SCREWTHREADS MEM18003C Fitting and Machining, Campbelltown College

SCREWTHREADS (VEE FORM) Definition: A screwthread is a helical structure used to convert between rotational and linear movement or force. Fitting and Machining, Campbelltown College

SCREWTHREADS USES To fasten parts together, as with bolts and nuts, set screws, studs or pipe joints. To adjust the position of parts. To exert pressure, as with a vice, screw press or screw jack. Transmit motion. e.g. Lathe cross slide. Measuring device. e.g. micrometer. Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Pitch Pitch is the distance, measured parallel to the axis, between corresponding points on adjacent thread forms. Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Major Diameter Major Diameter of a parallel tried is the diameter of a imaginary cylinder which just touches the crest of the external thread, and the root of an internal thread. The major diameter is the largest diameter of a screw or nut. Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Pitch Diameter Pitch Diameter of a vee thread is the diameter at which the thickness of the tread is equal to half the pitch. Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Minor Diameter Minor Diameter of a parallel thread is the diameter of an imaginary cylinder which just touches the crest of an internal thread. The minor diameter is the smallest diameter of the thread on a screw or nut. Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Crest Crest of the thread is the prominent part of the thread, whether the thread is external or internal. Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Angle Angle of thread or Included Angle is the angle between flanks measured in the axial plane. Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Root Root of the thread is the bottom of the groove between adjacent flanks. Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Flank Flank or side of thread is the straight side which connects the crest and the root. Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Depth Depth of the thread is the Radial Distance between it’s crest and root. Major Diameter – Minor Diameter 2 Depth of thread = Fitting and Machining, Campbelltown College

SCREWTHREAD TERMINOLOGY Hand of thread: Will either be ‘Right’ or ‘Left’ hand. Right hand Thread – Nut turns in a clockwise direction. Left hand Thread – Nut turns in a anti clockwise direction. Fitting and Machining, Campbelltown College

METRIC THREAD FORM To Calculate the Depth of Thread on a Bolt: .591 x Pitch Fitting and Machining, Campbelltown College

UNIFIED THREAD FORM To Calculate the Depth of Thread on a Bolt: .613 x Pitch Unified Thread Forms come in two series: UNC Unified National Course UNF Unified National Fine NPT National Pipe Thread Fitting and Machining, Campbelltown College

BRITISH STANDARD THREAD FORM To Calculate the Depth of Thread on a Bolt: .64 x Pitch British Standard Thread Forms: BSW British Standard Whitworth BSF British Standard Fine BSP British Standard Pipe Fitting and Machining, Campbelltown College

IMPERIAL THREAD FORM To calculate the pitch of imperial threads: T.P.I = Threads Per Inch e.g. Pitch of an 8 T.P.I. Thread = Pitch = .125 Depth of Thread = .125 x .54 = .0675” 1 TPI 1 8 Fitting and Machining, Campbelltown College

METHODS OF PRODUCING SCREW THREADS Screw threads may be formed by casting, rolling or metal removal process. The metal removal processes in common use are: Single point threading tool as used in the screw cutting lathe. Dies and chasers as used in die heads on capstan, turret and automatic lathes. Dies as used in hand stock and die. Hand taps and machine taps. A thread grinding process. Thread milling process. Fitting and Machining, Campbelltown College

HAND TAPPING PROCESS Hand taps are for the cutting of female threads. Hand taps are made in sets of three and comprise of: Taper Intermediate Plug or ‘Bottoming’ Fitting and Machining, Campbelltown College

CUTTING AN EXTERNAL THREAD With Hand Stock and Die Two types of dies are: Button Die Used for manufacturing new threads. Die Nut Used for repairing or cleaning existing threads. Fitting and Machining, Campbelltown College

EXAMPLES OF TAP WRENCHES AND STOCKS AND DIES Set of Tap Wrenches Tee-tap Wrench Set of Taps and Dies Fitting and Machining, Campbelltown College

Different types of fasteners that use Vee Threads Fitting and Machining, Campbelltown College