number.Be drawn about 0.25” from other dimensions.



1. Dimension LinesMechanical Style

Start and end with arrowheads.Continuous line with gap in centerDimension value is centered in the gap.

Architectural StyleStart and end with “tic” marks.Continuous line with NO gap in center.Dimension value is centered over the dimension line.



2. Arrowheads and Tic MarksAre placed at the ends of dimension lines.Show where a dimension begins and ends.Are used at the end of a leader to show where a note or dimension applies.Can be open or solid.Should be one size and shape in any one drawing.See Figure 6-4.

Mechanical – arrowheads Architectural - tics



3. Extension Lines Are thin linesExtend the lines or edges of the views or part.Are used to locate center points.Are used to provide space for dimension lines (Figure 6-5).Are not part of the views and should not touch the outline.Start about 0.06” from the part.Extend to about 0.12” beyond the last dimension line.



4. Numerals and NotesCAPITAL LETTERS ARE PREFERRED.Vertical or Inclined letters.About 0.125” high.Must be easy to read.Draw light guidelines first.



5. LeadersA thin lineDrawn from the note or dimension to the place where it applies.Drawn at an angle to the horizontal.

60o is preferred but 45o, 30o or other angles can be used.

Start with an 1/8” long dashEnd with an arrowhead.Use a dot if the leader is pointing to a surfacerather than to an edge (Fig 6-8).


LeadersLeaders

5. LeadersMany leaders drawn close togetherDraw them parallel to each other.Draw a leader to a circle or arc so that the arrowhead points to its center.Use one leader per dimension, (don’t try to economize).


LeadersLeaders

5. LeadersLeader “Don’ts”

Do not draw long leaders.Do not draw horizontally, vertically, or at a small angle.Do not draw leaders parallel to dimensions, extensions or section lines.


The Finish MarkThe Finish Mark


The Finish MarkThe Finish Mark

The finish mark or surface-texture symbol shows that a surface is to be machined

See Figure 6-7.The older symbol is still used but it is being replaced.The standard symbol now in general use is shown in Figure 6-7.The point of the symbol should touch the edge view of the surface.


Scale of A DrawingScale of A Drawing


Scale of A DrawingScale of A Drawing

The scale should be given near the drawing title.

If the drawing has views of more than onepart, and different scales are used,

Show the scale close to the views.


Units and Parts of UnitsUnits and Parts of Units

If parts require a great deal of accuracy use decimal dimensioning.



Decimal dimensioning is commonly used between

finished surfaces.center distances.pieces that must be held in a definite, accurate relationship to each other.



With customary measures, decimals are shown to two places where limits of +/- 0.01”are close enough.

Use decimals to three places where where limits smaller than +/- 0.01” are required.



Decimal dimensioning is used in most industries.

A dual dimensioning system is sometimes used in industries involved in international trade.

This system uses both the decimal inch and the millimeter.It is becoming more common to use the metric system alone.


Theory of DimensioningTheory of Dimensioning

Two kinds of dimensions existSize dimensionsLocation dimensions



Size dimensionsDefine the size of each piece.Show the dimensions of a number of simple shapes

Basic geometric formsPrismsCylindersPyramidsCones, etc.)

See Figure 6-16.



Location dimensionsShow the positions of each piece must also be given.Each piece is considered separately and then in relation to the other pieces.


Size Dimensions Size Dimensions -- PrismsPrisms



The Prism is the first basic shape we will considerFor a rectangular prism, the width, height, and depth are needed.



For any flat piece, give The thickness in the edge view.All other dimensions in the outline view.The Outline View is the one that shows the shape of the flat surface or surfaces.The front views in Fig 6-19 are outline views.



Methods for dimensioning other prisms are shown in Figure 6-20.


Size Dimensions Size Dimensions –– PrismsPrisms


Size Dimensions Size Dimensions -- CylindersCylinders

The cylinder requires two dimensionsthe diameter.the length.

For cylindrical pieces, give the diameter and the length on the same view.



The symbol Ø is always placed with the diameter.Example: Ø 1.625”

The abbreviation DIA may be found on olderdrawings instead of the symbol Ø.Notes are used to give hole sizes, and are placed in the outline view.



When parts of cylinders occur, such as filletsand rounds they are dimensioned in the view in which the curves show.



The other shapes:ConeFrustumSquare pyramid Sphere

can be dimensioned in one view.


Location DimensionsLocation Dimensions


Location DimensionsLocation Dimensions

Location dimensionsShow the relative positions of the basic shapes.Locate holes, surfaces and other features.

Location dimensions are needed in three mutually perpendicular directions.

Up / down.Crossways.Forward and backward.


Steps In DimensioningSteps In Dimensioning

Two basic steps1. Apply the size dimension

This gives the overall size of the object and the various features which make up the object.

2. Apply the location dimensionLocation dimensions are dimensions which locate various features of an object from some specified datum or surface.


Location Dimensions Location Dimensions -- PrismsPrisms

Prisms are located bySurfacesSurfaces and axesAxesThree dimensions are needed


Location Dimensions Location Dimensions -- CylindersCylinders

Cylinders are located byAxes and bases.Three dimensions are needed

Refer to Fig 6-30.


Datum Dimensioning Datum Dimensioning


Datum DimensioningDatum Dimensioning

Datum are points, lines and surfaces that are assumed to be exact

Datum are used to compute other dimensions.

Location dimensions are given from datum.When positions of a part are given, they are all located from the same datum.

A datum must be clear.


Rules for DimensioningRules for Dimensioning



1.Space dimension lines about 0.25” apart and 0.38” from the view outline.

2. Aligned system - dimensions read in line with the dimension lineRead aligned with part.Refer to Fig 6-12.



3. Unidirectional system - dimensions read in line with the dimension lineRead left to right from bottom of sheet. Refer to Fig 6-14.

4.Machine drawingsUse decimal inches or millimeters.



5.When all dimensions are in inches or millimeters, omit the symbol.

Add a note:“ALL DIMENSIONS ARE IN INCHES”.

6.Position all dimensions clearly.

7.Do not repeat the same dimension on different views.



8.Do not give dimensions that are not needed.

9.Place overall dimensions outside the smaller dimensions

See Figure 6-31.See Figure 6-32.



10. Avoid placing dimensions in the shaded areas when using the aligned system (Fig 6- 35).

11. Give dimensions from centerlines, finished surfaces or datums where needed.

12. Do not use a centerline or line of a view as a dimension line.



13. Do not draw a dimension line that extends from a line of a view.

14. Avoid crossing a dimension line with another line.



15. Give the diameter of a circle, not the radius. Use the symbol Ø before the dimension.

16. Place the abbreviation “R” before the dimension when giving the radius of an arc.

17. In general, dimensions should be placed outside the view outlines.



18. Do not draw extension lines that cross each other or cross dimension lines if possible.

19. Avoid dimensioning to a hidden line if possible.


Dimensioning a Detail DrawingDimensioning a Detail Drawing



A detail drawing is a drawing for a single part that includes all the information necessary to make that part.



Dimension a detail drawing in the following order:1.Complete all views first before adding any

dimensions or notes2. Think about the part first, and its

characteristic views3. Think about the size dimensions and the

related location dimensions. Draw the dimension lines, leaders and arrowheads

4. After considering any changes, add any dimensions and notes that may be needed.


Dimensioning an Assembly DrawingDimensioning an Assembly Drawing



An assembly drawing shows parts of a machine in their relative positions.

If an assembly drawing needs a complete description of size, the rules and methods of dimensioning apply.



1. If the drawing is to show arrangement of parts or appearance only, leave dimensions off.

2. If a drawing is needed to show the space a product requires, give overall dimensions.

3. If parts can be located in relation to each other without giving detail dimensions, center-to-center dimensions are usually given.


AbbreviationsAbbreviations

Examples of abbreviations from ANSI are listed in Appendix C of the text.


Limit Dimensioning Limit Dimensioning


Limit DimensioningLimit Dimensioning

Parts are generally manufactured with a tolerance. A tolerance is an allowable variation in the part dimension

The tolerance may be stated in a note “DIMENSION TOLERANCE +/- 0.01 UNLESS OTHERWISE SPECIFIED”

Limit dimensions give the max and min dimensions allowed

The max limit is placed over the min limitFig 6-47B and C


Limit DimensioningLimit Dimensioning

Consecutive dimensions are listed one after the otherProgressive dimensions are all given from a single surface (Figure 6-47 C). This is called baseline dimensioning


Expressing SizeExpressing Size

Size is a designation of magnitude.When a value is given to a dimension it is called the size of that dimension


Expressing SizeExpressing Size

Nominal size - used for general identification.Basic size - is the size to which allowances and tolerances are added to get the limits of size.Design size - the size to which tolerances are added to get the to get the limits of size.Actual size - is the measured size.Limits of size - is the max and min sizes.


Expressing PositionExpressing Position

Locating round holesx-y coordinatespolar coordinates“equally spaced” holesDatum line dimensionsPoint-to-point or chain dimensioningDatum dimensioning


ToleranceTolerance

A tolerance is the total amount a given dimensions may vary.

Express in the same form as its dimension.Express in the same number of decimal places.

If the dimension is written as fraction, the tolerance should be also written as a common fraction.


ToleranceTolerance

In a chain of dimensions with tolerances, the last dimension may have a tolerance equal to the sum of the tolerances between it and the first dimension.

Tolerances accumulate (they are added together).


ToleranceTolerance

In a datum dimensioning, tolerances do NOTadd together.


TolerancesTolerances

Unilateral tolerancesAllow variations in only one direction from a design size.

Bilateral tolerancesAllow variations in both directions from a design size.


TolerancesTolerances

The Limit SystemOnly the largest and smallest allowed dimensions are shown .The tolerance is the difference between the limits.

Sacramento City College EDT 300/ENGR 306 · 2013-04-02 · EDT 300/ENGR 306 - Chapter 6 Dimension...

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