1. Thread Manufacturing Terms Historical Perspective Thread Types
Specifications
Applications
Internal Threads
taps
boring
External Threads
Dies
Grinding
Forming - Grinding

2. Thread Terms - Nomenclature
Major Diameter aka outside diameter
Minor Diameter aka root diameter
Pitch Diameter
the diameter of an imaginary cylinder which passes through the thread profile at points where the width of the groove and the width of the thread are equal.

3. External Screw Thread

4. Terms Pitch Flank Crest Root Thread Angle Shoulder Undercut
Single Depth
Pitch Line
Lead

5. Historical Perspective
Archimedes
Draws Water
Individual fit
Henry Maudsley
England, 1797
Lead Screw
Whitworth: BSW
William Sellers USS
SAE Threads: 1911
American (National) thread system, 1935
Unified National Screw Threads: 1948
UNC, UNF, UNEF

6. Thread Types Acme Thread 29º Buttress Thread
Not as strong as square threads: but easier to cut
Transmit power
Buttress Thread
Transmits Power - Supports load

7. Thread Types Knuckle Thread Tapered Thread
American Standard Pipe Thread. Taper is 3/4”/foot on both the external and internal threads. Compound usually used.

8. Specification: 1/4 - 20 UNC - 2A
Major Diameter, Pitch, Thread Standard, Class Fit, and Form
A = external, B = internal
Multiple Leads
Direction of Thread

9. Classes of Unified Threads
Intentional loose fit
Class 2:
Free fit, commercial applications
Class 3:
Close fit, minimum tolerances

10. Tap Drill Size
The tap drill size is a actually a function of both the major diameter (D) and the length of engagement of the internal thread (L)
Standard tap drill sizes are typically used however. Those shown here are for #2 standard fit (75% thread)

11. Applications Transmit Power Fasten: Vibration Multiple Leads
LH Threads

12. ISO Standards M6 x 0.75 - 5g6g Metric, major diameter, pitch
5 = tolerance grade
g = tolerance position (pitch diameter)
6 = tolerance grade
g = crest diameter

13. Internal Threads
Taps: hand, plug, bottoming, tapered, gun, spiral (note “B” designation)
flutes, land area, taper, HSS
Boring: single point tool mounted on a strong bar
Forming: fluteless tool: inserts

14. External Threads: Dies: Adjustable, Split, Solid Single Point Cutting:
29º, half-nut, compound rest, single side depth of cut, gearbox pitch numbers

15. Thread Manufacture Grinding: Precision operation
Forming: Thread rolling: strength

16. Size Specifications: Tap drill sizes: Major Diameter class of fit
percentage of thread
Major Diameter
class of fit
percentage of thread
undercut size

17. Common Problems: Tapping: External Threads
Insufficient land area on tap
chip removal: cutting fluids
Improper hole size
Bottoming
External Threads
Material irregularities
Tool Alignment
Chip removal
Bottoming