Engineering Graphics

Chapter 3:DimensioningCopyright 2014 by K. PlantenbergRestricted use only

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Dimensioning: Basic TopicsSummary3-1) Detailed Drawings3-2) Learning to Dimension3-3) Dimension Appearance and Techniques.3-4) Dimensioning and Locating Simple Features.3-5) Dimension Choice.3-6) Dimensioning and Locating Advanced Features.

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DimensioningSummary

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Dimensioning SummaryWhat will we learn in Chapter 3?How to dimension an orthographic projection using proper dimensioning techniques.

Key points Dimensions affect how a part is manufactured.

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Dimensioning3-1) Detailed Drawings

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Detailed DrawingsOrthographic Projection: A shape description of an object (front, top, right side views).

Detailed Drawing: An orthographic projection, complete with all the dimensions and specifications needed to manufacture the object.

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Detailed Drawing Example

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Detailed DrawingsShould we dimension an object using the dimensions that we used to draw the object?No

These are not necessarily the same dimensions required to manufacture it.

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Dimensioning3-2) Learning To Dimension

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Learning to DimensionWhat is our goal when dimensioning a part?Basically, dimensions should be given in a clear and concise manner and should include everything needed to manufacture and inspect the part exactly as intended by the designer.

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Learning to DimensionProper dimensioning techniques require the knowledge of the following three areas. Dimension Appearance/Technique Dimensioning and Locating FeaturesDimension Choice

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Dimensioning3-3) Dimension Appearance and Techniques

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Learning to DimensionDimension Appearance/Technique: Dimensions use special lines, arrows, symbols and text.

The lines used in dimensioning.Types of dimensions.Dimension symbols.Dimension spacing and readability. Dimension placement.

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Lines used in DimensioningDimensioning requires the use of Dimension linesExtension linesLeader lines

All three line types are drawn thin so that they will not be confused with visible lines.

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Dimension LineDimension line: A line terminated by arrowheads, which indicates the direction and extent of a dimension.

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Dimension LineDimension line: Ideally, the dimension line is broken to allow for the insertion of the features size.

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Extension LineExtension line: An extension line is a thin solid line that extends from a point on the drawing to which the dimension refers.

Long extension lines should be avoided.

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Leader LineLeader Line: A straight inclined thin solid line leading to a note or dimension value.

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Leader LineLeaders may be terminated:with an arrow, if it ends on the outline of an object.

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Leader LineLeaders may be terminated:with a dot or nothing if it ends within the outline of an object.

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ArrowheadsArrowheads are drawn between the extension lines if possible. If space is limited, they may be drawn on the outside.

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Types of DimensionsDimensions are given in the form of linear distances, angles, and notes.

Linear distances: They are usually arranged horizontally or vertically, but may also be aligned with a particular feature of the part.

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Types of DimensionsDimensions are given in the form of linear distances, angles, and notes.

Angles: Used to give the angle between two surfaces or features of a part.

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Types of DimensionsDimensions are given in the form of linear distances, angles, and notes.

Notes: Used to dimension diameters, radii, chamfers, threads, and other features that can not be dimensioned by the other two methods.

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Exercise 3.3-1Dimension types

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Exercise 3.3-1Count the different types of dimensions.How many linear horizontal dimensions are there?

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Exercise 3.3-1Count the different types of dimensions.How many linear vertical dimensions are there?

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Exercise 3.3-1Count the different types of dimensions.How many angular dimensions are there?

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Exercise 3.3-1Count the different types of dimensions.How many leader line notes are there?

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LetteringLettering should be legible, easy to read, and uniform throughout the drawing. Upper case letters should be used for all lettering unless a lower case is required. The minimum lettering height is 0.12 in (3 mm).

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Dimensioning SymbolsDimensioning symbols replace text. The goal of using dimensioning symbols is to eliminate the need for language translation.

Why is it important to use symbols.How many products are designed in the United States?How many products are manufactured or assembled in the United States?

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Dimensioning Symbols

TermSymbolDiameternSpherical diameterSnRadiusRSpherical radiusSR

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Spacing and ReadabilityDimensions should be easy to read, and minimize the possibility for conflicting interpretations.

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Spacing and ReadabilityThe spacing between dimension lines should be uniform throughout the drawing.

Do not dimension inside an object or have the dimension line touch the object unless clearness is gained.

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Spacing and ReadabilityDimension text should be horizontal which means that it is read from the bottom of the drawing.

Dimension text should not cross dimension, extension or visible lines.

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Exercise 3.3-2Spacing and Readability 1

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Exercise 3.3-2List the dimensioning mistakes and then dimension the object correctly.

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Correctly Dimensioned

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Spacing and ReadabilityDimension lines should not cross extension lines or other dimension lines.Extension lines can cross other extension lines or visible lines.

Extension lines and centerlines should not connect between views.

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Spacing and ReadabilityLeader lines should be straight, not curved, and point to the center of the arc or circle at an angle between 30o 60o.

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Exercise 3.3-3Spacing and Readability 2

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Exercise 3.3-3List the dimensioning mistakes and then dimension the object correctly.

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Correctly Dimensioned

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Spacing and ReadabilityDimensions should not be duplicated or the same information given in two different ways. If a reference dimension is used, the size value is placed within parentheses (e.g. (10) ).

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Dimension PlacementDimensions should be placed in such a way as to enhance the communication of your design.

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Dimension PlacementDimensions should be grouped whenever possible.

Dimensions should be placed between views, unless clearness is promoted by placing some outside.

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Dimension PlacementDimensions should be attached to the view where the shape is shown best.

Do not dimension hidden lines.

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Exercise 3.3-5Dimension placement

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Exercise 3.3-5List the dimensioning mistakes and then dimension the object correctly.

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Correctly Dimensioned

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Dimensioning3-4) Dimensioning and Locating Simple Features

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Learning to DimensionDimensioning and Locating Features: Different types of features require unique methods of dimensioning.

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Dimensioning FeaturesA circle is dimensioned by its diameter and an arc by its radius using a leader line and a note.

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Exercise 3.4-1Circular and rectangular views

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Exercise 3.4-1Answer questions about the cylindrical and hole features of the part shown.

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Find the hole and cylinder.

Which view is considered the circular view and which is considered the rectangular view?

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Looking at just the top view, can you tell the difference between the hole and the cylinder?

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Why is the diameter of the hole given in the circular view and diameter of the cylinder given in the rectangular view?

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Dimensioning FeaturesHoles are dimensioned by giving their diameter and location in the circular view.

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Dimensioning FeaturesA cylinder is dimensioned by giving its diameter and length in the rectangular view, and is located in the circular view.

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Dimensioning FeaturesThe depth of a blind hole may be specified in a note and is the depth of the full diameter from the surface of the object.

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Dimensioning FeaturesIf a hole goes completely through the feature and it is not clearly shown on the drawing, the abbreviation THRU follows the dimension.

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Dimensioning FeaturesIf a dimension is given to the center of a radius, a small cross is drawn at the center.

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Dimensioning FeaturesWhere the center location of the radius is unimportant, the drawing must clearly show that the arc location is controlled by other dimensioned features such as tangent surfaces.

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Dimensioning FeaturesA complete sphere is dimensioned by its diameter and an incomplete sphere by its radius.

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Dimensioning FeaturesRepetitive features or dimensions may be specified by using the symbol X along with the number of times the feature is repeated. There is no space between the number of times the feature is repeated and the X symbol, however, there is a space between the symbol X and the dimension.

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Exercise 3.4-2Dimensioning and locating features

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Exercise 3.4-2Dimension the object shown.

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Completely dimensioned.

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Dimensioning4-6) Dimension Choice

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Learning to DimensionDimension Choice: Your choice of dimensions will directly influence the method used to manufacture the part. Units and decimal places. Locating feature using datums.

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Units and Decimal PlacesDecimal dimensions should be used for all machining dimensions.

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Units and Decimal PlacesOn drawings where all the dimensions are either in inches or millimeters the following note should be used.

UNLESS OTHERWISE SPECIFIED, ALL DIMENSION ARE IN MILLIMETERS (or INCHES)

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Units and Decimal PlacesIf an inch dimension is given on a millimeter drawing or visa versa, the abbreviations IN or mm should be placed after the dimension value.

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Units and Decimal PlacesMetric dimensions are given in mm and to 0 or 1 decimal place (e.g. 10, 10.2). When the dimension is less than a millimeter, a zero should proceed the decimal point (e.g. 0.5).

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Units and Decimal PlacesEnglish dimensions are given in inches and to 2 decimal places (e.g. 1.25). A zero is not shown before the decimal point for values less than one inch (e.g. .75).

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Locating Features Using DatumsConsider three mutually perpendicular datum planes. These planes are imaginary and theoretically exact.

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Locating Features Using DatumsMost of the time, features on a part are located with respect to a datum feature.

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Locating Features Using Datums

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Locating Features Using DatumsDatum dimensioning is preferred over continuous dimensioning.

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Locating Features Using DatumsDimensions should be given between points or surfaces that have a functional relation to each other Slots, mating hole patterns, etc...

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Application QuestionWhy is the distance between the two holes functionally important?

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Dimension AccuracyThere is no such thing as an "exact" measurement. Every dimension has an implied or stated tolerance associated with it. A tolerance is the amount a dimension is allowed to vary.

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Exercise 3.6-1Dimension Accuracy

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Exercise 3.6-1Which dimensions have implied tolerances and which have stated tolerances?

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Rounding OffThe more accurate the dimension the more expensive it is to manufacture. To cut costs it is necessary to round off fractional dimensions.

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Rounding OffHow do we round off? Lets round off to the second decimal place.1.1252nd decimal place3rd decimal place

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Rounding OffIf the third decimal place number is:less than 5, we truncate after the second decimal place. 1.1231.12

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Rounding OffIf the third decimal place number is:greater than 5, we round up and increase the second decimal place number by 1. 1.1261.13

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Rounding OffIf the third decimal place number is:exactly 5, whether or not we round up depends on if the second decimal place number is odd or even. If it is odd, we round up and if it is even, it is kept the same.1.1651.16

1.1351.14

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Exercise 3.6-2Rounding Off

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Exercise 3.6-2Round off the following fractions to two decimal places according to the rules stated above.(5/16) .3125

(5/32) .1562

(1/8) .125

(3/8) .375

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Exercise 3.6-3Dimension Choice

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Exercise 4.6-3List the dimensioning mistakes and then dimension the object correctly.

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What are the 5 dimensioning mistakes?

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Correctly Dimensioned

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Exercise 4.6-6Dimensioning 1

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Exercise 4.6-7Dimension the following object using proper dimensioning techniques.

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Exercise 4.6-8Dimensioning 3

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Exercise 4.6-8Dimension the following object using proper dimensioning techniques.

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Exercise 4.6-9Dimension the following object using proper dimensioning techniques.

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