Excerpted from ANSI B1.13M-1979:
The American Society of Mechanical Engineers.

Tolerance System

ISO System of Limits and Fits as applied to Screw Threads


General The international metric tolerance system is based on a system of limits and fits. The limits of the tolerances on the mating parts and their allowances (fundamental deviations) determine the fit of the assembly. For simplicity the system is described for cylindrical parts. Holes are equivalent to internally threaded surfaces and shafts to externally threaded surfaces.

Basic Size is the zero line or surface at assembly, where the interface of the two mating parts have a common reference.

Upper Deviation is the algebraic difference between the maximum limit of size and the basic size. It is designated by the French term “ecart superieur” (ES for holes and es for shafts).

Lower Deviation is the algebraic difference between the minimum limit of size and the basic size. It is designated by the French term “ecart inferieur” (El for holes and ei for shafts).

Fundamental Deviations (Allowances) are the deviations that are closest to the basic size. In Figure 1 they would be El and es. Fundamental deviations are designated by alphabetical letters.

Fits are determined by the fundarrtental deviation, which may be positive or negative, assigned to the mating parts. The selected fits can be clearance, transition, and interference. To illustrate fits schematically, a zero line is drawn to represent the basic size. By convention, the shaft always lies below the zero line and the hole lies above the zero line (except for interference fits). This makes the fundamental deviation negative for the shaft and equal to its upper deviation (es). The fundamental deviation is positive for the hole and equal to its lower deviation (El). See Figure 1.

Tolerance is defined by a series of numerical grades, each grade provides numerical values for nominal sizes corresponding to the standard tolerance.

The Tolerance Grade
This is indicated by a number. The system provides for a series of tolerance grades for each of the four screw thread parameters:



Tolerance Grade
D1
d
D2
d2
Minor diameter of internal threads
Major diameter of external threads
Pitch diameter of internal threads
Pitch diameter of external threads
4, 5, 6, 7, 8
4, 6, 8
4, 5, 6, 7, 8
3, 4, 5, 6, 7, 8, 9

The Tolerance Position

This is indicated by a letter. This position is the allowance (fundamental deviation). A capital letter is used for internal threads and a lower case letter for external threads. The system provides a series of tolerance positions for internal and external threads. These tolerance positions are as follows:

Internal Threads
External Threads
G, H
e, g, h




Coated or Plated Threads

Coated Threads Coating is one or more applications of additive material to the threads, including dry film lubricants, but excluding soft or liquid lubricants.







Material Limits for Coated and Plated Threads The pitch diameter on tolerance position H/h threads shall be within adjusted limits before coating, and after coating the threads shall not exceed the tabulated maximum material limits specified herein. On tolerance position g threads, unless otherwise specified, the tolerance position g on the external thread may be used to accommodate the coating or plating thickness and the threads after coating or plating shall not exceed the basic size. When the tolerance position g must be retained on coated or plated external threads the thread class designation shall be followed by the words: AFTER COATING or AFTER PLATING.




     
     

General

The complete designation of a screw thread gives the thread symbol. the nominal size and the thread tolerance class.

The tolerance class designation gives the class designation for the pitch diameter tolerance followed by a doss designation for the crest diameter (major diameter for external thread and minor diameter for internal thread) tolerances. The class designation Consists of a number indicating the tolerance grade followed by a letter indicating the tolerance position.

Examples:

External thread 4g6g

where:

4g 6g represents the Tolerance Class; and
4g represents the Thread Tolerance Class Designation for Pitch Diameter; and
6g represents the Thread Tolerance Class Designation for Major Diameter.

Internal thread 6H

where:

6H represents the thread class designation for pitch and minor diameter with identical tolerance class designations.

Designation of Standard Screw Threads

Metric screw threads are identified by letter (M) for the thread form profile, followed by the nominal diameter size and the pitch expressed in millimeters. separated by the sign (x) and followed by the tolerance class separated by a dash (—) from the pitch. The simplified international practice for designating course pitch M profile screw threads is to leave off the pitch. Thus a Ml 4 x 12 thread is designated just M14. To prevent misunderstanding, it is mandatory to use the value for pitch in all other designations. Unless otherwise specified in the designation, the screw thread helix is right hand.

Example:

External Thread M Profile, Right Hand:

M6 x 1—4g6g

where:

M represents the Metric Thread Symbol, ISO 68 Metric Thread; and
6 represents the Nominal Size; and
1 represents the Pitch; and
4g6g represents the Tolerance Class; while
  4g represents the Pitch Diameter Tolerance Symbol with
    4 representing the Tolerance Position; and
    g representing the Tolerance Grade; and while
  6g represents the Major Diameter Tolerance Symbol with
    6 representing the Tolerance Grade; and
    g representing the Tolerance Position.

Internal Thread M Profile, Right Hand:

M6 x 1—5H6H

where:

M represents the Metric Thread Symbol, ISO 68 Metric Thread Form; and
6 represents the Nominal Size; and
1 represents the Pitch; and
5H6H represents the Tolerance Class; while
  5H represents the Pitch Diameter Tolerance Symbol with
    5 representing the Tolerance Position; and
    H representing the Tolerance Grade; and while
  6H represents the Major Diameter Tolerance Symbol with
    6 representing the Tolerance Grade; and
    H representing the Tolerance Position.

Designation of Left Hand Thread

When left hand thread is specified. the tolerance class designation is followed by a space and LH.

Example:

M6 x 1—5H6H-LH

Designation for Equal Tolerance Classes

If the two tolerance class designations for a thread are identical, it is not necessary to repeat the symbols.

Example:

M6 x 1—6H

Designations Using All Uppercase Letters

When computer and teletype thread designations use all uppercase letters, the external or internal thread may need further identification. Thus the tolerance class identification is followed by the abbreviations EXT or INT in capital letters.

Examples:

M6 x 1—4G6G EXT
M6 x 1—6H INT

Designation for Thread Fit

A fit between mating threads is indicated by the internal thread tolerance class followed by the external thread tolerance class separated by a slash.

Examples:

M6 x 1—6H/6g
M6 x 1—6H/4g6g

Designation for Rounded Root External Thread

Modified ISO 68 Thread with Radius Root Rmin = 0.125P. For the mandatory condition external fasteners of property class 8.8 and stronger no special designation is required. Other parts requiring a 0.125P root radius must specify that radius.

Designation for Special Rounded Root Thread

When a special rounded root thread is required its external thread designation is suffixed by the minimum root radius value in millimeters and the letter R.

Example:

M42 x 4.5—6g-0.63R

where:

0.63R represents the Minimum Root Radius


























Designation of Threads Having Modified Crests

Where the limits of size of the major diameter of an external thread or the minor diameter of an internal thread are modified, the thread designation is suffixed by the letters MOD followed by the modified diameter limits.

Example: External thread M profile, major diam. reduced 0.075mm.

M6 x 1—4h6h MOD
Major dia. 5.745—5.925 MOD

Example: Internal thread M profile, minor diam. increased 0.075mm.

M6 x 1—4H5H MOD
Minor dia. = 5.101—5.291 MOD

Designation of Special Threads

Special diameter-pitch threads, developed in accordance with this standard, shall be identified by the letters SPL following the tolerance class. Below the designation shall be specified the limits of size for major diameter, pitch diameter and minor diameter.

Example: External thread

M6.5 x 1—4h6h-SPL
Major dia. 6.320—6.500
Pitch dia. = 5.779—5.850
Minor dia. = 5.163—5.386

Example: Internal Thread

M6.5 x 1—4H5H—SPL
Major dia. = 6.500mm
Pitch dia. = 5.850—5.945
Minor dia. = 5.417—5.607



Designation of Multiple Start Threads

When a thread is required with a multiple start, it is designated by specifying sequentially M for metric thread, nominal diameter size, x L for lead, lead value, dash, P for pitch, pitch value, dash, tolerance class, parenthesis, script number of starts, and the word starts, close parenthesis.

Examples:

M16 x L4—P2—4h6h (TWO STARTS)
M14 x L6—P2—6H (THREE STARTS)

Designation of Coated or Plated M Threads

Specify if the tolerance class is after coating or after plating. If no designation of after coating or after plating is specified, the tolerance class applies before coating or plating in accordance with ISO practice. After plating the thread profile shall not transgress the maximum material limits for the tolerance position H/h.

Examples:

M6 x 1—6h AFTER COATING or AFTER PLATING
M6 x 1—6g AFTER COATING or AFTER PLATING

Where the tolerance position Gig is insufficient relief for the application to hold the threads within product limits, the coating or plating allowance may be specified as the maximum and minimum limits of size for minor and pitch diameter of internal threads or major and pitch diameter of external threads before coating or plating.

Example: Allowance on external thread M profile based on 0.010mm min coating thickness.

M6 x 1—4h6h AFTER COATING
BEFORE COATING
Major dia. = 5.780—5.940
Pitch dia. = 5.239—5.290