7 Dimensioning Section 7.1 Basic Dimensioning Principles Section 7.2 Dimensioning Techniques Chapter Objectives • Add measurements, notes, and symbols to a technical drawing. • Apply ASME and ISO standards to dimen- sions and notes. • Differentiate between size dimen- sions and location dimensions. • Specify geometric tol- erances using symbols and notes. • Add dimensions to a drawing using board- drafting techniques. • Use a CAD system to add dimensions, notes, and geometric tolerances to a techni- cal drawing. Playing with Plastics Jonathan Ive says that engineers and designers can now do things with plastic that were previously impossible. What are the characteristics of plastic that give it this ability? 214 Drafting Career Jonathan Ive, Engineer What comes to mind when you think of a mobile phone that offers all these features: multimedia player, access to the Internet, camera, text messaging, and visual voicemail? Probably the iPhone designed by Jonathan Ive, senior vice president of industrial design at Apple Inc., and his product design team. Ive, recipient of many awards, is especially proud of what the iPod shuffl e represents. Originally shipped for $79, its aluminum body clips together with a tolerance of ±0.03 mm—remarkable precision. “I don’t think there’s ever been a product produced in such volume at that price … given so much time and care.… I hope that integrity is obvious.” Academic Skills and Abilities • Math • Science • English • Social Studies • Physics • Mechanical Drawing Career Pathways A bachelor’s degree in engineering is required for almost all entry-level engineering jobs. Some engineers must be licensed by all 50 states and the District of Columbia. Beginning engineers usually work under the supervision of experienced engi- neers. Communication skills are important. Go to glencoe.com for this book’s OLC to learn more about Jonathan Ive. 215 Ashley Troy Photography 7.1 Basic Dimensioning Practices Connect Symbols have a signifi cant function in drafting. Identify three symbols discussed in this section, and explain what they mean? Content Vocabulary • dimensioning • fi nish mark • unidirectional • datums • dimension line • aligned system system Academic Vocabulary Learning these words while you read this section will also help you in your other subjects and tests. • symbols • processes Graphic Organizer Use a table like the one below to organize notes about dimensioning. Basic Dimensioning Go to glencoe.com for this Lines and Symbols for Dimensioning Systems book’s OLC for a downloadable Dimensioning and Types version of this graphic organizer. Academic Standards English Language Arts NCTE National Council of Teachers of English Read a wide range of print and non-print texts to acquire new information (NCTE) NCTM National Council of Teachers of Mathematics Mathematics ADDA American Design Apply appropriate techniques, tools, and formulas to determine measurements (NCTM) Drafting Association ANSI American National Standards Institute Industry Standards ASME American Society of Mechanical Engineers ADDA Section 2 Dimensioning and Tolerancing (ANSI B4.1, ASME Y14.5M) 216 Chapter 7 Dimensioning LOCAL NOTE Lines and Symbols for LEADER O.38 .75 O1.25 Dimensioning 2 HOLES T 1.06 What information does a complete set of R.62 working drawings include? This chapter discusses how to show the size and shape of the objects you draw. It is important to understand the rules and prin- CENTERLINE ciples of size description, or dimensioning. 1.75 USED AS AN EXTENSION Dimensions, or sizes, are measured in either 3.50 LINE O2.12 U.S. customary or metric (SI) units. Decimal EXTENSION LINE divisions and metric units are now most com- monly used throughout the industry and are DIMENSION used exclusively in ASME Y14.5M (the draft- 1.62 ing standard on dimensioning). Notes and symbols that show the type .44 of fi nish, materials, and other information (4.75) needed to make a part are also included in ROUNDS AND dimensioning. A complete set of working REFERENCE DIMENSION FILLETS R.I2 drawings (the drawing or set of drawings from DIMENSION LINE GENERAL NOTE which the part is manufactured) includes Figure 7-1 shape description, measurements, notes, and symbols (see Figure 7-1). Chapter 13 Dimensioning includes measurements, notes, provides more information about working and symbols. drawings. Dimensions on working drawings must be as precise as necessary to allow the man- Standards, and the International Standards ufacturer to create the part or object. When Organization (ISO). dimensions must be precise, they are given in hundredths, thousandths, or ten-thou- Dimension Line sandths of an inch. If the metric system is A dimension line is a thin line that shows being used, the measurements may be in where a measurement begins and where it tenths, hundredths, or even thousandths of ends. It is also used to show the size of angles. a millimeter. The dimension line should have a break in The views on drawings describe the shape it for the dimension numbers. To keep the of an object. To ensure accuracy and effi ciency, numbers from getting crowded, dimension the drafter adds size information to the draw- lines should be at least .38″ (10 mm) from the ing using a system of lines, symbols, and lines of the drawing and at least .25″ (6 mm) numerical values. from each other. In general, a dimension line Lines and symbols are used on drawings to should be placed outside the view outlines. show where the dimensions apply, as shown in Figure 7-2. Professional and trade asso- Extension Line ciations, engineering societies, and certain An extension line is a thin line that extends industries have agreed on the symbols, so the lines or edges of views. It is used to locate people who use the drawings can recognize center points and to provide space for dimen- their meaning. The latest standards informa- sion lines. Because an extension line is not tion on drawings and symbols can be found part of the views, it should not touch the out- in publications from the American Society of line. Start the extension line about .03″ to .06″ Mechanical Engineers (ASME), the Society (1 to 1.5 mm) from the part, and extend it of Automotive Engineers (SAE), the Military about .12″ (3 mm) beyond the last dimension Section 7.1 Basic Dimensioning Practices 217 .38" (I0 mm) MINIMUM FROM LEADER VISIBLE LINE TO DIMENSION LINE 2X R.25 O..75 .25" (6 mm) MINIMUM BETWEEN DIMENSION LINES 2.25 R.88 Figure 7-2 SPACE 1.12 Dimensioning consists of lines, symbols, and placement techniques. 1.88I.88 3.50 SPACE 6.50 .75 EXTENSION LINE .12" (3 mm) R.12 R.25 FINISH MARK 1.25 DIMENSION LINE .62 .50 line as in Figure 7-2. Avoid drawing exten- Some industries use other means to point sion lines that cross each other or that cross out the endpoint of a dimension line or leader. dimension lines. Figure 7-3C shows some examples. These symbols do the same job as arrowheads. For Arrowhead example, slash marks are often used instead of An arrowhead is placed at each end of dimen- arrowheads in architectural drafting. For most sion lines to show where a dimension begins mechanical working drawings, however, the and ends. It is also used at the end of a leader arrowheads shown in Figure 7-3A and B are to show where a note or dimension applies to a preferred. Also, when space is limited, a dot drawing. Refer again to Figure 7-1. may be used to replace arrowheads as shown Arrowheads can be open or solid. Their in Figure 7-3D. shapes are shown enlarged in Figure 7-3A and in actual size in Figure 7-3B. In any The Finish Mark one drawing, they should all be the same size To dimension a drawing correctly, drafters and shape. In a small space, you may need to must know the correct symbols to include on make the arrowheads slightly smaller. drawings as well as the principles of dimen- sioning. Sometimes the symbols indicate which processes are needed. The fi nish mark, or surface-texture sym- THIS bol, shows that a surface is to be machined, 1 UNIT or fi nished. See Figure 7-4 for the standard 2-1 UNITS 2 fi nish mark now in general use, as well as two NOT THIS older forms of the symbol. The point of the fi nish mark symbol should AB touch the edge view of the surface to be fi n- .24 ished or an adjacent extension line. Also, it should be positioned to read from the bottom .30 of the sheet or from the right side of the sheet CD (see Figure 7-4). Leaders Figure 7-3 A leader is a thin line drawn from a note or Arrowheads dimension to the place where it applies (see Figure 7-5). Always place leaders at an angle 218 Chapter 7 Dimensioning 60° Dimensioning Systems .19 (5 mm) and Types .09 (2 mm) What are the two basic types of APPLICATION STANDARD FORM dimensions? 60° Various systems or types of dimensioning .12 (3 mm) 60° can be used and are discussed here. .19 (5 mm) OLD FORM Units OLD FORM When you use the U.S. Customary sys- tem, give the measurements in inches and Figure 7-4 decimals of an inch. When customary dimen- ″ The fi nish mark tells which surfaces are to be sions are in inches, omit the inch symbol ( ). machined. Add a note to the drawing: UNLESS OTHER- WISE SPECIFIED, ALL DIMENSIONS ARE IN INCHES. Sometimes, parts must fi t together with .12 (3 mm) extreme accuracy. In that case, the machinist Ø1.00 must work within specifi ed limits. If the mea- 60° surements are customary, the decimal inch is used.