From the beginning of the industrial revolution, there was a need to communicate the ideas of the designer to the people who were responsible for making the parts required. When the designer was also the owner and manager of the business word of mouth, aided by some sketches or models, was sufficient, but as businesses grew and outsourcing was increasingly used, the need for proper recording of design information increased. Use was made of scale drawings, generally outside views and frequently coloured or shaded to make them life-like, and wooden models and mock-ups of assemblies.

The first CAD systems developed in the 1970s were two dimensional (2D) and essentially replicated manual drawing methods on the computer. Plan, elevation and section views could be drawn on the computer to represent the object being designed. Using the computer had several significant advantages: modifying the drawing was very easy, common parts could be replicated by simple copy-and-paste or mirroring operations, and dimensioning was at least partly automated. The onus was still on the CAD operator to make sure that the various views were consistent with one another. When the design was complete, a paper copy was made and given to the workshop for manufacture using traditional machine tools. As computing power and software technology developed, the ability to visualise the 3D object was added to some 2D systems. Initially this was confined to wireframe pictures, but ultimately included the facilities of shade and illuminate surfaces. This ability was intended for visualisation of the object so that the user could check that the object being designed would match expectations. Creating and editing the design was still done on 2D views.

True 3D CAD systems are based on the object itself rather than 2D views of the object. The user interacts with and shapes the object directly. The design may start with a “primitive”, a simple shape such as a cylinder or a box. Further primitive shapes can be added to build up the final object. For example, an axle might start with a cylinder representing the part of the axle between the wheels. Further cylinders coaxial with the initial cylinder are added to the ends to represent the parts of the axle where the wheels are located. If the axle runs in outside bearings it extends beyond the wheels, and further cylinders are added to give the final shape.

If the shape is complicated, the starting point will be a sketch showing some key feature of the design. The sketch is made from lines, circles, arcs, polygons and other similar features, and is dimensioned to make it the correct size. A wheel might start with cross sections of a hub and a tyre, the latter shaped according to the Guild standard. The shapes defined by the sketch are then extruded (extended perpendicular to the sketch), or for the wheel example, rotated about the axis, to give a 3D shape.

The drawing of the wheel starts with a 2D sketch of the cross section of the tyre. This is made up of straight lines and circular arcs, and in this example it is to finescale dimensions. The user must place the various elements and define the dimensions that are shown in the sketch, including the radius of the tyre from the centreline of the wheel.

In the case of the wheel, further sketches on the outside surface of the hub and the inside surface of the tyre define the shape of a spoke at each end. The two sections are then joined by a lofting operation to give the spoke, and the spoke body can be copied and rotated about the centre of the wheel as many times as required.

In this way, “parts” or “components” are created. Parts can be put together in assemblies, for example, a wheelset is created from two wheels fixed in their correct positions to an axle. Joins between parts may be fixed, as in the wheelset, or may be allowed to move in defined ways. A complete steam locomotive valve gear can be animated by allowing rotary motion between the various rods and pins connecting them. The outcome of this process is a solid model that can be the basis for making the component using a numerically controlled manufacturing or machining process. If it is to be made by conventional, manual machining, 2D drawings can be made of the solid model. The key difference between 2D and 3D CAD is that in 3D CAD, the 2D drawings are generated from the solid model of the component. In 2D CAD, the component is defined by the 2D drawings.

This section lists the uses of CAD files in making railway models, or parts or components for models. The lists are not comprehensive or complete, and there may be considerable overlap between 2D and 3D CAD files. It is generally true to say that 3D CAD software will do anything that 2D CAD software will do, but the reverse is definitely not the case. The advantage of 2D is that many will find it easier to learn and use.

These files are used in the manufacture of components that are essentially flat plates, for example locomotive frames, footplates, carriage and wagon sides, sides of buildings, etc. Processes include:

• Chemical etching. The CAD file is used to mask the plate to separate areas to be etched from those to be left intact, before the etching process itself.
• Laser cutting. This includes cutting out components and adding surface detail to a component (door outlines, planking, etc) by controlling the power of the laser.
• Water jet cutting. Used for metal components.
• CNC machining.
• Dimensioned drawings for machining and manufacture by manual methods or the manual programming of computer controlled machine tools.

A variation on CNC machining is sometimes called “two and a half D” CAD, where the x and y dimensions along the surface of the plate are defined by the CAD file, and the “half D” refers to movements of the cutting tool in the z axis perpendicular to the plate. A thick plate may be machined in several repeated cuts at increasing depth of the tool. These are not included in the CAD file itself, and will be programmed by the machine operator.

3D CAD can, in principle, be used to create any 3D object, such as fittings for locomotives and rolling stock, large assemblies such as complete bodies, and scenic items. In addition to the 2D CAD applications listed above, 3D CAD output can be used for:

• 3D printing. For personal use, 3D printers are mostly based on stereolithography (SLA) and fused deposition modelling (FDM) technologies, and print a range of soft plastic materials. Industrially, many other technologies are used including selective laser sintering (SLS), multijet fusion (MJF) and direct metal laser sintering (DMLS). These allow a wider range of materials, including metals and hard plastics, to be printed.
• Numerically controlled machining (CNC) in a wide variety of metallic and non-metallic materials.

The concept of 2D CAD is easily understood by anyone trained in traditional mechanical or technical drawing, or used to reading drawings created in this way. The concepts themselves, of exterior views from different directions, showing hidden edges, and sectional views to show the interiors of components, are fairly straight forward.

The way that 3D CAD operates is quite different, in its focus on the component itself rather than views of the component, and those familiar with conventional 2D CAD and 2D engineering drawings will find it necessary to do some “unlearning” and approach the subject with an open mind uncluttered with preconceptions. In that respect, the initial steps in 3D CAD are often easier for those with no experience of traditional engineering drawing and design.

Once the concept is understood, much of the learning time will go into learning the package itself and how to use it to create the results required. Different packages have different approaches. Some are highly graphics-based, make extensive use of the mouse to select and manipulate objects and use the keyboard for little more than inputting dimensions. At the other end of the spectrum are packages that use extensive text input or “scripts”. Each point (e.g. the end of a line, centre of an arc or circle) is defined by x, y and z coordinates, and further input defines how the points are joined to form shapes. Different approaches will appeal to different people and it is worth taking time to understand how each one works to find the one that suits you best.

This screenshot is of a graphics-based 3D CAD package. The menus and toolbars at the top are used to define and control all the shapes and operations. On the left of the screen is a list of all the components that are part of the model.

By way of contrast, this screenshot is of a script-based package. The box on the left is the “script” that defines all the shapes and how they are used to create the complete model. The script is written by the operator, but it does not have to be done within the CAD package. It can be done externally using any text editor and then imported.

Some packages are free and can be tried out without any commitment. Those that are not may have a trial offer that is free for a limited time and may come with reduced functionality. Any package that has a significant user base will have, in addition to instructional material supplied by the vendor, instruction, examples and feedback in web forums, videos, etc, that can be studied.

The table below is a list of CAD packages that are potentially useable in railway modelling and have a significant user base. The list is confined to packages that are primarily aimed at mechanical design and omits those focused on electronic circuits, architectural modelling, animation and visual effects. It is also confined to those packages that are likely to be used by hobbyists or small businesses serving the hobby market. Full-featured products clearly aimed at large industrial organisations, offering complete support and coming with a large price tag, are not included. Some software suppliers offer their products in a series of packages including low-cost options with fewer features and limited technical support, and where it is relevant, this option is included.

The list is not comprehensive and contributions and updates are welcome.

1. For one-off lifetime purchase unless otherwise stated. Excludes VAT and import duty where applicable.

2. Design, modify and manipulate 2D drawings. Create 2D CAD files in a standard format, and dimensioned 2D drawings containing user-defined views such as elevations, plans and sections.

3. Design, modify and manipulate 3D objects including arbitrary surfaces. Create 3D CAD or solid model files of the object in a standard format.

4. The ability to combine individual parts into assemblies with rigid joints or joints with defined freedoms of movement. Create a 3D CAD file of the assembly in a standard format.

5. The 2D or 3D model generator can be run from a script, or a set of commands, rather than direct operator input. The script may be produced inside the CAD program or written separately using a text editor or similar and imported into the program. Scripts can also be exported.

6. In addition to exporting CAD files that can be used with additional processing in CAM systems, the CAD package contains specific CAM processing to create files that can operate machining or manufacturing tools directly, such as slicers for SLA and FDM printers, tool paths for CNC machines, etc.

2D CAD does not make great demands on computing power. Any computer made within the last 5 years is very likely to have a sufficiently fast processor and adequate RAM and disk storage. A large screen (about 24in. or larger) is a considerable advantage. It is possible to run 2D CAD on a laptop, but it is not convenient. A printer is only necessary if you wish to make paper copies of drawings as opposed to distributing and using the CAD files, and the ability to print on A3 paper is an advantage if the drawings are large or complicated.

3D CAD requires greater computing power and a recent-specification computer is advisable. Older and slower computers, and those with limited RAM, can be used but the software will run slowly and screen refreshing and updating will take noticeable time. A dedicated graphics card is a definite advantage. The same comments about monitors and printers as for 2D CAD apply here.

Software vendors web sites should be consulted for further information.

Written by Nick Baines, April 2021