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

Transcription

1 DIMENSIONING C H A P T E R E L E V E N Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 1

2 OBJECTIVES 1. Use conventional dimensioning techniques to describe size and shape accurately on an engineering drawing. 2. Create and read a drawing at a specified scale. 3. Correctly place dimension lines, extension lines, angles, and notes. 4. Dimension circles, arcs, and inclined surfaces. 5. Apply finish symbols and notes to a drawing. 6. Dimension contours. 7. Use standard practices for dimensioning prisms, cylinders, holes, and curves. 8. Identify guidelines for the dos and don ts of dimensioning. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 2

3 Three Aspects of Good Dimensioning Technique of dimensioning Placement of dimensions Choice of dimensions Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 3

4 Tolerance Tolerance is the total amount that the feature on the actual part is allowed to vary from what is specified by the drawing or model dimension. E X A M P L E S ALL TOLERANCES ±.02 INCH UNLESS OTHERWISE NOTED. A Title Block Specifying Tolerances. (Courtesy of Dynojet Research, Inc.) Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 4

5 Geometric Breakdown Engineering structures are composed largely of simple geometric shapes, such as the prism, cylinder, pyramid, cone, and sphere. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 5

6 LINES USED IN DIMENSIONING Dimension, Extension and Centerlines Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 6

7 Guidelines for USING DIMENSION AND EXTENSION LINES Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 7

8 ARROWHEADS When you are drawing by hand and using the arrowhead method in which both strokes are directed toward the point, it is easier to make the strokes toward yourself. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 8

9 LEADERS A leader is a thin, solid line directing attention to a note or dimension and starting with an arrowhead or dot. For the Best Appearance, Make Leaders near each other and parallel across as few lines as possible Don t Make Leaders parallel to nearby lines of the drawing through a corner of the view across each other longer than needed horizontal or vertical Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 9

10 DRAWING SCALE AND DIMENSIONING Many standard title blocks include a note such as: DO NOT SCALE DRAWING FOR DIMENSIONS Drawing scale is noted in the title block. The drawing should not be scaled for dimensions. (Courtesy of Dynojet Research, Inc.) Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 10

11 DIRECTION OF DIMENSION VALUES AND NOTES All dimension values and notes are lettered horizontally to be read from the bottom of the sheet, as oriented by the title block. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 11

12 DIMENSION UNITS A note stating ALL MEASUREMENTS IN MILLIMETERS or ALL MEASUREMENTS IN INCHES UNLESS OTHERWISE NOTED is used in the title block to indicate the measurement units (Courtesy of Dynojet Research, Inc.) Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 12

13 MILLIMETER VALUES The millimeter is the commonly used unit for most metric engineering drawings. One-place millimeter decimals are used when tolerance limits permit. Two (or more) place millimeter decimals are used when higher tolerances are required. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 13

14 DECIMAL-INCH VALUES Two-place inch decimals are typical when tolerance limits permit. Three or more decimal places are used for tolerance limits in the thousandths of an inch. In two-place decimals, the second place preferably should be an even digit. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 14

15 DUAL DIMENSIONING and COMBINATION UNITS Dual dimensioning is used to show metric and decimal-inch dimensions on the same drawing. Two methods of displaying the dual dimensions are: 1. Position Method 2. Bracket Method DIMENSIONS IN () ARE MILLIMETERS Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 15

16 DIMENSION SYMBOLS Dimensioning symbols are used to replace traditional terms or abbreviations. Form and Proportion of Dimensioning Symbols. (Reprinted from ASME Y14.5M-1994 (R2004),by permission of The American Society of Mechanical Engineers. All rights reserved.) Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 16

17 PLACING AND SHOWING DIMENSIONS LEGIBLY Rules for the placement of dimensions help you dimension your drawings so that they are clear and readable Fitting Dimension Values in Limited Spaces (Metric Dimensions) Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 17

18 PLACING AND SHOWING DIMENSIONS LEGIBLY continued Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 18

19 SUPERFLUOUS DIMENSIONS All necessary dimensions must be shown, but do not give unnecessary or superfluous dimensions. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 19

20 SUPERFLUOUS DIMENSIONS continued No unnecessary or superfluous dimensions. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 20

21 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 Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 21

22 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. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 22

23 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 Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 23

24 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. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 24

25 DIMENSIONING COUNTERBORES AND SPOTFACES WITH FILLETS Counterbored hole with a fillet radius specified. When a fillet radius is specified for a spotface dimension, the fillet radius is added to the outside of the spotface diameter, Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 25

26 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. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 26

27 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. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 27

28 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. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 28

29 DIMENSIONING CHAMFERS A chamfer is a beveled or sloping edge. It is dimensioned by giving the length of the offset and the angle. A 45 chamfer also may be dimensioned. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 29

30 DIMENSIONING KEYWAYS The preferred method of dimensioning the depth of a keyway is to give the dimension from the bottom of the keyway to the opposite side of the shaft or hole. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 30

31 DIMENSIONING KNURLS A knurl is a roughened surface to provide a better handgrip or to be used for a press fit between two parts. For handgrip purposes, it is necessary only to give the pitch of the knurl, the type of knurling, and the length of the knurled area. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 31

32 FINISH MARKS A finish mark is used to indicate that a surface is to be machined, or finished, as on a rough casting or forging. To the patternmaker or diemaker, a finish mark means that allowance of extra metal in the rough workpiece must be provided for the machining. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 32

33 SURFACE ROUGHNESS Surface finish is intimately related to the functioning of a surface, and proper specification of finish of surfaces such as bearings and seals is necessary. Surface quality specifications should be used only where needed, since the cost of producing a finished surface becomes greater as the quality of the surface called for is increased. Generally, the ideal surface finish is the roughest that will do the job satisfactorily. Surface Texture Symbols and Construction. (Reprinted from ASME Y14.36M-1996 (R2002), by permission of The American Society of Mechanical Engineers. All rights reserved.) Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 33

34 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. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 34

35 COORDINATE DIMENSIONING Coordinate dimensioning practices allow you to identify a corner of the part as the 0,0 location and dimension the remaining features relative to that position. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 35

36 TABULAR DIMENSIONS A series of objects having like features but varying in dimensions may be represented by one drawing. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 36

37 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. Giesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman 37