The document discusses different types of screw threads including Vee threads, buttress threads, square threads, and trapezoidal threads. It also covers screw thread terminology such as pitch, lead, and starts. Additionally, it describes methods of screw cutting including how to calculate the number of starts and testing threads using methods such as a test nut, thread calipers, or three wire method. Tracking ratios and various methods for tracking threads are also outlined.

1. Screw Threads
Screw Cutting

2. Screw Threads
 There are 4 basic Thread Forms
 Vee Threads
 Buttress
 Square
 Trapezoidal

3. Screw Threads
 Vee Thread

4. Screw Threads
 Buttress Thread

5. Screw Threads
 Square Thread

6. Screw Threads
 Trapezoidal Thread (30° Angle)

7. Screw Threads
 Thread Terminology
Pitch
Distance from a point on one
Thread Groove to the exact
same point on the next Thread
Groove

8. Screw Threads
 Thread Terminology
Lead
Pitch x Number of Starts

9. Screw Threads
 Thread Terminology
Starts
Grooves running side by side
on the same cylinder

10. Screw Threads
 Lead = 12 Lead = 12 Lead = 6
 Pitch = 3 Pitch = 6 Pitch = 6
 Starts = 4 Starts = 2 Starts = 1

11. Screwcutting
 How many Starts?
2
1

12. Screwcutting
 How many Starts?
3
2

13. Screwcutting
 How many Starts?
3 4

14. Screwcutting
 Job 1

15. Screwcutting
 Alternative to Angling Compound Slide
Compound
Slide moves
= ½ Cross
Slide
Movement

16. Screwcutting
 Methods of Testing Threads
 Test Nut

17. Screwcutting
 Methods of Testing Threads
 Go – No Go Thread Caliper Gauge

18. Screwcutting
 Methods of Testing Threads
 Three Wire Method

19. Screwcutting
 Three Wire Method requires
calculating:
Best Wire Size =
P
2 x Cos 30º
(ie. ½ included angle)

20. Screwcutting
 Three Wire Method requires
calculating:
Measurement Over Wires =
M = D – 1.5155 x P + 3 x W

21. Screwcutting
 External Threads

22. Screwcutting
 Full Form Carbide Insert

23. Screwcutting
 Internal Thread

24. Screwcutting
 Tracking Ratio
 Tracking Ratio =
 Usually expressed as the lowest whole
number ratio
 Eg. = 1 = Magic #
Lead of Work
Lead of Lead Screw
1
2
3
6
(The Magic # is used to
determine the
Starwheel or Chaser
Dial details)

25. Screwcutting
 Tracking Ratio
 Tracking Ratio =
 Usually expressed as the lowest
whole number ratio
 Eg. = 5 = Magic #
Lead of Work
Lead of Lead Screw
1.25
6
5
24

26. Screwcutting
 Tracking the Thread
 Tracking the Thread means to engage
into the same Thread groove on
successive pass of the tool.

27. Screwcutting
Methods of Tracking the Thread
 Starwheel
 Chaser Dial
 Reversing the Machine
 Repeat Distance Method

28. Screwcutting
Starwheel
 A thin gear like disc with teeth cut of a
size to engage with the Leadscrew of
the Lathe

29. Screwcutting
Chaser Dial
 Sometimes used in conjunction with a
Starwheel to assist correct engagement
of the Leadscrew

30. Screwcutting
Chaser Dial Details

31. Screwcutting
Reversing the Machine
 Reversing the Spindle is useful where a
Thread with an unusual Lead needs to
be produced
 Usually achieved by reversing the
direction of the Lathe spindle without
disengaging any of the drive gears

32. Screwcutting
Repeat Distance Method
 Repeat Distance is calculated by
multiplying the Tracking Ratio by the
Lead of the Leadscrew

33. Screwcutting
Threads
 Metric Threads
available by direct
selection

34. Screwcutting
 Change Gears - Simple Gear Train
 Standard
Leads

35. Screwcutting
Threads
• To produce an M40x4 select:
• Range II, Lever Position C6

36. Screwcutting
Threads
• To produce an M24x3 select:
• Range I, Lever Position A1

37. Screwcutting
Threads
• Sometimes Threads are NOT available
by direct selection, eg. 2.5 mm Lead

38. Screwcutting
Threads
• As 2.5 mm is not on the Chart of
available Leads we must improvise

39. Screwcutting
Threads
• We can cut Leads not normally
available on the chart by using:
• Formula
D D1 x D2
F F1 x F2

40. Screwcutting
 Compound Gear Train

41. Screwcutting
Threads
• We can cut Leads not normally
available on the chart by using:
• Formula
D Dn x R
F Fn x S

42. Screwcutting
Threads
• To cut a 2.5 mm Lead:
• Formula
D 30 x 2.5
F 60 x 3

43. Screwcutting
Threads
• To cut a 2.5 mm Lead:
• Formula
D 30 x 2.5
F 60 x 3
1 x 2.5
2 x 3
2.5 25
6 60

44. Screwcutting
 Change Gears - Simple Gear Train
 Change
the 30z
for a 25z
25
60

45. Screwcutting
Threads