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DIMENSIONING CHAPTER TEN

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DIMENSIONING ANGLES Angles are dimensioned by specifying the angle in degrees and a linear dimension. The coordinate method is better when a high degree of accuracy is required

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**DIMENSIONING ARCS, FILLETS AND ROUNDS**

A circular arc is dimensioned in the view where its true shape in seen by giving the value for its radius preceded by the abbreviation R. Individual fillets and rounds are dimensioned like other arcs. FILLETS R6 AND ROUNDS R3 UNLESS OTHERWISE SPECIFIED or ALL CASTING RADII R6 UNLESS NOTED or simply ALL FILLETS AND ROUNDS R6.

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**SIZE DIMENSIONS: CYLINDERS**

Cylinders are usually dimensioned by giving the diameter and length where the cylinder appears as a rectangle. Use “Ø” to indicate circular shape Dimensioning a Machine Part Composed of Cylindrical Shapes

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**SIZE DIMENSIONING HOLES**

The leader of a note should point to the circular view of the hole, if possible. Countersunk, counterbored, spotfaced and tapped holes are usually specified by standard symbols or abbreviations.

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**DIMENSIONING COUNTERBORES AND SPOTFACES WITH FILLETS**

When a fillet radius is specified for a spotface dimension, the fillet radius is added to the outside of the spotface diameter, Counterbored hole with a fillet radius specified.

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DIMENSIONING CURVES One way to dimension curves is to give a group of radii. Both circular and noncircular curves may be dimensioned by using coordinate dimensions, or datums. When angular measurements are unsatisfactory, you may give chordal dimensions

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**DIMENSIONING ROUNDED-END SHAPES**

For accuracy, in parts d–g, overall lengths of rounded-end shapes are given, and radii are indicated, but without specific values. The center-to-center distance may be required for accurate location of some holes. In part g, the hole location is more critical than the location of the radius, so the two are located.

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DIMENSIONING THREADS Local notes are used to specify dimensions of threads. For tapped holes, the notes should, if possible, be attached to the circular views of the holes.

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DIMENSIONING TAPERS A taper is a conical surface on a shaft or in a hole. The usual method of dimensioning a taper is to give the amount of taper in a note, such as TAPER ON DIA (with TO GAGE often added), and then give the diameter at one end with the length or give the diameter at both ends and omit the length. Taper on diameter means the difference in diameter per unit of length.

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**DIMENSIONING CHAMFERS**

Achamfer is a beveled or sloping edge. It is dimensioned by giving the length of the offset and the angle, as in Figure 10.51a. A 45° chamfer also may be dimensioned

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LOCATION DIMENSIONS After you have specified the sizes of the geometric shapes composing the structure, give location dimensions to show the relative positions of these geometric shapes.

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MATING DIMENSIONS Mating dimensions should be given on the multiview drawings in the corresponding locations.

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TABULAR DIMENSIONS A series of objects having like features but varying in dimensions may be represented by one drawing.

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**COORDINATE DIMENSIONING**

A set of three mutually perpendicular datum or reference planes is usually required for coordinate dimensioning. These planes either must be obvious.

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NOTES It is usually necessary to supplement the direct dimensions with notes. Notes should be brief and carefully worded to allow only one interpretation. Notes should always be lettered horizontally on the sheet and arranged systematically. They should not be crowded and should not be placed between views, if possible. Notes are classified as general notes when they apply to an entire drawing and as local notes when they apply to specific items.

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TOLERANCING C H A P T E R E L E V E N

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**OBJECTIVES Describe the nominal size, tolerance, limits, and**

allowance of two mating parts. 2. Identify a clearance fit, interference fit, and transition fit. 3. Describe the basic hole and basic shaft systems. 4. Dimension mating parts using limit dimensions, unilateral tolerances, and bilateral tolerances. 5. Describe the classes of fit and give examples of each. 6. Draw geometric tolerancing symbols. 7. Specify geometric tolerances. 8. Relate datum surfaces to degrees of freedom.

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**UNDERSTANDING TOLERANCE**

Tolerance is the total amount a specific dimension is permitted to vary. Tolerances are specified so that any two mating parts will fit together. To effectively provide tolerances in your drawings and CAD models, you must: • Understand the fit required between mating parts. • Have a clear picture of how inspection measurements are performed. • Be able to apply tolerance symbols to a drawing or model. • Apply functional tolerancing to individual features of the part. The inner workings of a watch are an example of parts that must fit precisely to work. (Courtesy of SuperStock, Inc.)

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Quality Control Before paying for parts, most companies have a process to quality certify (QC) the parts against the drawing or model. A tolerance must be specified for each dimension so that it can be determined how accurately the part must be manufactured to be acceptable. The tolerances that you specify are based on the part’s function and fit.

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**Definitions for Size Designation**

Definitions of size designation terms that apply in tolerancing. Feature Feature of size Actual local feature Nominal size Nominal size Allowance Allowance

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**Variations in Form and Envelope**

You can think of tolerance as defining a perfect form envelope that the real produced part must fit inside in order to be acceptable. Actual minimum material envelope This envelope is the counterpart to the actual mating envelope. You can sometimes notice variations in form by placing a machinists’ scale along the edge of the part and checking to see whether you can slip a feeler gage between the scale and the edge of the part.

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**Material Envelope continued…**

Actual mating envelope The envelope toward the outside of the material, in which the acceptable actual feature must fit. For external parts, like cylinders, this is the perfect feature at the largest permissible size; for internal features, like holes, this is the perfect feature at the smallest permissible size.

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Implied Right Angles Implied 90° angles have the same general tolerances applied to them as do any other angles covered by a general note. The exception is when a geometric tolerance is used for that feature. When geometric tolerances are specified, implied 90° or 0° angles between feature centerlines are considered basic dimensions to which no tolerance applies outside that stated by the geometric tolerance. The tolerance of plus or minus 1° applies to the implied 90° angles as well as to the dimensioned angles in the drawing.

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**Fits between Mating Parts**

Fit is the range of tightness or looseness resulting from the allowances and tolerances in mating parts. The loosest fit, or maximum clearance, occurs when the smallest internal part (shaft) is in the largest external part (hole),

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LIMIT TOLERANCES Limit tolerances state the upper and lower limits for the dimension range in place of the dimension values. Method of Stating Limits Note: The upper value is always placed above the lower value.

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**PLUS-OR-MINUS TOLERANCES**

• Unilateral when the tolerance applies in only one direction so that one value is zero; or, • Bilateral when either the same or different values are added and subtracted.

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Angular tolerances Angular tolerances are usually bilateral and given in terms of degrees, minutes, and seconds, unless geometric dimensioning and tolerancing is used. Plus/Minus–Toleranced Decimal Dimensions

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TOLERANCE STACKING A Chained dimension uses the end of one dimension as the beginning of the next. Tolerance stacking refers to the way the tolerance for one dimension is added to the next dimension in the chain and so on from one feature to the next, resulting in a large variation in the location of the last feature in the chain. Baseline dimensioning locates a series of features from a common base feature. Tolerances do not stack up because dimensions are not based on other toleranced dimensions.

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**Tolerances and CAD 3D models**

SolidWorks software makes it easy to select surface finish symbols. (Courtesy of Solidworks Corporation.) Tolerances can be added directly to a 3D model so that it can be used as the digital product definition. (Reprinted from ASME Y , by permission of The American Society of Mechanical Engineers. All rights reserved.)

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Geometric Tolerances J. M. McCarthy Fall 2003

Geometric Tolerances J. M. McCarthy Fall 2003

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