CAD- Geometric dimensioning and tolerancing

Geometric dimensioning and tolerancing

1. Datum Feature Symbol A datum feature is the tangible surface or feature of size (comprised of multiple surfaces or revolved surfaces) that is indicated by the datum feature symbol. In otherwords datum feature symbol is used to identify physical features of a part as datum features. The datum feature symbol consists of a capital letter enclosed in a square box. The square box is connected to a leader directed to the datum feature ending in a triangle. The triangle may be solid or open. The datum feature identifying letters may be any letter of the alphabet.

2. One of the 12 geometric characteristic symbols always appears in the first compartment of the feature control frame. The second compartment is the tolerance section. In this compartment, there is , of course, the tolerance followed by any appropriate modifiers. A diameter symbol precedes the tolerance if the tolerance zone is cylindrical. If the tolerance zone is not cylindrical, nothing precedes the tolerance. The final section of the feature control frame is reserved for datum features and any appropriate material boundary modifiers. If the datum feature is a feature of size , a material boundary applies; if no material boundary modifier is specified for a datum feature of size , regardless of material boundary automatically applies. Datum features are arranged in order of precedence or importance. The first letter to appear in the datum section of the feature control frame , the primary datum feature , is the most important datum feature. The second letter, the secondary datum feature , is the next most important datum feature, and the third letter, the tertiary datum feature, is the least important. Datum features need not be specified in alphabetical order. Feature Control Frame

3. 1. The geometric characteristic symbol 2. The controlled element of the feature (determined by the geometry of the feature shown on the drawing, not in the feature control frame ) 3. tolerance zone shape 5. tolerance zone size 6. Any tolerance modifier(s) 6. datum feature(s ) 1. The flatness tolerance requires that 2. All elements on the controlled surface 3. Must lie within a tolerance zone between two parallel planes 4. 004 apart 1. The perpendicularity tolerance requires that 2. The axis of the controlled feature 3. Must lie within a cylindrical tolerance zone 4. 010 in diameter 5. At maximum material condition ( MMC) 6. Oriented with a basic 90° angle to datum feature A

4. 1. Position tolerance 2. Axis of the controlled feature 3. Must lie within a cylindrical tolerance zone 4. 020 in diameter 5. At MMC 6. Oriented and located with basic dimensions to a datum reference frame established by datum feature A, datum feature B at its maximum material boundary, and datum feature C The feature control frame can be read as follows. The position control requires that the axis of the hole must lie within a cylindrical tolerance zone of 0.014 in diameter at MMC (circle M ). The tolerance zone is oriented and located with basic dimensions perpendicular to datum feature A , located up from datum feature B, and over from datum feature C. If the hole diameter is produced at its MMC size,  3.000 , the diameter of the tolerance zone is 0.014 . If the hole diameter is produced at  3.020 , the diameter of the tolerance zone is . 034.

5. Feature control frames may be attached to features with extension lines, dimension lines, or leaders . Feature control frames attached to features. Feature control frames with tolerance refinements.

6. Other Symbols Used with Geometric Tolerancing

7. The all around and between symbols are used with the profile control.

8. Geometric Characteristic Symbols Geometric characteristic symbols are the essence GD&T. 12 geometric characteristic symbols are divided into five categories: 1. Form 2. Profile 3. Orientation 4. Location 5. Runout Form controls are not related to other features. Features toleranced with orientation, location, or runout controls are related to datum features. Profile controls may relate features to a datum feature(s) or they may apply without datum features as required. Datums Datums are theoretically exact points, axes, lines, and planes or a combination thereof that are derived from datum features.

9. Form controls limit the flatness, straightness, circularity , or cylindricity of part surfaces. Form is a characteristic that limits the shape error of a part surface (or in some cases, an axis or centerplane) relative to i ts perfect counterpart. Form controls are used to define the shape of a feature in relation to itself; therefore, they never use a datum reference. The form characteristic of a planar surface is flatness. Flatness defines how much a surface can vary from its perfect plane . Cylindrical surfaces can have three different form characteristics: straightness , circularity, and cylindricity. Straightness defines how much a line element can vary from a straight line. Circularity defines how much circular elements can vary from a perfect circle. cylindricity defines how much a surface can vary from a perfect cylinder .

10. Straightness Straightness tolerance specifies a tolerance zone within which the line element of a feature must lie. Since straightness is applied to an individual feature, this tolerance does not need to be related to a datum. The figure below shows two views of a simple cylindrical pin. In the side view, the cylindrical surface has a straightness tolerance applied to it. Because the surface is cylindrical, this tolerance applies to both the top and bottom line elements. The boxed symbols can be read “the top and bottom of the cylinder must lie between two parallel planes 0.02 apart”.

11. Flatness Flatness references how flat a surface is regardless of any other datums or features . Flatness tolerance is always less than the dimensional tolerance associated with it . In GD&T flatness tolerance defines a zone between two parallel planes within which a surface must lie. Since flatness is applied to an individual surface, this tolerance does not need to be related to a datum.

12. Circularity Circularity is used to control the roundness of circular parts or features. Some examples of circular features include cylinders, spheres, and cones. Circularity helps ensure these parts move smoothly and wear evenly. Circularity applies for conical shapes as well. Because circularity is applied to an individual surface, this tolerance does not need to be related to a datum. The simple pin shown in the left figure has a circularity tolerance applied to it. The boxed symbols can be read “each circular cross section of this cylinder must lie between two concentric circles spaced 0.25 apart”. Fig. shows a sample cross section A-A and its possible roundness relative to the tolerance zone. Circularity applies to the entire length of the cylinder, and would therefore require multiple checks to verify the pin meets the specified tolerance over its entire length .

13. Cylindricity The Cylindricity symbol is used to describe how close an object conforms to a true cylinder. Cylindricity is a 3-dimensional tolerance that controls the overall form of a cylindrical feature to ensure that it is round enough and straight enough along its axis. Cylindricity is independent of any datum feature. Two concentric cylinders that run the entire length of the feature – one inner and one outer, in which all the points on the entire surface of the cylindrical feature must fall into. The entire length of the called out feature would be controlled.

14. Profile af a Line Profile of a line establishes a two-dimensional tolerance zone that controls individual line elements of a feature or surface. Profile of a line is usually applied to parts with varying cross-sections, or to specific cross sections critical to a part’s function. Examples of parts where profile of a line could be applied include aircraft wings and housings used to seal out dust or water . In the fig, profile of a line is applied to the top surface of a part. The boxed symbols can be read "each line element of this surface at any cross section must lie between two boundaries spaced 0.2 apart relative to datums A and B". The tolerance remains the same for the line element even though the cross-section shape is different. Two datums are needed to define the orientation of the cross section plane. Because profile of a line can require multiple measurements at any number of cross sections, it is customary to specify the number of cross sections to be checked on the drawing.

15. Profile of a surface Profile of a surface defines a uniform boundary around a surface within which the elements of the surface must lie. This applies in all directions regardless of the drawing view where the tolerance is specified. Profile of a surface is a complex tolerance that simultaneously controls a feature‘s form, size, orientation, and sometimes location. Therefore it is usually used on parts with complex outer shape and a constant cross-section like extrusions. An example of profile tolerance is shown below. The top figure shows the profile tolerance applied to a curved surface. The boxed symbols can be read “with respect to datum A, this surface must lie between two surface boundaries 0.8 apart and spaced equally about the true (or ideal) surface profile”. The bottom figure shows an example of a part that meets this tolerance.

16. Perpendicularity In production drawings, perpendicularity allows the designer to specify the degree to which the orientation of a right-angled part feature may vary. The perpendicularity symbol is often used on a drawing to ensure that mating features can be assembled. In most cases, the perpendicularity symbol is applied to a feature-of-size (FOS) with its base dimension. The tolerance zone is created perpendicular to the specified datum, and a part feature, axis, or center plane must lie within it . In the Fig. perpendicularity symbol and tolerance are used to control the center axis of a male feature.  The boxed symbols can be read “This axis must lie between two planes perpendicular to the surface at A and spaced 0.1 apart”. The tolerance zone created is indicated by the parallel lines in the Fig, and  applies for the entire length of the axis extending from datum plane A. Note that this example only controls the orientation in the plane drawn.

17. Parallelism parallelism allows the designer to specify the degree to which a feature‘s orientation may vary with respect to its referenced datum by creating a tolerance zone parallel to that datum. The relevant feature, axis, or center plane must then lie within this zone. The parallelism symbol is generally used to ensure features are aligned for proper function . In the Fig, the boxed parallelism symbol and tolerance are used to control the center axis of a hole. The boxed symbols can be read “This axis must lie between two planes parallel to the axis A and spaced 0.3 apart”. Note that if the boxed tolerance includes a diameter symbol, indicating a cylindrical tolerance zone, the tolerance would apply to all views. The tolerance zone created is indicated by the parallel lines and applies for the entire length of the axis.

18. Angularity Angularity allows the designer to specify the degree to which the orientation of an angled part feature may vary. The angularity symbol is often used to ensure that the part can properly mate with another. The degree of permissible variation is not specified as a tolerance on the angle. Rather an indirect method is used where one specifies a tolerance zone at a specified angle from a datum, within which a part feature, axis, or center plane must lie . In the Fig, boxed angularity symbol, tolerance and datum are used to control the center axis of an angled hole. The boxed symbols can be read “This axis must lie within two planes 0.5 apart , the planes inclined 60° to surface A”. T he tolerance zone created is indicated by the parallel lines. This form of angularity tolerance applies only in the drawing view in which the tolerance is specified, and requires the permissible variation to be defined for other views. If a diameter symbol were placed in front of the boxed 0.5, this would create a cylindrical tolerance zone which would then apply to all drawing views .

19. Position Position is a versatile tolerance that can be used to control location, coaxiality, orientation or axis offset of a part feature or axis. Position tolerance is generally applied to features important to assembly like holes or slots, and it is often included when performing a tolerance stack . Fig shows the position symbol applied to two holes. The boxed symbols can be read "relative to datums A, B, and C, the position of these hole centers shall lie within a cylindrical tolerance zone of diameter 0.3". In side view of the part, the orientation of the hole is controlled in the same manner as perpendicularity or parallelism. In the top view, the position of the hole is controlled by its axis location. The actual position of the hole’s axis (shown in red) must lie within a tolerance zone related to the true axis position (shown in black) specified on the drawing. Note that in this example, both location and orientation are controlled simultaneously in 3D.

20. Circular Runout Circular Runout is used to control the location of a circular part feature relative to its axis. This is different than circularity, which controls overall roundness. Runout is usually applied to parts with circular cross sections that must be assembled like drill bits, segmented shafts, or machine tool components. Runout helps to limit the axis offset of two parts to ensure they can spin and wear evenly . In the figure the runout symbol is applied to the angled surface. The boxed symbols can be read “each circular element of this surface must have full indicator movement (FIM) of less than 0.1 relative to datum A”. The bottom figure shows a sample measurement taken at one cross section, but multiple measurements are required to verify runout. Note that the indicator is applied perpendicular to the measured surface, and that this tolerance controls only individual circular elements and not the whole surface simultaneously (see total runout).

21. Total Runout Total runout is a complex tolerance that controls a feature‘s straightness, profile, angularity, and other geometric variation. Total runout is different than runout because it applies to an entire surface simultaneously instead of individual circular elements . The fig. shows a total runout tolerance applied to a horizontal surface. The boxed symbols can be read “this entire surface must have full indicator movement (FIM) of less than 0.1 relative to datum A”. The lower figure shows how total runout is verified. Note that the indicator is applied all along, and perpendicular to the surface to which the tolerance is applied.

22. Upper limit The maximum permissible size or largest acceptable size of the component is called the upper limit of size. Lower limit The minimum permissible size or smallest acceptable size of the component is called the lower limit of size . Nominal or basic size The nominal size is described as the size of the component about which the limits are fixed. It is also termed as basic size or size of zero deviation. Example: Shaft has to be manufactured to the diameter of 20 ± 0.02 mm The basic size of the shaft is 20 mm and it is acceptable if its diameter lies between the limits of size. The upper limit of the shaft diameter is given as: dmax = 20 mm + 0.02 mm =20.02 mm The lower limit of the shaft diameter is given as: Dmin = 20 mm−0.02 mm = 19.98 mm Therefore, the upper limit and lower limit of the shaft diameter are 20.02 mm and 19.98 mm respectively.

23. Unilateral tolerance Unilateral tolerance is described as the tolerance that allows variation in only one direction from the basic or nominal size. In unilateral tolerance, the size of the component is always either small or large from its basic size. In unilateral tolerance, both upper and lower limits are either above or below the basic size of the component. Example: The diagrammatical representation of the unilateral tolerance is shown as:

24. Bilateral tolerance Bilateral tolerance is described as the tolerance that allows variation in both directions from the basic or nominal size. In unilateral tolerance, the size of the component may be small or large from its basic size. In bilateral tolerance, the maximum limit is above the basic size, and the minimum limit is below the basic size. The combined plus or minus symbol is used with a single value to specify an equal variation in both directions. Example:

25. Clearance fit The clearance fit results when the shaft size is always less than the hole size for all possible combinations lying in the tolerance range. In a clearance fit, the relative motion between the shaft and hole is always possible. In this, the lower limit size of the hole is greater or equal to the upper limit size of the shaft. The min clearance is obtained at the max shaft size and the min hole size in the clearance fit. In contrast, the max clearance is obtained at the min shaft size and the max hole size . Fits FIT is the general term to signify the range of tightness or looseness among two mating parts resulting from the application of a specific combination of allowances and tolerances in the design Following are the classification of fits based on the combination between shaft and hole:  Clearance fit  Transition fit Interference fit

26. Transition fit The practical transition fits occur when the tolerance is so that the largest hole is greater than the smallest shaft and the largest shaft is greater than the smallest hole. In transition fit, the relative motion between shaft and hole is possible when clearance exists but not possible when interference exists .

27. Interference fit The interference fit results in the min shaft size being larger than the max hole size for all possible combinations in the given range. In interference fit, the relative motion between the shaft and hole is impossible. The max interference occurs at the min shaft size and max hole size in an interference fit. In contrast, the max interference occurs at the max shaft size and min hole size. The diagrammatical representation of the interference fit is shown as:

28. Allowance allowance is described as the difference between the max material limit of the mating parts or holes and the shaft. The min clearance is called a positive allowance , and max interference is called a negative allowance . The max material limit is defined as the max shaft size and min hole size. Hence, the difference between them is the max interference or min clearance . Max interference = Max shaft size − Min hole size Min clearance = Min hole size − Max shaft size

29. Tolerance Allowance Upper limit – Lower limit (of shaft/hole) MML of hole – MML of shaft Prescribed difference between two mating parts Permissible variation of the size/dimension Two mating parts are involved. Individual component is involved. Intentional difference between the sizes of the shaft and hole Difference between upper and lower limits of dimensions It is provided on the mating parts to get the desired functional requirement Influenced by the method of manufacture and provided because exact duplication is not possible May be positive or negative It is an absolute value Tolerance vs Allowance

30. Fundamental deviation The fundamental deviation is described as the distance from the basic size to the tolerance zone.

31. Hole Basis System Shaft basis system Basic hole with lower deviation zero is chosen Basic shaft with upper deviation zero is chosen Size of shaft is varied to get the desired fit Size of hole is varied to get the desired fit Limits on the hole are kept constant Limits on the shaft are kept constant Preferred for mass production for economic reasons Not suitable for mass production Easy to get the desired fit by varying shaft size Difficult to get the desired fit by varying hole size Gauging external features is easy Gauging internal features is difficult Hole Basis System vs Shaft basis system

32. Basic Shaft It is the shaft, whose upper deviation is zero or whose max limit of size is equal to basic size. Basic hole It is the hole, whose lower deviation is zero or whose min limit of size is equal to basic size. Basic Shaft Basic hole

33. Deviation It is the amount by which the size of a part deviates from its basic size. Hence, it is the algebraic difference between actual size and basic size. The upper deviation is the algebraic difference between the max limit and the basic size. The lower deviation is the algebraic difference between the min limit and the basic size. The mean deviation is the arithmetical mean between the upper and the lower deviations . Fundamental deviation Fundamental deviation is either upper deviation or lower deviation, which is nearest one to the zero line for either hole or a shaft.

34. Hole basis system In this system, the design size of hole, whose lower deviation is zero, is assumed as basic size and different clearances or interferences are obtained by varying the limits of the shaft to have different class of fit. It is very easy, convenient and less costly to make holes of standard sizes.

35. Shaft basis system In this system, the design size of a shaft, whose upper deviation is zero, is assumed as basic size and different clearances or interferences are obtained by varying the limits of the hole to have different class of fit. It is difficult to vary the hole sizes and therefore not suitable in mass production.

36. Interchangeability An interchangeability part is one which can be substituted for similar part manufactured to the same drawing. It is only possible when certain standards are strictly followed in various manufacturing units. Interchangeability reduced the cost of production and increases the rate of production.

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