The Design Rules of AM Metal Technology
Why designers need to lead the next wave of additive manufacturing adoption
In this article, I want to talk directly to product designers, as they hold the key to how Additive Metal Technology (AMT) gets adopted within their industry and product set. AMT offers significant advantages to the product design community to develop innovative solutions for the next generation of products. But product designers need to understand how these liberating options should to be approached.
Design for Manufacturing (DFM) procedures for AMT will become a huge paradigm shift for most designers, and it’s going to take supportive collaboration between product designers and AMT service providers before learning cycles are completed and experiences aligned so future designs benefit from AMT.
Why use AMT?
With the correct design efforts, AMT offers the following functional advantages that improve product design.
Light-weighting: Additive approaches yield far lighter parts. Mass reduction can be three or four times that of other technologies. This is because AM can grow much thinner wall structures and supports that would be difficult, and far more expensive, using subtractive technologies. Also, these solutions are performed without expensive tooling or casting expenditures.
Thermal management: AMT liberates designers to build in thermal management features by increasing surface area for the same part mass or topology.
Adding complex features: On the external and inside surfaces of parts, AM can, at almost no cost, add features that just cannot be done in conventional machining, as well as features that most casting tooling cannot do. Also, it can now offer casting or mold tooling designers internal cooling channels and pathways for both molding and casting tooling.
Part consolidation: AMT lets design engineers integrate multiple parts into one single component rather than make and assemble them together using conventional technologies. This may also eliminate assembly fit-up and integration issues associated with part interchange tolerance issues or, in the case of weldments, debilitating distortion due to excess fabrication. And we all know how much adding screws to a design attracts costs and reliability issues.
Tooling free rapid prototyping: These are always possible with AMT. Taking a typical subtractive part with excessive mass and applying AMT to it will provide a part that can be delivered far more rapidly with no tooling. However, your purchasing department may get sticker shock unless a panic delivery with no concern for price is what they have in mind.
Assembly fit up elimination: This may make AMT an advantage since AM structures have a homogeneous metallurgical structure. Parts machined from a solid block of material may not be able to provide this without quality and end-use reliability issues.
How to start with AMT
Ensure you start at the concept design stage, since a lot of the advantages of AMT rely on how you conceptualize the product design. Here is the most important design paradigm shift you need to undertake.
Traditionally, in conventional subtractive technology (milling turning, pressing etc.), we have learned that the more material we subtract, the more expensive the part. But, with AMT, the more material we add, the more expensive the part. In fact, most features can be added without additional cost.
This suggests that, to make this DfAM paradigm shift, you will need to adopt a design mentality of “wire frame thinking” rather than the huge trap of starting your design thinking with a block of metal and carving material off and drilling holes in it.
Here is an example. In the design below, you may be tempted to adopt a design like Fig 1. It’s either a milled block or a welded plate fabrication with holes bored through. The best way to proceed for an additive design is to think of the same design differently (Fig 2). It’s essentially 4 z-planes with 4 small holes between 2 planes, 2 small holes between 2 planes and 1 large hole between 2 planes.
Then you’d look at the part strength requirements and design in minimal wall sections to support the application. Remember that you can factor in stronger materials to achieve this minimum wall design thinking. This gets you to a design as in Fig 3. From this base design, you could then remove even more material as progressions (Figs 4 to 6). Just look at the design paradigm shift between Fig 1 and Fig 6. Both have the same Fit and Function, but not the same Form.
Of course, we all know that this won’t always work from a design strength point of view, but now you can add back material as gussets or radius where needed. Remember, these features are free in AMT, and the more material you can remove, the lower the cost.
This approach can sometimes be an advantage in terms of thermal mass and heat transfer, and of course overall weight improvement. Also, if the conventional approach was to make the parts in pieces and fasten or weld them together, then you have eliminated many process steps and quality issues with the one-piece AMT design. But it requires that you define the real mission of the part and forget manufacturing rules until you have got that mission clear.
AMT Design Constraints
Currently, AMT does have design constraints worth mentioning. Again, a suitable AMT provider will help you navigate the limitations. Most have documented DfAM guidelines and some provide reference samples to illustrate the constraints. In a nutshell, here are the major constraints.
Surface finish and accuracy is about the same as a casting finish, with tight tolerances being managed with stock added in the build stage and then a machining operation performed. As AMT evolves, this is always being improved, but smart design tolerancing can eliminate this constraint.
Down-facing surfaces have the worst surface finish and may be degraded (Fig 7). A slope above 30 degrees can be built without adding supports. Sometimes the provider can solve this problem by changing the part’s orientation during the build. Radius and gussets can also be added to create less “negative surfaces,” but striving to design all surfaces in one plane is optimum.
Holes and passages built in the horizontal build plane (Fig 8) are a challenge with the top of the hole being degraded due to unsupported powder at the top of the hole. Part orientation can be the solution but designs with holes in both horizontal and vertical planes will need attention. As the hole becomes smaller, however, this degradation effect is reduced.
If it’s a passage hole that doesn’t need to be round, one solution is to redesign the hole as a sharp apex. Once you realize that you don’t have to drill round holes or put them in a straight line, it’s amazing how much you can design using this apex hole concept.
Wall thickness is not a huge constraint and is typically much better than casting. Design limits will vary depending upon material used, but the typical minimum is 0.4mm, which is certainly more difficult to machine, especially when you have high complexity.
Threads, male or female, in any plane are not a good candidate for AMT; the “negative” surfaces will be too rough without at least a thread chasing process or the maximum or minimum diameter processed in the AMT build and then post threaded.
Some Do’s and Don’ts
Based on everything presented so far, here are some additional pointers:
- Do find a suitable AMT service provider who can consult on the AM technology. The best ones will have a strong engineering capability and a process to explain these new game changing DfAM rules. They will also be able to provide hands-on prototyping and early production capability.
- Don’t compare the costs associated with AMT vs subtractive technologies until you have looked at designing for AMT and also factored in all the commercial advantages of no tooling and rapid delivery.
- Do shift your design paradigm and start the transition to design in AMT at the concept stage of your product. Remember to define the mission of your parts so you fully understand how to leverage the advantages of AMT while avoiding the technology’s constraints.
- Don’t lock down the choice of materials until you understand which AMT powders are available. Keep an open mind and design your parts first. Then, based on the design requirements, decide which material will work best.
AMT is a rapidly emerging new tool in the designer’s kitbag. For some, it’s very exciting. Others may feel heavy pressure to change and manage new risk.
Remember, it’s a disruptive technology. You can either adopt it into your industrial sector effectively or be disrupted if your competitor takes the lead.
Nigel Southway is VP of Engineering at Additive Metal Manufacturing Inc. in Concord, Ontario.