drwing notes for al tech by hafeel sir

8/11/2019 Drwing Notes for AL Tech by Hafeel Sir

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C H A P T E R T E N

DIMENSIONING


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2Technical Drawing with Engineering Graphics, 14/e

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

2012, 2009, 2003, Pearson Higher Education,

Upper Saddle River, NJ 07458. All Rights Reserved.

OBJECTIVES

1. Use conventional dimensioning techniques to describe size andshape 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 donts of dimensioning.


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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 dimensioningmanufactured 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 FitsAutomatically Generated Dimensions.

Views and dimensions can be generated

automatically from a solid model.

(Courtesy of Robert Kincaid.)


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Three Aspects of Good Dimensioning

Technique of dimensioning

Placement of dimensions

Choice of dimensions


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ToleranceTolerance 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.

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

ALL TOLERANCES .02 INCH

UNLESS OTHERWISE NOTED.EX

AMPLES


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Geometric Breakdown

Engineering structures are composed

largely of simple geometric shapes, suchas the prism, cylinder, pyramid, cone,

and sphere. They may be exterior

(positive) or interior (negative) forms.


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8/438Technical Drawing with Engineering Graphics, 14/e




Guidelines for

USING DIMENSION

AND EXTENSION LINES






ARROWHEADS

When you are drawing by hand and using

the arrowhead method in which bothstrokes are directed toward the point, it is

easier to make the strokes toward yourself.






LEADERSA 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

Dont 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 SCALE

AND DIMENSIONING

Many standard title blocks include a

note such as:

DO NOT SCALE DRAWINGFOR DIMENSIONS

Drawing scale is noted in the title block. The drawing should not be scaled for

dimensions. (Courtesy of Dynojet Research, Inc.)


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DIMENSION UNITSA 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 VALUESTwo-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.


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Technical Drawing with Engineering Graphics, 14/e




RULES FOR DIMENSION VALUESGood 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.


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DUAL DIMENSIONING and

COMBINATION UNITS

Dual dimensioning is used to show metric and decimal-inch dimensions on the samedrawing. Two methods of displaying the dual dimensions are:

1. Position Method2. Bracket Method

DIMENSIONS IN () ARE MILLIMETERS


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DIMENSION SYMBOLSDimensioning symbols are used to replace traditional terms or abbreviations.

Form and Proportion of Dimensioning Symbols.(Reprinted fromASME Y14.5M-1994 (R2004),by permission of The American Society of Mechanical Engineers. All rights reserved.)


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PLACING AND SHOWING

DIMENSIONS LEGIBLYRulesfor the placement of dimensions help you dimension your drawings so

that they are clear and readable

Fitting Dimension Values in Limited Spaces (Metric Dimensions)


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PLACING AND SHOWING

DIMENSIONS LEGIBLY continued


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SUPERFLUOUS DIMENSIONSAll necessary dimensions must be shown, but do not give unnecessary or super f luous

d imens ions.


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SUPERFLUOUS DIMENSIONS

continued

No unnecessary or super f luous d imens ions.


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DIMENSIONING ANGLESAngles 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

ROUNDSA 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 NOTEDor simply

ALL FILLETS AND ROUNDS R6.

SIZE DIMENSIONS CYLINDERS


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SIZE DIMENSIONS: CYLINDERS

Use to indicate circular shape

Dimensioning a Machine Part Composed of Cylindrical Shapes

Cylinders are usually dimensioned by giving the diameter and length where the

cylinder appears as a rectangle.


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SIZE DIMENSIONING HOLES

Countersunk, counterbored, spotfaced and tapped holes are usually specified by standard

symbols or abbreviations.

The leader of a note should point to the circular view of the hole, if possible.


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DIMENSIONING COUNTERBORES AND

SPOTFACES WITH FILLETS

Counterbored hole with afillet radius specified.

When a fillet radius is specified for aspotface dimension, the fillet radius is

added to the outside of the spotface

diameter,


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2012, 2009, 2003, Pearson Higher Education,Upper Saddle River, NJ 07458. All Rights Reserved.

DIMENSIONING CURVESOne 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 measurementsare unsatisfactory, you may

give chorda l d imens ions


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DIMENSIONING ROUNDED-END

SHAPES

For accuracy, in parts

dg, overall lengths of

rounded-end shapes

are given, and radii are

indicated, but without

specific values. The

center-to-centerdistance 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 tappedholes, the notes should, if possible, be attached to the circular views

of the holes.


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DIMENSIONING TAPERSA 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 givethe 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 CHAMFERSAchamfer 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 45chamfer also may be dimensioned


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DIMENSIONING KNURLS

A knur lis 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 thepitch of the knurl, the type of knurling, and the length of the knurled area.


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36Technical Drawing with Engineering Graphics, 14/eGiesecke, Hill, Spencer, Dygdon, Novak, Lockhart, Goodman



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SURFACE ROUGHNESSSurface 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.)


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LOCATION DIMENSIONSAfter you have specified the sizes of the geometric shapes composing the structure, give

locat ion dimensions to s how the relative positions of these geometric shapes.


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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.

SHEET METAL BENDS


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SHEET METAL BENDS

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.

NOTES


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NOTESIt 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.

drwing notes for al tech by hafeel sir

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