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You are here: Home / Tools Comparisons / What does it mean: Tap’s Marking System

What does it mean: Tap’s Marking System

By Editorial Board Leave a Comment

What does it mean: Tap’s Marking SystemTaps are marked according to the Standard System of Marking Ground Thread Taps.

The standard mark consists of four groups of symbols. They contain information about the nominal size, the number of threads per inch or thread pitch, and the appropriate thread form symbol and pitch diameter or thread class symbol.

📖 Related Reading: Our reviews of the best tap and die sets

Tap’s Marking System pic 1

So, what does this mark means? And how to read taps?

Let’s find out.


Nominal Size of Tap

Tap’s Marking System pic 2

The nominal size marking differs for the metric and SAE systems.

Metric sizes

M3
M4
…
M16
and others

SAE sizes

⅛”
¼”
…
¾”
and others


Threads Per Inch

Tap's Marking System — Threads Per Inch

Actual for SAE sizes.
Starts from 11.

Tap's Marking System — Threads Per Inch


Threads Pitch

Tap's Marking System — Threads Pitch

Actual for Metric sizes.
Starts from 0.5 mm.

Tap's Marking System — Threads Pitch


Thread Form Symbol

Tap’s Marking System pic 4

ACME-C Acme Thread-Centralizing
ACME-G Acme Thread-General Purpose
AMO American Standard Microscope Objective Thread
ANPT Aeronautical National Form Taper Pipe Thread (Ground thread tap marked NPT)
BA British Association Standard Thread
BSF British Standard Fine Thread Series
BSP British Standard Pipe
BSPP British Standard Pipe (Parallel) Thread
BSPT British Standard Taper Pipe Thread
BSW British Standard Whitworth Coarse Thread Series
M Metric ScrewThread Series
N American National 8, 12, and 16 Thread Series (8N, 12N, 16N)
NBUTT American Buttress Screw Thread
NC American National Coarse Thread Series
NEF American National Extra Fine Thread Series
NF American National Fine Thread Series
NGO National Gas Outlet Thread
NGT National Gas Taper Thread (see “SGT”)
NH American National Hose Coupling and Fire Hose Coupling Threads
NPS For tap marking only (See NPSC, NPSM)
NPSC American National Standard Straight Pipe Thread in Pipe Couplings (Tap
marked NPS)
NPSF Dryseal American National Standard Fuel Internal Straight Pipe Thread
NPSH American National Standard Straight Pipe Thread for Hose Couplings
NPSI Dryseal American National Standard Intermediate Internal Straight Pipe Thread
NPSL American National Standard Straight Pipe Thread for Loose Fitting Mechanical Joints with Locknuts
NPSM American National Standard Straight Pipe Threads for Free-Fitting Mechanical Joints for Fixtures (Tap marked NPS)
NPT American National Standard Taper Pipe Thread (See ANPT, NPTR)
NPTF Dryseal American National Standard Taper Pipe Thread
NPTR American National Standard Taper Pipe Thread for Railing Joints (Tap marked
NPT)
NS American National Thread-Special
PG Panzer Gewinde
PTF Dryseal SAE Short Taper Pipe Thread
SGT Special Gas Taper Thread
SPL-PTF Dryseal Special Taper Pipe Thread
STI Special Thread for Helical Coil Wire Screw Thread Inserts
Stub Acme Stub Acme Thread
*UN Unified Constant Pitch Thread Series
*UNC Unified Coarse Thread Series
*UNEF Unified Extra Fine Thread Series
*UNF Unified Fine Thread Series
UNJ Unified Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJC Unified Coarse Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJF Unified Fine Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNM Unified Miniature Thread Series
UNR Unified Constant Pitch Thread Series with a 0.108P to 0.144P Controlled Root
Radius; Ext. thread only.
UNRC Unified Coarse Thread Series with a 0.108P to 0.144P Controlled Root Radius; Ext.
thread only.
UNRF Unified Fine Thread Series with a 0.108P to 0.144P Controlled Root Radius; External thread only.
*UNS  Unified Thread-Special
V A 60° “V” thread with Truncated Crest and Root. The theoretical “V” Form is usually flatted to the user’s specifications.
WHIT British Standard Whitworth Special Thread

 


Pitch Diameter Limit

Tap’s Marking System pic 5

All standard ground thread taps are marked with the letter “G” to designate Ground Thread.

The letter G will be followed by:
– the letter “H” to designate above basic;
– the letter “L” to designate below basic.

The number following H or L signifies the number of .0005″ steps above or below the basic pitch diameter.

Pitch Diameter Limits for taps to 1″ diameter
inclusive:
L1 = Basic to Basic minus .0005
H1 = Basic to Basic plus .0005
H2 = Basic plus .0005 to Basic plus .0010
H3 = Basic plus .0010 to Basic plus .0015
H4 = Basic plus .0015 to Basic plus .0020
H5 = Basic plus .0020 to Basic plus .0025
H6 = Basic plus .0025 to Basic plus .0030

Taps larger than 1″ dia. are ground to a .0010″ tolerance on the pitch diameter.
For example, H4 (Basic plus .0010″ to Basic plus .0020″ ).

Metric taps generally have more manufacturing tolerance than .0005 to the minus side.
U1 = Basic minus .0005 = min. tap P.D.
D1 = Basic plus .0005 = max. tap P.D.
D2 = Basic plus .0010 = max. tap P.D.
D3 = Basic plus .0015 = max. tap P.D.
D4 = Basic plus .0020 = max. tap P.D.
D5 = Basic plus .0025 = max. tap P.D.
D6 = Basic plus .0030 = max. tap P.D


Thread Class

Tap's Marking System — Thread Class

Screw thread classes are distinguished from each other by the amount of tolerance and allowance.

Class 1A and Class 1B. The combination of Class 1A  for external threads and Class 1B for internal threads are intended to cover the manufacture of threaded parts where the quick and easy assembly is necessary or desired, and an allowance is provided to permit ready assembly.

Class 2A and Class 2B. The combination of Class 2A for external threads and Class 2B for internal threads designed for screws, bolts, and nuts, is also suitable for a variety of other applications. A similar allowance is provided which minimizes galling and seizure encountered in assembly and use.

It also accommodates, to a limited extent, plating, finishes, or coatings.

Class 3A and 3B: The combination of Class 3A for external threads and Class 3B for internal threads are provided for those applications where the closeness of fit and accuracy of lead and angle of thread are important.

These threads are obtained consistently only by the use of high-quality production equipment supported by a very efficient system of gauging and inspection.


And finally, a complete scheme of tap’s marking system.

Tap’s Marking System pic 6


Editorial Board of ToolsLaboratory.

Table of Contents

  • Nominal Size of Tap
  • Threads Per Inch
  • Threads Pitch
  • Thread Form Symbol
  • Pitch Diameter Limit
  • Thread Class

Filed Under: Editorials, Hand Tools, Tools Comparisons Tagged With: Editorial, Tap and Die, What does it mean

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