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TAPS ENGINEERING DATA
TAP MARKING SYSTEM
STANDARD SYSTEM OF TAP MARKING
Taps, dies and other types of threading tools are marked according to the Standard System of Marking Ground Thread
Taps. Tools are marked with the nominal size, number of threads per inch (pitch), and the appropriate thread form symbol
and pitch diameter symbol. Symbols typically used are listed.
DRILLS 3/8-20 N S
HSS GH- 3
END MILLS NOMINAL SIZE - THREADS PER INCH THREAD FORM SYMBOL
TAP MATERIAL
See Listing Below
PITCH DIAMETER LIMIT
See Listing Below
ROUTERS PITCH DIAMETER LIMIT SYMBOLS
THREAD MILLS All standard ground thread taps are marked with the letter “G” to designate Pitch Diameter Limits for taps to 1” diameter inclusive:
& TAPS Ground Thread. The letter G will be followed by the letter “H” to designate L1 = Basic to Basic minus .0005
above basic; or the letter “L” to designate below basic. The number H1 = Basic to Basic plus .0005
ENGRAVERS following H or L signifies the number of .0005” steps above or below the H2 = Basic plus .0005 to Basic plus .0010
basic pitch diameter. For instance, the tap pictured above is a 3/8” dia. tap H3 = Basic plus .0010 to Basic plus .0015
with 20 threads per inch (pitch), and has a NS (American National Special H4 = Basic plus .0015 to Basic plus .0020
Thread) thread form. The tap is made from High Speed Steel, and the H5 = Basic plus .0020 to Basic plus .0025
GH-3 pitch diameter limit symbol indicates a Ground Thread tap with pitch H6 = Basic plus .0025 to Basic plus .0030
diameter limits .0010 to .0015 over basic.
Taps larger than 1” dia. are ground to a .0010” tolerance
on the pitch diameter and are, for example,
H4 (Basic plus .0010” to Basic plus .0020” ).
BORING BARS THREAD FORM SYMBOLS
ACME-C Acme Thread-Centralizing NPTF Dryseal American National Standard Taper
ACME-G Acme Thread-General Purpose Pipe Thread
AMO American Standard Microscope Objective Thread NPTR American National Standard Taper Pipe Thread
ANPT Aeronautical National Form Taper Pipe Thread for Railing Joints (Tap marked NPT)
(Ground thread tap marked NPT) NS American National Thread-Special
BA British Association Standard Thread PG Panzer Gewinde
BSF British Standard Fine Thread Series PTF Dryseal SAE Short Taper Pipe Thread
REAMERS BSP British Standard Pipe SGT Special Gas Taper Thread
BSPP British Standard Pipe (Parallel) Thread SPL-PTF Dryseal Special Taper Pipe Thread
BSPT British Standard Taper Pipe Thread STI Special Thread for Helical Coil Wire Screw
BSW British Standard Whitworth Coarse Thread Series Thread Inserts
M Metric ScrewThread Series Stub Acme Stub Acme Thread
N American National 8, 12 and 16 Thread Series *UN Unified Constant Pitch Thread Series
(8N, 12N, 16N) *UNC Unified Coarse Thread Series
NBUTT American Buttress Screw Thread *UNEF Unified Extra Fine Thread Series
SAWS NC American National Coarse Thread Series *UNF Unified Fine Thread Series
NEF American National Extra Fine Thread Series UNJ** Unified Thread Series with a 0.15011P to 0.18042P
NF American National Fine Thread Series Controlled Root Radius on External Thread only.
NGO National Gas Outlet Thread UNJC Unified Coarse Thread Series with a 0.15011P to
NGT National Gas Taper Thread (see “SGT”) 0.18042P Controlled Root Radius on External
NH American National Hose Coupling and Fire Hose Thread only.
TECHNICAL Coupling Threads UNJF Unified Fine Thread Series with a 0.15011P to
NPS For tap marking only (See NPSC, NPSM) 0.18042P Controlled Root Radius on External
NPSC American National Standard Straight Pipe Thread Thread only.
in Pipe Couplings (Tap marked NPS) UNM Unified Miniature Thread Series
NPSF Dryseal American National Standard Fuel Internal UNR Unified Constant Pitch Thread Series with a 0.108P
Straight Pipe Thread to 0.144P Controlled Root Radius; Ext. thread only.
NPSH American National Standard Straight Pipe Thread UNRC Unified Coarse Thread Series with a 0.108P
for Hose Couplings to 0.144P Controlled Root Radius; Ext. thread only.
INDEX NPSI Dryseal American National Standard Intermediate UNRF Unified Fine Thread Series with a 0.108P to 0.144P
Internal Straight Pipe Thread Controlled Root Radius; External thread only.
NPSL American National Standard Straight Pipe Thread for *UNS Unified Thread-Special
Loose Fitting Mechanical Joints with Locknuts V A 60° “V” thread with Truncated Crest and Root.
NPSM American National Standard Straight Pipe Threads The theoretical “V” Form is usually flatted to the
for Free-Fitting Mechanical Joints for Fixtures user’s specifications.
(Tap marked NPS) WHIT British Standard Whitworth Special Thread
NPT American National Standard Taper Pipe Thread
(See ANPT, NPTR) *Taps are not marked with “U” but with the symbol for the corresponding
American Standard thread form with which it is compatible.
** See page 183 for additional information on UNJ taps.
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