Page 26 - 3D Metal Printing Magazine Winter 2022

3D CAM Software Triggers Success with DED Metal Printing
of the basics for the process are built into the software, explains Rooks, such as rotat- ing start-stop points around the build, how the build differentiates from periph- eral to infill, and moving back and forth to fill in.
“We’re also pushing new ideas,” he says, “such as building rotationally symmetrical geometries, even with variable thickness. Traditionally, building these geometries requires numerous starts and stops, which can lead to discontinuities in the build. We recently developed an additive turning process where you essentially take a 2D slice and set the tool-axis orientation so that during post processing it becomes a true five-axis tool path. This keeps all of the tool-axis articulation to the vectors established in that 2D slice. This way, as the build progresses, even if it’s very com- plex, there’s only one start point and one stop. This avoids creating the disconti- nuities you would otherwise have within the build.”
“When you’re processing the pre-post- ed code and actually mapping it to the kinematics of the machine and you pass through the pole,” Levine adds, “the C- axis will whip around to keep it on one side of the rotary. We can program around that so that the secondary rotary will just position and transition through, rather than having that C-axis motion come around to the point.”
The result: The DED machine runs bet- ter, Levine and Rooks say. “If you have those constant motions where you hit the pole, all of the geometry is pulled around and whips fast, and you lose the real tra- verse speed of the AM head,” Rooks explains. “This is what causes overbuilds and underbuilds; the head is not travers- ing at its true standard rate and staying constant. By getting around that C-axis polar move, we can pivot all of the way through the raised rotary without having the whip-around move.”
Hybrid-Machine Advantages
Levine and Rooks also point to the advantages of applying the DED process in a hybrid machine, where subtractive and additive manufacturing occur in one environment. To illustrate, consider build- ing a part surface using AM by depositing five 1-mm layers.
“In the end, the final buildup will not be 5 mm; the process isn’t perfect,” Levine says. “When the process moves out of control, the ability to go back and machine the deposited material and then requalify the top surface—it may be 4.5 mm high, for example, after the five passes—allows us to then continue the process knowing exactly where we’re at, 4.5 mm rather than 5 mm. All of this affects build quality as the passes build up, because the laser focal position changes from optimal if
Manufacturers increasingly are interested in modifying their old tooling to produce the next generation of parts after a design change.
you don’t account for that precision with each pass. If laser position is not in con- trol, build quality will suffer.”
Also important is the ability to control the process parameters specifically for dif- ferent areas of the build. “This is a relatively new capability developed in response to customer feedback,” Rooks explains. “For example, when running a hybrid machine tool for DED and we need to turn a sharp corner, one of the common problems is that the machine tool must slow down in the corners and then accelerate out. This leads to uneven builds in the corners. A new software feature provides process- parameter control so that when navigating corners, we can program the DED process parameters to prevent these overbuilds in user-defined build zones.”
Here, full five-axis motion control becomes critical. “Machine-motion smoothness is paramount,” says Levine, “otherwise, with the rate of traverse of the head especially with rotaries in play, process consistency suffers. So, good five- axis motion control is critical to optimize results.”
Another example where five-axis motion pays off: polar moves or dwells. “In the subtractive mode you might just get a little witness mark on the part,” says Rooks. “But with AM, that dwell leads to overbuilds. Again, smooth motion is crit- ical during the DED AM process.” 3DMP
24 | 3D METAL PRINTING • WINTER 2022
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