Tolerancing principles

This Section establishes practices for expressing tolerances on linear and angular dimensions, applicability of material condition modifiers on geometric tolerance values, and interpretations governing limits and tolerances.

  1. Tolerances application
  2. Direct tolerancing methods
  3. Tolerance expression
  4. Interpretation of limits
  5. Single limits

Tolerances application

Tolerances may be expressed as follows:

(a) as direct limits or as tolerance values applied directly to a dimension. See para. 2.2.

(b) as a geometric tolerance, as described in Sections 5 through 9.

(c) in a note or table referring to specific dimensions.

(d) as specified in other documents referenced on the drawing for specific features or processes.

(e) in a general tolerance block referring to all dimensions on a drawing for which tolerances are not otherwise specified.

Positional Tolerancing Method

Preferably, tolerances on dimensions that locate features of size are specified by the positional tolerancing method described in Section 7. In certain cases, such as locating irregular-shaped features, the profile tolerancing method described in Section 8 may be used.

Basic Dimensions

It uses a symbolic language rather than words on engineering drawings that explicitly describes nominal geometry and its allowable variatiBasic dimensions may be indicated on the drawing in the following ways:

(a) applying the basic dimension symbol to each of the basic dimensions. See Fig. 7-1, illustrations (a) and (b).

(b) specifying on the drawing (or in a document referenced on the drawing) a general note such as: UNTOLERANCED DIMENSIONS ARE BASIC. See Fig. 7-1, illustration (c).

NOTE: Where using this method a plus/minus general tolerance is not allowed.

(c) For specifying and querying basic dimensions on models or digital drawings with models, see ASME Y14.41.

Implied 90° Angle

By convention, where center lines and surfaces of features are depicted on 2D orthographic engineering drawings intersecting at right angles, a 90° angle is not specified. Implied 90° angles are understood to apply. The tolerance on these implied 90° angles is the same as for all other angular features shown on the field of the drawing governed by general angular tolerance notes or general tolerance block values. See para 1.4(i).

Implied 90° or 0° Basic Angle

Where center lines and surfaces are depicted on 2D orthographic engineering drawings intersecting at right angles or parallel to each other and basic dimensions or geometric tolerances have been specified, implied 90° or 0° basic angles are understood to apply. The tolerance on the feature associated with these implied 90° or 0° basic angles is provided by feature control frames that govern the location, orientation, profile, or runout of features. See paras. 1.4(j) and (k).


Direct tolerancing methods

Limits and directly applied tolerance values are specified as follows.

(a) Limit Dimensioning. The high limit (maximum value) is placed above the low limit (minimum value). When expressed in a single line, the low limit precedes the high limit and a dash separates the two values. See Fig. 2-1.

(b) Plus and Minus Tolerancing. The dimension is given first and is followed by a plus and minus expression of tolerance. See Fig. 2-2.

(c) Geometric Tolerances Directly Applied to Features. See Sections 5 through 9.

Metric Limits and Fits

For metric application of limits and fits, the tolerance may be indicated by a basic size and tolerance symbol as in Fig. 2-3. See ANSI B4.2 for complete information on this system.

Limits and Tolerance Symbols

The method shown in Fig. 2-3, illustration (a) is recommended when the system is introduced by an organization. In this case, limit dimensions are specified, and the basic size and tolerance symbol are identified as reference.

Tolerance Symbol and Limits

As experience is gained, the method shown in Fig. 2-3, illustration (b) may be used. When the system is established and standard tools, gages, and stock materials are available with size and symbol identification, the method shown in Fig. 2-3, illustration (c) may be used.


Tolerance expression

The conventions shown in the following paragraphs shall be observed pertaining to the number of decimal places carried in the tolerance.

Millimeter Tolerances

Where millimeter dimensions are used on the drawings, the following apply.

(a) Where unilateral tolerancing is used and either the plus or minus value is nil, a single zero is shown without a plus or minus sign. In this example the 32 value is the nominal size.

(b) Where bilateral tolerancing is used, both the plus and minus values have the same number of decimal places, using zeros where necessary. In this example the 32 value is the nominal size.

(c) Where limit dimensioning is used and either the maximum or minimum value has digits following a decimal point, the other value has zeros added for uniformity.

(d) Where basic dimensions are used, associated tolerances contain the number of decimal places necessary for control. The basic dimension value observes the practices of para. 1.6.1.


Inch Tolerances

Where inch dimensions are used on the drawing, the following apply:

(a) Where unilateral tolerancing is used and either the plus or minus value is nil, its dimension shall be expressed with the same number of decimal places and the appropriate plus or minus sign.

(b) Where bilateral tolerancing is used, both the plus and minus values and the dimension have the same number of decimal places.

(c) Where limit dimensioning is used and either the maximum or minimum value has digits following a decimal point, the other value has zeros added for uniformity.

(d) Where basic dimensions are used, associated tolerances contain the number of decimal places necessary for control. There is no requirement for the basic dimension value to be expressed with the same number of decimal places as the tolerance.


Angle Tolerances

Where angle dimensions are used, both the plus and minus values and the angle have the same number of decimal places.


Interpretation of limits

All limits are absolute. Dimensional limits, regardless of the number of decimal places, are used as if they were continued with zeros.

Plated or Coated Parts

Where a part is to be plated or coated, the drawing or referenced document shall specify whether the dimensions apply before or after plating. Typical examples of notes are the following:

(a) “DIMENSIONAL LIMITS APPLY AFTER PLATING.”

(b) “DIMENSIONAL LIMITS APPLY BEFORE PLATING.”

(For processes other than plating, substitute the appropriate term.)


Single limits

MIN or MAX is placed after a dimension where other elements of the design definitely determine the other unspecified limit. Features, such as depths of holes, lengths of threads, corner radii, chamfers, etc., may be limited in this way. Single limits are used where the intent will be clear, and the unspecified limit can be zero or approach infinity and will not result in a condition detrimental to the design.


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