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### DIMENSIONING

C H A P T E R T E N

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. List practices for dimensioning a solid model as documentation.

9. Identify guidelines for the dos and don’ts of dimensioning.

UNDERSTANDING DIMENSIONING

The increasing need for precision manufacturing and interchangeability

has shifted responsibility for size control to the design engineer or detail drafter.

Practices for dimensioning architectural

and structural drawings are similar

in many ways to those for dimensioning

manufactured parts, but some practices

differ.

Refer to the following standards:

• ANSI/ASME Y14.5-2009 Dimensioning and Tolerancing

• ASME Y14.41-2003 Digital Product definition Data Practices

• ASME B4.2-1978 (R1999) Preferred Metric Limits and Fits

Automatically Generated Dimensions. Views and dimensions can be generated automatically from a solid model. (Courtesy of Robert Kincaid.)

Three Aspects of Good Dimensioning

Technique of dimensioning

Placement of dimensions

Choice of dimensions

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.

ALL TOLERANCES ±.02 INCH

UNLESS OTHERWISE NOTED.

E

X

A

M

P

L

E

S

A Title Block Specifying Tolerances. (Courtesy of Dynojet Research, Inc.)

Geometric Breakdown

Engineering structures are composed largely of simple geometric shapes, such as the prism, cylinder, pyramid, cone, and sphere. They may be exterior (positive) or interior (negative) forms.

LINES USED IN DIMENSIONING

Dimension, Extension and Centerlines

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.

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

DRAWING SCALEAND DIMENSIONING

Drawing scale is noted in the title block. The drawing should not be scaled for dimensions. (Courtesy of Dynojet Research, Inc.)

Many standard title blocks include a note such as:

DO NOT SCALE DRAWING

FOR DIMENSIONS

DIRECTION OF DIMENSIONVALUES 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.

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.)

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.

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.

RULES FOR DIMENSION VALUES

Good hand-lettering is important for dimension values on sketches. The shop produces according to the directions on the drawing so to save time and prevent costly mistakes, make all lettering perfectly legible.

Make all decimal points bold, allowing ample space. When the metric dimension is a whole number, do not show either a decimal point or a zero. When the metric dimension is less than 1 mm, a zero precedes the decimal point.

When the decimal-inch dimension is used on drawings, a zero is not used before the decimal point of values less than 1 in.

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:

Position Method

Bracket Method

DIMENSIONS IN () ARE MILLIMETERS

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.)

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)

SUPERFLUOUS DIMENSIONS

All necessary dimensions must be shown, but do not give unnecessary or superfluous dimensions.

SUPERFLUOUS DIMENSIONS continued…

No unnecessary or superfluous dimensions.

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

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.

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

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.

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.

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

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.

DIMENSIONINGTHREADS

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.

DIMENSIONING TAPERS

A taperis 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 0.167 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.

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

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.

DIMENSIONING KNURLS

A knurlis 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.

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.

The finish mark is shown only on the edge view of a finished surface and is repeated in any other view in which the surface appears as a line, even if the line is a hidden line.

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.)

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.

MATING DIMENSIONS

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

TABULAR DIMENSIONS

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

COORDINATE DIMENSIONING

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

SHEET METAL BENDS

In sheet metal dimensioning, allowance must be made for bends. The intersection of the plane surfaces adjacent to a bend is called the mold line, and this line, rather than the center of the arc, is used to determine dimensions.

Bends

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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