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Generative Design – A Fusion of Additive and Subtractive Manufacturing

Working in Fusion 360

Introduction

Using generative design (GD), we are able to engage in design exploration to assist us in seeking out new ways to solve engineering and product design problems. It provides us with the opportunity of exploring what lies beyond the boundaries of traditional designing, especially where light weight, high stiffness, and minimal material usage are important. Exploration and discovery in design can be helpful as we look to address the environmental pressures we are facing in the area of design and manufacturing.

However, many people still see generative design as a tool for creating designs that can only be produced using additive manufacturing. Find out how that's not the case, and how the tools that support other methods of manufacturing can be used in interesting ways.

"Complexity comes for free" is often a phrase used in conjunction with additive manufacturing; however, complexity is not always a desired outcome with some additive methods due to the constraints and limitations of the process. Yet delivering an optimized design outcome that's suitable for the chosen manufacturing method is always the goal of using generative design.

Related: Fusion 360 Introduction to Generative Design with Robert Savage

So, in these cases, how can you use GD to drive an outcome that is suitable for those additive manufacturing methods? See how subtractive objectives can be used to output an efficient, manufacturing-ready design for use with additive, and how generative design really can deliver a fusion of subtractive and additive manufacturing.

Multiple Manufacturing Methods

When it comes to engaging in the opportunities for design exploration that generative design offers, there can be a perception that it is only used for pushing the boundaries of additive manufacturing parts. That perception is often shaped by some of the more complex, high-profile examples of GD that people have seen, and the fact that when it first emerged it was initially focused on additive manufacturing as the method of delivering outcomes.

However, that is no longer the case. Generative design can now produce solutions that are far more inclusive when it comes to the types of manufacturing techniques that we intend to use to produce our final design.

GD is now capable of delivering designs that can be manufacturing-ready for methods such as 2D cutting, 2.5-axis, 3-axis and 5-axis CNC machining, as well as being able to produce outcomes for die-casting which can easily lend themselves to being used for manufacturing using other methods of moulding.

To illustrate that point, here are the results of a generative design study that returned a number of different solutions. All of them are valid design options, each tailored to a specific manufacturing method. All of them were produced from a single set-up in generative design.

One study with four manufacturing outcomes.
One study with four manufacturing outcomes.

That breadth of capability is interesting, since in the world of manufacturing, additive and subtractive techniques are often seen as mutually exclusive methods. There’s sometimes an element of cross-over in hybrid manufacturing applications where additive manufacturing is used to deliver a near-net shape and needs the assistance of subtractive methods to get to the final accurate form, but often those who are using additive manufacturing will see just the possibilities for using additive, whilst those using subtractive manufacturing focus on the opportunities for employing machining.

Exploring a Fusion of Manufacturing Methods

I would like to introduce how fusing together additive and subtractive manufacturing methodologies within generative design can be beneficial. It also serves as an example of how Fusion 360 lives up to its name through the way it brings both manufacturing methods together within the manufacturing and generative design work spaces that exist within it.

When using generative design, I would encourage you to consider that just because the manufacturing outcome for a study is targeted at one particular manufacturing method, it does not always have to mean that it is the sole preserve of that method. What follows is an example of a type of “manufacturing fusion” workflow.

Additive Manufacturing without Complexity

Additive manufacturing is often described as a way of making things where “complexity comes for free.” Whilst that's never been entirely true it's a statement that has served as an enduring strapline for those advocates of the technology. The truth of the matter though is that some additive manufacturing methods cannot deliver on that promise of high geometric complexity, yet they are still able to offer a very effective way of making things.

Additive manufacturing with some materials and with some techniques is restricted by the fact that unsupported overhangs are not only undesirable, but may not even be possible. There are two examples I will to use to illustrate that, the primary one involving Fused Filament Fabrication (FFF) 3D printing and the secondary one involving the Directed Energy Deposition (DED) method of additive manufacturing.

With FFF 3D printers, it is now possible to use material filaments that are densely loaded with metal powder carried within a polymer binder. Using these materials, parts can be 3D printed on desktop machines and then subsequently put through catalytic de-binding and sintering processes to get to a final metal part that is very close to being 100% dense. There are several examples of this kind of system available, but here I will concentrate on the use of BASF Forward AM Ultrafuse 316L material on an Ultimaker S5 (or S3) desktop 3D printer.

BASF Forward AM Ultrafuse 316L and Ultimaker S5 3D printer.
BASF Forward AM Ultrafuse 316L and Ultimaker S5 3D printer.

Ultrafuse 316L offers the opportunity to create 316L stainless steel parts through 3D printing but, at the time of writing, there is no effective removable support material that is compatible with that particular 3D printing filament. Overhanging areas can also be the cause of significant issues with part stability during the de-bind and sintering stage of the process. However, I still wanted to explore how that kind of material and additive manufacturing method could be used for creating a part that had been produced using generative design.

Generating a Fusion of Subtractive and Additive Manufacturing

So, I took a slightly different approach when it came to specifying the manufacturing outcome in order to do this. Because of the need to produce a design that was free from overhanging areas, the manufacturing objective I chose to use was 2.5-axis CNC machining since, by its very nature, it should generate a design that conforms to that requirement.

When specifying the manufacturing objectives for 2.5-axis CNC machining in the generative design study set-up there is the ability to define the minimum tool diameter that will be used. Whilst you would normally specify the real dimensions for the CNC tool that you are going to use, the software does not actually constrain you to using real-life tools. So, it doesn’t really matter for instance that you are unlikely to be using a 0.4 mm diameter tool for 2.5 axis machining but, if you are looking to create a solution for FFF 3D printing, that dimension could be used to represent the diameter of the material extrusion from the nozzle being used.

Specifying minimum machine tool diameter in Fusion 360 generative design.
Specifying minimum machine tool diameter in Fusion 360 generative design.

So, using that thinking, here's a study that I ran for the design of a gripper arm. It’s one that is based on the design that can be found in the Sample projects section in the Fusion 360 Data Panel:

( Project => Simulation Samples, Folder => 1 – Hands-On Exercises, Design => GripperArm).

Gripper arm set-up in Fusion 360 generative design.
Gripper arm set-up in Fusion 360 generative design.

In this case, I set up the study and included the 2.5-axis CNC machining manufacturing objective in the full knowledge that I intended to use the outcome instead for an additive manufacturing method. In this case, I set the tool diameter at a fairly modest 2 mm (even though a nozzle with a 0.6 mm diameter would be used for the 3D printing) and a wall thickness of 2.5 mm to provide a usable condition for 3D printing. Had I wanted an outcome with more complexity I could have reduced that specified tool diameter even further.

Specifying tool diameter and wall thickness in Fusion 360 generative design.
Specifying tool diameter and wall thickness in Fusion 360 generative design.

The tool direction was set at Z to ensure that no overhangs would be created for the 3D printer to deal with and 316L stainless steel was included in the materials selected for the study.

This is one of the solutions generated for a 316L stainless steel part using 2.5-axis machining which looked looked worthy of further investigation and potential for 3D printing. As you can see from the image, there are no overhangs in the design, so I decided to export this outcome and made only very minor changes to it before using it for 3D printing.

The selected generative design outcome.
The selected generative design outcome.

3D Printing and Post-Processing

What resulted was a straightforward, support-free 3D print using Ultrafuse 316L produced on an Ultimaker S5 machine.

3D printing the selected design.
3D printing the selected design.

 

Completed pair of Ultrafuse 316L 3D prints.
Completed pair of Ultrafuse 316L 3D prints.

These 3D prints were then put through the de-bind and sinter process to finish up with generatively designed 316L stainless steel parts that had been successfully manufactured using a desktop 3D printer.

3D printed part after de-bind and sinter process.
3D printed part after de-bind and sinter process.

Next Steps: Directed Energy Deposition

This same technique can also be applied to additive manufacturing using Directed Energy Deposition (DED) methods. Machines using this technology are now starting to emerge at a lower cost point and at a more user-friendly level than ever before, which means that we are starting to see the barriers for adoption of that technology being lowered. This has the potential benefit of accelerating its use as a manufacturing method.

Whilst DED can be deployed as part of a hybrid manufacturing approach, where the deposition head is used within a multi-axis CNC machine, it’s also being made available in stand-alone machines such as the Meltio M450. This offers DED (or in this case what is described as LMD - Laser Metal Deposition) in a way that is very similar to FFF 3D printing. However, in this application it has the same constraints of not wanting to produce overhanging areas, so the technique of using 2.5-axis generative design outcomes for additive manufacturing is valid for this process too, and will form part of my ongoing use and development of the workflow described above.

Meltio M450 DED (LMD) machine.
Meltio M450 DED (LMD) machine.

Conclusion

So what are you waiting for? Open up Fusion 360 and become a “design explorer” using all the tools that generative design offers. See how you too can combine additive and subtractive manufacturing objectives into your explorations and discover the potential benefits of using the capability of the multiple manufacturing objectives that generative design provides.

Generative design gripper arm assembly.
Generative design gripper arm assembly.

Steve Cox is a qualified, professional engineer with over 30 years of experience working in the automotive industry, having had engineering responsibility for many vehicle projects that involved meeting multi-million-pound investment budgets together with leadership of teams of up to 25 engineers. He is now a 3D technologies consultant and educator working across a wide range of sectors. Steve is very much involved with the future of making through his training activities with Fusion 360 as a Silver-level Autodesk Certified Instructor, together with promoting the use and applications of 3D printing and additive manufacturing. This is both within the education sector and industry where he is also up-skilling engineering and manufacturing users in the latest 3D digital engineering workflows. Steve also offers consultancy and services in the area of product design and engineering together with 3D printing/additive manufacturing.

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