1. Dimensioning Size dimensions Location and orientation dimensions
2D drawings (shop drawings) must show proper dimensions to fabricate parts
Size dimensions
Location and orientation dimensions
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

2. Dimensioning Terminology
Numerical value that defines the size and location
Dimension line
Gap
Extension line offset
1.25
Extension line
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

3. Accuracy 1.25 indicates ± .01 variation is acceptable (1.24 to 1.26)
or ± .001 variation is acceptable (1.249 to 1.251)
NOT
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
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

4. Placement of Dimensions
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

5. Do’s & Don’t’s of Dimensioning
Never dimension hidden lines
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

6. Do’s & Don’t’s of Dimensioning
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

7. Dimensioning Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

8. Avoid Over-Dimensioning
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

9. Avoid Over-Dimensioning
Θ = tan-1(12.7/7.1) = 60.8o
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

10. Basic Dimensioning Style
Continue dimensioning
Baseline dimensioning
Ordinate dimensioning
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

11. Tolerancing
Tolerancing is the technique of dimensioning parts within a desired range of variation.
Why tolerancing?
Parts made by different companies have to be interchangeable.
Mating parts have to fit precisely.
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

12. Bilateral 1.250 ± .003 Unilateral 1.250 + Limit Form 1.251 1.247
Size Tolerancing
Bilateral
1.250 ± .003 _
.003
.001
Unilateral
Limit Form
1.251
1.247
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

13. Mating Parts Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

14. 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.
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi

15. Example – Clearance Fit
Mechanical & Aerospace Engineering Dept., SJSU
Ken Youssefi