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Software 36 MoldMaking Technology May 2015 CONTRIBUTORS Jeff Jaje is a technical support manager for Vero Software and the WorkNC brand. normal orientation. While this avoids collisions, it may not be optimal when you also consider a machine's dynamics. For example, performing holder and spindle avoidance may introduce an excessive amount of directional changes in the mill's rotational axis. That is, the rotation may switch from clockwise to counter-clockwise and back several times during the toolholder avoidance stage as the algorithm attempts to maintain the rotational component of the tool normal to the geometry being milled. This is similar to what occurs in three- axis operations, when every time the direction of a machine axis is reversed, it affects the milling machine performance and surface finish in that location (see Figure 5). Instead, it can be beneficial to program the cutter path with only those transi- tions necessary to avoid collisions. This allows fewer reversals in axis direction and places higher importance on smooth transitions than on forcing the tool to be normal in that axis. Smooth Milling Options Milling machines benefit from smooth movements, so the rotational axis should remain in motion rather than repeat- edly starting and stopping. Imagine five-axis milling of a cube with rounded corners. On the straight portions of the cube, the part does not need to be rotated. It can be milled with just a simple, linear, three-axis movement. Yet, as the cutter approaches the corners to be rounded, the part rotates simultaneously with the linear axes. The changes in machine vibration as the axis starts and stops can often affect the sur- face finish of the part (see Figure 6). While it is theoretically possible to manually program and set the rotational value for a part, molds include complex, FOR MORE INFORMATION: Vero Software / worknc.com freeform shapes that require millions of CAM-programmed points. A fully manual method for controlling the rotational axis for complex mold shapes is impractical. CAM software programs specifically designed for moldmaking incorporate different methods for handling the issues that arise from axis reversals and stoppages. One example is a semi-automated method using a specific mill- ing algorithm that analyzes the part, avoids collisions and looks far enough ahead to smooth out transitions in the calculated cut- ter paths. Another method incorporates user- created wireframe shapes that help guide and direct the mill's continual rotational axis. A curve, for example, can be used to determine the general direction of a tool tilt and pro- vide input on how the continual axis rotates (see Figure 7). Input drive geometry enables the axis to remain in continu- ous motion without a large number of stoppages or axis reversals. This works by allowing the CAM algorithm to use the drive geometry as a guide for the part rotations, as long as there are no imminent tool collisions. If the input curve is continuous and smooth, the CAM algorithm will use that as a model for its output rotations. Smoother rotations with fewer changes in direction will result in faster performance and better finishes. Even when methods such as these are employed, complexi- ties associated with molds today demand that the algorithm of the CAM system places the most importance on tool- holder and spindle avoidance, and secondary importance on the input drive curve. Although there may be automatic algorithms for five-axis machining that allow toolholder and spindle avoidance to mill deeper with shorter tools, axis control is still an important consideration to make a five-axis machine run smoothly and efficiently. With an input curve, the axis can stay in motion and reduce reversal movements. FIGURE 7