7.2 Dimensioning and tolerancing

Engineering drawings come to life with dimensions and tolerances. These crucial elements define part sizes, shapes, and allowable variations. Without them, manufacturers would be lost, unable to create parts that fit and function properly.

Dimensioning techniques and tolerancing methods ensure clear communication between designers and manufacturers. From basic linear measurements to advanced geometric controls, these tools help create precise, functional, and interchangeable parts in the real world.

Dimensioning Techniques in Engineering Drawings

Linear and Angular Dimensioning

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• Dimensioning adds measurements to engineering drawings specifying size, location, and geometry of features
• Linear dimensions define lengths, widths, and heights of features
• Angular dimensions specify angles between lines or surfaces
• Baseline dimensioning uses a common reference line for multiple dimensions
• Chain dimensioning measures from one feature to the next in sequence
• Proper dimensioning employs appropriate line types, arrowheads, and text placement for clarity and readability

Dimensioning Rules and Techniques

• Avoid redundant dimensions to prevent conflicting information
• Use appropriate units consistently throughout the drawing (metric or imperial)
• Place dimensions outside the view where possible to maintain clarity
• Apply specific techniques for curved features (radii, diameters, chamfers)
• Select dimensioning method based on manufacturing process and functional requirements
• Utilize ordinate dimensioning for precision machined parts with many features
• Implement tabular dimensioning for repetitive features or complex parts

Advanced Dimensioning Considerations

• Account for thermal expansion in dimensioning when dealing with materials sensitive to temperature changes
• Apply dual dimensioning when both metric and imperial units are required
• Implement reference dimensions for non-critical features or derived measurements
• Use datum dimensioning to establish key reference points or surfaces
• Incorporate functional dimensioning to highlight critical interfaces or clearances
• Apply geometric dimensioning and tolerancing (GD&T) for complex form and position requirements
• Consider scaled dimensioning for very large or small parts where standard dimensioning would be impractical

Tolerances on Dimensions

Types of Tolerances

• Tolerances allow variations from nominal dimensions accounting for manufacturing imperfections and functional requirements
• Bilateral tolerances specify equal positive and negative deviations from nominal dimension (±0.1 mm)
• Unilateral tolerances allow variation in only one direction (+0.2 mm, -0.0 mm)
• Limit dimensioning states maximum and minimum allowable dimensions for a feature (10.2 mm - 10.0 mm)
• Fit tolerances define relationships between mating parts (clearance, transition, interference fits)
• Statistical tolerancing methods (Root Sum Square) optimize tolerance allocation in complex assemblies

Tolerance Considerations and Analysis

• Tolerance stacking occurs when multiple toleranced dimensions affect a critical feature
• Perform tolerance stack-up analysis to ensure proper assembly and function
• Consider manufacturing processes when specifying tolerances (machining, casting, 3D printing)
• Balance manufacturing costs with functional requirements and interchangeability considerations
• Implement tighter tolerances for critical features and looser tolerances for non-critical features
• Use geometric tolerancing for form and position control when linear tolerances are insufficient
• Apply tolerance analysis software for complex assemblies to optimize design and reduce costs

Tolerance Zones and Representations

• Tolerance zones define the allowable space for a feature to exist within specification
• Cylindrical tolerance zones apply to features like pins or holes
• Parallel plane tolerance zones apply to flat surfaces or thickness dimensions
• Angular tolerance zones define allowable variation in angular dimensions
• Represent tolerances as fractions, decimals, or percentages depending on industry standards
• Use symmetric or asymmetric tolerance representations based on functional requirements
• Implement profile tolerancing for complex curved surfaces with varying allowable deviations

Geometric Dimensioning and Tolerancing (GD&T)

Fundamental Concepts of GD&T

• GD&T defines and communicates engineering tolerances through symbolic language on drawings
• Feature control frames contain geometric characteristic symbol, tolerance value, and datum references
• Datums establish reference planes, axes, or points for GD&T measurements
• Material condition modifiers (Maximum Material Condition, Least Material Condition, Regardless of Feature Size) affect how tolerances apply
• GD&T improves communication between design, manufacturing, and inspection departments
• ASME Y14.5 serves as primary GD&T standard in United States
• ISO 1101 commonly used internationally for GD&T specifications

GD&T Characteristics and Applications

• Form tolerances control shape independent of size or location (flatness, straightness, circularity, cylindricity)
• Orientation tolerances control angular relationships (perpendicularity, parallelism, angularity)
• Location tolerances control position of features relative to datums (position, concentricity, symmetry)
• Runout tolerances control rotation of features around an axis (circular runout, total runout)
• Profile tolerances control complex surfaces relative to perfect form (profile of a line, profile of a surface)
• GD&T allows for specification of tolerances that cannot be expressed with traditional dimensioning
• Implement GD&T to reduce costs, improve quality, and increase part interchangeability

Advanced GD&T Concepts

• True Position defines the theoretically exact location of a feature relative to datums
• Virtual Condition represents the worst-case boundary of a feature accounting for size and geometric tolerances
• Bonus tolerance concept allows for increased geometric tolerance as feature size deviates from Maximum Material Condition
• Simultaneous requirements specify when multiple tolerance conditions must be met concurrently
• Composite tolerancing combines multiple levels of control for a single feature
• Statistical tolerancing in GD&T optimizes production by allowing controlled percentage of parts outside nominal tolerance
• Functional gauging verifies GD&T requirements using physical gauges simulating mating part conditions

Symbols and Notation in Dimensioning and Tolerancing

Basic Dimensioning Symbols

• Dimension lines indicate extent of a dimension with arrowheads at ends
• Extension lines extend from feature to dimension line without touching feature
• Diameter symbol (Ø) specifies circular features (Ø20 mm)
• Radius symbol (R) indicates curved features (R5 mm)
• Spherical radius (SR) and spherical diameter (SØ) symbols denote three-dimensional curved surfaces
• Countersink symbol specifies angled enlargement of hole opening (90° x Ø15 mm)
• Counterbore symbol indicates cylindrical enlargement of hole (Ø20 x 10 mm deep)

GD&T Symbols and Modifiers

• Geometric characteristic symbols represent specific tolerance types (flatness ◻, perpendicularity ⟂, position ⌖)
• Datum feature symbols (A, B, C) establish reference elements for measurements
• Datum targets specify precise points, lines, or areas on a datum feature
• Material condition modifiers affect tolerance application (Maximum Material Condition ⓜ, Least Material Condition ⓛ, Regardless of Feature Size ⓢ)
• Feature control frames contain complete GD&T specifications (characteristic symbol, tolerance value, datum references)
• All-around symbol (circled) applies tolerance to all instances of a feature
• Free state symbol (circled F) indicates part should be evaluated in unconstrained condition

Advanced Notation and Conventions

• Projected tolerance zone symbol extends tolerance zone beyond feature
• Translation symbol indicates a movable tolerance zone
• Tangent plane symbol modifies a flatness tolerance to control local variations
• Unequal bilateral profile tolerance represented by different values above and below profile symbol
• Unilateral profile tolerance shown with single value and direction indicator
• Statistical tolerance symbol (ST) denotes tolerances based on statistical analysis
• Continuous feature symbol applies tolerance to entire length of feature without interruption