Could the key to an efficient additive manufacturing production workflow be found in the build plate?

AM technology provider DMG MORI has worked with users of its laser powder bed fusion (LPBF) machines to implement a simple but significant change to build plates that streamlines production of small LPBF parts. Using modified build plates, parts are 3D printed onto separate round pucks that rest within the overall square plate. Each puck then travels with the printed part via robot loading into a CNC lathe, where the puck serves as the clamping feature for holding the workpiece in the chuck. With the part initially held this way, all CNC machining, including cutoff, can be performed in one cycle on a lathe featuring milling capability and dual spindles for complete turning of the part. That means no wire EDM is needed for part removal from the plate. It also means no build plate resurfacing is required, because the puck gets resurfaced via the lathe’s cutoff operation. DMG MORI says the process integration between 3D printing and machining made possible by the puck system, when applied to an additive build of 16 spine implants, reduces the total postprocessing time by 80% — from more than 6 hours for EDM plus finish machining down to just 1 hour and 20 minutes for machining with the puck system.

Lennart Tasche is additive intelligence consultant with DMG MORI Additive Solutions. He notes that the company is in a distinctive position when it comes to integrating metal 3D printing and machining: Beyond its AM offerings including LPBF and directed energy deposition (DED), DMG MORI is also long-established as a provider of machining automation.

“We want to improve the additive manufacturing process chain overall, and I think we are in a unique spot to do that,” Tasche says. “We have the machines and knowledge to bring automation to additive, to realize better process integration between additive and CNC. So, we are thinking: How can we move better from additive to subtractive? How can we make the total process leaner and faster?”

Some established technologies extend well to AM. For example, in machining, repeatable transfer from one machine tool to the next often relies on a “zero-point” clamping system in which clamping features on the underside of the fixture allows the fixture to quickly lock in place on multiple machines outfitted with the same receiver. For larger 3D printed parts, DMG MORI has applied zero-point clamping to its additive machines, allowing the part to go straight from LPBF or DED to CNC machining without setup time on the second machine. But for small additive parts, zero-point clamping is not the right fit, not just because of the space needed for the receiver, but also because the clamping force is not necessarily needed for 3D printing at the smaller size. Company engineers therefore expanded the process integration options that AM can employ. For smaller parts, the puck is in many ways an easier solution.

A Tray of Pucks

In this system, the puck rests precisely within a mating pocket machined into the build plate. Holding force is not needed. In LPBF, the forces tend to come from stresses within the parts as they grow, and the build plate in LPBF is secured to resist the cumulative effect of these stresses across the entire build area of the plate. But the potential distortion of a part as small as a medical implant is not enough to shift the location of one puck holding one such part. Therefore, gravity is sufficient to keep the puck in place, combined with a flat milled onto the puck to assure rotary alignment. The build plate then becomes like a rack or tray, holding these many pucks. In moving from LPBF to machining, the one manual intervention needed is removal of the completed build plate from the additive machine and placing this plate (with all its pucks) into a robot loading station. The robot can then lift pucks (with parts) out of the build plate for loading into the lathe one at a time.

Tasche says the resulting consolidation of AM postprocessing steps into one cycle represents a significant cost savings. The elimination of part removal from the build plate as a standalone operation is probably the chief example of this.

“Wire EDM, or bandsaw cutting — that’s a big step,” he says. “If wire EDM is needed, this takes time and the equipment is expensive. If the work is done with a bandsaw, then there is more work to remove additional supports and resurface the plate. The chance to do away with all this can be a relief.”

Plus, through a compressed air or coolant wash step, powder removal can be made a quick and automatic part of the same machining cycle as well.

One final compelling feature of this approach to additive/subtractive integration is the extent to which the idea, because it is so straightforward to implement, is both adaptable and scalable. The solution is being applied now by DMG MORI customers using “pucks” that were cut from round barstock. But pucks for future applications could be a different size or shape, as the application calls for. Unlike zero-point clamping, says Tasche, this idea will not require the company to stock or support any special hardware — the idea can advance just by teaching it to potential users. “Every manufacturer that has machining, and wants to integrate it with additive, should already have the capability they need to machine their own build plates and pucks,” he says.