CAD and Model Engineering

This page talks about Computer Aided Design (CAD) and how it can be useful for model engineers and hobbyists to design parts for projects. As well as part design, CAD can be used to check the fit of parts, to check for clashes in moving assemblies, to export 2D drawings for manufacture and to create files suitable for 3D printing.

CAD softwares are constantly evolving and new options appear, but at the time of writing this page (2023) some free options are; Creo elements express, Onshape, Sketchup, freeCAD, Fusion 360, Solid Egde and NanoCAD.

CAD software can be intimidating due to a high number of menus and options but most software works in a similar way. There are typically some 2D sketch tools to create profiles and then some 3D operations to create 3D parts from your sketch. Once you have the initial body created you can continue with more sketching and 3D operations to add details to your part, or there are a number of 3D functions such as tapers, ribs and chamfers that can be applied direct to the 3D model. This page explains what some of the more common functions are using some worked examples. The software commands are shown in bold italic text.

Basic Commands

This 2D sketch shows a circle with a 12mm diameter.

If we extrude the circle by 20mm, we get a cylinder. This could be the piston for a model steam engine.

A 10mm circle can be sketched on one end face.

This time, an extruded cut can be used to a depth of 19mm. This gives us a part which could be a cylinder of a small steam engine.

We can take the same starting sketch and add a vertical construction line, 25mm away.

Then the rotate or revolve function is available and we can produce a doughnut shape; an O-ring or a tyre perhaps.

The revolve function doesn't have to be 360°. Here the same profile has been rotated by 225° to create the basis of a hook.

The sketch doesn't have to be a regular shape either. Here a "U" is being extruded to create a channel section.

And this image shows a circle sketched on one side of the channel.

An extruded cut can then cut the sketched profile from the part to create a small bracket.

Fillets are 3D radii that can be added along any edge. Here they have been added to make the part look like sheet metal.

And this image shows two slots sketched on the base with an extruded cut applied, for mounting holes.

A lot can be achieved just using the extrude, rotate and cut commands, because actually the majority of machined parts in the home workshop, are cylindrical or rectangular.

The toy locomotive shown on the left was created just using cylinders, rectangles and sketched cuts. The software used fort his model is called Creo Elements Direct, and can be downloaded from :
https://www.ptc.com/en/products/creo/elements-direct/modeling-express
The software is free to download and use, but is limited to 60 parts in an assembly. Larger models would, therefore, have to be broken down into sub-assemblies.

The process to create a couple of the parts is shown below.

Something like the buffers can be created starting with a half profile sketch like the one shown above....

...followed by the revolve function about the horizontal line.

An extruded square with rounded corners sketched on the back face could make a mounting plate, with some holes cut for rivets.

Add some face colour to help you visualise the design.

The cab can be made by extruding the correct profile to the required depth.

The shell command is a powerful way to hollow out a shape. Just define the wall thickness and select any faces to remove.

An extruded cut on the front face could be used to create windows and a cut-out for the boiler.

Another extruded cut from a "P" shaped sketch on the cab side, will create the side profile. Add colours if you like.

Sketch planes

So far all the sketches have been placed on the face of a part. If a sketch is needed somewhere else, a custom sketch plane can be created. A sketch plane is like a piece of paper which the user can place anywhere in the workspace to drawn on. Some CAD software will give the user one sketch plane to start with, others will give X, Y and Z sketch planes as shown here.
However, if a sketch plane in a specific location or at a specific angle is needed, the user can define one to allow the sketch and subsequent 3D operation to have the desired affect. In the example below there is a need to model the angled hole in a cylinder casting to connect the valve to the steam inlet of the cylinder.

Here is the cylinder casting made from an extruded body with various extruded cut profiles where needed.

The part requires a hole at 20° to the end face. This can be done by setting up a sketch plane at 20° to one end face.

Now a circle can be sketched on the plane and the extruded cut operation will make a hole at the required angle.

With the extruded cut operation set to the correct depth, a cross-section view can be used to show the angled hole.

A sweep is similar to an extruded part, but in this case, the path of the extrusion is not linear, nor a circle, but a profile sketched by the user. For example, drawing a handrail for a locomotive cab.

For this part we could start with a sketch of a circle, the diameter of the grabrail. 2mm in this case.

Then a second sketch describing the path we want for the profile is needed. This sketch should start from the centre of the circle as shown.

Then the circle can be "swept" along the sketched profile using the sweep command.

Some circular extrusions could be added as mounting plates.

A loft adds another variable to the 3D operation. Like the sweep command you are moving a cross-section along a path, but this time the cross-section can vary too. In this case, the user will need to define more than one cross-sectional sketch, as well as the path for the loft and sometimes some guide curves to tell the software which vertices to connect.

Let's say we want to model a hammer head which is circular at one end, narrowing to a square where it joins the handle and then tapering to a rectangle along a curve for the claw.

A number of sketch planes will need to be set up in the correct place, each one with a profile sketched for that section of the hammer.

A guide curve may be needed to help the software understand which vertices should be connected. This gives the option to twist a part throughout the loft if desired.

The loft command will request the profiles to be used and any guide curves for the form.

A fillet can be added to the hammer head

And a "V" shape sketch on the horizontal plane can be used with an extruded cut to create the claw.

Assemblies

Finally, we should mention assemblies. When you have more than one part drawn up, you may want to put them together in an assembly to check the fit and alignment of features.
Most CAD software will have another environment where you can assemble parts. Relations or mates are used to define how parts go together. A piston should be concentric in a bore for example. You can also align faces and axis, make parts perpendicular, or place them a fixed distance or angle from each other.
Once an assembly has been created it is often possible to check for clashes and also subtract one part from another to create a matched socket from a plug for example.

CAD and Model Engineering

Basic Commands

Sketch planes

Advanced Functions

A sweep is similar to an extruded part, but in this case, the path of the extrusion is not linear, nor a circle, but a profile sketched by the user. For example, drawing a handrail for a locomotive cab.

Assemblies