How to Read Technical Drawings: A Comprehensive Guide
Understanding technical drawings involves the ability to interpret the lines and symbols of what you’re looking at. This knowledge isn’t just the best way to read an engineering drawing, it’s the only way to get the results you want. We’ve put together this guide to help you do just that. We’ll cover:
Understanding Drawing Basics: Lines, Dimensions, and Scales
Identifying Symbols and Notations: Drawing Conventions and References
Reading Tables and Charts: Data and Information in Drawings
What are the Types of Technical Drawings?
Understanding Drawing Basics: Lines, Dimensions, and Scales
Knowing how to read technical drawings is essential to designing a product. A technical drawing, also called a mechanical drawing or engineering drawing, communicates design information, specifications, and instructions. Think of it as the language of mechanical engineers. A technical drawing is not a replacement for a 3D Computer Aided Design (CADs), which is an excellent representation of a product. But CADs do not provide the information that technical drawings give you.
There are different parts of a technical drawing, with each employing various elements and techniques to convey accurate and detailed representations of the component being depicted. The engineering-drawing basics include lines, dimensions, and scales.
Lines: How to Read Drawings
Lines outline shapes, define edges, and convey different features. To interpret engineering drawings, you need to become familiar with the variations. Common types of lines used include:
Continuous, thick line: indicates outlines or edges.
Continuous, thin line: used for imaginary lines of intersection, projection lines, hatching, bending lines, outlines of revolved sections and short centre lines
Dashed, thin lines: can also be thick, but must be consistent within drawing. Indicates hidden lines and hidden edges
Dashed, thick lines with dots: indicates surface requirements
Dashed, thin lines with dots: axis lines in front of section planes.
Thin chain line with thick ends: identifies the plane in which the part is cut. If the cut line is on more than one plane, the directional change is designated with thicker ends
Continuous, thin zigzag line: signals a break when a part needs to be shortened for ease of visibility
Continuous free hand: indicates breaks or cuts.
Long, thin dash and double short-dash lines: show adjacent components and also alternative or extreme positions. Where bends are indicated, these lines represent the initial outlines before bending. Can also show parts in front of the cutting plane line
Section line: drawn at 45˚ use to show interior view of solid areas of cutting plane line
Dimensions and Scales in Engineering Drawings
Dimensions provide quantitative information about the size, shape, and location of various elements within a technical drawing. Dimensions are usually indicated using numeric values and symbols, ensuring accurate replication or manufacturing. How to read drawing dimensions is easy. For instance, "Ø" denotes diameter, "H" represents height, or "L" stands for length.
Scales in technical drawings are used to proportionally represent objects or structures within limited drawing space. They establish the relationship between the drawing and the actual size of the object. Some scales include 1:1, 1:2, 1:5, or 1:10, where the first number represents the drawing size, and the second number represents the actual size. Scales ensure that accurate measurements and proportions are maintained in the drawing.
Identifying Symbols and Notations: Drawing Conventions and References
Reading technical drawings correctly requires understanding the drawing conventions and references commonly used.
ISO 128 is the international standard for the visual representations used in technical drawings. Standardised symbols represent various elements, components, or materials. For example, a plus or minus – as shown here – communicates the precision of an approximation in the information box.
Abbreviations and notations convey information concisely. These can include abbreviations for materials. Reference tables or legend notes within the drawing provide explanations for these.
Technical Drawings Explained
Below is the drawing for a tapered cap, which protects profiles from damage and debris, moisture and liquid ingress during manufacturing, storage and transportation.
Let’s start with the information box on the right-hand side of the technical drawing. This is a technical drawing title block example, and tells you the product designer, which in this case is Essentra Components. On the bottom of the title block, the mechanical engineering drawing provides other information that tells you more about the cap. See below for help understanding engineering drawings.
The figures in the table are in millimetres
This is standard and means that you shouldn’t take measurements from the drawing itself. The stated dimensions give you the information you need.
Once manufactured, all parts vary slightly in their actual measurements. Linear tolerance is the amount of error acceptable in the dimensions of the product.
The projection in which the cap is shown. Only the U.S., UK, Japan, Australia and Canada use third-angle projection. All other countries use first-angle projection.
The angular tolerance of a dimension. Like linear tolerances, this tells you how much error is allowed.
When radii of bends or fillets (rounding of a part’s corner) are the same, or if any radius is predominant, it’s standard not to draw the radii dimensions on the image. Instead, it’s noted as unspecified radii.
A requirement for injection-moulding designs. Calculated in degrees, the draft angle is measured from the vertical axis of the mould to account for shrinkage. Note, the material shown in the box is LDPE, a popular thermoplastic.
How to Read Dimensions in Technical Drawings
Below is a section of the engineering technical drawing, along with the key. We already know that the measurements are in millimetres, so for example, simply match d1 (dimension 1) to the table, and we see that it measures 11.3 mm. D2 is 13.2 mm, and so on.
What is Linear Tolerance?
Know how to read the drawing when it comes to tolerances, which are a standard piece of information. In the title box of our technical engineering drawing, you’ll see three columns. Each column indicates the tolerance of a dimension, depending on the number of decimal places that the dimension has. So the column headed 0 means a dimension with no decimal places, while 0.0 is a dimension with one decimal place and 0.00, a dimension with two decimal places.
As an example, let’s take the dimensions for d1 in the drawing, which again, is 11.3 mm. The size puts it in the 10-30 mm range. To discover the tolerance, we go to the column headed 0.0, because 11.3 has just one decimal place. The tolerance is therefore more or less 0.20 mm.
What is Third-Angle Projection?
Understanding mechanical drawings involves knowing what third-angle projection means.
Third-angle projection tells an engineer that the top view of the product comes above the front view, and the right-side view is drawn to the right side of the elevation. Compare this to first-angle projection, where the top view comes below the front view and the right-side view is drawn to the left side of the elevation.
While the technical-drawing in our example above shows the symbol and states that this is a third-party projection, some designers simply show the symbol without labelling it.
Reading Tables and Charts: Data and Information in Drawings
How to use and extract information from engineering drawings is straightforward. Read engineering drawings carefully to extract data and information related to dimensions, specifications, or other relevant details. Tables and charts often organise data in a structured manner. Look for patterns or relationships within the table, such as ascending or descending order, grouping of similar items, or categories.
Reading drawings includes column headers, which describe the content of each column. These headers provide a clear indication of the data or parameters presented in the table. Take care to also read each row in the table, which corresponds to a specific piece of information.
Pay attention to the units of measurement specified in the table. The units may be included in the column headers or indicated in a separate note or legend.
Tables and charts in technical drawings may refer to other sections, figures, or notes within the drawing or accompanying documentation. Cross-referencing enables you to find additional information or context related to the data in the table.
Check for any accompanying notes, legends, or footnotes that provide explanations, clarifications, or additional information about the table entries. These notes can help interpret specific terms, abbreviations, or symbols used in the table.
Tables or charts may indicate specific ranges, tolerances, or limitations relevant to the data presented. Reading mechanical drawings means paying attention to these details to understand the scope or applicability of the information.
What are the Types of Technical Drawings?
Finally, there are different types of technical drawings used in various fields to represent aspects of designs or systems. Regardless of the type, reading engineering drawings correctly involves understanding the relevant symbols and tables.
Blueprint: a detailed technical drawing that focuses on architectural or engineering structures. It provides precise information about the dimensions, materials, construction methods, and other specifications necessary for building or manufacturing.
Diagram: A simplified visual representation that illustrates the structure, components, or relationships within a system. It’s often used to explain concepts, processes, or complex ideas. They’re widely used in engineering to convey information visually.
Schematic: A schematic focuses on electrical or electronic systems. It shows the connections, components, and electrical relationships within a circuit. Schematics use standardised symbols to represent various elements, such as resistors, capacitors, transistors, and wires.
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