Modern Machine Shop

SEP 2018

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Page 42 of 220

ADDITIVE INSIGHTS MMS SEPTEMBER 2018 40 Understanding Industrial 3D Printing Milling Studies Cover Additive-Part Supports TIMOTHY W. SIMPSON | COLUMNIST Data reveal that milling thin-walled, additively manufactured part supports requires particular caution. Last month, we detailed the machining of the metallic support structures used to anchor addi- tively manufactured parts to their build plates and prevent warping and distortion. We learned about how these structures tend to collapse during mill- ing rather than shearing cleanly away, and about how trapped powder can further complicate sup- port removal. This month, we will focus on how different support-structure configurations can impact tool life and, by extension, overall milling performance. Studies show that cutting forces are higher in fully dense material than "block-type" supports with thin walls and hollow sections. Compare fig- ures 1a and 1b, which show in-plane milling forces in the X and Y directions throughout 10 rotations in fully dense and thin-walled supports, respec- tively. Figure 1a shows forces peaking six times per rotation, corresponding to each of the six teeth on the fluted end mill. In contrast, Figure 1b shows force varying as the tool travels across the support structure because each of the cutter's rotating teeth encounters a different geometry (either a thin- walled section, a hollow section or a combination of the two). Although the amplitude of the force is lower when compared to cutting fully dense mate- rial, the frequency is higher. The difference between the support types is even more evident when we take a moving average of the cutting forces. As shown in Figure 2a (page 42), the average cutting force in the fully dense material remains flat in both X and Y directions, which is consistent with the homogeneity of the material. This is markedly different to the data in Figure 2b, which shows the moving average of the cutting force when machining the thin-walled structures. The peaks and valleys clearly show how cutting forces increase when removing the thin-walled structures and decrease quickly thereafter. Based on these averages, we can compute the corresponding cutting energy over time. In doing so, we find that the specific cutting energy for the thin-walled support structures is only 12 percent of the energy for the fully dense material. If we normalize this by the associated density of the material being cut, then that value jumps to 43 percent, indicating that the thin-walled support structures require less than half as much energy to be removed. Based on this result, we would think that would be good for the tool, right? Not so. In one tool t=0.36 sec (b) Thin-walled support structures (a) Fully dense material Figure 1: In-plane milling forces during 10 rotations.

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