Engineering Graphics: Dimensioning PDF

LESSON 7 DIMENSIONING TARGET This lesson will be dealing with dimensioning. After going through this lesson, you should be able to: 1. Use conventional dimensioning techniques to describe size and shape accurately...

LESSON 7 DIMENSIONING TARGET This lesson will be dealing with dimensioning. After going through this lesson, you should be able to: 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. Identify guidelines for the dos and don’ts of dimensioning. REMEMBER To make this module really help you, do not just read the lesson like reading a novel or pocketbook. Reading alone will make you forget easily. Instead, draw by following the step-by-step instruction in specially in tips in sketching and the like. ANCHOR You have learned from our previous lessons how to show internal details of objects without the need for hidden lines by using section views, imagine a variety of objects cut apart along a cutting plane line, show section lining (hatching) to indicate the solid parts of the object that would be cut by the cutting plane, check that you are not showing hidden lines where they are no longer needed because the internal surfaces are exposed when the object is imagined cut, use section-lining symbols to indicate the material of the object, leave the section lining off ribs, webs, and spokes that are sectioned lengthwise, revolve symmetrical features so the section view depicts the part’s symmetry, use conventional breaks on drawings to show object details when they would appear too small at a scale where the entire object would be shown on the sheet, and interpret assembly drawings that include section views.  IGNITE Dimensioning It is essential to describe not only the shape of the features you design but also their sizes and locations. Dimensions and notes define the size, finish, and other requirements to fully define what you want manufactured. Standards organizations prescribe how dimensions should appear and the general rules for their selection and placement in the drawing and in digital models, but it takes skill and practice to dimension drawings so that their interpretation is clear and unambiguous. Three Aspects of Good Dimensioning Dimensions are given in the form of distances, angles, and notes regardless of the dimensioning units being used. For both CAD and hand drawing, the ability to create good- dimensioned drawings requires the following: 1. Technique of dimensioning The standards for the appearance of lines, spacing of dimensions, size of arrowheads, and so on, allow others to read your drawing. Note the strong contrast between the visible lines of the object and the thin lines used for the dimensions. The dimensions are easily read because they follow the standards for dimensioning technique. 2. Placement of dimensions Use logical placement for dimensions according to standard practices so that they are legible, easy to find, and easy for the reader to interpret. Notice that when dimensions are placed between two views, it is easier to see how the dimension relates to the feature as shown in each view. 3. Choice of dimensions The dimensions you show affect how your design is manufactured. Dimension first for function and then review the dimensioning to see if you can make improvements for ease of manufacturing without adversely affecting the final result. 3D CAD models can be transmitted as all or part of a digital product definition, but this method still requires a thorough understanding of the sizes and relationships between the part features. A drawing released for production should show the object in its completed state and contain all necessary information for specifying the final part. As you select which dimensions to show, provide functional dimensions that can be interpreted to manufacture the part as you want it built. Keep in mind: The finished piece. The function of the part in the total assembly. How you will inspect the final part to determine its acceptability. Production processes. Also, remember the following points: Give dimensions that are necessary and convenient for producing the part. Give sufficient dimensions so that none must be assumed. Avoid dimensioning to points or surfaces inaccessible to the worker. Do not provide unnecessary or duplicate dimensions. Tolerance When a finished part is measured, it will vary slightly from the exact dimension specified. 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. A good understanding of tolerance is important to understanding dimensioning, especially when choosing which dimensions to show. For now, keep in mind that tolerance can be specified generally by giving a note on the drawing such as ALL TOLERANCES ±.02 INCH UNLESS OTHERWISE NOTED. Example. One way to consider dimensioning of engineering structures involves two basic steps: 1. Give the dimensions showing the sizes of the simple geometric shapes, called size dimensions. 2. Give the dimensions locating these elements with respect to one another, called location dimensions. Note that a location dimension locates a 3D geometric element and not just a surface; otherwise, all dimensions would have to be classified as location dimensions. LINES USED IN DIMENSIONING A dimension line is a thin, dark, solid line terminated by an arrowhead, indicating the direction and extent of a dimension (Figure 11.4). In a machine drawing, the dimension line is usually broken near the middle to place the dimension value in the line. In structural and architectural drawing, the dimension figure is placed above an unbroken dimension line. As shown in Figure 11.5, the dimension line nearest the object outline should be spaced at least 10 mm (38-) away. All other parallel dimension lines should be at least 6 mm (14-) apart, and more if space is available. The spacing of dimension lines should be uniform throughout the drawing. An extension line is a thin, dark, solid line that extends from a point on the drawing to which a dimension refers (Figure 11.5). The dimension line meets the extension lines at right angles, except in special cases. A gap of about 1.5mm (116-) should be left where the extension line would join the object outline. The extension line should extend about 3 mm (18-) beyond the outermost arrowhead. A centerline is a thin, dark line alternating long and short dashes. Centerlines are commonly used as extension lines in locating holes and other symmetrical features (Figure 11.6). When extended for dimensioning, centerlines cross over other lines of the drawing without gaps. Always end centerlines using a long dash. DIMENSION LINES, EXTENSION LINES , ARROWHEADS AND LEADERLINE Dimension lines and extension lines should follow the guidelines shown in Figure 11.7a. The shorter dimensions are nearest to the object outline. Dimension lines should not cross extension lines, as in Figure 11.7b, which results from placing the shorter dimensions outside. Note that it is perfectly satisfactory to cross extension lines (Figure 11.7a), but they should not be shortened (Figure 11.7c). A dimension line should never coincide with or extend from any line of the drawing (Figure 11.7d). Avoid crossing dimension lines wherever possible. Dimensions should be lined up and grouped together as much as possible, as in Figure 11.8a, and not as in Figure 11.8b. In many cases, extension lines and centerlines must cross visible lines of the object (Figure 11.9a). When this occurs, gaps should not be left in the lines (Figure 11.9b). Dimension lines are normally drawn at right angles to extension lines, but an exception may be made in the interest of clarity, as in Figure 11.10.  Arrowheads, shown in Figure 11.11, indicate the extent of dimensions. They should be uniform in size and style throughout the drawing, not varied according to the size of the drawing or the length of dimensions. Sketch arrowheads freehand so that the length and width have a ratio of 3:1. The length of the arrowhead should be equal to the height of the dimension values (about 3mm or 18- long). For best appearance, fill in the arrowhead, as in Figure 11.11d. Figure 11.12 shows the preferred arrowhead styles for mechanical drawings. Most CAD systems allow you to select from a variety of styles. A leader is a thin, solid line directing attention to a note or dimension and starting with an arrowhead or dot. A leader should be an inclined straight line drawn at a large angle, except for the short horizontal shoulder (about 3–6 mm or 18–14-) extending from the center of the first or last line of lettering for the note. A leader to a circle should be a radial line, which is a line that would pass through the center of the circle if extended. Figures 11.13a–d show examples of leader lines; radial lines are shown in Section 11.22. Use an arrowhead to start the leader when you can point to a particular line in the drawing, such as the edge of a hole. Use a dot to start the leader when locating something within the outline of the object, such as an entire surface (see Figures11.13e and f). 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 SCALE AND DIMENSIONING Drawing scale is indicated in the title block. The scale is intended to help you visualize the object by giving an approximate idea of its size, but is not intended to communicate dimensions. Never scale measurements from drawings to find an unknown dimension. Many standard title blocks include a note such as DO NOT SCALE DRAWING FOR DIMENSIONS, as shown in Figure 11.14. Draw a heavy straight line under any single dimension value that is not to scale (Figure 11.15). Before CAD was widely used, if a change made in a drawing was not important enough to justify correcting the drawing, the practice was simply to change the dimension value. If a dimension does not match the appearance in the drawing, the part is made as dimensioned, not as pictured. If there seems to be an error, many manufacturers check to confirm that the drawing is correct; however, it is your responsibility to specify exactly what you want built. If the entire drawing is not prepared to a standard scale, note NONE in the scale area of the title block.  You may see the abbreviation NTS on older drawings, meaning “not drawn to scale.” When you create a drawing using CAD, make sure to define dimensions according to the proper standards. Because it is easy to edit CAD drawings, you should generally fix the drawing geometry when making changes and not merely change dimension values. If you are using a digital model as the sole definition for the part, the model dimensions must be represented accurately. All dimension values and notes are lettered horizontally to be read from the bottom of the sheet, as oriented by the title block. Figure 11.16 shows the direction for reading dimension values. The exception is when dimensioning from a baseline as in coordinate dimensioning. Then dimension figures may be aligned with the dimension lines so that they may be read from the bottom or right side of the sheet as shown in Figure 11.17. In both systems, general notes on the sheet and dimensions and notes shown with leaders are always aligned horizontally to read from the bottom of the drawing.  Dimension values are shown using metric or decimal-inch values. Millimeters and decimal inches can be added, subtracted, multiplied, and divided more easily than fractions. When 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, no units are needed with the dimension values. When indicating dimensions: Millimeters are indicated by the lowercase letters mm placed to the right of the numeral, as in 12.5 mm. Meters are indicated by the lowercase m, as in 50.6 m. Inches are indicated by the symbol “ placed slightly above and to the right of the numeral. Feet are indicated by the symbol ‘ similarly placed. It is customary in feet-inch expressions to omit the inch mark. It is standard practice to omit millimeter designations and inch marks on drawings and note the units in the title block except when there is a possibility of misunderstanding. For example, 1 VALVE should be 1” VALVE. 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. Typical values are shown to two decimal places even when they represent a whole number (e.g., use 2.00 instead of 2). Correct decimal- inch dimension values are shown in Figures11.21a–e. DIMENSIONING SYMBOLS A variety of dimensioning symbols are used to replace traditional terms or abbreviations (see Figure 11.23). The symbols are preferred because (1) they take less space in the drawing and (2) they are internationally recognized and therefore do not have translation issues if the part is manufactured in a country where a different language is spoken. RULES FOR PLACING DIMENSIONS LEGIBLY Never letter a dimension value over any line on the drawing; if necessary, break the line. In a group of parallel dimension lines, the dimension values should be staggered, as in Figure 11.24a, and not stacked up one above the other, as in Figure 11.24b. Do not crowd dimension figures into limited spaces, making them illegible. There are techniques for showing dimension values outside extension lines or in combination with leaders (Figure 11.25). If necessary, add a removed partial view or detail to an enlarged scale to provide the space needed for clear dimensioning. Place dimensions between views when possible, but attached to only a single view. This way it is clear that the dimension relates to the feature, which can be seen in more than one view. When a dimension must be placed in a hatched area or on the view, leave an opening in the hatching or a break in the lines for the dimension values, as shown in Figure 11.26b. Dimensions should not be placed on a view unless it promotes the clarity of the drawing, as shown in Figure 11.27. In complicated drawings such as Figure 11.27c, it is often necessary to place dimensions on a view. Avoid dimensioning to hidden lines (see Figure 11.28). Do not attach dimensions to visible lines where the meaning is not clear, such as the dimension 20 in the top view shown in Figure 11.29b. Notes for holes are usually placed where you see the circular shape of the hole, as in Figure 11.29a, but give the diameter of an external cylindrical shape where it appears rectangular. This way it is near the dimension for the length of the cylinder. Give dimensions where the shapes are shown—where the contours of the object are defined—as is shown in Figure 11.29. Locate holes in the view that shows the shape of the hole clearly DIMENSIONING ANGLES Angles are dimensioned by specifying the angle in degrees and a linear dimension, as shown in Figure 11.31a. Coordinate dimensions can also be given for two legs of a right triangle, as shown in Figure 11.31b. The coordinate method is better when a high degree of accuracy is required. Variations in degrees of angle are hard to control because the amount of variation increases with the distance from the vertex of the angle. Methods of indicating angles are shown in Figure 11.31.  In civil engineering drawings, slope represents the angle with the horizontal, whereas batter is the angle with the vertical. Both are expressed by making one member of the ratio equal to 1, as shown in Figure 11.32. Grade, as of a highway, is similar to slope but is expressed in percentage of rise per 100 feet of run. Thus a 20’ rise in a 100’ run is a grade of 20%. In structural drawings, angular measurements are made by giving the ratio of run to rise, with the larger size being 12”. These right triangles are referred to as bevels. DIMENSIONING ARCS 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 (Figure 11.33). The centers is marked with small crosses to clarify the drawing, but not for small or unimportant radii or undimensioned arcs. When there is room enough, both the radius value and the arrowhead are placed inside the arc. If not, the arrowhead is left inside but the value is moved outside, or both the arrowhead and value are moved outside. When section lines or other lines are in the way, the value and leader can be placed outside the sectioned or crowded area. For a long radius, when the center falls outside the available space, the dimension line is drawn toward the actual center, but a false center may be indicated and the dimension line “jogged” to it (Figure 11.33f). DIMENSIONING HOLES Figure 11.39 shows standard symbols used in dimensioning holes. The order of items in a note corresponds to the order of procedure in the shop in producing the hole. The leader of a note should point to the circular view of the hole, if possible. When the circular view of the hole has two or more concentric circles, as for counterbored, countersunk, spotfaced or tapped holes, the arrowhead should touch the outer circle. Draw a radial leader line, that is, one that would pass through the center of the circle if it were extended.  ,. EXPLORE For the whole class: NONE IN A NUTSHELL Dimensioning is also an effective way of communicating with all members of the design team. Pointers to be digested about dimensioning. Dimensions and notes are placed on drawings according to prescribed standards. Good placement practices are essential to making drawings easy to read. It is important to learn to dimension basic shapes. Complex drawings are usually made of many simpler features. Finish symbols and notes are a part of describing how the part is to be manufactured. They should be shown on the drawing as needed. Dimensioning dos and don’ts can provide guidelines for dimensioning practices, but overall, the dimensions that are necessary for the part to be manufactured and function in the assembly as intended must be shown. Special dimensioning techniques are used for surfaces that have been machined by one of the manufacturing processes. To increase clarity, dimensions and notes are added to a drawing to describe size, location, and manufacturing process precisely.