Unit 55 Threads and Threading

Unit 55 Threads and Threading

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THREADS

1. Define thread.

2. List four purposes of threads.

THREAD TERMINOLOGY

3. Define pitch diameter, pitch, lead, root, and crest.

4. How is the pitch designated for: a. UNC threads?b. Metric threads?

5. Why is the diameter of the blank for a rolled thread equal to the pitch diameter?

6. How may a right-hand thread be distinguished from a left-hand thread?

THREAD FITS AND CLASSIFICATIONS

7. Define fit, allowance, tolerance, and limits.

8. How are external UNC threads classified?

9. Name and describe three classifications of UNC fits.

10. How are thread fits designated for the ISO threads?

11. Why was the ISO metric system of threads adopted?

12. Describe a thread designated as M 56 X 5.5.

13. Name five thread forms used in North America and state the included angle of each.

14. What are the principal differences between the American and Unified threads?

THREAD CALCULATIONS

15. For an M 20 x 2.5 thread, calculate:

A. Pitchd. Width of crest

B. Depthe. Width of root

C. Minor diameter

16. Sketch a UNC thread and show the dimensions of the parts.

17. How does a Brown and Sharpe worm thread differ from an Acme thread?

18. For a 1 in.—8 NC thread, calculate:

A. Minor diameter b.Width of toolbit point c. Amount of compound rest feed

19. What is the purpose of the quick-change gearbox?

20. Describe a thread-chasing dial and state its purpose.

21. The lead screw on a lathe has 6 tpi; at what point or points on the thread-chasing dial may the split-nut lever be engaged to cut the following threads: 8, 9, 11½, 12?

22. List the main steps required to set up the lathe for cutting a 60° thread.

23. List the main steps required to cut a 60° thread.

24. It has been necessary to remove a partially threaded piece of work from the lathe and to finish it at a later time. Describe how to reset the threading tool to “pick up” the

thread.

25. A. what two change gears are required to cut a metric thread on a standard lathe?

B. Where are these gears mounted?

26. Describe how a lathe (having a quick-change gearbox) is set up to cut a 2.5-mm thread.

27. What precaution should be taken when cutting a metric thread?

THREAD MEASUREMENT

28. Make a sketch to illustrate how a thread is measured using the three-wire method.

29. Calculate the best wire size and the measurement over the wires for the following threads:

a.¼ in.—20 NC b. 5/8 in.—11 NC c. 1¼ in.—7 NC

30. What is the purpose of a multiple thread?

Multiple threads are used when it is necessary to obtain an increase in lead and a deep, coarse thread cannot be cut

31. If the pitch of a multiple thread is 1/8 in., what will be the lead for a double-start thread? A triple-start thread?

32. List three methods by which multiple threads may be cut.

33. Name two factors that affect the helix angle of athread.

34. Calculate the leading and following angles of a square threading toolbit required to cut a

1½ in.—6 square thread.

35. List the main steps required to cut a square thread.

ACME THREADS

36. If the width of the root of an Acme thread is O.3707P at minimum depth, why is the Acme threading tool ground to O.3707P — 0.0052?

37. Describe how an Acme thread may be measured.

INTERNAL THREADS

38. List the steps required to cut a 1¼ in.—7 UNC thread 2 in. deep in a block of steel 3 in. X 3 in. x 3 in. Show all necessary calculations.

Tap drill size: Major diam. - 1/N = 1.25 - (1/7) = 1.1071 Tap drill size