GEOMETRIC DIMENSIONING AND TOLERANCING (GDT)

GEOMETRIC DIMENSIONING AND TOLERANCING (GD&T)

GD&T is an engineering product definition standard that geometrically describes design intent. It also provides the documentation base for the design of quality and production systems. Used for communication between product engineers and manufacturing engineers , it promotes a uniform interpretation of a component's production requirements.

This interpretation and communication are of interest to those who are about to undertake the DFSS baton. Without the appropriate and applicable interpretation of the design and without the appropriate communication of that design to manufacturing, problems will definitely occur.

Therefore, in this section we will address some of the key aspects of GD&T in a cursory manner. We will touch on some of the definitions and principles of general tolerancing as applied to conventional dimensioning practices. The term conventional dimensioning as used here implies dimensioning without the use of geometric tolerancing. Conventional tolerancing applies a degree of form and location control by increasing or decreasing the tolerance.

Conventional dimensioning methods provide the necessary basic background to begin a study of geometric tolerancing. It is important that you completely understand conventional tolerancing before you begin the study of geometric tolerancing.

When mass production methods began , interchangeability of parts was important. However, many times parts had to be "hand selected for fitting." Today, industry has faced the reality that in a technological environment, there is no time to do unnecessary individual fitting of parts. Geometric tolerancing helps ensure inter-changeability of parts . The function and relationship of a particular feature on a part dictates the use of geometric tolerancing.

Geometric tolerancing does not take the place of conventional tolerancing. However, geometric tolerancing specifies requirements more precisely than conventional tolerancing does, leaving no doubts as to the intended definition. This precision may not be the case when conventional tolerancing is used, and notes on the drawing may become ambiguous.

 

When dealing with technology, a drafter needs to know how to properly represent conventional dimensioning and geometric tolerancing. Also, a technician must be able to accurately read dimensioning and geometric tolerancing. Generally , the drafter converts engineering sketches or instructions into formal drawings using proper standards and techniques. After acquiring adequate experience, a design drafter, designer, or engineer begins implementing geometric dimensioning and tolerancing on the research and development of new products or the revision of existing products.

Most dimensions in this text are in metric. Therefore, a 0 precedes decimal dimensions less than one millimeter, as in 0.25. When inch dimensions are used, a 0 will not precede a decimal dimension that is less than one inch. For review of decimals and their operations, refer to Volume II of this series.

Most dimensions in this text are in the metric International System of Units (SI). The common SI unit of measure used on engineering drawings is the millimeter. The common U.S. unit used on engineering drawings is the inch. (The reader may want to review the discussion and conversions of the SI system in Chapter 20 of Volume II of this series.) The actual units used on your engineering drawings will be determined by the policy of your company. The general note "UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN MILLIMETERS" (or "INCHES") should be placed on the drawing when all dimensions are in either millimeters or inches. When some inch dimensions are placed on a metric drawing, the abbreviation "IN." should follow the inch dimensions. The abbreviation "mm" should follow any millimeter dimensions on a predominantly inch-dimensioned drawing. Angular dimensions are established in degrees ( °) and decimal degrees (X.X °), or in degrees ( °) minutes (') and seconds (").

The following are some rules for metric and inch dimension units (for a more detailed discussion see Volume II of this series):

Millimeters

• The decimal point and zero are omitted when the metric dimension is a whole number. For example, the metric dimension "12" has no decimal point followed by a zero.

• When the metric dimension is greater than a whole number by a fraction of a millimeter, the last digit to the right of the decimal point is not followed by a zero. For example, the metric dimension "12.5" has no zero to the right of the five. This rule is true unless tolerance values are displayed.

• Both the plus and minus values of a metric tolerance have the same number of decimal places. Zeros are added to fill in where needed.

• A zero precedes a decimal millimeter that is less than one. For example, the metric dimension "0.5" has a zero before the decimal point.

• Examples in ASME Y14.5M show no zeros after the specified dimension to match the tolerance. For example, 24 ± 0.25 or 24.5 ± 0.25 are correct. However, some companies prefer to add zeros after the specified dimension to match the tolerance, as in 24.00 ± 0.25 or 24.50 ± 0.25.

Inches

• A zero does not precede a decimal inch that is less than one. For example, the inch dimension ".5" has no zero before the decimal point.

• A specified inch dimension is expressed to the same number of decimal places as its tolerance. Zeros are added to the right of the decimal point if needed. For example, the inch dimension ".250 ± .005" has an additional zero added to ".25" to match the three-decimal tolerance.

• Fractional inches may be used but generally indicate a larger tolerance. Fractions may be used to give nominal sizes such as in a thread callout.

• Both the plus and minus values of an inch tolerance have the same number of decimal places. Zeros are added to fill in where needed.

• Zeros are added where needed after the specified dimension to match the tolerance. For example, 2.000 ± .005 and 2.500 ± .005 both have zeros added to match the tolerance.

The following rules are summarized from ASME Y14.5M. These rules are intended to give you an understanding of the purpose for standardized dimensioning practices. Short definitions are given in some cases:

• Each dimension has a tolerance except for dimensions specifically identified as reference, maximum, minimum, or stock. The tolerance may be applied directly to the dimension, indicated by a general note, or located in the title block of the drawing.

• Dimensioning and tolerancing must be complete to the extent that there is full understanding of the characteristics of each feature. Neither measuring the drawing or assumption of a dimension is permitted. Exceptions include drawings such as loft, printed wiring, templates, and master layouts prepared on stable material. However, in these cases the necessary control dimensions must be given.

• Each necessary dimension of an end product must be shown. Only dimensions needed for complete definition should be given. Reference dimensions should be kept to a minimum.

• Dimensions must be selected and arranged to suit the function and mating relationship of a part. Dimensions must not be subject to more than one interpretation.

• The drawing should define the part without specifying the manufacturing processes. For example, give only the diameter of a hole without a manufacturing process such as "DRILL" or "REAM." However, there should be specifications given on the drawing, or related documents, in cases where manufacturing, processing, quality assurance, or environmental information is essential to the definition of engineering requirements.

• It is allowed to identify (as non-mandatory) certain processing dimensions that provide for finish allowance, shrink allowance, and other requirements, provided the final dimensions are given on the drawing. Non-mandatory processing dimensions should be identified by an appropriate note, such as "NON-MANDATORY (MFG DATA)."

• Dimensions should be arranged to provide required information arranged for optimum readability. Dimensions should be shown in true profile views and should refer to visible outlines.

• Wires, cables, sheets, rods, and other materials manufactured to gage or code numbers should be specified by dimensions indicating the diameter or thickness . Gage or code numbers may be shown in parentheses following the dimension.

• A 90 ° angle is implied where centerlines, and lines displaying features, are shown on a drawing at right angles and no angle is specified. The tolerance for these 90 ° angles is the same as the general angular tolerance specified in the title block or in a general note.

• A 90 ° basic angle applies where centerlines of features are located by basic dimensions and no angle is specified. Basic dimensions are considered theoretically perfect in size , profile, orientation, or location. Basic dimensions are the basis for variations that are established by other tolerances.

• Unless otherwise specified, all dimensions are measured at 20 °C (68 °F). Compensation may be made for measurements taken at other temperatures .

• All dimensions and tolerances apply in a free state condition except for non-rigid parts. Free state condition describes distortion of the part after removal of forces applied during manufacturing. Non-rigid parts are those that may have dimensional change due to thin wall characteristics.

• Unless otherwise specified, all geometric tolerances apply for full depth, length, and width of the feature.

• Dimensions apply on the drawing where specified.

To appreciate GD&T, the following definitions must be understood to be successful in your dimensioning practices:

• Actual size ” The measured size of a feature or part after manufacturing.

• Diameter ” The distance across a circle measured through the center. Represented on a drawing with the symbol "0." Circles on a drawing are dimensioned with a diameter.

• Dimension ” A numerical value indicated on a drawing and in documents to define the size, shape, location, geometric characteristics, or surface texture of a feature. Dimensions are expressed in appropriate units of measure.

• Feature ” The general term applied to a physical portion of a part or object. A surface, slot, tab, keyseat, or hole are all examples of features.

• Feature of size ” One cylindrical or spherical surface, or a set of two parallel plane surfaces, each feature being associated with a size dimension.

• Nominal size ” A dimension used for general identification such as stock size or thread diameter.

• Radius ” The distance from the center of a circle to the outside. Arcs are dimensioned on a drawing with a radius. A radius dimension is preceded by an "R." The symbol "CR" refers to a controlled radius. A controlled radius means that the limits of the radius tolerance zone must be tangent to the adjacent surfaces, and there can be no reversal in the contour. The use of CR is more restrictive than R (where reversals are permitted). The symbol "SR" refers to a spherical radius.

• Reference dimension ” A dimension, usually without a tolerance, used for information purposes only. This dimension is often a repeat of a given dimension or established from other values shown on the drawing. A reference dimension does not govern production or inspection. A reference dimension is shown on a drawing with parentheses. For example, (60) would indicate a reference dimension.

• Stock size ” A commercial or premanufactured size, such as a particular size of square, round, or hex steel bar.

A tolerance is the total amount that a specific dimension is permitted to vary. A tolerance (not to be confused with TOLERANSING) is not given to values that are identified as reference, maximum, minimum, or stock sizes. The tolerance may be applied directly to the dimension, indicated by a general note, or identified in the drawing title block.

The limits of a dimension are the largest and smallest numerical value that the feature can be. For example: A dimension is stated as 12.50 ± 0.25. This is referred to as plus-minus dimensioning. The tolerance of this dimension is the difference between the maximum and minimum limits. The upper limit is 12.50 + 0.25 = 12.75 and the lower limit is 12.50 - 0.25 = 12.25. So, if you take the upper limit and subtract the lower limit you have the tolerance: 12.75 - 12.25 = 0.50.

The specified dimension is the part of the dimension from which the limits are calculated. The specified dimension of the example above is 12.5. A dimension on a drawing may be displayed with plus-minus dimensioning, or the limits may be specified as 12.75 and 12.25. Many companies prefer this second method because the limits are shown and calculations are not required. This is called limits dimensioning.

A bilateral tolerance is permitted to vary in both the + and the - directions from the specified dimension. An equal bilateral tolerance is where the variation from the specified dimension is the same in both directions. An unequal bilateral tolerance is where the variation from the specified dimension is not the same in both directions.

A unilateral tolerance is permitted to increase or decrease in only one direction from the specified dimension.