Reading technical drawings: symbols, tolerances and views
Reading technical drawings is an essential skill for anyone working in the technical sector. Whether you work in metalworking in the Netherlands or in other technical fields, the ability to interpret technical drawings correctly determines the quality of your work. A technical drawing is the universal language of engineering, in which complex three-dimensional objects are represented on a two-dimensional plane using standardised symbols, views and dimensions.
This skill is becoming increasingly important at a time when trends in the manufacturing industry focus on precision and automation. From CNC programmers to assembly technicians, from quality inspectors to project managers — they all need to be able to read technical drawings in order to carry out their work effectively.
Fundamentals of technical drawings
A technical drawing is a standardised representation of an object that contains all the information necessary for production, assembly and inspection. These drawings follow international standards such as ISO standards to ensure that they are interpreted the same way worldwide. The basis of every technical drawing consists of lines, symbols and dimensions, each with its own meaning.
Learning to read technical drawings begins with understanding the different types of lines. Solid thick lines indicate the visible contours of an object, while dashed lines show hidden edges. Centre lines, represented by alternating long and short dashes, indicate the centreline of circular parts. Dimension lines with arrows at both ends indicate distances between two points.
The projection method used in technical drawings is based on the principle of orthogonal projection. This means that the object is viewed from different directions, with each view perpendicular to the viewing plane. This method ensures that all details of an object are represented fully and without distortion.
In addition to geometric information, technical drawings also contain technical specifications such as material types, surface finishes and tolerances. This information is crucial for the production and quality control of the part.
The three main views in technical drawings
Every technical drawing uses three main views: the front view, the top view and the right side view. Together, these three views provide a complete description of the three-dimensional object. The front view usually shows the most characteristic shape of the object and is often taken as the starting point for the other views.
The front view is chosen based on the function and shape of the object. For a cylindrical part, the front view will usually show the rectangular shape, while the top view shows the circle. For more complex shapes, the view is chosen that offers the most detail and the best orientation for understanding the function.
The top view shows the object seen from above and provides information about widths, lengths and positions of holes or protrusions. This view is essential for understanding the layout of the object and the relationships between different parts.
The right side view shows the depth and height of the object seen from the right. These three views are positioned according to a fixed layout: the top view sits above the front view, and the right side view sits to the right of the front view. This standard layout makes it possible to switch quickly between the different views and build a mental image of the object.
| View | Position on drawing | Shows information about | Typical applications |
|---|---|---|---|
| Front view | Central, main view | Height and width, main shape | Characteristic shape, functional elements |
| Top view | Above the front view | Width and depth, plan layout | Hole patterns, contours from above |
| Right side view | Right of the front view | Depth and height, side details | Wall thicknesses, profile shapes |
Understanding tolerances and fit notations
Tolerances in technical drawings indicate the permissible deviations from the nominal dimension. These are essential for the functionality of the part and determine how accurately a part must be produced. The most common notation is the ISO system with letters and numbers, such as H7/g6 for the fit between a shaft and a hole.
The tolerance system works with a nominal dimension around which a tolerance zone lies. The letter indicates the position of the tolerance zone relative to the nominal dimension, while the number determines the size of the tolerance zone. Capital letters are used for holes (A to Z), where H stands for a hole whose tolerance zone begins at the nominal dimension. Lower-case letters are used for shafts (a to z), where g stands for a clearance fit.
The combination H7/g6 is one of the most commonly used fit notations in the manufacturing industry in the Netherlands. Here the hole has an H7 tolerance and the shaft a g6 tolerance, resulting in a clearance fit in which the shaft is always smaller than the hole. This fit is widely used for bearings, bushings and other moving parts.
In addition to the letter-number notation, direct tolerance specifications also occur, in which the upper and lower limits are stated explicitly. For example: 50 +0.025/-0.000 means that the dimension must lie between 50.000 and 50.025 mm. This notation is often used for critical dimensions or when standard tolerance classes are not applicable.
Surface finish and roughness symbols
Surface finish is indicated by symbols that specify the required roughness of the surface. Roughness is usually expressed in Ra values, measured in micrometres (μm). A low Ra value means a smooth surface, while a high value indicates a rough surface.
The basic symbol for surface finish is a v-shape placed on the contour line. The Ra value is stated above this symbol, for example Ra 0.8 for a relatively smooth surface. In addition, further symbols can be added to indicate the machining method, such as a circle for turning or parallel lines for milling.
Different machining methods result in characteristic surface roughnesses. Rough operations such as sawing or flame cutting give Ra values of 25–50 μm, while finer operations such as turning yield Ra values of 1.6–6.3 μm. Grinding operations can achieve Ra values of 0.1–1.6 μm, and polishing can even achieve Ra values below 0.1 μm.
The choice of the correct surface finish depends on the function of the surface. Sealing surfaces require smooth surfaces for good sealing, while friction surfaces need a certain roughness for grip. The cost of machining rises exponentially with the required smoothness, so a correct specification is important for cost control.
ISO standards for technical drawings
The ISO 2768 standard specifies general tolerances for linear and angular dimensions without an individual tolerance indication. This standard makes it possible to simplify drawings by not providing every dimension with an individual tolerance, while still making clear what deviations are permitted.
ISO 2768 has four tolerance classes: fine (f), medium (m), coarse (c) and very coarse (v). These classes apply to different dimensional ranges and give progressively wider tolerances. For a dimension of 30 mm, tolerance class ‘m’ means a tolerance of ±0.2 mm, while class ‘f’ allows ±0.1 mm.
In addition to linear dimensions, ISO 2768 also covers angular tolerances. The standard angular tolerance for tolerance class ‘m’ is ±0.5° for short sides up to 10 mm, increasing to ±0.2° for longer sides. These tolerances are based on practical experience and are sufficiently accurate for most applications.
Other important ISO standards for technical drawings are ISO 128 for line types, ISO 5459 for geometric tolerances and ISO 1302 for surface texture. These standards ensure that technical drawings are interpreted the same way worldwide, which is essential for international collaboration in engineering.
| ISO 2768 Tolerance class | Dimensional range (mm) | Linear tolerance | Angular tolerance |
|---|---|---|---|
| Fine (f) | 0.5 – 3 | ±0.05 mm | ±1° |
| Fine (f) | 3 – 6 | ±0.05 mm | ±0.5° |
| Medium (m) | 0.5 – 3 | ±0.1 mm | ±1° |
| Medium (m) | 3 – 6 | ±0.1 mm | ±0.5° |
| Coarse (c) | 0.5 – 3 | ±0.2 mm | ±1.5° |
| Very coarse (v) | 0.5 – 3 | ±0.5 mm | ±3° |
Interpreting cross-sections and detail drawings
Cross-sections show the internal structure of an object by virtually cutting through it and drawing the cut face. This is essential for understanding complex internal geometries that are not visible in ordinary views. The cutting line is indicated by a thick dashed line with arrows showing the viewing direction.
There are various types of cross-sections: full sections, where the entire object is cut through; half sections, where only half is shown; and local sections for specific details. The hatched surfaces in a cross-section represent the cut faces, with different materials receiving different hatching.
Detail drawings enlarge specific parts of the main drawing to make small but critical details clear. These details are usually marked with a circle on the main drawing and worked out separately at a larger scale. This is especially useful for small features such as threads, chamfers or complex profile shapes.
Interpreting cross-sections requires spatial insight to understand how the two-dimensional section relates to the three-dimensional object. It is important to understand that a cross-section shows only one plane and that other parts of the object may remain hidden.
Modern developments in technical drawings
The digital transformation has had a major impact on the way technical drawings are created and read. Computer Aided Design (CAD) systems have largely replaced manual drawings and offer new possibilities for visualisation and collaboration.
3D models are increasingly used alongside traditional 2D drawings. These models provide a more intuitive understanding of the object, but the skill of reading traditional technical drawings remains essential. A lot of production information is still best conveyed through 2D drawings with tolerances and specifications.
Augmented Reality (AR) technology is being used experimentally to overlay technical drawings onto real objects. This can be especially useful for assembly and maintenance work. Industrial automation also makes use of digital technical information for robot-controlled production processes.
Cloud-based platforms make it possible to share technical drawings worldwide and collaborate on technical projects. Version management and change control are automated, which improves the quality and traceability of technical documentation.
Practical tips for reading technical drawings
Always start by reading the title block and the general information before studying the details. The title block contains essential information such as scale, material, tolerance class and version number. This information provides context for interpreting the rest of the drawing.
First study the main views to get an overall picture of the object. Try to mentally visualise the three-dimensional object based on the different views. This helps with understanding more complex details and cross-sections later in the process.
Pay attention to the different line types and their meanings. Thick solid lines show visible contours, dashed lines show hidden edges, and thin lines are used for dimensioning and construction aids. Recognising these different line types is fundamental to correct understanding.
Always check the units and scale of the drawing. Dutch drawings usually use millimetres as the standard unit, but this may differ for specific applications. The scale indicates how the drawing relates to the actual size of the object.
If you are unsure about the interpretation of symbols or notations, consult the relevant norms or standards. Most companies also have internal guidelines for reading and creating technical drawings. Training and experience are indispensable for developing expertise in reading technical drawings.
Frequently asked questions about reading technical drawings
What does the notation H7/g6 mean in a technical drawing?
H7/g6 is an ISO tolerance notation that indicates a clearance fit between a hole (H7) and a shaft (g6). The H7 means that the hole has a tolerance in which the lower limit is equal to the nominal dimension. The g6 means that the shaft is smaller than the nominal dimension by a certain tolerance. This combination always provides clearance between the parts, which is suitable for moving connections such as bearings.
How do I recognise the different line types in a technical drawing?
Different line types have specific meanings: thick solid lines (0.7 mm) show visible contours, thin solid lines (0.35 mm) are used for dimensioning and details, dashed lines show hidden edges, and centre lines (alternating long and short dashes) indicate lines of symmetry. Dimension lines have arrowheads and indicate distances between two points.
What is the meaning of Ra values in surface finish?
Ra stands for Roughness Average and is expressed in micrometres (μm). It indicates the average roughness depth of a surface. Ra 0.8 means a relatively smooth surface suitable for sealing faces, while Ra 25 indicates a rough surface such as after sawing or flame cutting. The lower the Ra value, the smoother and more expensive the surface is to produce.
Why are three views used in technical drawings?
Three views (front, top and right side) together provide a complete description of a three-dimensional object on two-dimensional paper. Each view shows two of the three dimensions without distortion through orthogonal projection. This combination makes it possible to represent all details, dimensions and shapes of an object fully and without ambiguity.
How do I interpret cross-sections in technical drawings?
Cross-sections show the internal structure of an object by virtually cutting through it. The cutting line is indicated by a thick dashed line with letters (A-A, B-B) and arrows for the viewing direction. The section view shows all cut faces hatched and all parts lying behind the cutting plane unhatched. This helps in understanding internal geometries that would otherwise remain hidden.
What does ISO 2768-m mean on a technical drawing?
ISO 2768-m refers to general tolerances according to the ISO 2768 standard in tolerance class "medium". This means that all dimensions without a specific tolerance indication fall under these standard tolerances. For dimensions between 3–6 mm, this means a ±0.1 mm tolerance. This standard simplifies drawings by removing the need to tolerance every dimension individually.
How do I read the scale of a technical drawing?
The scale is stated in the title block and indicates the ratio between the drawing and reality. Scale 1:2 means that the drawing shows half of the actual size, scale 2:1 means that the drawing is twice as large as reality. At scale 1:1 the drawing corresponds to the actual size. Note that only unmarked dimensions are scaled; marked dimensions are always the actual measurements.
What information do I find in the title block of a technical drawing?
The title block contains essential information such as: part name and number, material specification, scale, draughtsman and date, company details, version/revision number, tolerance class (for example ISO 2768-m), surface finish if not otherwise indicated, and approvals. This information provides context for the interpretation of the entire drawing and is crucial for correct production.
Mastering the reading of technical drawings is a fundamental skill that remains essential in our increasingly digitalised world. From traditional production techniques to modern automated processes, the principles of technical communication through drawings form the basis for precision and quality in the technical sector.
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