New reasons to consider combining additive and subtractive machining in one platform
Manufacturing Engineering last covered the pluses and minuses of combining additive and subtractive machining in detail in July 2017. The basic motivation remains the same: No matter which additive approach you use, producing a satisfactory part almost always requires follow-on machining. And while there remain situations in which it makes more sense to use a separate mill or lathe for that post-processing, a hybrid machine costs less than multiple machines, takes less space, requires less programming, and reduces handling and work in process. Plus a hybrid gives you the ability to inspect and machine interior features as you build them. But we knew all this three years ago. What’s changed?
New Materials Broaden the Applications
One exciting advance in hybrids is the ability to combine materials, thereby creating specialized alloys or transitional zones. For example, LASERTEC series machines from DMG Mori USA, Hoffman Estates, Illinois, have twin powder feeders that can deposit two different materials throughout the build, precisely metering the flow. As Nils Niemeyer, manager of sales and service for the company’s additive manufacturing division explained, “functionally graded materials have been discussed in academia for a while, but we’ve commercialized it. For example, you might use 100 percent tool steel in one area of the part, and then if there is a need for an area of higher hardness, blend in some high-speed steel. Layer by layer, the process transitions from that one material to the other until there is perhaps 100 percent high speed steel and 0 percent tool steel.” In the past, he added, operators would have to manually write NC code for the powder feeders to change their feed in every layer. But DMG Mori and Siemens partnered to add a feature to Siemens NX software that enables the user to program a seamless transition curve.
Niemeyer offered another benefit to the ability to control the transition between materials: Conventional methods for hard-facing a part create a stress riser in the area between the hard and soft material. “But now, because we have the capability of easily grading it, we can create one or two layers of transition, with a smooth transition from the soft material to the hard material. Suddenly you decrease that stress riser.”
He admitted that because it’s still a disruptive technology, use cases for graded materials (including grading in magnetic behavior) are still being investigated. One real-world success story is a roller bearing manufacturer that worked with DMG Mori to build bearing cages with different degrees of hardness. Specifically, a cage has very high stiffness and hardness along the line traveled by the bearing, but blends into a less stiff, more ductile material on the side to better absorb any shocks.
Extended Life of Used Parts
Just as in 2017, hybrid machines are most easily justified for the repair and modification of existing parts. But while the turbine blade repair business appeared to be the most important market then, Niemeyer said mold and toolmaking has a much bigger potential now. For one thing, he observed, turbine blade geometries are simpler “and there are very specialized niche applications for repairing them.” There is a huge geometric variation among tools and molds and “no specialized process. The industry has been manually repairing them, and the quality of the weld is not consistent.”
For example, Bodine Aluminum, Troy, Mo., uses the LASERTEC 65 3D hybrid to repair aluminum die-cast molds and has tripled the life of a repaired die compared to the manual process, equaling the lifetime of a new tool. What’s more, these repairs are faster than the conventional method, and generally right the first time. The process for repairing an aluminum die cast mold at Friedrich Deutsch Metallwerk GmbH, Innsbruck, Austria, involved two hours of preheating, eight hours of manual welding, two hours of cool down, and 30 minutes of milling. The four process steps stretched over two days. Doing the repair on a LASERTEC 65 3D takes one setup, with one hour of programming and two hours of laser deposition and machining—a time savings of 80 percent.
Jason Jones, PhD, co-founder and CEO of Hybrid Manufacturing Technologies, McKinney, Texas, echoed Niemeyer’s assessment that mold and die repair is big business now. In 2017, ME reported that the powder bed fusion (PBF) technique was better than directed energy deposition (DED) for mold applications, owing to PBF’s higher part density. But both Niemeyer and
Jones said their DED systems achieve the needed part densities of greater than 99 percent (depending on material and parameters).
Jones added that the ability to mix discrete materials is inherent to the powder-fed DED process and doesn’t have to be limited to two, saying “because we can vary the powder that flows through the nozzles on-the-fly, it could be any number of materials” and that any given machine setup could include two, three, four or even more powder supplies.
Hybrid doesn’t build CNC machines. It offers a selection of interchangeable DED heads (branded AMBIT) that fit into any standard machine tool spindle. The company has partnered with end users that want to add AM capability to their machines and with machine tool OEMs that want to offer the capability. The latter category has grown to 10 companies in the last few years, including Haas Automation, Mazak, Hurco, Sugino, Georg Fischer, ELB, and ROMI.
Another change since 2017 is an increase in the range of possible bead sizes. AMBIT heads now go from 0.5 to 6 mm (~0.02 to 0.24"), and Jones pointed out that build volume increases by the square of the change in width.
New, Multi-Material Parts
In some cases, the ability to mix and tailor materials make hybrid machines the right solution for new part creation as well. For example, Jones said, some customers “don’t just want to repair molds. They want to make molds that are better than new. They’re used to making molds out of single materials and now you can use multiple materials.”
Jones said he’s seen many cases of mold builders using the technology to add hard facing along complex edges, “like the parting line or other wear areas. And where a high-performance material is applied locally to those areas, it can often double or triple the lifetime of the mold.”
Niemeyer pointed to a case study in which a customer builds die-cast molds with tool steel over a bronze core. The better thermal conductivity of the core improves the mold’s cooling performance by roughly 20 percent without degrading the lifespan of the mold, boosting throughput.
Niemeyer said the space industry is also showing increased interest in hybrid machines, in part due to material costs. “Take Inconel. It’s hard to machine. So instead of machining a lot of material from billet, you can produce near-net shape and only finish the surface where it’s needed.” Copper alloys used in spacecraft are another good example, he added, and they are sometimes combined with Inconel, such as rocket nozzles with Inconel cladding over copper.
“The lead time for these rocket nozzles can be nine months,” Niemeyer explained. “If you’re suddenly able to build a nozzle within four weeks it allows [engineers] to improve the design of that rocket nozzle. That results in increased productivity and efficiency of the nozzle. You won’t see cost savings in the machining, but you will see increased part efficiency, which eventually leads to cost reduction in the lifetime cycle.” Niemeyer argues that the entire supply chain must be considered when deciding if a hybrid machine is a wise investment. “Isolating the machine costs is not enough. You need to look end to end, and take things like long lead times for importing parts into consideration.”
An addition to Hybrid Manufacturing’s latest offerings is printing polymers and polymer composites. As Jones put it, every additive process has to make a fundamental choice between the volumetric deposition rate and the fidelity of the surface to the actual model, and hybrid machines are designed to resolve this issue. But although the plastics side of AM had everything from “desktop-style machines to room-scale machines for polymer extrusion that can literally print a car in,” there weren’t any efficient hybrid solutions. Jones’ team created a head and control solution that deposits “between 200 and 2,000 times faster than typical polymer extrusion machines. It takes what is on a desktop machine now and scales it up to fit in a CNC machine spindle or on a robot.” This means printing meter-scale parts rapidly and surface finishing with conventional machining.
One key application is creating jigs and fixtures around or into a metal part. “Suppose you’re machining a part that’s tall and thin and tends to chatter or vibrate,” said Jones. “Normally it would be fixtured with something tough, strong, and rigid and there would be worries about cutters running into that. Instead, we can print polymer around the part to reduce vibration and not worry about the cutter running into it. We make a sacrificial component we intend to get milled away.” Jones added that turbine blades and blisks are ideal for this approach.
Build Volumes Expand
Options to manufacture larger parts are another recent change to the hybrid machine universe. DMG Mori’s new LASERTEC 125 3D Hybrid has a 49 × 29" (1,244 × 737-mm) build volume and a maximum table load of 4,409 lb (2,000 kg), while the LASERTEC 6600 3D Hybrid mill/turn handles parts up to 41" (1,050 mm) in diameter and up to 19.7' (6 m) between centers.
Hybrid Manufacturing participated in the European Union’s (EU’s) Open Hybrid project and partnered with German gantry manufacturer Güdel to create what Jones described as a “room-scale machine. We worked on Weir pump component castings that are several meters across.”
Hybrid also developed a wire-fed deposition system for this venture, as opposed to its usual powder-fed system. Jones said wire is desirable, especially in larger machine formats, because it can cost less than powder feedstock, and is safer. “With wire, there are no powders that you have to worry about breathing in, no possibility of explosion from reactive materials like titanium or aluminum,” he said.
Not big enough? Talk to the folks at the European Federation for Welding, Joining and Cutting (EWF), Porto Salvo, Portugal. They led the EU’s three-year, €4 million ($4.5 million) Large Additive and Subtractive Modular Machine (LASIMM) project. Concluded in 2019 and now being commercialized, LASIMM combined the efforts of machine tool builders, academic institutions, and end-users to create a machine concept of near limitless size.
As Dr. Eng. Eurico Assunção, deputy director of EWF explained, the AM portion relies on a proprietary wire and arc additive (WAAM) DED method developed by Cranfield University and capable of deposition rates of up to 8.8 lb (4 kg) of metal per hour. “It’s cold metal transfer, which is a MIG/MAG process,” Assunção added. “But depending on the application we can also deliver a plasma process.” The effort also validated the ability to provide local shielding throughout the build to enable the deposition of reactive materials like titanium. The list of viable metals stretches from various steels to Inconel, aluminum, titanium, tungsten, molybdenum, bronze, copper, and others.
The subtractive function is handled by a LOXIN Tricept module. Now part of the Aritex group, LOXIN refers to this as a parallel kinematics machine (PKM) because it’s based on a tripod of three parallel moving actuators that join over the end of a central sliding tube. A rotational head and wrist at the end bring the unit up to five-axis interpolation. “The Tricept module combines extreme dynamics, high stiffness, and a very long reach. In fact, it’s rigid enough to enable high-pressure cold rolling and peening to refine the microstructure of the metal,” said Assunção. Also, the Tricept unit can use an exchangeable head to switch between machining and either WAAM or plasma welding AM.
David Barbosa, project manager for EWF, added that “the long reach of the PKM allows for necessary space for separate robots for additive and subtractive machining. And the PKM’s many degrees of freedom make it possible to reach various points with the right rigidity and precision for subtractive machining.”
The LASIMM demonstration machine built at LOXIN incorporates two giant articulating FANUC robots on a set of rails (all engineered by Global Robots), one PKM Tricept unit on a traveling column (parallel to the robot rails), and one part rotator with two columns and synchronized drives situated between the robots and the PKM. The robots are each equipped with WAAM heads and the PKM is configured as described earlier.
However, this is just one possible configuration. The LASIMM team envisions even longer assembly lines with several sets of part rotators, and giant systems with eight or more robots on two sets of rails, plus the PKM unit on an overhead gantry moving down the line. “The project taught us that one machine architecture doesn’t fit all needs,” said Assunção. “The machine is modular, so we can add and remove robots or features. LOXIN specializes in these integrated systems, so we are ready to build tailored solutions.”
Barbosa said EWF and Autodesk developed software that operates the additive and subtractive functions from one user interface. “The software includes a digital model of the machine. You set up the part on a computer and the post-processor creates a file to control the machine. Basically, it’s PowerMill plus the LASIMM,” he said.
What’s the business case for a LASIMM? Assunção said it depends on the part geometry. And like smaller machines, one key justification might be whether there is a need to machine features while building a part. Another might be part volume and throughput. Said Barbosa, “BAE Systems was one of the end users in the consortium. They wanted to reduce the manufacturing time and cost of their products, including things like big wing spars, and they wanted the ability to produce parts on demand, tailor made for a specific application. Hybrid is the best approach for both goals.”
In-Process Quality Control
Hybrid machining providers have also improved the ability to monitor and maintain part quality. Niemeyer said DMG Mori’s machines include a sensor that tracks the distance between the nozzle and the part and warns the operator if adjustment is needed. Built-in algorithms also detect potential nozzle adhesion. The powder feed is automated and monitored to ensure constant flow. The machines monitor ambient temperature plus the temperature of the part, work table, spindle and equipment inside the machine. “We’ve taken what we’ve learned over the last seven years and built it into software and a sensor kit,” he said.
Jones of Hybrid Manufacturing takes it a step further and argues that hybrid machining offers the underutilized opportunity to validate both the part geometry and the micro-structure in situ. In traditional CNCs, part microstructure is mostly determined by the steel mill. “From a traditional CNC cutting perspective, you’re really only influencing [and then measuring] the outer 10 or 15 microns of the part surface,” he said. In AM, the machine creates both the part form and microstructure as it builds. So one of Hybrid Manufacturing’s goals is to inspect part density in the machine, both in-process and after completion. It uses an eddy current probe to do this, which creates an electric field and measures the conductivity through the material.
“The head just skims across the surface,” said Jones. “Where cracks or voids exist there is no continuous path for the electrons to flow, so they have to go around, and the system measures this.” For a typical mold, the process takes 20-30 seconds compared to 20-30 minutes for a conventional die penetrant test.
Safely Operating a Hybrid Machine Tool
Additive machines require new safety considerations and procedures. Okuma’s LASER EX machines—a combination of a standard Okuma five-axis machining center (MU series) or five-axis multitasking machine (MULTUS series) incorporate a Trumpf laser for laser metal deposition. In addition to standard laser safety considerations provided by Trumpf, Okuma has created the following safety checklist:
- Training: Operators are trained on the safe use of machinery or tools, including labels and manuals.
- Lockout/Tagout: Maintenance personnel need to power off the machine and put a lock with a tag on it when adjusting and maintaining the machine.
- Fire Suppression: Machines are connected to alarm system; operators understand inspection requirements.
- Floor Markings: Proper markings for safety awareness (based on OSHA requirements).
- HVAC/ Humidity: Room is controlled within powder requirements. Make sure filters are changed regularly.
- SOP: Written procedures must be in place.
- Jewelry: Not worn while operating machinery.
PPE (Personal Protection Equipment)
- Clothing: Anti-static, with the ability to keep metal powder
- Eye Protection: Face shield, goggles, or safety glasses.
- Hearing Protection: Ear plugs or earmuffs.
- Respirator: Ability to filter out metal powder – only required when loading powder into the canisters.
- Flammable Storage Cabinet: When required, based on
- Gas Cylinders and Gages: Properly secured to avoid any leaks.
- Safety Equipment: Eyewash station or safety shower per OSHA/ANSI code.
- Floor Mats: Anti-static mat.
- Wet/Dry Vacuum: “Explosion-Proof” with the ability to handle combustible powder.
- Network Connectivity: Machine and laser allow for transfer of files and updates to system.
—Michael Lail, QA Compliance Manager, Okuma America Corp.
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