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 = Pitch = Pitch = 6
Starts = Starts = Starts = 1

11. Screwcutting
How many Starts? 1 2

12. Screwcutting
How many Starts? 2 3

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 – 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 3 6 1 2
(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 5 24
1.25 6

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 x
F x 3

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

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

45. Screwcutting
Threads