General Principles of Technical Dimensioning

Introduction and Purpose of Technical Dimensioning

Dimensioning (acotación) represents a critical component of technical plan normalization, serving as the basis for a universal language that allows for the precise interpretation of measurements and annotations regarding an industrial object. This language is utilized by engineers and designers to formalize a product functionally and truthfully before it is created or manufactured. During the developmental phase of a new object, dimensioning aids the technical office and engineering departments in verifying measurements, making adjustments based on specific machinery or processes, and calculating associated costs and production times. As an essential part of technical documentation, it is an indispensable element for both the formal and functional interpretation of a design project. Furthermore, in the context of product redesign, dimensioning serves as the starting point for modifying technical aspects designated by the Design Department or the Technical Office.

Regulatory Framework and General Norms

The standards governing dimensioning described in this curriculum are based on the AENOR regulations, specifically those detailed in the standard UNE-1039:2004, among others. A central principle is that dimension figures must always indicate the real value of the dimension, regardless of the scale used in the drawing. These values are expressed in the same unit, which in technical drawing is conventionally millimeters ($mm$); if a different unit must be used, it must be explicitly noted next to the dimension. The inscription of dimensions must never compromise the clarity of the representation; therefore, dimensions should be placed outside the drawing, view, or piece whenever possible. While internal placement is permitted if it improves readability, the primary goal is to ensure the quantitative definition of the object without cluttering the visual field. Functional dimensions must be expressed for direct reading rather than being deduced from other values or calculated via scale application. Redundancy is to be avoided, and only the precise number of dimensions required to define the finished piece should be included.

Objectives and Strategic Orientations of Dimensioning

In manufacturing plans, the size of an object is as vital as its form. According to the standards found in DIN 406 and UNE 1-039-94, dimensioning can be approached through three distinct orientations. Functional dimensioning (Acotación dirigida a la función) focuses on those dimensions essential for the correct operation of the object, which must be represented directly. Manufacturing dimensioning (Acotación dirigida a la fabricación) and verification dimensioning (Acotación dirigida a la verificación) focus on dimensions used during specific production phases or during the inspection and quality control of the pieces. In practice, a single drawing may incorporate all three types, as a specific dimension may simultaneously serve a functional purpose, guide a manufacturing process, and provide a point for professional verification.

Essential Components and Graphic Representation of Dimensions

A complete dimension consists of several standardized elements: dimension lines, reference lines (auxiliary lines), dimension text (figures), and complementary letters or symbols. The dimension lines are continuous lines drawn parallel to the measurement being described, upon which the numerical value is placed. These lines are typically capped with arrows. Each arrow should be an isosceles triangle with an unequal angle of approximately $15^\circ$, filled with ink. The length of the arrow should be approximately $4$ to $5$ times the thickness of the lines used for the object's edges. Reference lines (lineas de referencia) are continuous lines that sit perpendicular to the dimension line, extending slightly past the dimension line but originating from the boundaries of the measured area. The dimension text should preferably be placed in the center and slightly above the dimension line. If space is insufficient between the arrows, the text is ideally placed on the right side above a continuation of the dimension line. Complementary signs, such as those for radius, diameter, or square sections, are added to the text to clarify the geometry if any ambiguity exists.

Technical Standards for Distribution and Placement

Dimensions must be distributed among the various views of a product to maximize clarity, with a tendency to group all dimensions related to a specific element in the same view or the most representative one. Dimensions defining the location of symmetrical elements must always refer to their centers (axes) rather than their contours. Holes (taladros) are specifically dimensioned relative to their axes of symmetry. A fundamental rule in technical drawing is to avoid dimensioning hidden edges (aristas ocultas); instead, the dimension should be placed on a view where the edge is visible as a continuous line, or through the use of sections, cuts, or breaks. Each characteristic of a piece should only be defined by a single dimension on the drawing. When placing dimensions on the plan, the first dimension line should be separated from the drawing by at least $8\,mm$. Subsequent parallel dimension lines should maintain equal spacing from each other, with a minimum separation of $5\,mm$. Dimension figures must have a consistent nominal height throughout the drawing, with recommended heights of $3\,mm$ or $4\,mm$, and a strict minimum of $2.5\,mm$.

Methodologies of Quantitative Representation

There are several systems for arranging dimensions depending on the complexity of the piece. Series dimensioning (Acotación en serie) involves placing dimensions one after another, where each measurement starts from the previous one; however, because this can lead to an accumulation of errors, it is often necessary to provide a total dimension. Parallel dimensioning (Acotación en paralelo) is used when multiple dimensions in the same direction share a common reference element, such as a face or an axis. Combined dimensioning (Acotación en combinado) is a mixture of both series and parallel systems and is the most frequently used method in industrial practice. Additionally, coordinate dimensioning (Acotación mediante coordenadas) can simplify complex pieces by referencing all dimensions to a common origin point ($X, Y$), using dimension lines with single arrows pointing in the positive direction from that origin. When dimensions are not drawn to scale due to errors or modifications, the dimension figure must be underlined to alert the reader.

Specific Geometric Symbols and Notations

Symbols are used to simplify representations and save the need for additional views. The diameter sign ($\emptyset$) is used when the circular nature of a surface is not identifiable in the current view, but it is omitted if the circle is shown in its true magnitude. The radius sign ($R$) is used for arcs, with a single arrow at the end of the dimension line; the "$R$" is only necessary if the center of the radius is not represented. For square-based prismatic surfaces not visible in the current view, the square symbol (a square with side length equal to $5/7$ of the text height) is used. The "San Andres Cross" (Cruz de San Andrés) refers to diagonal fine lines drawn across a surface to indicate it is flat, which is particularly useful for identifying flat faces on cylindrical parts (referred to as "entrecaras"). Spherical shapes are designated with the notation "$\text{esf}$" before the dimension figure. For angles, the dimension lines are curved arcs centered at the vertex of the angle.

Rules for Arcs, Holes, and Chamfers

The dimensioning of circular elements follows specific rules based on their angular extent. An arc is dimensioned by its diameter if it is greater than $180^\circ$ and by its radius if it is less than $180^\circ$. Semicircles are also dimensioned by the radius. For repeated elements, such as a series of equidistant identical holes, the representation can be simplified using the formula: number of holes $\times$ distance between centers $=$ total distance (e.g., $5 \times 80 = 400$). Chamfers (chaflanes) are dimensioned by providing the angle and one of the leg lengths; for $45^\circ$ chamfers, a simplified notation like $3 \times 45^\circ$ is permitted. To ensure central positioning for pieces made from raw materials with coarse tolerances (like casting or stamping), equality signs ($=$) can be used on distribution dimensions alongside total or center-to-center dimensions.

Reading Orientations and Section Hatching (Rayado)

Dimensions must be positioned so they can be read from the bottom for horizontal dimensions and from the right for vertical dimensions. For oblique dimension lines, the text follows the transition between these orientations, though dimensioning should be avoided in the $30^\circ$ zones adjacent to the vertical axis. In the context of sections and cuts, the intersecting surface is represented by hatching (rayado), which consists of thin, continuous parallel lines typically inclined at $45^\circ$ relative to the horizontal or the piece's symmetry axes. The spacing of these lines depends on the size of the piece. Crucially, the hatching must be interrupted to provide a clear rectangular space for any dimension figure placed inside it. In assembly plans, different pieces should have different hatching orientations to distinguish them, and very thin pieces like plates or profiles may be completely filled in black, with white space left between them if they are mounted together.