Technical Drawing Dimensions Quiz

Dimensions

• Dimensions are numerical values defining part or feature attributes in technical drawings.
• Features include size, geometric characteristics, location, and surface texture.
• Main goals of dimensioning are: using only essential dimensions to describe a part, to select and arrange for function and mating relationships, avoiding manufacturing method specifications, ensuring readability, and assuming 90-degree angles unless otherwise stated.

Basic Terminology

• Angular Dimension: A measurement specifying angles between lines or edges in design.
• Reference Dimension: Dimensions providing information or guidance only.
• Dimension Value: A numerical value indicating size or distance in technical drawings.
• Arrow: Marks dimension line ends, indicating measurement limits.
• Radius Symbol: Geometric notation for distance from a circle's center to its outer edge.
• Diameter Symbol: Represents the total distance across a circle, passing through its center.

Dimensioning Principles

• Use thin lines with 2 projection lines marking their start and end points to indicate measurements.
• Projecting lines are perpendicular to dimensioned elements, without touching.
• Size dimensions show geometric shape sizes.
• Location dimensions show element positioning relative to one another.

Manufacturing Specifications

• Tolerances (positive/negative deviations from a target size, geometrical controls).
• Surface finishes.
• Materials.
• Number of components required.

Dimensioning Techniques

• Chain Dimensioning: A method used only when cumulative tolerances won't harm a part's function.
• Baseline/Parallel Dimensioning: Basic dimensions based on a datum (reference point), often left-hand side of the drawing.
• Running Dimensioning: A simplified method of baseline dimensioning, taking less space.
• Combined Dimensioning: A method combining chain and baseline dimensioning, arranging dimensions in a straight line.
• Auxiliary Dimensioning: Simplified baseline dimensioning, requiring less space. This method underlines a particular dimension with a wide line.
• Theoretically Exact Dimension: Used with geometrical control.
• Dimensioning Squares/Flat Surfaces: A square on a shaft/surface so it can be turned by a spanner.
• Dimensioning Holes: M10, 1.5 Pitch, 6H (nominal diameter size, distance between adjacent threads, tolerance values).

Drafting & Dimensioning Screw Threads

• Male Thread (External): Found on bolts, screws, studs.
• Female Thread (Internal): Found inside holes.
• Dimensioning Counterbores: Create recesses for screw heads and flat surfaces (spotfacing), with dimensions showing counterbore details.

Dimensioning Countersunk Holes

• Used at angles generally 60° and 90°.
• Unidirectional Dimensions: Writing parallel with the bottom of the drawing sheet.
• Aligned Dimensions: Writing parallel with the related dimension line, readable from bottom or right-hand side of the drawing.
• Superfluous Dimensioning: Indicating overall length (called "REFERENCE DIMENSION").

Guidelines for Drawing Construction

• Dimension and extension lines are narrow, continuous lines, leaving a 2-3mm gap.
• Arrowheads should be triangular.
• Adequate spacing should be between rows of dimensions.
• Center lines are not dimension lines.
• Figures should be placed to be readable from the bottom.

General Rules of Dimensioning

• Dimensions should be placed on the clearest view (orthographic) and outside the part's outline.
• Round features are dimensioned from the front.
• When dimensions are placed on one side, the shortest measures are placed closest to the part.

Tolerances

• Tolerances are the allowable variations for a feature from the specified drawing or model dimension.
• Designers need to set maximum and minimum limits (e.g., 20 ± 0.1 mm).
• Milling machines produce parts within tolerance ranges.
• Nominal Dimensions: Target dimensions.
• Limits: Maximum and minimum sizes for a feature.

Tolerance Stacking

• Cumulative tolerance values affect a surface location.
• Chained/Continuous Dimensioning: Can lead to tolerance stacking, useful for the relative position between features and ensures mating with other parts
• Baseline Dimensioning: Avoids tolerance stacking, useful for CNC programming. Useful to limit dimensional variation from a single datum
• Coordinate Dimensioning: More suitable for ensuring mating with another part and precision holes

Limits & Fits

• Fits concern how parts engage with each other, controlled by upper and lower limits of size.
• Each part must be within the limits
• Components must fit as designed.
• Clearance Fit: Shaft is smaller than the hole.
• Interference Fit: Shaft is larger than the hole.
• Transition Fit: Shaft might be either larger or smaller than the hole.

Selecting Hole-basis System

• Fundamental deviation of the hole is zero, shaft varies to suit.
• Only be chosen for general use.
• Easier to machine a shaft to any specific size.

Geometric Dimensioning and Tolerancing (GD&T)

• GD&T uses symbols to indicate tolerances based on feature geometry.
• Tolerance Zone: The space containing deviation permitted for a feature to lie within.
• Important Considerations: Applied over and above normal dimensional tolerances to control shape and form of a part.
• Should only be applied when design requires
• Dimensions should be as wide as possible, within design limits.

Terminology Used in Geometric Tolerancing

• Feature: Identifies parts/portions of a component (e.g., surface, axis).
• Datum: A point, line, plane, or surface; used for referencing dimensions/tolerances.
• Virtual Size: The outline of a perfect form that touches the highest point of a feature.
• Maximum Material Condition (MMC): The size where a feature has the most material, defined by its largest possible value (e.g., largest shaft or smallest hole).
• Least Material Condition (LMC): The size where a feature has the least material, as defined by its smallest possible value (e.g., smallest shaft or largest hole).

Indicating Geometric Tolerances on Drawings Method

• Tolerance Values: Preceded by Ø (circular or cylindrical), SØ (spherical).
• Datums and Datum Systems: Foundation for geometric relationships.
• Datum Surface: A surface that should be accurately finished.
• Datum Systems: Primary, secondary and tertiary datums for various components in a system.
• Virtual Size: Overall size encompassing the perfect form touching a feature's highest point.

Applying Tolerance Frame to Toleranced Feature Methods

• Tolerances indicate geometric tolerances per shape.
• Datums and Datum Systems: Foundation for geometric relationships.
• Datum Surfaces: Surfaces that should be accurately finished.

Straightness, Flatness, Circularity/Roundness, Cylindricity, Parallelism, Perpendicularity, Angularity, Circular Run-out, Total Run-out, Position, Concentricity & Coaxiality

• Define and describe tolerances related to shape and form of features

Maximum Material Condition (MMC) and Least Material Condition (LMC)

• Maximum Material Condition (MMC): Size features contain maximum material.
• Least Material Condition (LMC): Size features contain minimum material.

Requirement for Positional Tolerances

• Bigger hole, flexibility in hole's center position
• Other design considerations and fabrication related factors.

Tolerancing for Function

• Selecting datum features.
• Controlling datum features
• Dimensioning feature Sizes
• Geometrically controlling functional features
• Refine dimensioning and geometric tolerances for both functional and non-functional limits

Surfaces in Engineering

• Forming process vs machining process
• Types of surfaces
• Importance of surface finishing (friction, wear, optical properties, assembly, aesthetics)

Surface Texture

• Roughness, waviness, and flaws in surfaces.
• Surface Profile Parameters to measure roughness. Using Cutoff Length and Ra as examples.

Methods of Measuring Surface Roughness

• Contact Stylus Method: Uses a stylus to measure surface variations.
• Optical Method: Uses light to measure surface details.

What is the Definition of a Feature in Geometric Tolerancing or Datum in Geometric Tolerancing?

• Feature: The general term for a part of or portion of a component
• Datum: Point, line, surface, from which sizes are measured, or for feature geometric relationship

What are Theoretically eXACT Dimensions (TEDs)?

• Dimensions that define true positions of features.
• Dimensions in boxes or rectangular frames.

Which of the Following Are Dimensioning Techniques?

• Baseline/Parallel Dimensioning
• Combined Dimensioning
• Dimensioning of Extrusions
• Dimensioning Squares

Based on the Following Image, What Are the Maximum and Minimum Limits of the Dimension?

• A numerical value for a dimension, given its upper and lower limits.

Additional questions

• List the types of fits
• Explain the concept of GD&T
• Explain the general rules of dimensioning
• Explain Tolerance Stacking
• List standards used by engineers