1. Threads and Threading

2. Types

3. 1. Translation threads
square
acme
buttress

4. 2. American National thread form
WWII - US military equipment did not interchange with equipment made in Britain and Canada

5. 3. Unified thread form
After WWII countries agreed on need for interchangeability
Essentially the same as the American National thread form except Unified has a rounded root and either a rounded or flat crest
Is interchangeable with the American National thread form
Has 60 deg thread angle
Two major thread series - UNC and UNF

6. 4. Metric threads

7. 5. Right hand vs. left hand

8. Screw thread Nomenclature (external)

9. Major Dia: largest dia

10. Minor Dia: smallest dia

11. Pitch Dia: imaginary point where width of groove and thread are equal

12. Root: bottom surface connecting 2 sides of a thread

13. Crest: top

14. Pitch: linear distance from corresponding points on a thread

15. Depth: Distance from crest to root perpendicular to axis of thread on one side

16. Flank: Sides of a thread that connect crest to root

17. Helix Angle Distance of movement compared to each revolution
The Lead of the helix

18. Lead: Distance of 1 revolution (lead = pitch on a single lead thread)

19. Thread angle: included angle between flanks of thread

20. Unified Screw thread designation (5 components)

21. 1/2-13-UNC-2A 1/2 = major dia 13 = threads per inch
UNC = Unified National Course
2 = Class of fit
A = External Thread form (B = internal)

22. Classes of fit
Class 1 - largest mfg tolerances, used for ease of assembly
Class 2 - used on largest percentage of threaded fasteners
Class 3 - smallest mfg tolerances, threads will be tight when assembled

23. Methods of manufacturing threads
taps and dies
lathe
milling
grinding - used when material cannot be machined
rolling - most common

24. Thread measuring instruments-regardless of method, pitch dia is always measured or compared
1. mating part - simplest, no measurement involved
2. comparator micrometer - does not measure pitch, only compares to a known standard
3. thread micrometer - each micrometer measures a range of TPI
(8 - 13), ( ), ( ), ( )

25. Measuring instruments (cont.)
4. three wire system (most accurate)
5. go / no go thread gages - used in production where quick gaging is necessary
6. optical comparator - light beam shows a profile of the thread for checking thread form, helix angle, and depth (external threads)

26. Taps and Dies Taps - create internal threads
Dies - create external threads
Usually made of high speed steel

27. Standard set consists of:
Taper tap - used for starting a tapped thread square with the hole
Plug tap - most common
Bottoming tap - to produce threads almost to the bottom of a blind hole

28. Types of taps:
interrupted thread taps - used for tough materials - alternate teeth reduce friction
spiral pointed (gun) - chips are forced ahead of the tap
spiral fluted - helical flutes to draw chips out of the tap
thread forming taps - fluteless taps that do not cut, they displace the material to form the threads - ductile materials
Tapered pipe taps

29. Tapping procedures
by hand with a tap wrench
by machine

30. Drilling the proper hole diameter
tap drill size
selected from a chart
hole should be reamed before tapping
Tapping problems (Table B-3)

31. Types of dies
Round split adjustable or (button) - allow for small adjustments in size
2 piece split die. - blanks are placed in cap with guide

32. Hand threading procedures
always start the die on the leading (throat) side
use lathe, drill press, or mill to start the die squarely
use lubricant
chamfer the end of the rod
reverse the die (or tap) after each full turn to clear chips