Mechanical Drawing (MDP 115)

Transcription

1 Mechanical Drawing (MDP 115) FirstYear, Mechanical Engineering Dept., Faculty of Engineering, Fayoum University Dr. Ahmed Salah Abou Taleb

2 Threads and Fasteners Topics Exercises 2

3 Threads & Fasteners: Topics Summary 1) Fasteners 2) Screw Thread Definitions 3) Types of Thread 4) Manufacturing Screw Threads 5) Drawing Screw Threads 6) Unified Threads 7) Metric Threads 8) Drawing Bolts 9) Bolt and Screw Clearances 3

4 Threads & Fasteners: Exercises Exercise 1: Screw thread features Exercise 2: Unified national thread note components Exercise 3: Unified national thread note Exercise 4: Metric thread note components Exercise 5: Metric thread tables Exercise 6: Fastener tables and clearance holes 4

5 Threads and Fasteners Summary 5

6 Summary What will we learn in this topic? How to represent threads on an engineering drawing. How to calculate bolt and screw clearance holes. Key points Threads are represented by thread symbols, not by a realistic drawing. 6

7 Threads and Fasteners 1) Fasteners 7

8 Fasteners Fasteners include: bolts and nuts (threaded) set screws (threaded) washers keys Pins Fasteners are not a permanent means of assembly such as welding or adhesives. 8

9 Fasteners Fasteners and threaded features must be specified on your engineering drawing. Threaded features: Threads are specified in a thread note. General Fasteners: Purchasing information must be given to allow the fastener to be ordered correctly. 9

10 Threads and Fasteners 2) Screw Thread Definitions 10

11 Thread Definitions Screw Thread: A ridge of uniform section in the form of a helix. 11

12 Thread Definitions External Thread: External threads are on the outside of a member. A chamfer on the end of the screw thread makes it easier to engage the nut. Chamfer 12

13 Thread Definitions External Thread: An external thread is cut using a die or a lathe. 13

14 Thread Definitions Internal Thread: Internal threads are on the inside of a member. An internal thread is cut using a tap. 14

15 Thread Definitions Major DIA (D): The largest diameter (For both internal and external threads). Minor DIA (d): The smallest diameter. Depth of thread: (D-d)/2 Pitch DIA (d P ): The diameter at which a line cuts the spaces and threads equally. 15

16 Exercise 1 Screw thread features 16

17 Identify the Major, Minor & Pitch diameters and the Thread Depth

18 Identify the Major, Minor & Pitch diameters and the Thread Depth Thread Depth Minor d dp Major 8 D

19 Thread Definitions Crest: The top surface. Root: The bottom Surface. Side: The surface between the crest and root. 19

20 Identify the Crest, Root and Side Thread Depth Minor d dp Major D

21 Identify the Crest, Root and Side. 1 2 Crest Root Thread Depth Minor d dp Major D 10 Side 21

22 Thread Definitions Pitch (P): The distance from a point on a screw thread to a corresponding point on the next thread (in/threads). Angle of Thread (A): The angle between the threads. 22

23 Thread Definitions Screw Axis: The longitudinal centerline. Lead: The distance a screw thread advances axially in one turn. 23

24 Identify the Pitch, Screw Axis and Thread Angle. 1 2 Crest Root Thread Depth Minor d dp Major D 10 Side 24

25 Identify the Pitch, Screw Axis and Thread Angle. Crest Axis Pitch Root Thread Depth Minor d dp Major D Angle Side 25

26 Thread Definitions Right Handed Thread: Advances when turned CW. (Threads are assumed RH unless specified otherwise.) Left Handed Thread: Advances when turned CCW. 26

27 Application Question 1 Name an example of a left handed thread. Left peddle of a bike 27

28 Threads and Fasteners 3) Types of Thread 28

29 Types of Thread There are many different types of thread forms (shape) available. The most common are; Unified Metric 29

30 Types of Thread Thread form choice depends on; what it will be used for length of engagement load etc 30

31 Types of Thread (Form) Thread Name Figure Uses Unified screw thread ISO metric screw thread Square General use. General use. Ideal thread for power transmission. 31

32 Types of Thread (Form) Thread Name Figure Uses ACME Stronger than square thread. Buttress Designed to handle heavy forces in one direction. (Truck jack) 32

33 Threads and Fasteners 4) Manufacturing Screw Threads 33

34 Manufacturing Threads Internal Threads First a tap drill hole is cut with a twist drill. 34

35 Manufacturing Threads Internal Threads Then the threads are cut using a tap. 35

36 Manufacturing Threads Internal Threads Chamfers are sometimes cut to allow for easy engagement. 36

37 Manufacturing Threads External Threads You start with a shaft the same size as the major diameter. 37

38 Manufacturing Threads External Threads The threads are then cut using a die or on a lathe. 38