Design of Machine Elements-I DEPARTMENT OF MECHANICAL

1. Design of Machine Elements-IDEPARTMENT OF MECHANICAL ASST.PROF. KIRAN KADAM M.E.[DESIGN]B.E.[MECHANICAL]

2. GD&T • Datums in GD&T • Symbol:

3. Feature Control Frame • Example Frame: 

4. Parts of the Feature Control Frame

6. Leader Arrow – This arrow points to the feature that the geometric control is placed on. If the arrow points to a surface than the surface is controlled by the GD&T. If it points to a diametric dimension, then the axis is controlled by GD&T. The arrow is optional but helps clarify the feature being controlled. • Geometric Symbol – This is where your geometric control is specified. See our page on GD&T symbols or the sidebar for a description of each symbol. • Diameter Symbol (if required) – If the geometric control is a diametrical tolerance then the diameter symbol (Ø) will be in front of the tolerance value. • Tolerance Value – If the tolerance is a diameter you will see the Ø symbol next to the dimension signifying a diametric tolerance zone. The tolerance of the GD&T is in whatever unit of measure that the drawing is written in. All of our examples on GD&T basics are metric units. • Feature of Size or Tolerance Modifiers (if required) – This is where you call out max material condition or a projected tolerance in the feature control frame. See the Modifiers section of the GD&T Symbols page for further clarification on these features. • Primary Datum (if required) – If a datum is required, this is the main datum used for the GD&T control. The letter corresponds to a feature somewhere on the part which will be marked with the same letter. This is the datum that must be constrained first when measuring the part. Note: The order of the datum is important for measurement of the part. The primary datum is usually held in three places to fix 3 degrees of freedom • Secondary Datum (if required) – If a secondary datum is required, it will be to the right of the primary datum. This letter corresponds to a feature somewhere on the part which will be marked with the same letter. During measurement, this is the datum is fixated after the primary datum. • Tertiary Datum (if required) – If a third datum is required, it will be to the right of the secondary datum. This letter corresponds to a feature somewhere on the part which will be marked with the same letter. During measurement, this is the datum is fixated last.

7. Definition: • A datum is theoretical exact plane, axis or point location that GD&T or dimensional tolerances are referenced to. You can think of them as an anchor for the entire part; where the other features are referenced from. A datum feature is usually an important functional feature that needs to be controlled during measurement as well.

8. All GD&T symbols except for the form tolerances (straightness, flatness, circularity and cylindricity) can use datums to help specify what geometrical control is needed on the part. When it comes to GD&T, datum symbols are your starting points where all other features are referenced from.

9. How Datum Features are Shown on a Drawing • The datum features on a drawing are denoted with a series of capital letters. These letters are in boxes and tied to the datum feature with a black triangle.  This letter will also show up in any feature control frame that uses this datum feature as a reference. A feature control frame can reference multiple datums at once and each one can be referenced as many times as needed.

11. Notation is Important on Drawings • As stated before, datums can be located on points, axes,edges, and surfaces. It is important though that they is called out correctly on the drawing to control the right type of feature. Here is how different types of features are called out on engineering drawings.

13. On a surface

14. On an Axis

15. True Position • True Position: • Symbol: • Relative to Datum:Yes • MMC or LMC applicable: Yes (common)

16. True center position of a hole (RFS w/ 2 Datums)

17. Position of a hole under MMC (3 Datums)

18. GD&T Tolerance Zone:

20. Circularity • GD&T Symbol:  • Relative to Datum: No • MMC or LMC applicable: No

22. GD&T Tolerance Zone: • Two concentric circles, one inner and one outer, in which all the points within the curcular surface must fall into. The tolerance zone lies on a plane that is perpendicular to the central axis of the circular feature.

24. Example:

25. Parallelism • GD&T Symbol:  • Relative to Datum: Yes • MMC or LMC applicable: Yes • GD&T Drawing Callout:

27. Gauging / Measurement:

28. Parallelism Example:

30. Cylindricity • GD&T Symbol: • Relative to Datum: No • MMC or LMC applicable: No • Drawing Callout:

31. GD&T Tolerance Zone: • 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.

33. Example:

34. Profile of a Surface • GD&T Symbol:  • Relative to Datum: Optional • MMC or LMC applicable: No

35. Gauging / Measurement:

36. Example:

37. Runout • GD&T Symbol:  • Relative to Datum: Yes • MMC or LMC applicable: No • Drawing Callout

38. Gauging / Measurement:

39. Example:

40. Concentricity • GD&T Symbol:  • Relative to Datum: Yes • MMC or LMC applicable: No • Drawing Callout:

41. Gauging / Measurement:

42. Example:

43. Example:

44. Perpendicularity • GD&T Symbol:   • Relative to Datum: Yes • MMC or LMC applicable: Yes • GD&T Drawing Callout: • Surface Perpendicularity:

45. Surface Perpendicularity:

46. Axis Perpendicularity:

47. GD&T Tolerance Zone: Surface:

48. Axis:

49. Gauging / Measurement: • Surface:

50. Axis: