advertisement

Precision Dimensioning Engineering II Dimensioning Rectangular Prisms Dimensioning Cylinders • The diameter of cylinders should be dimensioned in the rectangular view (not the circular view). • Cylinders without a hole passing through them only require one view. Dimensioning Cones Dimensioning Spheres Rectangular Coordinate Dimensioning • Used when computer-controlled production machines are used to manufacture parts. • The designer should consult with personnel in manufacturing to ensure that the origin is located in an appropriate position. • Two types of rectangular coordinate dimensioning: – Coordinate Dimensioning with Dimension Lines – Coordinate Dimensioning Without Dimension Lines Coordinate Dimensioning with Dimension Lines Coordinate Dimensioning without Dimension Lines (Baseline Dimensioning) Tabular Dimensioning • Tabular dimensioning is used when a series of parts consists of the same features or geometry but vary in dimension. • Letters are used in place of dimension values, and the values are then placed in a table. • Most standard parts are dimensioned this way in catalogs, the machinery handbook, and in the back of most textbooks. Tabular Dimensioning Dual Dimensioning – Position Method • Millimeter value is placed above (or below) the inch value or separated by a dash. Dual Dimensioning – Bracket Method • Millimeter value is enclosed in square brackets. A note should be placed on the drawing such as: DIMENSIONS IN [ ] ARE MILLIMETERS. Tolerance Dimensioning • Why do we need tolerance dimensioning? – Interchangeable parts manufacturing – Parts are manufactured at widely separate localities – Effective size control – Modern industry relies on it for subcontracting and replacement parts • Accuracy is Expensive, however Reading Dimensions .1 One tenth of an inch .01 One hundredth of an inch .001 One thousandth of an inch .0001 One ten-thousandth of an inch .00001 One millionth of an inch Specification of Tolerances Limit Dimension Unilateral Bilateral-Equal Bilateral-Unequal Tolerance • Tolerance is the total amount a specific dimension is permitted to vary (difference between the maximum and minimum limits). • The dimension below has a tolerance of .0003. Maximum Material Condition • When specifying tolerance dimensions, the maximum material condition (MMC) means the product or part contains the maximum amount of material specified by the tolerance. • The heaviest part. Maximum Material Condition • For the part shown here the MMC is 1.4996 since that size would yield the most material. Allowance • Allowance is the minimum clearance or maximum interference intended between the maximum material condition (MMC) of mating parts. • The allowance for the system below is: 25.000 - 24.890 = 0.110 More Terminology • Nominal Size - General identification in fractions (ex. 1-1/2 for 1.500). • Basic Size - General identification in decimal (ex. 1.500). • Actual Size - Measured size. • Limits - Maximum and minimum sizes indicated by the tolerance dimensions. Clearance Fit • Space is always left between parts. • What is the allowance in this case? • 1.5000 – 1.4988 = .0012 Interference Fit • Always an interference of material. • What is the allowance in this case? • 1.5000 – 1.5013 = -.0013 or just .0013 Transition Fit • Fit might result in clearance or interference. Line Fit • Clearance or surface contact may result at assembly. Basic Hole System (Hole Basis) • The minimum size hole is taken as the basic size. • Used when standard tools are used to produce holes (reamers & broaches). Basic Shaft System (Shaft Basis) • The maximum shaft size is taken as the basic size. • When several parts having different fits, but one nominal size are required on a single shaft. Specifying a Fit - Inches Class RC 1 Standard Limits Nominal Size Range Inches Limits of Clear. Over Hole H5 Shaft g4 To 0-0.12 0.1 0.45 +0.2 –0 –0.1 –0.25 0.12-0.24 0.15 0.5 +0.2 –0 –0.15 –0.3 0.24-0.40 0.2 0.6 +0.25 –0 –0.2 –0.35 0.40-0.71 0.25 0.75 +0.3 –0 –0.25 –0.45 0.71-1.19 0.3 0.95 +0.4 –0 –0.3 –0.55 1.19-1.97 0.4 1.1 +0.4 –0 –0.4 –0.7 • Determine type of fit and find corresponding table • Determine basic size • Find size range on table • Determine tolerances for Hole and Shaft • Remember values are in thousandths of an inch. Specifying a Fit - Inches Class RC 1 Standard Limits Nominal Size Range Inches Limits of Clear. Over Hole H5 Shaft g4 To 0-0.12 0.1 0.45 +0.2 –0 –0.1 –0.25 0.12-0.24 0.15 0.5 +0.2 –0 –0.15 –0.3 0.24-0.40 0.2 0.6 +0.25 –0 –0.2 –0.35 0.40-0.71 0.25 0.75 +0.3 –0 –0.25 –0.45 0.71-1.19 0.3 0.95 +0.4 –0 –0.3 –0.55 1.19-1.97 0.4 1.1 +0.4 –0 –0.4 –0.7 • RC1 - Close Sliding Fit • Basic size of 1.500 • Upper tolerance on hole is +0.4, which is really +0.0004 • Lower tolerance on hole is -0. • Upper tolerance on shaft is -0.0004 • Lower tolerance on shaft is -0.0007 Specifying a Fit - Inches Class RC 1 Standard Limits Nominal Size Range Inches Limits of Clear. Over Hole H5 Shaft g4 To 0-0.12 0.1 0.45 +0.2 –0 –0.1 –0.25 0.12-0.24 0.15 0.5 +0.2 –0 –0.15 –0.3 0.24-0.40 0.2 0.6 +0.25 –0 –0.2 –0.35 0.40-0.71 0.25 0.75 +0.3 –0 –0.25 –0.45 0.71-1.19 0.3 0.95 +0.4 –0 –0.3 –0.55 1.19-1.97 0.4 1.1 +0.4 –0 –0.4 –0.7 Specifying Fits - Metric Loose Running Basic Size Hole H11 Shaft c11 Fit 1 Max Min 1.060 1.060 0.940 0.880 0.180 0.060 20 Max Min 20.130 20.000 19.890 19.760 0.370 0.110 25 Max Min 25.130 25.000 24.890 24.760 0.370 0.110 • Determine type of fit and find corresponding table • Determine basic size • Find size range on table • Determine tolerances for Hole and Shaft Specifying Fits - Metric • Loose Running Fit • Basic size of 25 Loose Running Basic Size Hole H11 Shaft c11 Fit 1 Max Min 1.060 1.060 0.940 0.880 0.180 0.060 20 Max Min 20.130 20.000 19.890 19.760 0.370 0.110 25 Max Min 25.130 25.000 24.890 24.760 0.370 0.110