Tips for Designing Multipart Assemblies

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Insight: Tips for Designing Multipart Assemblies


Hi, it’s Friday which means it’s time for another Insight video from me.

Each week we take a look at a different aspect of additive manufacturing. In this week's video we look at material properties and what you need to have in mind when considering the role your part has to play.

Two of the most basic tenets of designing parts for manufacturing and assembly are shrink to the bill of materials and make products easy to assemble. If that sounds easier said than done… well, you aren’t quite wrong.

However, there are several ways to make your next multipart design less like a trip to the dentist and more like a trip to the park. And by several, we mean eight.

Number one - symmetrical parts.

It can be a tough thing to wrap your mind around at first, but many products that are usually made from two unique components can actually be made from a pair of identical parts.

Think of a housing box made up of two halves. Conventional wisdom might say to put bolt holes in one half, threaded bosses in the other, then use fasteners and washers to stick it all together. It works, but it also needs two different moulds. Instead, we can just make the two halves symmetrical, distributing the holes and threads in each part equally and opposite one another.

Same function, half the tooling costs.

Tip number two: use self-mating parts.

Take the housing we were just talking about – it’s nice and symmetrical, but it still requires manual installation of threaded inserts, never mind keeping track of all the screws and washers. Too much hassle by half.

Instead, you can use a simple system of mating clips and hooks and allows the two halves to snap together. You can take things even further by replacing even the clip mechanisms with a living hinge; now you have a single-piece design.

The amount of materials you need has been slashed, assembly time is cut to zero and you just need one mould. Can’t really ask for more.

Number three: family moulds.

These are a great way to produce a whole series of parts with similar shapes and sizes. You take, say, the two mating parts we mentioned earlier, develop them in the same mould and you get the entire thing done in one run instead of two. Any group of parts that make up a complete assembly can take advantage of this, so long as they are made of the same material and fit in the volume you’re working with. A great way to save time and money.

Next up, tip number four: use standard hardware.

This one seems obvious, but when you’re making something new it can be easy to get caught up and try to reinvent the wheel – by which I mean try to manufacture your own specialist screws, shafts and other bits and pieces.

This is a road to madness and frustration, trust me. Better to redesign your whole product around standard hardware than try to go it alone.

We’re halfway through now, with tip number five: use monoform parts.

Essentially, this means seeing if you can find a way to make your part out of just one piece. The properties that make plastics such as polypropylene and styrene butadiene excellent candidates for living hinges also makes them suitable for other flexible components; this provides clever part designers with opportunities to simplify assemblies and shrink those bills of materials. In some cases, a bit of clever design can reduce what was once a complex assembly down to just a single piece.

Next up, tip number six – make use of overmoulding and insert moulding.

You often find that two or more components can be moulded together. Rather than welding, gluing, or screwing a rubber grip onto a plastic instrument case, overmoulding creates a permanent bond with no need for secondary assembly processes. The two plastics must be chemically compatible, but that leaves you with a pretty broad spectrum of material combinations and colours.

Onto the final stretch with tip number seven – think about ease of assembly during the design phase.

In an ideal world you have as little of it as possible, but sometimes the design is just going to need putting together and there’s no way around it.

If a product must be assembled, make it easier by orienting the various screws and connectors in such a way that all are accessible from the same side. Use common hardware so the worker doesn’t have to reach for different tools. And mistake-proof the assembly by designing in alignment pins and guiding features to avoid mismatched parts.

Also, try and spend a little time in the assembly room. Knowing the issues that emerge at this stage often yields big improvements in part design.

Finally, tip number eight. Look for alternatives.

It’s a slightly strange tip, but if you find that the technology you’re working with isn’t quite meeting your requirements, don’t be afraid to branch out.

Technologies and materials we could only dream of a few decades ago are now turning the design and engineering world on its head.  A good example is industrial 3D printing. Quantities permitting, parts that once required construction of an injection mould or setup and programming of a CNC machine tool can now be 3D printed in a fraction of the time and cost.

Of course, some multipart assemblies are best done the old-fashioned way, by machining individual pieces and assembling them. Yet even here, the rules of designing for manufacturability and assembly apply. Keep things simple, don’t make what you can buy off the shelf, explore all your manufacturing options, and understand the rules of each. Stick close to the guidelines and you’ll be designing cost-effective and efficient multipart assemblies in no time.

Right that’s more than enough for this week, so I’ll say goodbye and see you again next Friday.

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