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3D Design

A very light introduction to 3D CAD

For what seems like a long time now 3D printers have been around.they are incredibly useful pieces of kit for fabricating all sorts of useful things, from enclosures to fabrication tools and jigs and even to morse keys. The distinctive look of the the common additive layer technique is just about everywhere. I even have a fitting for my shower screen that I drew up and printed in ABS and it’s still there 5 years later, long after the original part broke.

The method of printing has been described in the past in Radcom and to some extent those introductions have helped bring 3D printing to amateur radio. But if we take a step back and look at the tools used to design the parts we can see how technology has moved on and how Computer Aided design (CAD) has moved from 2D to 3D. Digital twinning, where an electronic version of a physical object, like a plane or a building, exist as 3D models. So, how does one go about designing your own parts in a 3D design package?

Concepts

There are some key concepts that we need to discuss. Firstly is the terminology that surrounds how we actually design things. There are surface modelling tools (like SketchUp) which allow you to develop simple models but as the name suggests they tend to work with surfaces. This means that the objects that are drawn can lack fundamental relationships and one of those is that they generally aren’t solids, just the outside face. To explain this imagine a dice. If you look at it you can see the outside face with the dots on it. It can either be hollow or filled. Surface tools assume it is hollow. This is great for imprecise sketches but lacking when we need precise dimensions and relationships as well as tools to add and subtract parts to them.

The other tools are parametric modelling tools. These software tools work off parameters like length, depth, breadth for example as well as the relationship,or constraints, that object may have with its surroundings. Its a bit tricky to understand and each software package will do it in a slightly different way but there are just a few fundamental concepts.

1. Sketches

A sketch is the starting point for most design work. This is where the initial shape of an object is drawn on a plane. Using our dice analogy if we wanted to draw a dice with sides of 10mm then we could sketch out the initial square looking from top or either of the sides, it doesn’t matter for this example as it is a simple cube. So we would simply draw a square with constraints of 10mm for sides as if we were looking down from the top.

Most software packages will allow you to draw either the individual lines or a rectangle (in this case with equal sides). This just speeds things up. We have completed the first sketch, all that needs to be done to turn this into a cube is to give it some volume. This is done by extruding the square to give it some volume

2. Extrusion

To extrude the part we simply pull it in the direction we want for 10mm. The method of extrusion is largely the same for each software package. Now we have a cube. But what about the indents for the numbers? Well this is extrusion again but in this case we are removing material rather than adding it. Like just about any engineering problem (of which this is a pretty small one) there is more than one way to achieve this. For the ‘one’ you could sketch a circle on the centre of any face (it doesn’t matter for the first one but remember that a dice has a specific orientation)and then use the extrude method to remove some material but you would be left with a cylinder.

An additional step would be needed to round off, or filet, the cylinder to make a nice dimple shape.

Another option might be to draw a sphere that intersects the plane on the face of the sketch and take one part away from the other, a Boolean cut. The end result is the same but the method is different.

Behind these simple concepts are some complex bits of thinking. Which part is constrained to which and the relationship to planes is all done in the software. Pretty neat?

Parts and assemblies

A part is simple that, a nut or a bolt for example. Each can be drawn individually but to put the nut on the bolt you need to assemble the parts. 3D CAD does this too. Lets take a different example from a dice, a more radio focussed one that can illustrate another important aspect. So I recently looked at a small project that needed an enclosure, but which is the right one and will all the parts I have fit in it? I could draw everything out, but I don’t need to. Many companies supply CAD data in common formats. The most common being STEP. I downloaded the file, imported it into the drawing and therefore had half the job done. I could do the same for an RF connector, an SMA with a bulkhead fitting in this case. That’ s 2 parts that I didn’t have to draw.

Putting the connector on the enclosure was done as an assembly. Each software package will do this slightly differently but again each part will have constraints against its mating face. A useful thing to know is that in most packages you can draw (or import it) once and then use it many times. You can change the part if you wish and all the parts will change. Handy but worth keeping an eye on if you only need to change one.

The method of mating can be different for different parts. Our nut can be threaded onto our bolt for example and used in video documentation.

Software

Ask a dozen radio amateurs what’s the best antenna for 40m and you’ll get at least a dozen answers. It’s the same for mechanical design packages. I have steered away form explaining how to do things in certain packages because what works for me might not work for all. There are some obvious ones to look at. There is also no need to spend a fortune on ‘professional’ software. CAD packages can cost a lot more than you think. But for most of us having a computational fluid dynamics package isn’t going to get used. Although it might be for an F1 team!

1. Open Source Tools – FreeCAD is the leader. It works off work benches and has excellent support and tutorials.

2. Software Packages – Autodesk Inventor is a professional tool which plays very nicely with Eagle if you wanted to get a physical view of a circuits that you’ve designed in Eagle. Simpler tools like TinkerCAD are ideal for the beginner and Autodesk Fusion for those with a bit more skill.

3. Browser based – Onshape is one of a newer trend in browser based CAD that needs no software to be installed.

The hard part is choosing the right tool for you. Once you’ve decided on where you’d like to start then spend the time to understand the software, use it’s tutorials and the more time you spend with it the simpler it’ll become.

Conclusion

3D CAD has been around for years now, it has come a long way from the monster tool it used to be and with the ability in a lot of packages to export data. Feature rich packages are available for free and with a bit of learning the basics can be understood very quickly. I have used it for designing things from mast brackets to tools to align LCD screens in enclosures (and the shower part of course). 3D CAD and 3D printing are extremely handy tools for any ham who likes to make their own stuff.

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