Modern Machine Shop

OCT 2017

Modern Machine Shop is focused on all aspects of metalworking technology - Providing the new product technologies; process solutions; supplier listings; business management; networking; and event information that companies need to be competitive.

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66 MMS October 2017 mmsonline.com ADDITIVE INSIGHTS Understanding Industrial 3D Printing Assuming you've already taken the necessary precautions that will allow you to safely handle parts additively built from metallic powder and feedstock, your next question likely is: What is the best method for removing their metal suppor t structures? For parts manufactured in laser-based powder-bed systems, those support structures will be of the same material as the part itself, whether that is a titanium alloy such as Ti-6Al-4V (Ti64) or a nickel chromium alloy like Inconel 718. Logic might dictate that you use the same feeds and speeds to machine away the support structures as you would to machine any other part made from these materials, but the jury is still out on this case for two important reasons: 1) In subtractive machining, you start with a material that has the properties you want and then create the shape you want from it. In additive manufacturing, it is the reverse: You build the shape you want and then heat treat, age and solution- anneal the material to get the properties you want. So depending on the postprocessing heat treat- ment (or lack thereof), the microstructure of an additive part will vary, which means you may not be post-machining the grade of material you think you are. In short, you will not know exactly what material you are machining when you are removing metal support structures from an AM part. 2) Support structures are often hollow, which means you are not continuously machining metal as you cut through them. The images in Figure 1 show what you will likely encounter: a thin-walled, mostly solid support, then air, then another thin wall, then air, and so on. Furthermore, most sup- ports are not fully dense, particularly in the build direction. Since the supports will be removed from the finished part, it seems unnecessary to make them fully dense, so laser-path planning algorithms often skip every other layer of the supports, saving material and reducing build time. Figure 2 (on page 68) shows the hatch spacing (laser-scan path) for a part and its supports. The tightly spaced gray lines represent the laser path for the part, while the sparser pink lines indicate the laser path for the supports. The tighter hatch spacing yields a fully dense part, while the coarser spacing will lead to slightly porous supports. Figure 1 also illustrates the challenges in remov- ing internal support structures by different machin- ing methods. These images were taken by a cap- stone engineering design team at Pennsylvania State University that was tasked with finding ways to remove the supports from a stainless steel piston crown. As you can see by the top left image, a Drill, Mill, Grind . . . What is the best way to remove metal support structures from AM parts? TIMOTHY W. SIMPSON PAUL MORROW PROFESSOR OF ENGINEERING DESIGN & MANUFACTURING PENNSYLVANIA STATE UNIVERSITY Fig. 1. These images show the results of attempts to remove stainless steel support structures from an AM part using different machine tools. (Photo courtesy of Logan Brown, Penn State University.)

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