Hybrid manufacturing—using one machine to perform both additive and conventional subtractive manufacturing processes—is gaining traction across manufacturing sectors, allowing companies to leverage the benefits of both.
Additive machines can print more intricate, complicated and exotic shapes. But compared with subtractive processes, additive offers less final precision. To achieve required specifications, many manufacturers already use a combined additive and subtractive approach: an additive machine creates a complicated part and then the part is moved to another machine for final milling or turning.
Increasingly, though, manufacturers are moving to a true hybrid system architecture where the same machine performs additive functions and the final finish work.
“Most additive processes print near-net shape,” Fabrisonic President Mark Norfolk said. “You can get in the ballpark but they will not get you that perfect shape every time. With a hybrid approach, you can stop at any point and clean up.”
Reducing the number of times a part is moved eliminates errors and improves precision.
“For a lot of parts, the hybrid process makes the difference,” Norfolk said. “Any time you move a part from machine A to machine B, you introduce the possibility of error. To clean up a 3D part, you’re moving it at least twice if not three or four times. Having that same part always held in an open fixture on one machine means you’re not getting an error stack up from moving.”
“With hybrid machines, you have a big advantage,” said Nils Niemeyer, product manager for additive at DMG Mori USA. “You can build larger-volume parts very rapidly and finish-machine the parts on the same machine. There’s less handling of the parts, smaller buffers. You get a finish-built product out of one setup. You integrate your supply chain completely into one machine.”
Using the same machine for additive and subtractive also means operators can mill the inside of the part, which isn’t possible on a separate finishing machine that can finish mill only the part exterior, Norfolk said.
“When you 3D print a part, you can always go back and mill the outside of the part on another machine but you can not mill the internal features,” he said. “Hybrid manufacturing on one machine allows you to clean up the internal features as you go.”
Envision an additively manufactured box with dimensions of 1 inch by 1 inch. On the inside, you need a channel that is .5 inch by .5 inch. With additive, you might get a channel that is .45 inch by .45 inch. With a hybrid machine, you can stop at any point, mill and clean up the surface to get the dimensions to exactly .5 inch, Norfolk said.
Beyond dimension, “It’s also about surface finish,” he said. “After 3D printing, a part may feel like a piece of sandpaper. After final milling, it will feel like a piece of glass. That finish is as important as precise dimension.”
In many applications, a smoother finish impacts performance.
For example, additive manufacturing often is used to create pipes with complex channels for cooling, Norfolk said. If the inside of the part is really rough, as can happen with additive processes alone, the roughness will create backpressure that makes it difficult to pump fluid through the channel.
“The better the surface finish, the easier it is to pump liquid through that cooling channel,” he said.
Fabrisonic’s hybrid process also makes it possible to embed sensors, Norfolk said. The material is built up additively to the point of insertion; CNC milling is used to make a precise cavity for the sensor; and then additive is used to build up further and fully embed the sensor in the finished product. Since Fabrisonic’s process is low temperature, the sensor is not damaged.
The hybrid approach poses a few potential disadvantages: below average capital utilization, the complexity of the hybrid machine, and less final resolution in the finished product.
Ideal capital utilization calls for performing the process on the least expensive machine capable of reliably performing the task.
“You’re buying this hybrid machine that costs hundreds of thousands of dollars because you want that additive capability,” Norfolk said. “But when you use that expensive hybrid machine for CNC milling, you’re running a very expensive machine to do what a much less expensive CNC machine could do. You’re using your most expensive equipment when you could be using something at half the cost.”
A more complex hybrid machine also is more difficult to maintain.
“Having both capabilities in one machine makes the machine more complex and a little more difficult to maintain,” he said. “There are more things to break. Generically, it’s a concern that everyone talks about. In our experience, we haven’t seen it to be a problem.”
Using hybrid processes can mean trading better speed for less resolution.
“We typically use the hybrid system for larger volume, bigger applications,” Niemeyer said. “We have higher deposition rates. We build up quicker than with powder bed systems. That sacrifices the resolution. Resolution is certainly something that can be limiting on hybrid systems.”
Overall, “in our opinion, the advantages strongly outweigh the disadvantages,” Norfolk said.
DMG Mori is working with Siemens to develop an additive/hybrid bundle that allows manufacturers to program machines for the hybrid manufacturing process, Niemeyer said.
Design tools favor subtractive method
Earlier software was not designed for optimizing the topology of parts produced with additive or hybrid manufacturing, said Dale Tutt, head of aerospace and defense industry at Siemens.
“If you’re machining a part, you are limited to the capabilities of the machine,” he said. “Design tools are built around the subtractive method.”
Another challenge of using different machines for additive and subtractive processes has been that software often has been disconnected.
For example, designers would create the geometry in a CAD system and then have to convert the CAD design to whatever software package the subtractive machine used, Tutt said.
“You design in one system, take the design to an intermediate file and take it to a different machine,” he said. “There are handoffs from additive to subtractive. Every conversion equals a possibility for a translation error—we always had to check it. You don’t have that full connectivity.”
With a true hybrid system and Siemens’ NX design software, “you don’t lose any content in the part,” Tutt said. “With our software setup, you can use the design files all the way through and maintain the linkage from form to finished part.”
Through its NX software and other tools, Siemens is offering convergent modeling to enable manufacturers to put material exactly where they want it on a part.
The designs for hybrid processes generated by NX are often well outside the box compared with what human designers come up with, he said.
“The design comes up and you think, ‘Wow that’s not what I expected’,” Tutt said. “But when you do the analysis, it’s right on target.”