Guided Tour Learning Objectives clearly identify the knowledge and skills to be obtained when the chapter is completed. 136 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. much work to implement a minor revision. For this reason, there is a method for showing a dimension that is out of scale relative to the drawn geometry. If a dimension on a manually created drawing is revised by more than .030″ and the geometry is not revised, the dimension is out of scale. An out-of-scale dimension must be underlined with a straight solid line to indicate that it is out of scale. See Figure 4-70. This practice should only be used on revisions to manually created drawings. This practice must not be used for drawing views generated from models. PRO TIP When to Revise the Model Geometryy When using CAD to complete a product defi nition based on a model, all model geometry must match the dimensions applied to the geometryy. Any time a dimension revision is needed, the model geome- try must also be revised. Surface Texture Specifi cation Surface texture requirements for a part are deter- mined according to the part’s function. Examples of surface texture requirements determined by function are bearing surfaces and the outside surfaces of a gearbox. Bearings require a smooth surface fi nish to reduce friction. The outer surface of a gearbox hous- ing can be relatively rough. Surface texture is a function of the fabrication process. A sand cast part has a surface texture that is affected by the size of sand used in the mold. The tex- ture on a lathe-turned part is affected by the cutting tool shape, machine spindle speed, and feed rate of the tool. The texture on a ground part is relatively smooth because of the small grain size in the grind- ing wheel, the high speed of the wheel, and a rela- tively slow movement of the grinding wheel across the surface. W b y a lied y y Specifi cation of size and geometric tolerances on dimensions has an indirect effect on surface texture. As tolerances become small, the fabrication processes become more exacting. More exacting fabrication processes usually result in better-quality surface tex- tures. A tolerance of ±.0002″ on a shaft diameter may require the part to be ground or lapped. Grinding to achieve a tolerance this small will probably result in a surface roughness of 32 microinches or better. (A microinch is .000001″.) In effect, the ±.0002″ size tolerance forced a relatively smooth surface texture. However, the size tolerance specifi ed did not require meeting a specifi c surface texture requirement. Where the surface texture is of importance, require- ments need to be shown. Surface texture requirements may be specifi ed by showing appropriate symbols applied to a surface in a model or on the profi le of the surface in an orthographic view. In an orthographic view, the sur- face texture symbol is typically applied where the surface appears as a line. Generally, the minimum information specifi ed will include a surface roughness value. Surface roughness is applied per ASME Y14.36 and interpreted in accordance with ASME B46.1. STANDARDS ADVISORY Revisions of Standards Specifi cation of surface texture requirements is advancing and becoming more complex. Relevant ASME standards are in revision and signifi cant changes in the standards are expected.p Care must be exercised not to over-specify re- quirements because of the potential to increase cost of manufacturing and cost of inspection. Manufacturing costs increase as surface texture requirements increase. Choice of the manufacturing methods used to fabricate a product can be impacted by the degree of surface texture control specifi ed. In order to keep costs down, surface texture must not be over-specifi ed. It is important to only apply the surface texture requirements needed to meet the design function. Surface Texture Conditions Surface texture conditions include variations known as roughness, waviness, and lay. Surface roughness is the height of small peaks and valleys on a surface. See Figure 4-71. Waviness is a larger variation in surface texture than roughness. Roughness varia- tions are superimposed on the waviness variations. R Sp req A sig t ex Line indicates an out-of-scale dimension Revision letter indicates the dimension was changed at revision B Goodheart-Willcox Publisher Figure 4-70. Out-of-scale dimensions are not allowed on models and should be avoided on manually created drawings. 91 Copyright Goodheart-Willcox Co., Inc. CHAPTER 4 Dimension Application and Limits of Size Objectives Information in this chapter will enable you to: → Clearly apply dimensions in annotated models and multiview drawings by complying with the stated general dimensioning guidelines. → Apply dimensions to any of the geometric shapes commonly found on mechanical parts. → Cite the standardized types of fi ts for inch dimensions and describe the general condition associated with each type. → Calculate and apply limits of size for mating features. → Explain Rule #1 and Rule #2 of the ASME Y14.5-2018 standard. → Provide examples of the effects that dimensions and tolerances have on manufacturing. → Complete a surface texture specifi cation when provided the allowable variations. Technical Terms allowance apex offset arc tangents base diameter basic hole system basic shaft system basic size bilateral tolerance blind holes centerdrill chamfer cone height counterbore countersink diameter features of size irregular features of size joggle keyseat lay limit dimensions limits of size mate drilling minor radius nominal size parallelepiped regular features of size removed view revisions right square pyramid roughness roughness cutoff roughness width roughness width cutoff sampling length sharp diameter single limit dimensions spotface standard machine taper surfaces taper tolerance unilateral tolerance waviness Introduction A description of part geometry can be given in a model or through the orthographic views, detail views, section views, and auxiliary views of a multiview drawing. Pictorial views may also be used. The size, location, and other values affecting the part geometry are given through the application of dimensions and tolerances. Part defi nition is complete when all aspects of each geometric feature on a part, including dimension and tolerance requirements, are defi ned. As explained in Chapter 3, each part can be visualized as a composite of geo- metric shapes, and each of those shapes dimensioned and toleranced. Methods for applying dimensions on various geometric shapes are included in this chapter. The dimensioning methods explained and illustrated in this chapter are applicable to annotated models and orthographic views unless otherwise indicated. 197 Chapter 6 Datums and Datum Feature References Copyright Goodheart-Willcox Co., Inc. the degrees of freedom of the tolerance zones on a part relative to the datum reference frame. Some tol- erances require constraining a limited number of the degrees of freedom and others require constraining all six degrees of freedom. Tolerances must include adequate datum feature references to constrain the appropriate degrees of freedom. There are three translational and three rota- tional degrees of freedom. Translational degrees of freedom are identifi ed as x, y, and z and are parallel to the X, Y, and Z axes of the datum reference frame axes. Rotational degrees of freedom are identifi ed as u, v, and w, and they exist around the X, Y, and Z axes. The u rotation is around the X axis, v rotation is around the Y axis, and w rotation is around the Z axis. Refer to Figure 6-8. The relationships of the X, Y, and Z axes are eas- ily remembered using the thumb and two fi ngers on the right hand. If the thumb and index fi nger are held at 90° to one another while pressed against a sheet of paper, the thumb pointing to the right will represent the X axis, the index fi nger pointing to the top of the paper will represent the Y axis, and the middle fi nger pointing up from the paper will represent the Z axis. See Figure 6-11. Another method is to point the right thumb of an open hand in the direction of the +Z axis. Closing the fi ngers of the right hand, they wrap around the Z axis with the fi ngers closing from the X axis toward the Y axis. The positive direction of rotation around each axis is determined by pointing the right thumb in the direction of the positive axis and wrapping the fi ngers around the axis. The fi ngers wrap in the direction of the positive rotation. NOTE The constraints on a tolerance zone or toler- ance zone framework relative to a datum refer-- ence frame are always explained in regard to “translational and rotational degrees of freedom.” These are the terms that appear in the ASME Y14.5 standard and are used to accurately express the impact of the datum feature references. “Tolerancing terms” such as and orienta- tion are used in regard to the control and variation of part features within the tolerance zones. Datum Feature Identifi cation Datum features must be identifi ed through the application of datum feature symbols or datum targets. The following sections explain methods for applying datum feature symbols and datum targets. PAST PRACTICE Old drawings based on standards to 1982 are permitted to have implied datums. Implied datums are no longer and are mentioned only because they could be encountered on old product documentation.. Datum Feature Symbol A datum feature symbol is a square or a rectangle connected by a short line to a triangle. The datum fea- ture symbol is used to identify surfaces and features of size as datum features. See Figure 6-12. The datum feature symbol typically has a square containing one letter to identify a datum feature. The square may be replaced with a rectangle where two letters are used to identify the datum feature. The square or rectan- gle is connected by a short leader line to a triangle. The leader line may include a corner. In a model or drawing, the triangle may be attached to a part feature, a dimension line, a leader line, or a feature control frame in a manner that T T a ear y exp im act pposition p pprior ppermitted y y X Y X Y Z Goodheart-Willcox Publisher Figure 6-11. The directions of the X, Y, and Z axes may be remembered by using your right hand as shown. Introduction provides an overview and preview of the chapter content. Technical Terms list the key terms to be learned in the chapter. Pro Tips provide you with guidance and direction that is especially applicable for use in industry. Standards Advisory notes provide additional information about requirements specifi ed in current and former standards. Notes alert you to important aspects of a topic. Past Practice notes identify and explain discontinued practices based on previously used standards. The instructional design includes student-focused learning tools to help students succeed. 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