CE En 112 Engineering Drawing with CAD Application Chapter 4: Dimensioning and Tolerancing...

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CE En 112 Engineering Drawing with CAD ApplicationChapter 4: Dimensioning and Tolerancing Practices

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Lecture Outline

• Dimensioning (4.1) (page 170 – white number in a black circle)

• Size and location dimensions (4.2) (page 171 – white

number in a black circle)

• Detail dimensioning (4.3) (page 177 – white number in a black

circle)

• Dimensioning techniques (page 170 – white number in a black

circle)

• Tolerancing (4.4-4.5) (page 181– white number in a black circle)

• Next class

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Dimensioning (4.1)

• Geometrics: the science of specifying and tolerancing the shapes and locations of features on objects

• Why do we need dimensions?– Once the shape of

a part is defined with an orthographic drawing, the size information is added in the form of dimensions to physically build the part

An engineering drawing will have graphics, dimensions, and words or notes to fully define

the part

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Dimensioning (con’t)

• Two types of dimensioning:

1) Size and Location Dimensions

2) Detail Dimensions

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Size and Location Dimensions (4.2)

• Units of measure:– Should be in accordance with policy of the user– Generally use Traditional (English) units, commonly

inches, or feet and inches if over 72 inches (general rule)– Most countries outside of

U.S. use metric or international system of units (SI), commonly millimeters (mm)

– Two options available for dual dimensioning (bothtraditional and SI units)

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Size and Location (con’t)

• Units of measure (con’t):– Angular dimension are

shown in either decimal degrees, or in degrees, minutes, and seconds

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• Terminology: be familiar with the terms in section 4.2.2

(Theoretical exact size) (Information only – not used in fabrication of part)

(Numerical value that defines the size, shape, location, surface texture, or geometric characteristics of a feature)

(1 mm or 1/16”)

(Maximum material conditionLeast material condition)

(General rule: Full circle or >180° use Ø, otherwise use R)

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• Dimensioning standards:

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• Ask yourself: “What information is necessary to manufacture or construct the object?”– Answer: Size and location

dimensions (horizontaland vertical)

Dimensions showing the location and orientation

of features

Dimensions showing the size of features

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• Standard practices:

Dimension Text PlacementMinimum Dimension

Line Spacing

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Group DimensionsDo not use object lines

as part of dimension

• How should we place multiple dimensions?

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Stagger Dimension Text

View DimensionsKeep dimensions off of

the view

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Repetitive Features

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Extension Lines

• What are the extension line basic standards?

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Reading Direction

• How should we align the dimension text?

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Detail Dimensioning (4.3)

• The importance of accurate, unambiguous dimensions cannot be overemphasized.

• The primary guideline is clarity: – whenever two guidelines appear to conflict, the method

that most clearly communicates the size of information shall prevail

• The next slides outline some of the dimensioning guidelines from the text – please review all guidelines in the text and be familiar with them

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Detail Dimensioning (con’t)

• Guideline 2: Double dimensioning of a feature is not permitted

This guideline is related to tolerance

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• Guideline 3: Dimensions should be placed in the view that most clearly describes the feature being dimensioned (contour dimensioning)

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• Guideline 4: Maintain a minimum spacing between the object and the dimension and between multiple dimensions

• Guideline 5: A visible gap shall be placed between the ends of extension lines and the feature to which they refer

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• Guideline 7: Avoid placing dimensions within the boundaries of a view, whenever practicable

• Guideline 9: Unless otherwise specified, angles shown in drawings are assumed to be 90 degrees

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• Guideline 10: Avoid dimensioning hidden lines

• Guideline 13: Leader lines for diameters and radii should be radial lines

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Dimensioning Techniques

• The dimensioning process:– Step 1: Front view – locate

ends of angled surfaces using dimension lines – place numeric values

– Step 2: Add dimension lines for overall dimensions: height, width, and depth

– Step 3: Locate the center of the hole using horizontal and vertical dimension lines. Extend center lines.

– Step 4: Draw a radial leader line from the hole to a convenient location, add extension or shoulder, then add dimension

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Tolerancing (4.4-4.5)

• Used to control the variation that exists on all manufactured parts

• Total amount a dimension may vary and is the difference between the maximum and minimum limits

• Example: Tolerance = 4.650 ± .003– Final measurement of machined part can be anywhere

from 4.653 (upper limit) to 4.647 (lower limit)– Lower and upper allowable sizes are referred to as the

limit dimensions (upper and lower), and the tolerance is the difference between the limits (in this case, tolerance = 0.006)

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Tolerancing (con’t)

• Tolerances can be expressed as follows:– Direct limits, or as tolerance values applied directly to a

dimension (ASME Preferred Method)– Geometric tolerances– Notes referring to specific conditions– A general tolerance note in the title block(e.g. All Decimal Dimensions to Be Held To ± .002”)

Geometric Tolerances

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Tolerancing (con’t)

• Important terms – review these and be familiar with the basic concept of tolerances

• Tolerances are used more in mechanical engineering than in civil engineering applications

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Next Class

• Chapter 4. Engineering Geometry (p.191 – p. 231) – It’s the last chapter in the manual drawing textbook

CE En 112 Engineering Drawing with CAD Application Chapter 4: Dimensioning and Tolerancing...

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