3D Printing and 5-Axis Machining Combined in One Machine
James Anderton posted on October 24, 2014 |

Hybrids are all the rage these days; in automotive engineering, they deliver a useful compromise between electric car efficiency and internal combustion engine range. It’s a practical solution to “range anxiety” for motorists, but in advanced manufacturing, hybrid systems promise something more: a synergistic combination of 3-D printing and advanced five-axis machining that should allow the user to design and make parts that are uneconomic or simply impossible to produce any other way. But can these radically different processes be combined in one machine?

 

Machine tool heavyweight DMG MORI builds a broad range of multi-axis vertical and horizontal centres. Now they have taken the firm’s DECKEL MAHO monoblock five-axis milling machine design and added a powder deposition articulating nozzle for on-part laser sintering, creating the LASETEC 65 3D.  

DMG/MORI claims that the nozzle-based powder deposition process is up to 10 times faster than part generation with a conventional powder bed system. Speed isn’t important only for productivity reasons, it can define both part geometry and the production process.

Current SLS-based additive manufacturing in metals requires extremely thin powder layers on the order of tens of microns, making the process extremely slow when compared to subtractive manufacturing standards. Cycle times for parts of modest size can be measured in hours or even days, frequently one or two orders of magnitude longer than machining processes.

With an “either/or” approach to part making, conventional, machineable features must be built with expensive 3-D processes, or complex internal structure must be sacrificed for machining speed. DMG/MORI expects to bridge the gap with their hybrid machine.

 

What’s behind the fast deposition rate?

DMG/MORI’s additive strategy with the LASERTEC 65 3D is to deposit and fuse the metal powder dynamically onto a moving part substrate. The process looks similar to plasma-spray processes, but this one’s not about rebuilding bearing journals.

 

 

It’s a true additive process, but takes advantage of the machine spindle to address the root of the deposition rate problem with powder bed SLS processes: resolution. With the 20 or 30 micron layers needed for good surface finishes in metal 3D parts, the physics of the process leaves no available shortcut to build times that are slow, so slow they’re reminiscent of the pre-CNC era of machining.

Unless there’s no other way to make a part, additive build rates in metals are, in a word, unacceptable. With the new hybrid technology however, the majority of the part can be machined with conventional 5-axis technology.

Operators familiar with coding tool paths in conventional milling machines will be comfortable here, mainly because the LASERTEC 65 3D is based on familiar technology. The software switches form cutting and deposition processes automatically, which is one secret to its speed.

Not only is the deposition rate relatively quick with the flying head, but the ability to finish machine the built-up region where accessible means there is no need for very fine layer control: “build-up, then cut away the excess”, is faster than  fine deposition from and delivers superior surface finish in the bargain. Depending on the laser/nozzle selection, wall thicknesses between 0.1 to 5 mm are possible. 

An added benefit is the elimination of the support structure needed in purely additive processes in almost all applications. Conventional work holding and pallet-based automation solutions are usable. On the shop floor, operators should see little difference between this machine and other 5-axis centers.

 

Laser deposition adds options

The laser is a solid state diode unit operating at 2kW inside a powder feed nozzle that appears similar to the shielding gas feed commonly seen in laser welding. The 1-micron wavelength is typical for solid state lasers and the spot diameter has two available choices: 3mm is standard with 1.6 mm optional. DMG/MORI claims a typical deposition rate of abut 1kg/hr, but the actual rate is highly dependent on the build parameters and material choice.

The laser head rests outside the tool magazine but is picked up and fitted to the spindle with a dedicated swingarm similar to conventional toolholding. The head parks well clear of chips and coolant streams during machining operations.

 

 

The laser head interfaces with the spindle using the popular European HSK standard, in this case the HSK-A63. It’s a shorter taper holder that’s controversial with some users in North America who have standardized on NMBT or ISO types but there are several advantages to HSK. It’s a 1:10 taper, about half the overall length of a typical CNC taper and is hollow, clamping from the inside. The shank walls are designed to flex slightly; in addition to the clamping force of the drawbar, centrifugal force adds to the shank/spindle grip force. While this aspect isn’t a factor in retention of the laser head, it does allow positional accuracy from the spindle that’s equivalent to magazine-fed cutting tools, without a separate laser head drive system. Compared to powder bed systems, the ability of the LASERTEC 65 3D to orient the part for optimal deposition (generally vertical) allows conical, tapered and even undercut geometries that are difficult to reproduce with good resolution at speed.

Materials currently available include tool steels, “chromoly”, stainless and bronze alloys as well as some exotics such as niobium alloys, but two options open interesting possibilities: tungsten carbide and Stellite. Hardfacing during part build is an obvious option, as is part remanufacturing for high value components such as gas turbine hot section blades.

An optional dual powder tank facilitates hardfacing while keeping base material alloys on hand. This option is important, as material changeover in SLS processes is time consuming and carries risk if residual powder is allowed into the stream.

While “done-in-one” is always the goal of any machining operation, the use of advanced equipment like the LASERTEC for reman applications will require either very high value component work, considerable volumes or both. Addition of in-machine gauging using a Renishaw or similar probe might make a reman application profitable for processes needing customer qualification, like aerospace applications. Other markets would include medical, tool/die/mold and energy, particularly downhole applications.

Bigger, heavier parts

 

The other major advantage of hybrids like the LASERTEC is their ability to handle CNC mill-sized parts. The machine is based on the firm’s DMU 65MonoBLOCK machine and as a result offers a similar working envelope, with a 500mm by 500mm working area with 350mm of height. It’s substantial, as is the 600 kg maximum part weight. A 1.4 meter wide opening allows direct access to the working envelope, as expected for a machine capable of heavy weight parts.

Although DMG/MORI has not yet offered turnkey automation packages for the unit, the base DMU 65 is available with robotics and pallet changers…an attractive proposition for high flex operations contemplating production runs mixed with hard surfacing/remanufacturing jobs. The LASERTEC 65 3D is based on a CNC mill and that’s how users will regard it: a machining centre with additive feature capability. The majority of the machine code and cycle time is likely to be used in conventional machining, making “conventional” machine specifications as important as the deposition rate and resolution of the 3D build.

The standard magazine carries 30 tools, with options for 60 or 90 tools, while the machine spindle delivers a peak 13kW of power at a 40% duty cycle, 9kW at 100%. Maximum torque, which many feel is a better indicator of milling capability, is 83/57Nm at the 40/100 duty cycles. Standard spindle RPM is 10,000 with a 14000 RPM option. From a “chip” perspective, machine parameters are equivalent to the firm’s 5-axis mill offerings.

 

 

Controls are key

 While 5-axis machining control technology is highly advanced, integrating 3-D printing into a control system design for subtractive processes is a new field. DMG/MORI uses the firm’s CELOS system and are ERGOline control with Operate 4.5 software running on a Siemens 840 D solutionline CNC control. The Siemens architecture is well proven, and entire components can be designed using Siemens’ NX CAD product. NX handles the milling programming, while the integrated SAUER module in the NX software handles the additive portion. A useful feature is the ability to interrupt the 3-D process with milling steps and vice versa, optimizing machine utilization and allowing 3-D geometries with undercuts that cannot be made by either process alone.

In both the machining and additive manufacturing communities, hybrid technology is controversial. Many engineers believe that hybrid solutions do neither process well, a criticism which could be true if powder bed processes were combined with a conventional spindle.

One secret to DMG/MORI’s approach with the LASERTEC 65 3D is a strong emphasis on the machining side of the hybrid part making process. With the laser sintering head relieved of the need to produce high-quality surface finishes, deposition rates can be an order of magnitude higher than conventional SLS processes, since there is little penalty to adding a “cleanup” cut post-deposition. DMG/MORI in fact describes the laser deposition process as “laser deposition welding”, a subtle nod to traditional fast-rate build processes like plasma spray.

Like any advanced part making process, one set-up is the efficiency goal. With hybrid machines however, that age-old rule may not always apply. Will it make sense to use a specialized machine for parts that are 90% machined, or palletize the process and handoff to the hybrid after roughing or intermediate operations? Much will depend on the value of the parts, and in the case of very close tolerance work in medical and aerospace, the additional gauging burden of multiple setups may decide the issue. For gas turbine hot section blades for example, a dedicated machine purely for remanufacturing, while simpler, but extremely high wear components in the downhole or cutting tool markets might also justify a machine like the LASERTEC 65, possibly with automation at higher volumes.

DMG/MORI has invested heavily in hybrid technology and has a substantial global user base comfortable with their equipment. Will it work? Yes. Is there enough specialized work in the market to justify hybrid machines? That remains to be seen.

 

 

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