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

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

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

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

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

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

Dimensioning Mechanical & Aerospace Engineering Dept., SJSU Ken Youssefi

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

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

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

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

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

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

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

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