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Questions and Answers
Which type of fit is characterized by maintenance of constant bore pressures throughout the range of sizes?
Which type of fit is characterized by maintenance of constant bore pressures throughout the range of sizes?
What is the primary advantage of baseline dimensioning compared to chain dimensioning?
What is the primary advantage of baseline dimensioning compared to chain dimensioning?
What governs the shape and size of a contour when a datum is not specified in profile tolerances?
What governs the shape and size of a contour when a datum is not specified in profile tolerances?
What does a feature control frame include?
What does a feature control frame include?
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Which type of fit provides a snug fit for locating stationary parts and allows for easy assembly and disassembly?
Which type of fit provides a snug fit for locating stationary parts and allows for easy assembly and disassembly?
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What is the minimum distance dimension lines should be from any view to avoid clutter?
What is the minimum distance dimension lines should be from any view to avoid clutter?
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In what scenario would chain dimensioning be most appropriate?
In what scenario would chain dimensioning be most appropriate?
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Which of the following statements regarding decimal points for dimensions is true?
Which of the following statements regarding decimal points for dimensions is true?
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Where are notes typically located on a drawing?
Where are notes typically located on a drawing?
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What represents a counterbore on a drawing?
What represents a counterbore on a drawing?
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Study Notes
Dimensioning and Tolerancing
- Dimensioning Systems: Drawings conform to the SI or US Customary system.
- Metric Dimensions (mm): For dimensions less than 1 mm, a zero precedes the decimal point. Whole millimeters show no decimal point or zero. Exceeding a whole number by a decimal fraction keeps the last digit. Commas and spaces are not used to separate digits in millimeter dimensions.
- Inch Dimensions: A zero is not used before the decimal point for values less than 1 inch
- Dimensioning Holes: Dimension the distance between holes, not from the end of the part.
- Chain Dimensioning: Features are dimensioned relative to each other, useful when features must be related.
- Dimension Line Spacing: Dimension lines should be at least 10 mm from any view and at least 6mm between dimension lines, and at least 1mm between extension lines and object geometry. Extension lines extend 2-3 mm past dimension lines.
- Drawing Symbols: |___| = counterbore; / = countersink; notes typically in lower right-hand corner.
- Dimension Applicability: A dimension on one drawing level is not required on another.
- Free State: All dimensions/tolerances apply in a free state for non-rigid parts.
- Bilateral Tolerances: Deviations above and below the basic size, equally or unequally distributed.
- Baseline Dimensioning: Eliminates tolerance accumulation; maximum variation is the sum of tolerances from the origin to each feature. Minimizes overall tolerance accumulation.
- Statistical Process Control: Monitoring manufacturing with statistical process control allows better evaluation of tolerance needs and potential cost reduction.
- Transition Fits: Tolerances can be increased for less expensive manufacturing of individual parts.
- Tolerance Definition: The total allowable variation in a size or location dimension.
Geometric Dimensioning and Tolerancing (GD&T)
- Form Tolerances: Control individual feature shape (straightness, flatness, circularity, cylindricity), irrespective of datums.
- Profile Tolerances: 2D or 3D (defined by contours). With datums, shape, size, or location are controlled. Without datums, only shape is controlled. Inspection methods include optical comparators and mechanical gaging.
- Orientation Tolerances (e.g., Parallelism, Perpendicularity, Angularity): 3D tolerance zones for surfaces or axes relative to a datum. Surface tolerance is between planes; axis tolerance is within a cylinder
- Position Tolerances: Control orientation and location of a center, axis, or plane of a feature.
- Symmetry: Controls location of central plane relative to axis or datum; difficult to inspect.
- Circular Runout/Total Runout: 2D controls surface relative to datum axis (circularity/roundness).
- Inspection: Form tolerance inspections can use optical comparators and mechanical gaging, which compares a part guide with measured part.
- Feature Control Frame: Contains geometric characteristic symbol, tolerance value, and datum references.
Fit Types
- Clearance Fit: Space between mating parts.
- Interference Fit: Material interference between mating parts.
- Transition Fit: Either clearance or interference can exist after assembly.
- Loose Running Fits, Running Fits (RC): Used for large tolerances and heavy use with lubrication.
- Sliding Fits: Intended for moving/turning with accurate location.
- Locational Clearance Fits (LC): Stationary parts, freely assemble/disassemble.
- Locational Interference Fits (LN): Location of prime importance and rigidity.
- Medium Drive Fit: Suitable for ordinary steel/cast iron parts.
- Force Fit: High stress/shrink fits, heavy pressure.
- Selective Assembly: Matching parts measured after machining
- Allowance: The difference between maximum material limits of mating parts.
- Basic Size: The size from which limit dimensions are derived.
- Hole Basis/Shaft Basis: Basic size for hole lower/shaft upper limit.
Additional Details
- Datum References: Primary established by 3-point contact; secondary by 2-point contact perpendicular to primary; tertiary by 1-point contact perpendicular to primary and secondary.
- Maximum Material Condition (MMC): Maximum material within size limits.
- Dimensioning Conventions: Dimensions appear once, typically expressed to the same decimal place as the tolerance; dimension to visible lines when possible; dimension circles as diameter, arcs as radius; chamfers by length/angle or two distances; line with downward arrow indicates depth; 90° angles indicated by basic dimensioning and center lines at right angles to the feature.
- Metric vs. Inch Dimensioning: Metric uses zeros before decimals and tolerance is expressed the same number of decimal places as the basic size; Tolerances for inches are expressed the same number of decimal places as the basic size, along with an upper and lower limit.
- Conventional vs. Statistical Tolerancing: Conventional controls only size; statistical tolerancing assigns based on statistical practices.
- Drawing Requirements: Every dimension must have tolerance, unless otherwise specified. Best practice: dimension holes as circles; cylindrical features as rectangles.
- "TYP" Symbol: Indicates a feature is repeated.
- Form Tolerances: Control individual shape irrespective of datum references.
- Units Conventions: For wires, cables, etc., indicated by linear dimensions for diameters or thicknesses.
- Dimensioning Limitations: Dimensions apply only on drawings on specified level.
- Temperature and Pressure: Drawings assume room temperature, standard pressure.
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Description
Test your knowledge on dimensioning systems, including metric and inch dimensions. Understand the importance of spacing, dimensioning features, and the use of symbols in technical drawings. This quiz will enhance your understanding of proper dimensioning practices in engineering drawings.
