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
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 3
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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Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 6
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
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 7
Placement of Dimensions
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 8
Do’s & Don’t’s of Dimensioning
Never dimension hidden lines
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 9
Do’s & Don’t’s of Dimensioning
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 10
Dimensioning
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 11
Avoid Over-Dimensioning
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 12
Avoid Over-Dimensioning
Θ = tan-1(12.7/7.1) = 60.8o
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 13
Basic Dimensioning Style
Continue dimensioning
Baseline dimensioning
Ordinate dimensioning
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 14
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?
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 15
Size Tolerancing
• Limit Form
1.2511.247
• Bilateral1.250 ± .003
_ .003.001
• Unilateral
1.250 +
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 16
Mating Parts
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 17
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.
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 18
Example – Clearance Fit
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 19
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
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 21
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
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 23
Geometric TolerancingGeometric tolerancing is a system that specifies the form, profile, orientation, and location of part’s features using the ANSI standards.
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 24
Geometric Tolerancing - Examples
Form tolerancing
Flatness Straightness
Profile tolerancingLine
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Geometric Tolerancing - ExamplesOrientation tolerancing
Perpendicularity
Parallelism Angularity
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 26
Geometric Tolerancing - ExamplesLocation tolerancing
Concentricity
Symmetry
Ken Youssefi Mechanical & Aerospace Engineering Dept., SJSU 27
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.