1. Tolerances
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B406

2. Content Tolerances Plus- and –minus tolerance Fits and allowances
Geometric tolerances
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3. Tolerances
Exact dimensions and shapes could not be atttained in the manufacture of materials and products
Tolerances are the permissible variations in the specified form, size or locations of individual surfaces of part from that shown on the drawing
The finished form and size into which material is to be fabricated are defined on a drawing by various geometric shapes and dimensions
Generally most parts require only a few features to be held to high accuracy
In order to assembled parts function properly and to allow for interchangeable manufacturing it is necessary to permit only a certain amount of tolerance on each of the mating parts and certain amount of allowance between them
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4. Terminology Actual size: measured size
Basic size: theoretical size from which the limits for that dimension are derived by the application of the allowance and tolerance
Design size: refers to the size from which the limits of size are derived by the application of tolerances
Limits of size: maximum and minimum sizes permissible for a specific dimension
Nominal size: the designation used for the purpose of general identification
Tolerance: the total permissible variation in the size of a dimension
Bilateral tolerance: variation is permitted in both directions from the specified dimension
Unilateral tolerance: variation is permitted only one direction from the specified dimension
Maximum material size: limit of size of a feature that results in the part containing the amount of material. Thus it is the maximum limit of size for a shaft or an external feature or athe minimum limit of size for a hole or internal feture
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5. Tolerancing
All dimensions required in the manufacture of a product have a tolerance, except those who are identified as reference, maximum, minimum or stock
Tolerances may be expressed in one of the following ways:
As specified limits of tolerances shown directly on the drawing for a specified dimension
As plus and minus tolerancing
Combining the dimension with a tolerance symbol
In a general tolerance note referring to all dimensions on the drawing for which tolerances are not utherwise specified
In the form of a note referring to specific dimensions
Tolerances on dimensions that locate features may be applied directly to the locating dimensions or by the position tolerancing method
Tolerancing applicable to the control of form and runout referred to as geometric tolerancing
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6. Direct tolerancing methods
A tolerance applied directly to a dimension may be expressed in 2 ways
Limit dimensioning
Plus-and-Minus Tolerancing
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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7. Plus-and-Minus Tolerancing
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8. Tolerance accumulation
It is necessary to consider the effect of each tolerance with respect to other tolerances and not to permit a chain of tolerances to buid up a cumulative tolerance
When the position of a surface in any one direction is controlled by more than one tolerance the tolerances are cummulative
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9. Dimension method comaprison
Chain dimensioning: the maximum variation between any 2 features is equal to the sum of tolerances on the intermediate distances. Greatest tolerance accumulation
Datum dimensioning: the maximum variation between any 2 features is equal to the sum of tolerances on the 2 dimensions from the datum to the feature. Reduced tolerance accumulation
Direct Dimensioning: The maximum variation between any 2 features is controlled by the tolerance on the dimension between the features. Least tolerance accumulation
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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10. Dimension chain tolerance calculation
Dimension tolerance calculations are necessary in case of:
Product design: to check if the dimensions are inside the given tolerances
Manufacturing design: check reference dimensions and their tolerances
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11. Dimension chain tolerance calculation
𝐴 1 = 100 −
𝐴 2 = 20 −
𝐴 0𝑚𝑎𝑥 = 𝐴 1max − 𝐴 2min =100.2−19.7=80.5
𝐴 0𝑚𝑖𝑛 = 𝐴 1𝑚𝑖𝑛 − 𝐴 2max =99.9−20.2=79.7
𝐴 0 = 80 −
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12. Fits and allowances
In order that assembled parts function properly and to allow for interchangeable manufacturing it is permit only a certain amount of tolerance on each of the mating parts and a certain amount of allowance between them
Fits: the fit between 2 mating parts is the relationship between them with respect to the amount of clearance or interference present when they are assembled
Allowance: an international difference between the maximum material limits of mating parts
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13. Terminology
Basic size: the size to which limits or deviations are assigned
Deviation: The algebraic difference between a size and the corresponding basic size
Upper deviation: The algebraic difference between the maximum limit of size and the corresponding basic size
Lower deviation: The algebraic difference between the minimum limit of size and the corresponding basic size
Tolerance: The difference between the maximum and minimum size limits on a part
Tolerance zone: A zone representing the tolerance and its position in relation to the basic size
Fundamental deviation: The deviation closest to the basic size
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14. Terminology
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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15. Fundamental deviation - shafts
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16. Fundamental deviation - holes
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17. International tolerance grade
Establishes the magnitude of the tolerance zone
Identified by prefix IT + grade number (e.g. IT6)
The smaller the grade the smaller the tolerance zone
Grade 1-4: very precise grades intented primarily for gage making and similar precision work
Grade 5-16: a progressive series suitable for cutting operations, such as turning, boring, grinding, milling and sawing
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
scan
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18. Calculating tolerance
𝑇=𝑞∙𝑖
𝑖= 𝐷 D
𝑞 𝑛 = 𝑛−1
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19. Example
∅20𝑠7
𝐷= 18∙30 =23.24
𝑖=0,45 3 𝐷 +0,001𝐷=0, ,24 +0,001∙23.24
𝑖=1.3
𝑞= −1 =15.85
𝑇=𝑞𝑖=15.85∙1.3=20.6≈21 𝜇
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20. Example N=20 FD=0,035 LD=FD=0,035 UD=FD+T=0,035 +0,021=0,056
Max size=20+ 𝑈𝐷=20+ 0,056=20,056
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21. Dimensions with tolerances
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22. Dimensions with tolerances
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23. Geometric tolerances
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24. Summary Tolerances Direct tolerancing methods Fits
Tolerance calculations
Next week: Surface finish

25. Thank You for Your attention!