1. Geometric tolerances
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B406

2. Content
Tolerances
Geometric tolerances
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3. Engineering drawing
It must contain all the information necessary for the part to be correctly manufactured
It must enable an inspector to make a precise determination of whether the part is acceptable
Drawings must convey 3 essential information
The material to be used
Size or dimensions of a part
Shape or geometric characteristics
Must specify permissible variations for each of these aspects in the form of tolerance or limits
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4. Engineering drawing - tolerances
Information on engineering drawing
Material
Size
Shape and geometric characteristics
Usually covered by separate specifications or supplementary documents
The drawings need only make reference of these
Specified by linear or angular dimensions
Tolerances may be applied directly to dimensions
May be specified by general tolerance note
Described by views supplemented to some extent by dimensions
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5. Terminology
Dimension: a geometric characteristic the size of which is specified, such as diameter, length, angle, location or center distance. Indicates the value of a dimension
Tolerance: the total permissible variation in its size
Actual size: measured size of an individual part
Nominal size: the designation of size used for purposes of general identification
Specified size: the size specified on the drawing when the size is associated with a tolerance.
Design size: the size in relation to which the tolerance for that dimension is assigned. This is the size on which the design of the individual feature is based- it should be specified on the drawing
Deviations: the differences between the basic size and the minimum and maximum sizes
Basic (exact) dimensions: the theoretical exact size or location of a feature
Feature: a specific, characteristic portion of a part such as surface, slot, screw thread, profile
Axis: a theoretical straight line about which a part or circular feature revolves or could be considered to revolve
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6. Basic size Theoretical exact size or location of a feature
The basis from which permissible variations are established by tolerances
They are shown without tolerances
Each basic dimension is enclosed in a rectangular frame to indicate that the tolerances in the general tolerance note does not apply
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7. Dimensioning –point to point dimensions
when datums are not specified linear dimensions are intended to apply on a point-to-point datum
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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8. Dimensioning – datum dimensioning
Datum is a theoretical exact feature from which dimensions may be taken
There are often cases when the basic rules for measurements on a point-to-point basis cannot be applied, because the originating points, lines or surfaces are offset in relation to the features located by the dimensions
General rules of selecting datums
If the dimension refers to 2 parallel edges or planes the longer edge or larger surface which has the greatest influence in the measurement
If only one of the extension lines refers to a straight edge or surface the extension of that edge or surface the extension of that edge or surface
If both extension lines refer to offset points rather than to edges or surfaces generally it should be assumed that the datum line is running through one of these points and parallel to the line or surface to which it is dimensionally related
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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9. Geometric tolerancing
By themselves toleranced linear dimensions or limits of size do not give specific control over many variation of form, orientation and position
These variations colud be errors of parallelism or perpendicularity or deviations caused by bending of parts, lobing and eccentricity
Geometric tolerances are added to ensure that parts are not only within their limits of size but are also within specified limits of geometric form, orientation and position
Geometric tolerance is the maximum permissible variation of form, profile, orientation, location and runout from what indicated or specified on a drawing
The tolerance value represents the width or diameter of the tolerance zone within which the point, line or the surface of the feature must lie
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10. Geometric tolerancing
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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11. Geometric tolerancing
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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12. Material Condition Symbols (Modifiers)
MMC (Maximum material condition): when a feature or part is at the limit of size, which results in its containing the maximum amount of material
LMC (Least material condition): refers to that size of feature that results in the part containing the minumum amount of material
RFS (regardles of feature size) If neither of these symbols is shown it means RFS. Geometric tolerance applies to any feture that lies within its size tolerance
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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13. Projected tolerance zone
The application of the projected tolerance zone concept is recommended, when the variation in perpendicularity of threaded or press-fit holes could cause fastenerssuch as screws, studs or pins to interfere with mating parts
An interference can occur when a positional tolerance is applied to the depth of threaded or press-fit holes and the hole axes are inclined within allowance limits
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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14. Feature control frame Given in feature control frame
Feature control frame is divided into compartments containing the geometric tolerance symbol and the geometric tolerance value
When datums are used they are used in a separate compartment added to the frame
Modifiers are shown in the tolerance and datum compartments
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15. Feature control frame - placement
Running a leader from the frame to the feature
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16. Feature control frame - placement
Running a leader from the frame to the dimension line
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17. Feature control frame - placement
Running a leader from the axis of the shaft
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18. Feature control frame - placement
When a tolerance is applied to a specific portion
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19. Form tolerances
Control straightness, flatness, circularity, cylindricity and profile
They are applicable to single (individual) features or elements of single features
Do not require locating dimensions
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20. Straightness
A condition in which the elements of a surface or a median line is a straight line
Specifies a tolerance zone within which the considered element of the surface or median line must lie
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21. Flatness
Flatness of a surface is a condition in which all surface elements are are in one plane
Flatness tolerance is applied to a line representing the surface of a part
Means that all points on the surface should be contained within a tolerance zone consisting of the space between 2 parallel planes that are separated by the specified tolerance
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22. Circularity
Refers to a condition of a circular line or the surface of a circular feature in which all points on the line or on the circumferrance of a plane cross section of the feature are the same distance from a common axis or center point
It is wrapped around a circular cross section
The measurement plane is any plane perpendicular to the axis or center line
Errors of circularity may occur as ovalty, as lobbing or as random irregularities
Circularity tolerance specifies the width between 2 circular rings for a particular cross section within which the circular line or the circumference of the feature should lie
Each circular element of the surface must be within the specified limits of size
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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23. Cylindricity
A condition of a surface in which all points of the surface are the same distance from a common axis
Cylindricity tolerance is a composite control of form that includes circularity, straightness and parallelism of the surface elements of a cylindrical feature
Specifies a tolerance zone bounded by 2 concentric cylinders within which the surface must lie
The tolerance applies simultaneously to both circular and longitudinal elements of the surface
Must be less than size tolerance
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24. Profile tolerancing
Specifies a uniform boundary along the true profile within which the elements of a surface must lie
A line profile
is the outline form or shape of a line or surface
A line profile may be the outline of a part or feature as depicted in a view on a drawing
It may represent the edge of a part or it may refer to line elements of a surface in a single direction such as the outline of cross sections through the part
Elements may be straight lines, arcs or other curved lines
Surface profile
Outlines the form or shape of a complete surface in 3 dimensions
Elements may be flat surfaces, spherical surfaces, cylindrical surfaces or surfaces composed of various line profiles in 2 or mode directions
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25. Datums for geometric tolerancing
Datum is a theoretical point, line, plane or other geometric surface from which dimensions are measured when so specified or to which geometric tolerances are referenced
Datum feature is a feature of a part such as surface that forms the basis for a datum or is used to establish its location
Datums are theoretical, but are considered to exist (in reality they are subjest to manufacturing errors and variations)
Geometric tolerances (straightness, flatness) refer to unrelated lines and surfaces do not require the use of datums
Orientation and locational tolerances refer to related features – they control the relationship of features to one another or to a datrum or datum system
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26. Three-plane system
There are 3 datums and they are are mutually perpendicular
Primary datum
Secondary datum
Tertiary datum
Source: C. Jensen, J. D. Helsel, D. R. Short: Engineering Drawing&Design
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27. Identification of datums
Datum feature symbols are required
To identify the datum surface or feature on the drawing
To identify for reference purposes the datum feature
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28. Identification of datums
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29. Orientation tolerances
Orientation refers to the angular relationship that exists between 2 or more lines, surfaces or other features
Control angularity, parralelism and perpenducularity
A tolerance of form or orientation may be specified when the tolerance of size and location do not provide sufficient control
The general geometric characteristic for orientation is termed angularity. This term may be used to describe angular relationship of any angle between straight lines or surfaces with straight line elements
Orinetation tolerance specifies a zone within which the considered feature, its line elements, its axis or its center plane must be contained
Orientation tolerance indicates a relationship between 2 or more features
The feature to which the controlled feature is related should be designated as a datum
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30. Angularity tolerance
Angularity is the condition of a surface or axis at a specified angle (other than 90o or 0o) from a datum plane or axis
An angularity tolerance for a flat surface specifies a tolerance zone, the width of which is defined by 2 parallel planes at a specified basic angle from a datum plane or axis. The surface of the considered feature must lie within this tolerance zone
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31. Perpendicularity tolerance
Perpendicularity is the condition of a surface at 90o to a datum plane or axis
Specifies a tolerance zone defined by 2 parallel planes perpendicular to a datum plane or axis
The surface of the considered feature must lie within this tolerance zone
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32. Parallelism tolerance
Parallelism is the condition of a surface equidistant at all points from a datum plane
Specifies a tolerance zone defined by 2 planes or lines parallel to a datum plane or axis
The line elements of the surface must lie within this tolerance zone
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33. Positional tolerancing
The location of features is one os the most frequently used applications of dimensions on technical drawings
coordinate tolerances applied to the dimensions
geometric (positional) tolerancing
Especially useful, when it is applied on an MMC (maximum material condition) or pattern of holes or other small features in the mass production of parts
Location of round holes: most commonly used applications
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34. Positional tolerancing
Coordinate tolerancing: refers to tolerances applied directly to the coordinate dimensions or to applicable tolerances specified in a general tolerance note
Positional tolerancing: refers to a tolerance zone within which the center line of the hole or shaft is permitted to vary from its true position
Can be further classified according to the type of modifying symbol associated with the tolerance
Positional tolerancing regardless of feature size (RFS)
Positional tolerancing, maximum material condition basis (MMC)
Positional tolerancing, least material condition basis (LMC)
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35. Positional tolerancing
Defines a zone within which the center, axis or center plane of a feature of size is permitted to vary from true (theoretically exact) position
Ensures achivement of design requirements, offers greater production tolerances and allows the use of functional gages
Indicated by the position symbol, a tolerance and appropriate datum references placed in the feature control frame
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36. Correlative tolerancing
Refers to tolerancing for the control of 2 or more features intended to be correlated in position or attitude
They are coplanarity, positional tolerance at MMC, symmetrical relationships, concentricity, coaxiality and runout
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37. Concentricity
A condition in which 2 or more features such as circles, spheres, cylinders, cones or hexagons have a common center or axis
Example is a round hole through a center of a cylindrical part
Controls the permissible variation in position or eccentricity of the controlled feature in relation to the axis of the datum feature
Specifies a cylindrical tolerance zone having a diameter equal to the specified tolerance whose axis coincides with a datum axis
The center of all cross sections normal to the axis of the controlled feature must lie within this tolerance zone
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38. Symmetry
A condition in which a feature or features are positioned about the center plane of a datum plane
The concept of symmetry and concentricity are the same except that they apply to different shapes
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39. Runout
a composite tolerance used to control the functional relationship of one or more features of a part to a datum axis
The types of feature controlled by runout tolerances include those surfaces constructed around a datum axis and those constructed at right angles to a datum axis
Each feature must be within its runout tolerance when rotated about the datum axis
Types
Circular runout
Total runout
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40. Circular runout Provides control of circular elements of a surface
Does not provide control in any other directions
Applied independently at any usual measuring position as the part is rotated 360o
When applied to surfaces constructed around a datum axis circular runout controls variations such as circularity and coaxiality
When applied to surfaces constructed at right angles to the datum axis controls wobble at all diametral positions
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41. Total runout
Concerns the runout of a complete surface, not merely the runout of each circular element
For measurement purposes the checking indicator must traverse the full length or extent of the surface while the part is revolved around its datum axis. Measurements are made over the whole surface without resetting the indicator
Total runout is the difference between the lowest indicator reading in any position and the highest reading in that or in any other position on the same surface
The tolerance zone is the space between 2 concentric cylinders separated by the specified tolerances and coaxial with the datum axis
More costly to verify than circular runout and thus is not used as often as circular runout
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42. Geometric tolerancing – when to use it
It is not necessary to use geometric tolerances for every feature on a part drawing
In most cases it is to be expected that if each feature meets all dimensional tolerances, form variations will be adequately controlled by the accuracy of the manufacturing process and equipment used
Necessary when
parts are of such size or shape that bending to other distorsion is likely to occur
Errors of shape or form must be held within limits other than those that might ordinarly be expected from the manufacturing process and as a means of meeting functional or interchangeability requirements
Where possible manufacturing variations are not known
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43. Summary Geometric tolerances Form tolerances Orientation tolerances
Positional tolerances
Correlative tolerances
Next week: fits and allowances

44. Thank You for Your attention!