Page 29 - 3D Metal Printing Spring 2016

Designing for Additive Manufacturing 3D
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As-Built Surface Tumbled Surface
Dimensional Profile Allowance
relief, but even more so on parts that span the build area.
EBM may be a better choice in general for large parts in both the horizontal and vertical orientations, because the stress- relief step is not required. If geometric distortion is still an issue for a part, a few options can help it hold its shape through thermal processing. For instance, adding a sacrificial gusset or frame around thin- walled components can provide stability during thermal processing.
Compensate for Surface-Finish Needs
Printed parts exhibit rougher surface finish than castings when using powder- bed-fusion technologies, and finish also depends upon the orientation of the part surface. Top surfaces can be fairly smooth, vertical surfaces normally have a rough but consistent finish, and surfaces orien- tated at angles less than 90 deg. from the build plate can be the roughest. Generally, laser powder-bed systems have optimized powder and laser parameters that enable a smoother surface finish. EBM has focused on optimizing for cost via build speed, and therefore surfaces generally are rougher—a build-time trade-off.
Keeping surface finishes as-built is optimal for cost, but structural and flow requirements or part aesthetics can drive the need for various surface-finish meth- ods. Here, the ultimate goal is to under- stand the material-removal rate and com- pensate for that in the as-built model. For example, if a tumbling process removes 0.005 in. from the surface, add a 0.005- in. envelope to the as-built CAD file so that, following post-processing, the fea- tures will conform to required thicknesses.
When attempting to compensate for surface-finish methods, designers must also consider the physics of the process itself. For example, corners will exhibit a more aggressive material-removal rate than flat surfaces in a vibratory bowl, so profile tolerances may have to be opened in those locations (Fig. 4). For critical fea- tures or tolerances, different masking techniques during the surface-finish process may aid in maintaining the as-
Fig. 4—When attempting to compensate for surface-finish methods, designers must consider the physics of the process itself. For example, corners will exhibit a more aggressive material-removal rate than flat surfaces in a vibratory bowl, so profile tolerances may have to be opened (dashed line) in those locations.
built surface so that material does not degrade, or can be machined later.
In addition to external surface-finish methods, internal cavities often require finishing to smooth channels for flow requirements. Similarly, designers must consider the material-removal rates of those methods and compensate for them in the model.
Design for Machining
As discussed previously, most parts must be machined before becoming final products. Critical features and tight tol- erances that can’t be achieved by the print- ing process usually must be brought into conformance by conventional CNC processes. Design considerations up front make the CNC process more effective.
Design and manufacturing engineers should collaborate to understand how the part will be held during CNC-process setups. Utilizing advantages of additive manufacturing, datum features can be added to the as-built model to reduce custom tooling, or at minimum provide consistent tooling across a part family of similar geometries. Tabs, pins, holes, slots or even a temporary handle can be added to the printed part to help align any fix-
Fig. 5—You can see the difference between final part geometry (a) and as- built geometry (b) with holes filled in and bottom stock added for post-process machining.
tures and tooling, and can be later cut off. Having such conversations up front reduces development iterations and quickens setup and development time.
Wrap stock represents another design consideration with regard to machining. Just as with surface finishing, some mate- rial removal is required to bring critical tolerances into conformance. Adding wrap stock to those features ensures an ade- quate amount of material for removal via the CNC process. The amount of wrap stock added should be enough to com- pensate for removal but still hold toler- ances, and limited so as to not add exces- sive time and cost to the CNC process. This varies depending on the material, part geometry and the CNC technology used (Fig. 5).
Not Your Father’s Design Process
In summary, additive-manufacturing designers must wear multiple hats. They must understand the full manufacturing value stream, cost analysis, and even structural and materials engineering. One step better: Have experts available in a collaborative atmosphere to provide insight into the design process as it pro- ceeds. 3DMP
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SPRING 2016 • 3D METAL PRINTING | 27
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