Engineering Drawings and Design Development
Drawing Types and Scales
In engineering, most design information is shown on drawings. Today, drawings are generally not drawn by hand. They are produced on a computer, using CAD (Computer-Aided Design) systems.
A key factor on a drawing is the scale – that is, the size of items on the drawing in relation to their real size. When all the items on a drawing are shown relative to their real size, the drawing is drawn to scale and can be called a scale drawing. An example of a scale is 1:10 (one to ten). At 1:10, an object with a length of 100 mm in real life would measure 10 mm on the drawing.
Most engineering designs consist of a set of drawings (a number of related drawings):
- General arrangement (GA) drawings show whole devices or structures, using a small scale. This means objects on the drawing are small, relative to their real size (for example, a 1:100 drawing of an entire building).
- Detail drawings show parts in detail, using a large scale, such as 1:5 or 1:2. Small parts are sometimes shown in a detail as actual size (1:1), or can be enlarged to bigger than actual size (for example, 2:1).
For electrical circuits, and pipe and duct networks, it is helpful to show designs in a simplified form. In this case, schematic drawings (often referred to as schematics) are used. An everyday example is the map of a train network.
Notes: When written, drawing is often abbreviated to dwg. CAD is pronounced as a word: /kæd/.
Design Development
Initial Design Phase
A structural engineer from a firm of consulting engineers has sent an email to a more senior colleague, with an update on a project for a new airport terminal.
Stefan,
We had our first design meeting with the airport authority and the architect yesterday. As you know, the client just gave the architect a short list of essential requirements for the terminal, so the design brief was pretty open. As a result, the ideas he’s come up with form quite an adventurous concept. However, things are still at an early stage – there are no scale drawings yet, just eight sketches showing roughly what he wants the building to look like. So it wasn’t possible to assess the design in detail. The next step is for the architect to develop the sketches into preliminary drawings. These are due at the end of April.
Collaborative Development
When a design team consists of engineers and consultants from different organizations, the design development process needs to be carefully coordinated.
Before the first draft (version) of a drawing is sent to members of the team, a decision is made about who needs a copy. Sometimes, a drawing will only be issued to certain specialists in the team. Sometimes, it will be circulated to all the team members.
After team members have received a drawing, they can comment on it, and may ask for the design to be changed. Following these comments, the drawing will be revised– that is, drawn again with the requested changes made to it. Every drawing is numbered, and each time a drawing is amended (revised), the letter next to the drawing number is changed. Therefore drawing 110A, after a revision, becomes 110B. When revision B is issued, it becomes the current drawing, and A is superseded. With each new revision, written notes are added to the drawing. These describe the amendments that have been made.
When engineers revise drawings during the early stages of the design process, they may have to go back to the drawing board (start again), and redesign concepts completely. For later revisions, the design should only need to be refined slightly.
After a preliminary drawing has been finally approved (accepted), a senior engineer can sign off (authorize) the drawing as a working drawing– that is, one that the production or construction team can work to. However, this does not always mean the drawing will be final. Often, working drawings go through more revisions to resolve problems during production.