1Mechanical & Aerospace Engineering Dept., SJSUKen Youssefi

Dimensioning&

Tolerancing

Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 2

Dimensioning

• Size dimensions

2D drawings (shop drawings) must show proper dimensions to fabricate parts

• Location and orientation dimensions

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Dimensioning Terminology

1.25

Numerical value that defines the size and locationDimension line

Extension line

Gap

Extension line offset

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Pull down menu Dimension Style

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Accuracy

1.25 indicates ± .01 variation is acceptable (1.24 to 1.26)

Requires twice as much time to machine.

Use fractional dimensions when accuracy is not important, 2 ¼ , 5 ½ ,…..

Architectural dimensioning: use combination of feet and inches, 7’ – 3

1.250 indicates ± .01 variation is acceptable,

or ± .001 variation is acceptable (1.249 to 1.251)

NOT

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Placement of Dimensions

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Do’s & Don’t’s of Dimensioning

Never dimension hidden lines

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Do’s & Don’t’s of Dimensioning

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Dimensioning

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Avoid Over-Dimensioning

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Avoid Over-Dimensioning

Θ = tan-1(12.7/7.1) = 60.8o

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Basic Dimensioning Style

Continue dimensioning

Baseline dimensioning

Ordinate dimensioning

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Tolerancing

• Parts made by different companies have to be interchangeable.

• Mating parts have to fit precisely.

Tolerancing is the technique of dimensioning parts within a desired range of variation.

Why tolerancing?

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Size Tolerancing

• Limit Form

1.2511.247

• Bilateral1.250 ± .003

_ .003.001

• Unilateral

1.250 +

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Mating Parts

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Type of Fits for Mating Parts

• Clearance Fit – Results in a space between the mating parts.

• Interference Fit – Results in an interference between two parts (no space). It requires force to assemble parts (force fit).

• Transition Fit – May results in either interference

or clearance fit.

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Example – Clearance Fit

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ANSI Tables for FitsClearance fits (running and sliding) – RC1 to RC9

0.75 +1.2

+0.0

-.8 -1.6

Class RC 4

Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 20

ANSI Tables for FitsClearance fits (Locational) – LC1 to LC11

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ANSI Tables for FitsInterference fits (Force and Shrink fits) – FN1 to FN5

0.75 +.8 +0.0

+1.9

+1.4

Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 22

Geometric Tolerancing

Geometric Dimensioning and Tolerancing (GDT)

GDT defines the features of a part more efficiently than just the size. It also defines the standards for verifying the specified size and form.

ANSI Y14.5-1994

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Geometric TolerancingGeometric tolerancing is a system that specifies the form, profile, orientation, and location of part’s features using the ANSI standards.

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Geometric Tolerancing - Examples

Form tolerancing

Flatness Straightness

Profile tolerancingLine

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Geometric Tolerancing - ExamplesOrientation tolerancing

Perpendicularity

Parallelism Angularity

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Geometric Tolerancing - ExamplesLocation tolerancing

Concentricity

Symmetry

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Exam 1 – Friday Oct. 6, 9-10 (E-341)

Type of problems

2. 3D views - given the three standard views, draw an oblique or isometric views.

1. 2D views - given an isometric view of an object, draw the three standard views (front, top and right side).

3. Missing line problems - given standard views of an object with some features missing, draw the missing features.

All views are drawn freehand, no straight edge. Bring soft pencil and eraser, paper will be provided.

4. Section views – definition and standards.

5. Dimensioning standards and conventions.

6. 3D visualization.