Drawing Standards, Protocols and Conventions
Drawing Standards, Protocols and Conventions
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Technical drawings are made using specific standards, rules, and conventions to keep everything clear, accurate, and consistent. In the UK, these follow guidelines set by the BSI (British Standards Institution). These standards make sure that everyone, whether they're engineers, designers, or manufacturers, is working from the same set of instructions. They cover things like line types, scales, and how to show internal features clearly. Sticking to these conventions helps avoid mistakes and ensures that drawings can be easily read and understood by anyone involved in the project.
Orthographic Drawings
Orthographic Drawings
An orthographic drawing is a type of production drawing used to show detailed views of a product. It includes different views, such as the elevation (front), end elevation (side), and plan (top), to give a clear and accurate representation of the model. These drawings also include important information like dimensions and features, which help manufacturers create the product correctly.
3rd Angle Projection Symbol
3rd Angle Projection Symbol
The third-angle projection Symbol shows the projection (layout) of the drawing. In technical drawings, this symbol indicates that the views follow the third-angle projection method, where the plan is placed above the elevation, and the end elevations are positioned on the corresponding sides.
The symbol represents a view of a truncated cone (end elevation), while the circle represents the top of the cone (elevation). You must be able to draw the symbol and explain why it is used.
Layout of the Drawing
Layout of the Drawing
ElevationThis is the ‘front’ view. This view shows the most detail of the product.
Plan This shows the top surface of the product. The details are projected from the elevation.
End Elevation This shows the side of the product. The details are projected from the elevation and the plan.
Plan This shows the top surface of the product. The details are projected from the elevation.
End Elevation This shows the side of the product. The details are projected from the elevation and the plan.
Line Types
Line Types
Outline
A thick, continuous line is used to show visible outlines and edges.
Projection line
A thin continuous line is used to project between views when 2D sketching.
Hidden Detail
A dashed line used to show internal details/edges that can’t be seen from outside the product.
Centre Line
A dot dash line indicates the centre point on arcs and circles.
Fold Line
A dash double dot line indicates folds on surface developments.
Cutting Plane
A dot dash with thick ends shows where the view has been sectioned/cut to allow the internal details to be shown.
Springs
The following drawings illustrate different views of a spring: a regular view, a sectional view, and a simplified view.
Examples of line types
Examples of line types
Outlines - Shows the outer edge of the part
Hidden Detail - Shows the internal features that cannot be seen from the outside view
Cutting plane - Shows where the drawing has been sectioned to show the internal features on the elevation
Centre Line - Shows the centre point of the circles and curves
Hidden Detail - Shows the internal features that cannot be seen from the outside view
Cutting plane - Shows where the drawing has been sectioned to show the internal features on the elevation
Centre Line - Shows the centre point of the circles and curves
Title Block
Title Block
Features on a title block
Features on a title block
A title block is an important part of a technical drawing, providing key information about the part or model. It is usually found in the bottom right corner of the drawing and contains details such as the drawing title, scale, date, author, tolerance, and measurement.The title block ensures that anyone using the drawing has the necessary information to accurately understand and produce the design. It also helps with organisation and traceability, making it easier to track changes and ensure the correct version of the drawing is used in manufacturing.
Scale
Scale
Scale is used in technical drawings to represent a product at a suitable size while still showing all necessary details. The scale chosen depends on three key factors:
- the size of the paper
- the size of the product
- the amount of detail required.
Sections
Sections
Section drawings are used in technical drawings to show internal details of a product that would not be visible in a standard view. They include a cutting plane indicating where the object has been sliced and a hatched view representing the solid material inside.
Section drawings help designers and manufacturers understand how parts fit together and ensure that all internal features are correctly designed. You are expected to know how to create sectional plans, elevations, and end elevations, which provide different views of a product’s internal structure.
Section drawings help designers and manufacturers understand how parts fit together and ensure that all internal features are correctly designed. You are expected to know how to create sectional plans, elevations, and end elevations, which provide different views of a product’s internal structure.
Hatching
Hatching
Hatching lines are used in section drawings to represent the solid material that has been cut through.
- The section is hatched at 45°, with the lines approximately 5mm apart. The sectioned view is titled according to the cutting plane, i.e. sectional end elevation A-A
- If the cutting plane passes through a hole, the hole itself is not hatched since it is empty space.
- Nuts, bolts, and webs should not be hatched, even if they are cut by the section.
- When a product has multiple parts, the hatching lines can be drawn at different angles to help distinguish between the different components.
- For threads, hatching lines should be added to the outer edge of the thread but should not go inside the hole.
Understanding Sections
Understanding Sections
All circles on the plan resemble holes. We know this as we can see the hidden detail going through the elevation/end elevation. Therefore, we would not hatch them.
The section line cuts horizontally through the middle.
There is a web. That means it doesn’t get hatched.
Types of Sections
Types of Sections
Full section
Full section
When the plane cuts across the whole product without changing direction.
Half section
Half section
When the plane only cuts half the product. It creates a cut at a 90-degree angle.
Stepped section
Stepped section
Cuts through the object on two planes, meaning we can see the details clearly on different parts of the component.
Part section
Part section
The thin, irregular (wavy) line shows that only part of the model has been sectioned.
Revolved section
Revolved section
No cutting plane is used. It shows a cross-section that has been cut. It helps to keep the page from being cluttered.
Removed section
Removed section
Like the revolved section, this shows a cross-section of the product. However, the section views are placed away from the drawing this time.
Pitch Circle Diameter
Pitch Circle Diameter
The Pitch Circle Diameter is the size of the imaginary circle that a radial pattern is arranged around. It’s used to measure and position repeated features (like holes or shapes) that follow a circular layout. The number of items in the pattern doesn’t change the size of the PCD, it simply tells you where the features are placed in relation to the centre.
Knurling
Knurling
Knurling is a textured pattern added to a surface to make it easier to grip. It is often found on metal parts like tool handles or bottle caps. The pattern is small diamonds or straight lines, which helps prevent slipping when holding the object.
Flat Surface - A cross is drawn on a surface to show that it is flat and has no pattern.
Diamond- When a diamond pattern has been cut into the material to create texture.
Straight - When a straight pattern has been into the material.
Flat Surface - A cross is drawn on a surface to show that it is flat and has no pattern.
Diamond- When a diamond pattern has been cut into the material to create texture.
Straight - When a straight pattern has been into the material.
Threads
Threads
Internal Thread
When the thread has been cut into a hole/product,Internal Thread
The ‘thread’ is drawn OUTSIDE of the circle.
The lines of the ‘thread’ are also OUTSIDE the shape. These should be hatched like the example if sectioned.
External Thread
When the thread has been cut into a bolt or screw and is on the outside.External Thread
The ‘thread’ on the left is INSIDE the circle.
The lines of the ‘thread’ on the right are also INSIDE the shape.
Tolerances
Tolerances
Tolerances are the minor acceptable differences in a product’s size. Since making parts exactly the same every time is impossible, tolerances set a range that the measurements can be within while still working correctly. For example, if a hole needs to be 10mm, a tolerance might allow it to be between 9.98mm and 10.02mm. This ensures parts fit together properly, even with tiny variations in manufacturing.
Common
This way shows the overall maximum and minimum. The ‘math’ has already been worked out.
Asymmetrical
This way, the tolerances are shown separately. It shows that 0.25 can be added or 0.5 taken away.
Symmetrical
This method only works when both tolerances are the same. It shows that 0.25 can be added or removed.
Functional
A dimension that is essential to the function of a component or space.
Non Functional
A dimension that is not essential to the function of a component or space.
Dimensioning
Dimensioning
Dimensioning is the process of adding measurements to a technical drawing to show the exact size and position of a product’s features. It ensures that the design can be accurately manufactured and assembled. Dimensions include lengths, widths, diameters, angles, and hole placements, all shown using clear lines, numbers, and symbols. Correct dimensioning follows British Standards (BSI) to avoid confusion and ensure consistency across different designs.
Rules of Dimensioning
Rules of Dimensioning
- Only include the necessary dimensions needed to manufacture the product. Avoid duplicate dimensions, as they can make the drawing unclear and difficult to read.
- All measurements should be in millimetres (mm), but you do not need to write "mm" after each number.
- Dimensions should be written in their simplest form (e.g. 20, not 20.0).
- Do not cross projection lines with dimension lines, and avoid crossing dimension lines with other lines in the drawing.
Always follow British Standards Institution (BSI) rules when adding dimensions to ensure clarity and consistency:
- Dimension lines should not touch the drawing but should be aligned with the measured part.
- Arrowheads should be filled, touching the dimension lines and pointing outwards.
- Numbers should be placed above the dimension line for easy readability.
Types of Dimensions
Types of Dimensions
Parallel Dimensioning Each dimension starts from the same datum point or edge making it less likely to accumulate dimensioning errors, but it does take up a lot of space.
HorizontalA dimension that measures horizontally (left to right).
VerticalA dimension that measures vertically (up and down)
AngularA dimension that measures angles.
Dimensioning a Square If a square is drawn as a symbol next to a number, then you can assume the shape is a square and that all sizes are the same.
Major Axis
This is the distance between the shortest centre line on an ellipse.
Metric (screw thread)
The letter M (for Metric) is used to show the dimension of the thread that has been cut. This can be used for external or internal threads.
Chain Dimensioning Dimensions run one after the other and in line. This method can lead to accumulated dimensioning errors during manufacture. It takes up less space than parallel dimensioning
Running Dimensioning Each runs after the other and in line, like chain dimensioning. However, each dimension is measured from a single datum.
DiameterThe size from one side of the circle to the other (through the centre). Circles must always use diameter and not radius.
RadiusThe size from the centre point to the edge of the curve
Dimensioning a HexagonThere are two ways to dimension a hexagon: AF (across flats) is when we measure from a flat side to another flat side.
AC (across corners) is when we measure from one corner to another.
AC (across corners) is when we measure from one corner to another.
Major AxisThis is the distance between the longest centre line on an ellipse.
Examples
Examples
Apply the British Standard convention for an external thread on the end elevation of the fixing screw. (1 mark)
Answers
Apply hatching to Section A-A in accordance with British Standard conventions. (5 marks)
Answer
1 mark for each area correctly hatched
A combination of chain and parallel dimensioning has been used in the drawing. State an advantage of using chain and parallel dimensions in orthographic drawings. (2 marks)
Answer
Chain dimensions clearly show the dimensions from one feature to the next. (1 mark)Parallel dimensions void risk of accumulated error (1 mark)
Different methods of dimensional tolerancing have been applied to the drawing of the roof bracket. These are labelled method 1 and method 2. (2 marks)
Answer
Method 1: common tolerance Method 2: symmetrical tolerance
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