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Software 34 MoldMaking Technology May 2015 This illustration shows how the tool moves from position 1 to 2 to 3 to 4 in order to remain as normal to the surface as possible and still remain collision-free. This method causes large reversals of the rotating axis (or back and forth movements). reduced tool vibration, which yields better and more accurate finishes (see Figure 1). For practical purposes and programming ease, a ballnose tool typically is used for many five-axis finish cuts, because the tool can rotate around the center point of the ball using five-axis programming and still maintain the same contact point with the part. Tool diameter is used to calculate the surface feet per minute (sfm). As the contact point moves closer and closer to the tip of the ball, the effective cutting diameter continues to shrink and this cutting speed measurement starts to approach zero. When this happens while milling normal to the surface, it can be difficult to utilize feed rates and spindle speeds recommended by the tool manufacturers. This, com- bined with difficulty in creating a quality cutting edge at the very tip of the tool, can contribute to a poor surface finish. To work around this difficulty, many programmers will insert a lead angle into the cutting program. Instead of cut- ting perfectly normal or perpendicular to the geometry, the tool is tilted forward to cut at a set angle. This moves the actual contact point away from the cutter tip to another por- tion of the tool, which then allows for cutting speed calcula- tions that are not near zero when milling away from the tip. Using a lead angle does introduce some lateral forces, but LEARN MORE Visit our Software Zone for more information on CAD, CAM, CAE, fow analysis and data management software. Go to moldmakingtechnology.com/zones for a complete list. the majority of forces are still axial and directed along the tool axis, resulting in no deflection or vibration. Lead angle five-axis machining therefore is often preferred when mill- ing a part's flat and/or shallow, sloped areas (see Figure 2). However, actual milling machine kinematics may sometimes not support lead angle machining. Alternatives for Impractical Applications When milling relatively steep areas, such as pockets or inserts, it is not practical to mill truly normal to the surfaces, as the rotations necessary to accomplish this task often are out- side the limits of the machine. It also requires the part to be mounted at an impractical height to ensure the head does not collide with the table (see Figure 3). An alternative to milling normal for steep areas is using a constant rotational axis (often the C axis of a rotating table), FIGURE 5 FIGURE 4 This tool is normal to the face of the part in one axis, but it can still cause collisions.