1. Mechanical Engineering Drawing MECH 211/M
Lecture #10 Chapter 13
Dr. John Cheung

2. Screws, Nuts and Bolts Used to assemble machine parts
Used to transmit power / motion
The threads are cut or rolled
Bolts and nuts must be matched

3. Screw and Thread Terminology
Screw Thread - A ridge of uniform section in the form of a helix on the external or internal surface of a cylinder.
Major Diameter - The largest diameter of a screw thread.
Minor Diameter - The smallest diameter of a screw thread.

4. Screw and Thread Terminology
Axis – the longitudinal center line of the original work (blank) or hole
Chamfer – the angular relief at the beginning or end of the thread to allow easier engagement with the mating part
Crest – the peak of the top of a thread
Depth – the distance between the crest and the root

5. Screw and Thread Terminology
External thread – the screw thread on the outside of a cylindrical surface
Internal thread – the screw thread on the inside of a cylindrical surface
Lead – the distance that a screw will travel along the axis when turned by 360°

6. Screw and Thread Terminology
Pitch – the distance between corresponding points on adjacent thread forms, measured parallel to the axis expressed in 1 divided by the number of pitch in one inch
Pitch diameter – the diameter of an imaginary cylinder that is located equidistant between the major and the minor diameter

7. Screw and Thread Terminology
Root – the bottom of the screw thread cut in a cylinder
Thread angle – the angle between the surfaces of two adjacent threads
Thread series – the number of threads per inch for a given diameter

8. Tap and Die

9. Thread Specifications – Imperial System
ANSI Y
Thread form
Thread series
Major diameter
Class of fit
Threads per inch

10. Screw Thread Forms Imperial & Metric have same proportion
Sharp V was original American national thread
American National now has flattened root and crest to increase strength
Unified thread is agreed as standard in US, Canada and Britain, the crest may be flat or rounded but the root is rounded. Otherwise similar to American national

11. Screw Tread Forms
ISO Metric is the most common of all. The depth is smaller than that of Unified National thread
Knuckle thread is rolled or cast (used in light bulbs and sockets)
Square and Acme threads are used for transmitting power
Buttress thread takes pressure on one side ( to the axis). Unidirectional motion.

12. Power Transmission & Measurement Screws
Controls and positioning applications

13. Measuring Thread Pitch
Pitch is the distance along the axis between corresponding points in adjacent threads (i.e crest to crest or root to root)
Metric: Measured in millimeters (i.e. M10 X 1.5)
Imperial: Measured in threads per inch
Thread Pitch is measured with scale or a thread pitch gage

14. Thread Types
Series: Varies with the pitch and the major diameter of the thread
Coarse Series (UNC)
General purpose usage
Fine Series (UNF)
Used when a great deal of force is necessary
Extra Fine Series (UNEF)
Used when the length of engagement is short and there is a high degree of stress
8-Pitch (8UN), 12-Pitch (12UN), 16-Pitch (16UN)
Does not depend on the diameter

15. Metric Thread Series Simplified compared to Unified Series
Only two series
Coarse (General Purpose)
Fine

16. Left & Right Handed Threads
Threads are assumed to be right handed (RH)
A nut advances when turned clockwise
Left handed (LH) threads turn counter clockwise to tighten

17. Single and Multiple Thread Forms

18. Thread Symbols Three Types: Simple, Schematic or Detailed
For major diameters >1” Detailed is preferred

19. Simplified & Schematic Internal Symbols Normal & Section Drawings

20. Thread Clearance Class 1
Loose fit where quick assembly is required and play between parts is acceptable
Class 2
High quality general purpose commercial class of fit for bolts, nuts and screws used in mass production
Class 3
Very high quality threaded fasteners with a close fit used for precision assembly subjected to vibrations
A is for external threads and B is for internal threads

21. Metric Thread Class of Fit
Tolerance grade
3 to 9 for external threads
4 to 8 for internal
Tolerance position
Letter between A and H
Capitals for internal threads and lowercase letters external threads
Tolerance Class
Tolerance grade plus a tolerance position,
First for the pitch diameter
Second major diameter
Example, “4g6g”

22. Imperial Thread Notes

23. Thread Identification
Thread form symbol – M
Nominal size
Pitch size
Tolerance Class

24. Basic Metric Thread Note

25. Complete Metric Threading Note
For example, grade 4 < grade 6 < grade 8 tolerances
In addition to the tolerance grade, a positional tolerance is required
For external threads:
Tolerance position e (large allowance)
Tolerance position g (small allowance)
Tolerance position h (no allowance)
For internal threads:
Tolerance position G (small allowance)
Tolerance position H (no allowance)

26. How to Represent a Threaded Hole

27. Threads Per Inch for Various Forms
ANSI

28. Screws, Nuts and Bolts

29. Large variety of bolts (dimensional, head shape, etc.)
Material, quality, finishing
Grade

30. Unfinished bolts are not machined anywhere except for the thread portion
Finished bolts have machined face for washer holding or flush location on parts

31. Fastener Locking

32. Fastener Locking
Durlock nut
Castellated nut
Jam nut

33. Screw Types Head can have different shapes
Pass through a clearance hole to screw into member
Socket screws are used while in crowded condition

34. Screw Types - Continued
Similar to cap screws but smaller in size.
General diameters between .06 to .75 inches
Hex head (not shown here) may be slotted if desired
Other heads are available as either slotted or recessed
Generally used for screwing into thin materials

35. Bolt Representation

36. Cap Screw Representation

37. Screw Assembly Representation

38. Bolt Assembly Representation

39. Non-Permanent Fasteners Snap Rings

40. Non-Permanent Fasteners Pins

41. Once assembled, the parts of the assembly (including the fastener) would be destroyed to disassemble the assembly.
Rivets, soldering, brazing, welding

42. Used to permanently fasten mechanical components

43. Riveting process

44. Fastening With Rivets

45. Common Riveted Joints

46. Self-Piercing Rivets
Blind rivets

47. Blind Rivets

48. Joining of two part using a third component – filler that joins the parts when in liquid state
Soldering and brazing – low temperature binding materials – Sn-Pb, Cu-Ag alloys
Limited capability to face thermo-mechanical loading

49. Very well regulated activity
It requires license to practice
Welders bear significant responsibility
The activity is based on rigorous rules and regulations
Designer prescribe welding based on mechanics of materials calculations
Symbols indicate the type of welding

50. Basic Welding Note

51. Basic Welding Symbols

52. Basic Welding Symbols

53. Basic Welding Symbols

54. Compression Springs Extension Springs Torsion Springs
Designed to store energy when deflected and return the same amount of energy when released
Basically divided as Helical and Flat springs
Helical springs are sub divided as
Compression Springs
Extension Springs
Torsion Springs

55. Helical Springs
Compression, Extension & Torsion

56. Spring Representation
Detailed representation
Schematic representation
Working drawing of a compression spring

57. Gears

58. Gear Nomenclature

59. Gear Representation
A table containing cutting data must accompany the representation
This gear is not sectioned unless
something inside should be shown

60. Gear Representation
A table containing cutting data must accompany the representation

61. Gear Representation

62. Rack and Pinion Rack representation (the cutting data is included)
Transmission of liner motion to circular or vice versa

63. Worm and Gear representation
Transmission of motion between out of plane, perpendicular axes

64. Bevel Gear Bevel Gear representation
Transmission of motion between perpendicular axes

65. Cam and Follower

66. Clutch
Reduce the high stress when
power is coupled to an idle shaft

67. Ball Bearings

68. Ball and Roller Bearings

69. Ball and Roller Bearings