58 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. The same methods may be used on metric draw- ings to show a reference inch value. The metric value is shown as a fi rm requirement, and the inch dimen- sion is shown as a reference value in parentheses. The reference inch value is placed below the metric value. A note such as the following is added to the product defi nition that states: VALUES SHOWN IN PARENTHESES ARE INCHES AND ARE FOR REFERENCE ONLY. Conversion of dimension values from one sys- tem to the other must be approached with caution. The two values are supposed to be equal, but round- ing off values will cause some differences. There should be equal values for both the dimension value and any tolerance directly applied to the dimension. Conversion from one measurement system to the other requires the use of an accurate conver- sion factor. Rounding off converted values after fi nal calculations will minimize differences caused by rounding. Converted numbers that included rounded-off values generally should not be used in calculations, manufacturing, or inspection. Inch Dimension Values Dimension values shown in inches should be shown as decimal inches. Decimal inch values may be displayed as whole numbers or in tenths, hundredths, thousandths, or even smaller increments, depending on design requirements. HISTORY BRIEF Fractional Dimensions The use of common fractions was included in USASI Y14.5-1966. Fractions were widely used in industry and some old drawings may contain them. Fractions should not be used on new product defi nition that is intended to comply with current standards.. Applications in today’s industry often require that a high degree of accuracy be maintained in the manufacture of products. The required degree of accuracy makes fractions impractical. As an example, installation of some bearings requires that shaft sizes be produced with variations less than .0005″ (fi ve ten-thousandths) of diameter. Specifying dimen- sional control of this precision does not permit the use of fractions. The use of decimal dimensions per- mits specifi cation of allowable part variation to any necessary degree of accuracy. Of course, the specifi ed l y y y ply s tolerance should never be smaller than what is needed to achieve the intended design function. The number of decimal places used for inch val- ues is normally two or three but more or less may be used where needed. The number of decimal places does not affect the accuracy of the specifi ed value when the additional digits are zeroes. The decimal values .23, .230, and .23000 are exactly the same. The following rules apply for decimal inch values: ■ A zero is not placed in front of decimal inch values less than one inch. ■ Decimal inch values are shown with the same number of decimal places when: ■ a single plus or minus (±) tolerance value is applied immediately following a dimension value. ■ two tolerance values (one plus tolerance and one minus tolerance) follow a dimension. ■ limit dimension values are shown. Examples: ■ .250 not 0.250 ■ .375 ±.010 not .375 ±.01 ■ .500 +.010 –.002 not .500 ■ not Metric Dimension Values As previously mentioned, the standard unit of measurement for drawings using the metric system is the millimeter. One thousand millimeters equals one meter, and 25.4 millimeters equals one inch. Values less than 1 millimeter are shown using decimals. One-place decimals are common. See Figure 3-15 for examples of millimeter values and how they are displayed. The following rules apply for millimeter values: ■ A leading zero is used for any value less than one. ■ A whole number value shows no decimal point or trailing zeroes. ■ A tolerance value of zero is shown with no plus or minus sign and no decimal point. ■ Plus and minus tolerance values less than one have a leading zero and an equal number of decimal places. The number of decimal places may not be the same as that used for the dimension value. ■ Limit dimension values have an equal number of decimal places. +.01 –.002 .510 .502 .51 .502 370 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. Datum feature A is large enough to permit establishing a reliable and repeatable datum axis that serves as the axis of rotation. When the part is pro- duced, a datum simulator such as a chuck or collet can clamp on datum feature A while cutting the two small diameters. The part will be stable in the manu- facturing setup, and the forces applied while cutting the small diameters will not move the part within the chuck or collet. It should be understood that the part may not be held by the datum feature during fabrication. The datum feature may be fabricated in the same setup as the features with the runout tolerances. However, the requirement of the tolerances that are specifi ed relative to the datum feature must be met for the part to be acceptable. Improper selection of a datum feature could make it diffi cult to produce and inspect a part. Selecting the wrong feature to establish the datum axis may increase product cost or even make the tolerances impossible to verify. Consider selection of the smallest diameter on the given part as the datum feature. If this were done, the datum axis would be diffi cult to establish accurately because of the large size of the part relative to the small diameter and length of the selected fea- ture. The mass of the part alone may tend to cause the part to move in the chuck or collet. Model Orthographic views Interpretation Chain line 1.20 1.20 Chain line FIM Position dimension not needed .30 ≤ .009 Goodheart-Willcox Publisher Figure 10-8. A limited zone of application for a runout tolerance is indicated by supplementary geometry in a model or a heavy chain line in an orthographic view. 418 Copyright Goodheart-Willcox Co., Inc. GD&T: Application and Interpretation Application Problems Each of the following problems requires that a sketch be made. All sketches should be neat and accurate. Apply all required dimensions in compliance with dimensioning and tolerancing requirements. Show any required calculations. 25. Apply a bilateral surface profile tolerance of .020″ relative to datum feature A primary and datum feature B secondary. Ensure the tolerance applies between the noted locations. 26. Apply an unequally disposed surface profile tolerance of .015″ with the full .015″ zone lying to the outside of the material (adds material). Reference datum feature A primary and datum feature B secondary. Do not use the past practice alternate method. Ensure the tolerance applies between the noted locations. Extent 1 Extent 2 Extent 1 Extent 2 27. Apply a surface profile tolerance of .018″ all around the cutout. Relate the tolerance to datum features A primary, B secondary, and C tertiary. Datum features B and C should be RMB. 28. Apply a surface profile tolerance of .020″ relative to datum feature A primary and datum feature B secondary. Limit the application to the surface area between the indicated points. Label the points. Copyright Goodheart-Willcox Co., Inc. 416 Chapter Summary → Profi le of a line is the control of line elements in one direction on a surface. → Profi le of a surface simultaneously controls all points on a surface. → Profi le tolerances establish requirements for the full extent of the surface to which they are applied unless explicit limits of extent are defi ned. → An all-around symbol is required where a profi le tolerance extends all the way around the profi le of the part. → The all-over symbol indicates the profi le tolerance is applicable to all surfaces. → Profi le tolerance zones are equally disposed bilateral unless indicated otherwise by the application of the unequally disposed profile symbol. → An unequally disposed profi le symbol may be used to indicate that a profi le tolerance applies in one direction relative to the true profi le. → Levels of control that may be achieved with surface profi le tolerances include form, form and orientation, and form, orientation, and location. Form control also includes size control in some situations. → Coplanarity requirements may be specifi ed with a profi le tolerance. → Coplanarity, orientation, and location of multiple features may be achieved with profile tolerances. → Profi le is the only geometric tolerance that defi nes in one tolerance value the size, form, orientation, and location requirements. → A dynamic profi le tolerance may be used to indicate that the specifi ed tolerance value controls form but not size. → A nonuniform profi le tolerance zone varies in width across the feature. → Composite profi le specifi cations may be used to indicate a relatively large profi le location tolerance and a smaller profi le feature tolerance to control form, size, and orientation. → Multiple single-segment profi le tolerances may be applied to a single feature or a pattern of features. → A profi le tolerance and a position tolerance may be applied to the same feature. Review Questions Answer the following questions using the information provided in this chapter. Additional review questions and problems are available in the Study Guide. Multiple Choice 1. Single-segment profile tolerance specifications may include _____ datum feature references. A. no B. one C. two or three D. All of the above. 2. Profile tolerance values are always applied _____. A. RFS B. MMC C. LMC D. None of the above. 3. A profile tolerance with _____ datum feature references only controls form or possibly form and size. A. no B. one C. two or three D. None of the above. 4. Variations measured on one surface element are independent of the variations on adjacent elements on the same surface if a _____ tolerance is used. A. flatness B. line profile C. surface profile D. composite profile 5. When a profile tolerance is applied to a portion of a feature, _____ must be indicated. A. datums B. a surface profile tolerance C. a line profile tolerance D. extent of application History Brief notes help you understand formerly used practices encountered on documentation from the past. Illustrations have been designed to clearly and simply communicate the specifi c topic. Illustrations have been thoroughly updated for this edition. Many new illustrations have been added. Chapter Summary provides an additional review tool and reinforces key learning objectives. Review Questions allow you to demonstrate knowledge and comprehension of chapter material. Application Problems extend your learning and allow you to apply knowledge and skills.