Threads and Threading

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), (14 - 20), (22 - 30), (32 - 40)

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