The following is adapted from Robb Hudson’s presentation on combining additive and subtractive machining techniques into a single machining center.
When considering additive vs. subtractive manufacturing for a part, conventional wisdom has been to consider complexity. Additive is a natural choice for high-complexity components because there is complete geometric freedom to build components with sophisticated internal structures and features. CNC-based subtractive machining is highly productive for conventionally shaped parts, able to machine very tight tolerances. So the more complex the part, the better suited it is for additive technologies – with the caveat that parts with tighter tolerance specs need conventional approaches.
However, that logic fails to address advantages and limitations to both methodologies.
With laser-sintered additive manufacturing, essentially you’re taking an often expensive powder and driving a laser beam over a tool path to fuse powders, creating a part one layer at a time. This process is at least one order of magnitude slower than traditional CNC metal removal.
Getting up to the speed of subtractive systems involves tradeoffs that all come down to the thickness of each layer in the 3D build. You can increase productivity with much higher energy input, yielding thicker layers of powder and much faster builds, but the surface finish suffers. Or you can accept the slow process, producing smaller layers, creating a better surface finish (see Figure 1, pg. 16).
Even with the slower process – producing thinner layers for better surface finish – you’re still going to need post-deposition processing on some type of CNC machine platform. This is where the struggle has always been between additive and subtractive manufacturing. These have been seen as competing technologies instead of being complementary to one another. That gap is now bridged (see Figure 2, pg. 16).
Hybrid is the bridge between those two, where we can merge the technologies together to print material, add metal to existing components, grow parts from nothing, and then be able to machine them in that same envelope.
The freedom of additively built parts can’t eliminate traditional machining requirements in most cases. There are going to be features that require surface finishing or tolerances that cannot be achieved through straight material deposition. Adding directed energy deposition to a CNC machine tool allows the complexity of printed metal and yields the geometric and surface finishes of conventional CNCs.
Combining the systems can maximize production of the additive system by allowing thicker deposition layers because you will be able to improve surface finish within the same machine. By taking a very accurate machine tool and adding an adapted deposition nozzle, the machine can print from nothing or add material to existing workpieces, then machine as the process dictates. You can print a little bit, exchange the nozzle for a milling tool, machine the feature, do some surface prep for the next layer to be added, and return to the processing nozzle for the next layer of material. Or you can print to completion and then machine the component.
At Mitsui Seiki, our hybrid system is a 5-axis machining center platform that has sub 15µm accuracy, a 15,000rpm to 30,000rpm CAT or HSK spindle, and is coolant-equipped. By taking a spindle-adapted nozzle, fiber laser supply, and powder feed system, and integrating those in the machining center, we get a system that can change back and forth easily between conventional CNC and additive.
The nozzle loads directly into the tool changer just like any conventional milling tool, and multiple nozzles can support different powder flow rates and laser processing approaches. Other nozzles could potentially be maximized for localized heat treatment, surface cleaning, part drying, burning coolant away, surface preparation for the next layer, and even laser drilling or laser cutting.
With the hybrid approach, you can potentially build parts out of nothing, printing an entire part, or adding material to existing parts – for wear resistance, to reclaim worn parts, or to repair damaged components.
The low-hanging fruit is aero-engine turbine repair – such as high-pressure turbine compressor blades, low-pressure blades, blisks, large impellers, and high-pressure turbine segments (see Figure 3 below). This has been done for as many as 30 years in some cases. However, it has not been able to effectively repair component and perform post-weld machining on the same platform. We can add in the additive process by using an inspection system outside the machine to scan an existing component.
That scan will generate point-cloud data that can be compared against a CAD model version. Tool paths can then be automatically generated to jump back and forth between prepping a part for additive and subtractive machining. Our goal is to be able to do this eventually without any engineering intervention, with the ability to step from one process right to the next.
There’s also the potential to recover parts that would otherwise be scrapped in traditional machining environments because a machine crashes or tool breaks. Instead of scrapping a component that’s worth hundreds of thousands of dollars, much as with the repair scenario, a company could compare scan data of a damaged workpiece to the CAD file and generate additive/subtractive toolpaths to repair any damage.
Early additive days
The whole additive process is still in its infancy. Many companies are spending significant money on additive without an end-game solution is in mind. Let’s not fall naively for the hype. The laser industry had issues in its early days when it overpromised and underdelivered. Once developers created industrialized solutions, they had process control and successful systems. In the beginning, though, lasers and optics failed. Let’s avoid that troublesome phase in the additive world.
At Mitsui Seiki, we’ve always been focused on removing material. That’s been our job. We do it very well, and we do it very accurately. We see a day, through additive, that our current market space will evolve as we develop more creative solutions involving these new methodologies. Fifteen to 20 years from now, it will evolve significantly.
Mitsui Seiki USA Inc.
IMTS 2016 Booth #S-8519
About the author: Robb Hudson is business and technology development manager at Mitsui Seuki’s soon-to-be-released, additive/subtractive machine platform. He can be reached at email@example.com.