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

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.

External Screw Thread

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

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

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

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.

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

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

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)

Applications Transmit Power Fasten: Vibration Multiple Leads LH Threads

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

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

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

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

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

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