1. Metalwork
Screw Threads

2. Screw Threads
A screw thread is a specially shaped ridge (inclined plane) around a cylinder.

3. Screw Threads Definition
“A ridge that moves around a cylinder while at the same time it moves along the cylinder”.

4. Screw Threads
Definition

5. Screw Threads The correct name for this form is a helix.
A screw thread is in fact a helix.

6. Screw Threads
Helix

7. Screw Threads
The screw thread gives us a very quick method of assembling parts.
It can also be used as a power source as in a car jack or a vice.

8. Screw Threads
Power Source

9. Screw Threads Screw Thread Terms
There are several terms associated with screw threads the main ones are as follows:

10. Screw Threads Crest Root Flank Pitch Lead Major Diameter
Minor Diameter

11. Screw Threads
Screw Thread Terms

12. Screw Threads Screw Thread Terms Crest
This is the most prominent part of the thread where its flanks meet.
Root
This is the bottom of the thread groove where the flanks meet.

13. Screw Threads Screw Thread Terms Flank
This is the straight part of the thread between the root and the crest.
Pitch
This is the distance between corresponding points on adjacent threads, measured parallel to the axis.

14. Screw Threads Screw Thread Terms Lead
This is the distance the screw moves in one revolution along its axis. On a single start thread the lead is equal to its pitch.

15. Screw Threads
Screw Thread Terms

16. Screw Threads Screw Thread Terms Major Diameter
This is the largest diameter of the screw thread when measured over the crest of the thread.

17. Screw Threads Screw Thread Terms Minor Diameter
This is the smallest diameter at the root of the screw thread. It is equal to the major diameter minus twice the thread depth.

18. Screw Threads
Screw Thread Terms

19. Screw Threads Types of Screw Threads
The main types of screw threads used are:
ISO Metric Threads
Buttress Threads
Square Threads
ACME Threads

20. Screw Threads ISO Metric Threads
This is an international screw thread system which replaced the British Standards (BSW, BSS and BA threads).

21. Screw Threads ISO Metric Threads
ISO metric threads are designated by the letter M, followed by the nominal size and pitch. Both of these sizes are given in millimetres.
E.G.: M10 x 1.25: Nominal size 10mm and pitch of 1.25mm.

22. Screw Threads
ISO Metric Threads

23. It is used in the screws of quick release vices.
Screw Threads
Buttress Thread
This thread is used to apply thrust in one direction only and is often used as a power source.
It is used in the screws of quick release vices.

24. Screw Threads
Buttress Thread

25. Screw Threads
Buttress Thread

26. Screw Threads Square Thread
This thread is suitable for applying thrust in both directions.
It is used in screw jacks, bench vices and valve spindles.

27. Screw Threads
Square Thread

28. Screw Threads
Square Thread

29. Screw Threads ACME Thread
The sloped flanks of this thread allow easy engagement of half nuts.
These threads are often used in the leadscrews of lathes.

30. Screw Threads
ACME Thread

31. Screw Threads
ACME Thread

32. Questions What is the correct name for a screw thread?
Make a neat sketch of a screw thread and label the following, Crest, Root, Pitch, Flank and Thread angle.
Make a neat sketch of an ACME thread.
What is the main advantage of a square thread.