Basic gd&t datums


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Basic gd&t datums

  1. 1. Data Analysis Lab ME 288 L7 abc
  2. 2. GD & T • Is a method (systems of symbols) for defining a part’s geometry – it goes beyond the form description – based simply on tolerance dimensions – as directed under rule #1 • A basic dimension is a theoretical perfect size, location or orientation dimension. 2h 12 Varies Basic Dimension
  3. 3. These symbols are used when dimensioning. h 2h 90° h h h 0.6h Counterbore 60° Depth (or deep) Countersink 0.5h h h 0.5h h 2h 30° Square Conical Taper Dimension Origin 0.3h 1.5h ( ( ) 0.3 h Arc Length 15° h 1.5h Slope Reference 2h 12 X Places or By R Radius 2.5h 0.8h Varies Basic Dimension Ø Diameter SR Spherical Radius ST 1.5h 60° Statistical Tolerance (dimensional) SØ Spherical Diameter CR Controlled Radius
  4. 4. These symbols are used when dimensioning. h 2h 90° h h h 0.6h Counterbore Countersink 0.5h 0.5h h 60° Depth (or deep) h h 2h 30° Dimension Origin h = height of lettering Conical Taper Square
  5. 5. These symbols are used when dimensioning .3h 1.5h ( ( ) 1.6 Arc Length 15° h 1.5h Slope Reference 2h 2.5h X 12 Places or By Varies 0.8h ST 1.5h 60° Basic Dimension Statistical Tolerance (dimensional) R Ø SR SØ Radius Diameter Spherical Radius Spherical Diameter CR Controlled Radius
  6. 6. A feature control frame (the backbone of GD & T) modifies a part’s geometry. Geometric tolerance(characteristic) symbol (position) Diameter symbol Numeric tolerance (feature tolerance) Modifier 0.2 A B C 2h Tertiary datum Secondary datum with modifier Primary datum
  7. 7. Two additional examples of feature control frames modifying a part’s geometry. Numeric tolerance Diameter symbol Geometric tolerance (perpendicularity) 0.08 Numeric tolerance Geometric tolerance (flatness) (A) 0.05 Modifier Datum (B) A
  8. 8. Geometric characteristic(tolerance) symbols are categorized in two ways. Geometric Characteristic Symbols Symbol Type of tolerance Description Flatness Circularity Form Straightness Perpendicularity Parallelism Position Concentricity Symmetry * * Runout Circular Runout Total *Either filled or unfilled Individual or related features No datum or datums needed Depending on the situation Orientation Angularity Location Profile surface Runout Profile line Profile Cylindricity Individual features Related features A datum or datums are required
  9. 9. GD&T geometric characteristic symbols illustrated. 0.6h 2h 1.5h Straightness 1.5h h Concentricity 1.5h h Parallelism 1.5h Circularity Flatness 1.5h M 0.8h Modifier
  10. 10. GD&T geometric characteristic symbols illustrated. 2h 1.2h 1.5h h 0.5h Symmetry h 60° 1.5h Position Cylindricity 2h h All round h 2h h Profile surface Profile line
  11. 11. GD&T geometric characteristic symbols illustrated. 3h 1.5h 30° 2h Perpendicularity 0.8h * 0.8h * 0.6h 1.5h Angularity h 45° 0.6h Runout Circular Between * 1.5h 1.1h Runout Total * Filled or unfilled
  12. 12. Table lists GD&T geometric characteristic symbol modifiers. Modifiers M L Maximum Material Condition Least Material Condition P Projected Tolerance Zone F T Free State Variation Tangent Plane ST Statistical Tolerance (geometric) Between * *Filled or unfilled
  13. 13. Modifiers Maximum material condition, MMC (M) Least material condition, LMC (L) • It should be taken literally. • It should be taken literally. • The geometric feature or size is as large as it can be. • In the case of a hole, as small as it can be. • The geometric feature or size is as small as it can be. • In the case of a hole, as large as it can be.
  14. 14. 7B
  15. 15. Rule #1: “Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent of which variations in its geometric form, as well as size, are allowed.” ANSI Y14.5 – 1994
  16. 16. When only size tolerance is specified the object’s form can vary within the stated size limits. Ø12.2 Ø12.2 (B) 12.2 Ø 11.8 12.2 Ø 11.8 Ø11.8 (A) at Ø12.2 MMC must be perfect form Ø11.8 Ø12.2 Ø12.2 at Ø11.8 MMC must be perfect form (C) Ø11.8 Ø11.8 for entire length External dowel plug Ø11.8 Ø11.8 for entire length Internal hole
  17. 17. A cylinder can have a variety of shapes yet stay within the limits of size. Max Max Min (A) Max Min (B) Min (C)
  18. 18. The rectangular prism can vary in shape as long as it stays inside the volume of the limits of size. 22 20 18 16 16 14 11 9 9 7 8 (A) 7 (B)
  19. 19. Checking the size limits envelope: A ring gage and plug gage are used to check the geometric form of a pin and hole. 8.0 Ø 7.8 Ø7.8 LMC along entire length of dowel 16.0 15.8 Ring gage Ø8.0 MMC (A) Checking geometric form with ring gage Ø 7.0 6.8 Ø7.0 LMC Plug gage Ø6.8 MMC (B) Checking geometric form with plug gage
  20. 20. Standard Stock Item • Items whose geometry are already controlled by established industrial or government standards – bars, sheet stock, tubing or structural shapes
  21. 21. Datums • A datum is an exact surface, line, point ,axis or cylinder from which measurements are taken. • Ex. A surface plate or a polished slab of granite (simulated datum). • 3 points define a plane. • Datum feature – is the surface of the part in contact with the simulated datum.
  22. 22. A height gage measures the height of an object from the simulated datum surface of a surface plate. Dial face Probe Up Down Measured height Simulated datum (surface plate) Datum feature Note: the datum feature rests on the simulated datum. the height is measured from the simulated datum and not from the datum feature.
  23. 23. Degrees of freedom (12) allow movement in two directions along each axis and rotation about each axis (clockwise and counterclockwise). +Z -X -Y +Y +X In order to measure geometric features • part motion must be restricted. -Z
  24. 24. A datum reference frame consists of three intersecting planes at 90° to each other. The part can still move in the positive X, Y and Z directions “part motion must be restricted” What if we temporarily clamped the part? → Measurements can be taken from the simulated datums.
  25. 25. Datum surfaces must be indicated on the drawing Varies H 2H H A 60° Filled or unfilled 2H for single lettering 4H for double lettering (A) H = height of lettering E G F 16 H (B) Applications
  26. 26. 7C
  27. 27. The datum symbol is applied to solid cylinders. Ø14 J -or- Ø14 J J Ø8 K
  28. 28. The datum symbol is applied to holes. Ø12 Ø0.1 M F G H N Ø12 M R Ø6 P Ø6 Note: letters “I”, “O” and “Q” are not used to indicate datums because they may be confused with numbers one(1) and zero(0). Double letters can be used e.g. AA, BB etc.
  29. 29. Order of Datums: Primary datum S has 3 pts of contact, secondary datum T has 2 pts and tertiary datum U 1pt of contact. 2X Ø6 ± 0.2 Ø0.1 M S T U 8 (A)This drawing symbology U 6 12 10 U 1 pt of contact 3 pts of contact (B) Means this 2 pts of contact S T T S
  30. 30. The order of the datums is critical! 2X Ø6 ± 0.08 Ø0.1 M V W X X (1 pt) V (3 pts) W (2 pts) X (1 pt) True height True width W (2 pts) Correct inspection procedure V (3 pts) X (2 pts) False height False width V (3 pts) W (1 pt) Incorrect inspection procedure
  31. 31. The datum axis is formed by two intersecting planes. 8 8 4X Ø4±0.5 0.5 G H Datum axis Intersecting planes perpendicular to G 8 Primary datum G Ø24 H 8 G (A) This drawing (B) Means this
  32. 32. The plug gage establishes the datum axis. C Ø10.8±0.1 (A) This drawing Simulated datum cylinder C - largest Ø that fits into hole Datum axis Plug gage (B) Means this Datum feature C
  33. 33. The smallest circumscribed cylinder establishes the simulated datum and the datum axis. D (A) This drawing 19±0.5 Simulated datum D Datum feature D Datum axis (B) Means this
  34. 34. The simulated central datum plane is established by the center plane of the largest block that fits into the groove. 12.2 11.8 0.4 Y Z Z A) This drawing Y B) Means this Simulated datums at maximum separation Central datum plane Datum features 24.2 23.8
  35. 35. The simulated central datum plane is established by the center plane located by the two blocks at minimum separation. 18.2 17.8 0.6 A B B A A) This drawing B) Means this Simulated datum planes at minimum separation Central datum plane