DIMENSIONING
Dimensioning a technical drawing refers to the process of adding measurements and numerical values to indicate the size, location, and specifications of various elements in the drawing. These measurements provide important information for understanding the object's size, proportions, and how its components relate to each other.
Dimensioning is crucial in technical drawing for several reasons:
Clarity and Communication: Dimensioning adds clarity to the drawing by providing precise measurements. It allows others, such as engineers, fabricators, or manufacturers, to understand the intended size and specifications of the object accurately. It ensures that everyone interprets the drawing consistently, minimizing errors or misunderstandings.
Design Intent: Dimensioning communicates the designer's intent by defining critical dimensions and tolerances. It guides the manufacturing process, ensuring that the final product matches the designer's vision and requirements. It helps maintain consistency and accuracy during production.
Fit and Assembly: Dimensioning aids in determining how different components fit together or align in an assembly. It specifies the necessary clearances, tolerances, and spatial relationships between parts, ensuring proper functionality and ease of assembly.
Quality Control: Dimensioning provides a reference for quality control inspections and evaluations. It allows inspectors to verify that manufactured parts meet the specified dimensions and tolerances, ensuring the desired level of precision and quality.
Maintenance and Repair: Accurate dimensioning aids in maintenance and repair activities by providing essential information for replacing or repairing components. It allows technicians to identify the correct size, shape, and location of parts, facilitating efficient repairs and replacements.
Overall, dimensioning is essential for clear communication, precise manufacturing, efficient assembly, and quality control. It ensures that technical drawings effectively convey the necessary information, leading to accurate and successful realization of the designed object.
PARTS OF A DIMENSION
DIMENSION LINE: A line that normally has an arrow at one end of it that points to an extension line and the value on the other.
LEADER: A line with an arrow pointing towards the center of a feature and generally has a bend in it so that your text is horizontal or vertical. Used for dimensioning holes, as well as call outs and notes.
EXTENSION LINE: A line extending from a feature of the drawing to a little past where you are placing the dimension line. There is a small gap to ensure that any one looking at your drawing knows that it is not part of the actual drawing.
VALUE: The actual measurement of the feature that is being dimensioned. It will generally be centered between two dimension lines.
DIMENSIONING RULES
Each dimension should be given clearly so that it can only be interpreted in one way.
Dimensions should not be duplicated or the same information given in multiple ways.
No unnecessary dimensions should be used- only those needed to produce or inspect the part.
Follow the CONTOUR rule: dimensions should be attached to the view that best shows the contour of the feature to be dimensioned.
Avoid dimensioning to hidden lines and features.
Avoid dimensioning over or through object lines.
A dimension should be attached to one view only.
Whenever possible, locate dimensions between adjacent views and not near borders.
Try to avoid crossing extension lines, but do not break them when they do cross.
In general, a circle is dimensioned by its diameter and an arc by its radius.
Holes should be located and sized in the view that shows the feature as a circle.
Dimension lines like their space. Do not cross a dimension line with an extension line and avoid crossing dimension lines with leader lines.
Leader lines point toward the center of the feature (for instance the center of a circle) and should not occur horizontally or vertically.
Dimension values should be centered between arrowheads, except when using stacked dimensions and then the numbers should be staggered.
Keep in mind there may be a case where you need to break a standard rule to give clarity to the drawing.
DIMENSIONING TYPES
There are a few different dimensioning formats that are used to dimension a drawing. The reason there are different types is based on who the drawing is intended for. If you are sending your part out to be machined and tolerance is important or if you are drawing a architectural drawing you may want to use a different dimensioning format to ensure that the information is clearly displayed. I am only going to go through two of these styles now but know that there are more.
Chain Dimensioning
In chain dimensioning, dimensions are applied sequentially from one feature to the next, forming a chain-like sequence. Each dimension is referenced to the preceding one, which creates a linear progression of measurements. This method is useful when dimensions are related or when there is a logical sequence of features. However, it can become complex and confusing if there are many interconnected dimensions.
This method of dimensioning can lead to tolerance issues the further down the part you go, with variances in machining being compounded each dimension down the line. To many this is the easiest dimension style to draw though because you are measuring actual feature values.
Baseline Dimensioning
The second type of dimensioning style is baseline dimensioning. With baseline dimensioning you will continuously pull your dimensions off of the same baseline. This is helpful when machining parts because variations due to tolerance will not be compounded since each dimension is independent of the other dimensions in your drawing.
Datum Dimensioning
Datum dimensioning, also known as datum reference dimensioning, relies on a set of reference points, lines, or planes called datums to establish a coordinate system for dimensioning and tolerancing. Dimensions are typically referenced to these datums, providing a consistent and standardized frame of reference for measurement. Datum dimensioning is commonly used in geometric dimensioning and tolerancing (GD&T) to specify the location and orientation of features relative to a datum reference frame.
Other Dimensioning Types
Coordinate Dimensioning: In coordinate dimensioning, each feature's location is defined by Cartesian coordinates (X, Y, Z), usually referenced to a common origin or a specific datum. This method is particularly useful for specifying the precise location of features in three-dimensional space, especially in computer-aided design (CAD) environments.
Ordinate Dimensioning: Ordinate dimensioning involves establishing a zero reference point (usually at the intersection of two datum lines) and then dimensioning features using the distance from this reference point along orthogonal lines (X and Y directions). It's often used for linear or rectangular features, providing a clear and consistent method for dimensioning.
Direct Dimensioning: Direct dimensioning, also known as aligned dimensioning, involves placing dimensions directly on the object, aligned with the features being dimensioned. This method is straightforward and intuitive, as dimensions are positioned adjacent to the features they describe. Direct dimensioning is commonly used for simple parts with few features or for dimensioning in 3D CAD models.
GD&T
Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language used on engineering drawings and 3D CAD models to specify the geometric tolerances and permissible variations in form, size, orientation, and location of features. It provides a precise and standardized way to communicate the design intent to manufacturers and ensures that the parts produced conform to the designer's requirements.
GD&T is widely used in industries such as automotive, aerospace, and manufacturing, where precise dimensional control is essential for ensuring the interchangeability and functionality of mechanical parts. It offers several advantages over traditional dimensioning and tolerancing methods, including clearer communication of design intent, improved manufacturing efficiency, and better product quality and reliability.
Key Components of GD&T
Symbols: GD&T uses a set of symbols, such as squares, circles, triangles, and lines, along with letters and numbers, to define the tolerance zones and geometric characteristics.
Feature Control Frames (FCF): These are the basic building blocks of GD&T. A feature control frame consists of geometric characteristic symbols, tolerance values, and modifiers that specify the acceptable limits of a feature's variation.
Datum Reference Frames (DRF): Datums are reference points, lines, or planes on a part used to establish a coordinate system for dimensional measurements. A datum reference frame consists of datum features and datum planes that establish a set of reference points for dimensioning and tolerancing.
Modifiers: GD&T uses various modifiers to further refine the tolerance zones and control the relationship between features. Common modifiers include maximum material condition (MMC), least material condition (LMC), and regardless of feature size (RFS).
Form Controls: These specify the shape of a feature, such as flatness, straightness, circularity, and cylindricity.
Orientation Controls: These control the orientation of features relative to a specified datum, such as parallelism, perpendicularity, and angularity.
Position Controls: These specify the allowable deviation of a feature's location from its theoretical position, taking into account both the size and orientation tolerances.
Profile Controls: Profile tolerance specifies the allowable deviation of a surface from its true profile within a specified boundary. It is often used to control the overall shape of complex features.