CE En 112 Engineering Drawing with CAD Application Chapter 4: Dimensioning and Tolerancing Practices

Lecture Outline Dimensioning (4.1) (page 170 – white number in a black circle) Size and location dimensions (4.2) (page 171 – white number in a black circle) Detail dimensioning (4.3) (page 177 – white number in a black circle) Dimensioning techniques (page 170 – white number in a black circle) Tolerancing (4.4-4.5) (page 181– white number in a black circle) Next class

Dimensioning (4.1) Geometrics: the science of specifying and tolerancing the shapes and locations of features on objects Why do we need dimensions? Once the shape of a part is defined with an orthographic drawing, the size information is added in the form of dimensions to physically build the part An engineering drawing will have graphics, dimensions, and words or notes to fully define the part

1) Size and Location Dimensions Dimensioning (con’t) Two types of dimensioning: 1) Size and Location Dimensions 2) Detail Dimensions

Size and Location Dimensions (4.2) Units of measure: Should be in accordance with policy of the user Generally use Traditional (English) units, commonly inches, or feet and inches if over 72 inches (general rule) Most countries outside of U.S. use metric or international system of units (SI), commonly millimeters (mm) Two options available for dual dimensioning (both traditional and SI units)

Size and Location (con’t) Units of measure (con’t): Angular dimension are shown in either decimal degrees, or in degrees, minutes, and seconds

Size and Location (con’t) Terminology: be familiar with the terms in section 4.2.2 (General rule: Full circle or >180° use Ø, otherwise use R) (Maximum material condition Least material condition) (Numerical value that defines the size, shape, location, surface texture, or geometric characteristics of a feature) (1 mm or 1/16”) (Theoretical exact size) (Information only – not used in fabrication of part)

Size and Location (con’t) Dimensioning standards:

Size and Location (con’t) Ask yourself: “What information is necessary to manufacture or construct the object?” Answer: Size and location dimensions (horizontal and vertical) Dimensions showing the location and orientation of features Dimensions showing the size of features

Size and Location (con’t) Standard practices: Minimum Dimension Line Spacing Dimension Text Placement

Size and Location (con’t) How should we place multiple dimensions? Group Dimensions Do not use object lines as part of dimension

Size and Location (con’t) How should we place multiple dimensions? View Dimensions Keep dimensions off of the view Stagger Dimension Text

Size and Location (con’t) How should we place multiple dimensions? Repetitive Features

Size and Location (con’t) What are the extension line basic standards? Extension Lines

Size and Location (con’t) How should we align the dimension text? Reading Direction

Detail Dimensioning (4.3) The importance of accurate, unambiguous dimensions cannot be overemphasized. The primary guideline is clarity: whenever two guidelines appear to conflict, the method that most clearly communicates the size of information shall prevail The next slides outline some of the dimensioning guidelines from the text – please review all guidelines in the text and be familiar with them

Detail Dimensioning (con’t) Guideline 2: Double dimensioning of a feature is not permitted This guideline is related to tolerance

Detail Dimensioning (con’t) Guideline 3: Dimensions should be placed in the view that most clearly describes the feature being dimensioned (contour dimensioning)

Detail Dimensioning (con’t) Guideline 4: Maintain a minimum spacing between the object and the dimension and between multiple dimensions Guideline 5: A visible gap shall be placed between the ends of extension lines and the feature to which they refer

Detail Dimensioning (con’t) Guideline 7: Avoid placing dimensions within the boundaries of a view, whenever practicable Guideline 9: Unless otherwise specified, angles shown in drawings are assumed to be 90 degrees

Detail Dimensioning (con’t) Guideline 10: Avoid dimensioning hidden lines Guideline 13: Leader lines for diameters and radii should be radial lines

Dimensioning Techniques The dimensioning process: Step 1: Front view – locate ends of angled surfaces using dimension lines – place numeric values Step 2: Add dimension lines for overall dimensions: height, width, and depth Step 3: Locate the center of the hole using horizontal and vertical dimension lines. Extend center lines. Step 4: Draw a radial leader line from the hole to a convenient location, add extension or shoulder, then add dimension

Tolerancing (4.4-4.5) Used to control the variation that exists on all manufactured parts Total amount a dimension may vary and is the difference between the maximum and minimum limits Example: Tolerance = 4.650 ± .003 Final measurement of machined part can be anywhere from 4.653 (upper limit) to 4.647 (lower limit) Lower and upper allowable sizes are referred to as the limit dimensions (upper and lower), and the tolerance is the difference between the limits (in this case, tolerance = 0.006)

Tolerancing (con’t) Tolerances can be expressed as follows: Direct limits, or as tolerance values applied directly to a dimension (ASME Preferred Method) Geometric tolerances Notes referring to specific conditions A general tolerance note in the title block (e.g. All Decimal Dimensions to Be Held To ± .002”) Geometric Tolerances

Tolerancing (con’t) Important terms – review these and be familiar with the basic concept of tolerances Tolerances are used more in mechanical engineering than in civil engineering applications

Next Class Chapter 4. Engineering Geometry (p.191 – p. 231) – It’s the last chapter in the manual drawing textbook