Geometric Dimensioning & Tolerancing
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- Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language used to specify the function and tolerances of part features. It consists of concepts, tools, rules, and processes described in industrial standards. - The document discusses various GD&T concepts including: basis systems, types of fits, datum features, form, profile, orientation, locational, and runout tolerances, and maximum/least material conditions. - Control of part dimensions is achieved through the use of features control frames specifying tolerances for geometric characteristics, zones, locations, material conditions, and datums.
Geometric Dimensioning & Tolerancing
- 2. •Geometric Dimensioning &Tolerancing (GDE&T) Is a symbolic language for researching, refining, and encoding the function of each feature of a part. •It consists of concepts, tools, rules, and processes,which are described in various industrial standards, and are set forth in production drawings in abbreviated form.
- 3. Let us consider the steps involved in creating a mechanical device to solve a given problem. • The first step is conceptual development! product design (the design stage). • Draft !detail the plans for each part (the drawing stage) • Then the individual parts are machined. • Next we layout an assembly plan, finally the device is assembled.
- 4. It is almost impossible (and sometimes uneconomical) to maintain the strict degree of accuracy due to inevitable inaccuracy of manufacturing methods. • Due to interchangeability! mass production. • It is impossible for an operator to make perfect settings. In setting up machine .., i.e. in adjusting the tool and work piece on the machine, some errors are likely to creep in. Usually, the dimensional tolerance is decided at the design stage and a Machinist must take care to apply the required dimensional tolerance and to ensure that discrepancies are not introduced as a result of poor workmanship of measuring techniques. The tolerance is a compromise between accuracy required for proper functioning and the ability to economically produce this accuracy.
- 5. Tolerance is the total amount that a specific dimension is permitted to vary. It is the difference between the maximum and the minimum limits for the dimension. Tolerance may be specified in 3 places: • Directly on (with) the specified dimension • In a generaI note • In title block (tolerance block) For example a dimension given as 1.625 ± .002 means that the manufactured part may be 1.627 or 1.623, or anywhere between these limit dimensions.
- 7. •Nominal Size: It is the designation used for general identification .For example: 7/8 inch shaft, 25 mm shaft etc. •Basic Size or Basic dimension: It is the theoretical size from which limits of size are derived by the application of allowances and tolerances. •Actual Size: is the measured size of the finished part. •Limits: The two extreme permissible sizes between which the actual size lines are called limits. •Max Limit: It is defined as the maximum permissible size for a given basic size. In fig. the max limit for the basic size of Dia30 is = Dia30 + 0.035 = Dia30.035mm. •Min Limit: It is defined as the minimum permissible size for a given basic size. In fig. the min limit for the basic size of Dia30 is = Dia30 - 0.215 = Dia29.785mm. •Tolerance: It is defined as the amount of variation permitted to a basic size. The difference between the max and min limits of a basic size are called tolerance. In fig. the tolerance is = Dia30.035 - Dia29.785 = 0.2Smm.
- 8. •Deviation: is the difference between the basic size and the hole or shaft size. •Upper Deviation: is the difference between the basic size and the permitted maximum size of the part. •Lower Deviation: is the difference between the basic size and the minimum permitted size of the part. •Zero Line: Since the deviations are measured from the basic size, to indicate the deviations graphically, the basic shaft, the min shaft, the actual shaft and the max shaft are aligned at the bottom and a straight line, called zero line is drawn through the top generator of the basic shaft as shown in fig. This is called zero Line.
- 9. Fit is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts. Clearance Fit In clearance fit an internal member fits in an external member (as a shaft in a hole) and always leaves a space or clearance between the parts Interference Fit In interference fit the internal member is larger than the external member such that there is always an actual interference of material. The smallest shaft is 1.2513" and the la rgest hole is 1.2506", so that there is an actual interference of metal amounting to at least o.ooo7”
- 10. Transition Fit Transition fit result in either a clearance or interference condition. In the figure below, the smallest shaft 1.2503" will fit in the largest hole 1.2506", with 0.003" to spare. But the largest shaft, 1.2509" will have to be forced into the smallest hole, 1.2500" with an interference of metal of 0.009':
- 12. •Hole Basis System In this system the different types of fits are obtained by associating shafts of varying limit dimensions with a single hole, whose lower deviation is zero. When the lower deviation of the hole is zero, the minimum limit of the hole is equal to its basic size, which is taken as the base for computing all other limit dimensions. Shaft Basis System In this system the different types of fits are obtained by associating holes of varying limit dimensions with a single shaft, whose upper deviation is zero. When the upper deviation of the shaft is zero, the maximum limit of the shaft is equal to its basic size, which is taken as the base for computing all other limit dimensions.
- 13. Imagine the control of dimensions of this part shown here. Feature control frame has the following: •A geometric characteristic symbol •A tolerance zone descriptor •A tolerance of location •A material condition symbol •Primary, secondary, and tertiary datums
- 14. Maximum Material Condition is the condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size. This means that the tolerance is at the extreme that would result if too little material was cut off, and the maximum material remains. e.g. minimum size hole, or a maximum size shaft Least Material Condition is the condition in which a feature of size contains the least amount of material everywhere within the stated limits of size. This means that the tolerance is at the extreme that would result if too much material was cut off, and the minimum material remains.
- 15. A constant boundary generated by the collective effects of a size features specified MMC & the geometric tolerance for that material condition. It is used for analyzing mating parts, To find gauge dimensions & to check extreme conditions. Virtual Condition for external feature
- 16. Virtual Condition for internal feature Datum simulators A datum feature simulator is the manufacturing or inspection equipment contacting the datum feature of the part.( Surface plate,Gage surface,Mandrel). The simulated datum can be point,Axis or plane established from the actual surface of the datum feature locator.
- 17. Datum Targets Datum targets are specific portions of a surface, line or point that may be used for datum referencing. Sometimes due to the configuration of a part, its function in assembly or its rough or warped surfaces, it becomes desirable to use only a portion of the surface as a datum. The portion may be designated as a point or points, a line or lines, or an area or areas.
- 18. The datum target symbol consists of a circle cut in to two halves. The top tier contains the target area size that can be placed either internally or externally as shown. The lower tier contains a datum identifying letter with a target number. The symbol is placed outside the part outline with a radical (leader) line directed to the target. The use of solid radial line indicates that the datum target is on the rear surface. The use of a dashed radial line indicates that the datum target is on the far (hidden) surface.
- 19. Form tolerances Flatness A two dimensional tolerance zone defined by two parallel planes within which the entire surface must lie. Basically all the surface elements are constrained to lie within two parallel planes, separated by the tolerance zone Straightness A condition where an element of a surface or an axis is a straight line. One of the surface elements is constrained to lie within two parallel surface planes separated by the tolerance
- 20. Form tolerances Circularity All of the points on a cylindrical surface are constrained to lie within two circles. It is a 2-D surface form control. Cylindricity It is an extension to circularity that specifies the tolerance along the cylinder. It is a 3-D form control which controls roundness (circularity), straightness and taper.
- 21. Profile tolerances Profile of a Line The amount of deviation that is allowed for a surface to float within a certain dimensional range while maintaining the shape or form of each line elements that makes up that surface. Profile of a Surface It is the amount of deviation that is allowed for a surface.
- 22. Orientation tolerances Angularity It requires that all points on a specified feature must form an angle with a datum. Perpendicularity It requires that all points on a specified feature must be perpendicular with a datum.
- 23. Orientation tolerances Parallelism The condition of a surface or axis which is equidistant at all points from a datum of reference. All points on a surface are to be parallel to a given datum, within a specified tolerance.
- 24. Location tolerances True Position A zone within which the center, axis, or center plane of a feature of size is permitted to vary from its true (theoretically exact) position. Concentricity A cylindrical tolerance zone whose .axis coincides with the datum axis and within which all cross sectional axes of the feature being controlled must lie
- 25. Locational tolerances Symmetry Symmetry is that condition where the median points of all opposed or correspondingly located elements of two or more feature surfaces are congruent with the axis or center plane of datum feature.
- 26. Runout tolerances Circular Runout A composite tolerance used to control the relationship of one or more features of a part to a datum axis during a full 360 degree rotation about the datum axis. Total Runout All surface elements across the entire surface of the part must be within the runout tolerance.