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Nov 2017

Techspex provides metalworkers free research and analysis tools to help them find the right machine for their job.

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T H I N K I N G A B O U T F I V E - A X I S C N C M A C H I N I N G ? 2 about 5 percent of all machined parts require processing by full five-axis machines. Five-axis machining is most prevalent in the aerospace industry, where parts tend to be non-prismatic (not box-like), with complex surfaces, such as aerospace turbines, impellers or airfoil blisks. These complex designs require all five axes of the machine moving at the same time to create the shape, thus enabling the cutting tool to take a multidirectional approach to the part surface. Five-axis machining is also extensively used in moldmaking, where the need to produce complex geometry and surfaces is ever-present. Later in this article, we will cover some of the benefits of using a five-axis machine even if your parts aren't that complex. Processing Power In full five-axis machining, all five axes can move and cut simultaneously. This type of machine requires highly responsive servodrives to move instantaneously while responding to thousands of move/position commands. High-accuracy rotary tables are needed to position the part precisely, despite the forces created by the cutting process. To handle these computing tasks adequately, a machine control must have high processing power to calculate, keep track of and control the tool center point. (The tool center point is critical, because its position and orientation determine how material is removed to produce the desired shape.) Five-axis program code includes instructions for the X, Y and Z linear axes; the two rotational A and B axes working in unison to keep the tool normal (perpendicular) to the part surface when contouring; and I, J and K vectors for tool cutter offsets. This constant calculation includes all fixture offsets, tool offsets, tool cutter compensation and translation of all work planes, regardless of which axis or rotary table is moving. To do this, the control must be able to respond instantly to the servodrives, process motion commands in blocks, and interpret large quantities of CAM data while "looking ahead" to where the tool is going next. All of this computing must be done accurately, so that no dwell marks are left on the surface of the part. Avoiding dwell marks is especially critical in mold work. This capability also enables probing to be used during the machining cycle to check the results. To support this processing power and ability, the cost of a full five-axis machine is substantially higher than three-axis machines. Positional Machining with Five Axes Although it may appear that the next step forward from three-axis machining is full five-axis, there's a transitional step called 3+2 machining. This in-between mode gets the job done, but with a less-expensive machine. Just because a non-prismatic part has holes, angles or features that are not normal to the surface or machine axis does not mean that full five-axes capability is required. To machine the necessary features with 3+2 machining, the machine tilts the tool or rotates the fourth and fifth axes to a fixed position, then executes a three-axis program, moving in X, Y and Z. This is sometimes called five-axis positional machining, rather than full five-axis machining in which the tool is continually being manipulated in all linear and rotary axes at once. While the 3+2 control can track the initial origin of the part, it's a somewhat slower, one- move-at-a-time process. Unlike with full five- axis control, the 3+2 control does not need to be capable of tracking the movement of all axes, translating and updating work offsets, tooling offsets and coordinate systems during the cut. With 3+2, only one work plane is computed at a time. That is, the control "thinks" of the tool moving in relationship

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