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Five-Axis Machining For Toolmakers
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Five-axis machining has been used in aerospace applications for many years, but until recently the technology has made little impact on toolmakers. However, new developments in CAM software have made programming for five-axis operations much easier with the result that the toolmaking industry is showing an increasing interest. Other reasons have also contributed to the change in attitude. First, the price of five-axis machines has been falling steadily, and this trend is forecast to continue. Five-axis capabilities that were once only available on large-scale machines are now becoming available on the smaller machining centers used for toolmaking. A second factor has been the increased use of high speed machining. Conventional high speed machining has limitations when milling deeper cores or cavities. The length of the cutters that have to be used with a three-axis machine can result in chatter and poor surface quality. Thus the toolmaker can be forced to run at conventional speeds for part of the job in order to maintain the required accuracy and finish. Positional five-axis machining, or 3+2 machining, provides an alternative. On a pivoting-head machine, positional five-axis machining enables shorter cutters to be used since the head can be lowered toward the job and the cutter oriented towards the surface. As a result, higher cutting speeds can be achieved with no loss in accuracy. Another benefit is the ability to machine complex shapes in a single setup. This obviously saves time compared to performing the job in a series of setups. Furthermore, with multiple setups, there is always a possibility of incorrect alignment each time the part is moved. The combined benefits have made positional five-axis machining the preferred approach for most toolmakers. A study undertaken last year by CIMdata found that around three quarters of toolmakers with five-axis capabilities used this technique. Continuous five-axis machining is less popular in the toolmaking industry, although there are now signs of increased use. With this technique, there can be simultaneous movement in all five axes. The main challenge for the toolmaker using this approach is to ensure that the head does not collide with the job as the orientation is changing. This is less difficult when machining a core, but it can provide a major challenge when machining inside a small but complex cavity. One of the most common techniques promoted for continuous five-axis machining is based on keeping the cutter approximately normal to the surface being machined. While this approach has benefits in the aerospace industry, it is less appropriate for toolmaking, since the parts being machined-and the individual details within them-are typically much smaller. First, machining normal to the surface will require the machine head to travel a greater distance than necessary when machining a convex surface and will increase machining times. It may be found that the normal position is inaccessible by the machine head, while, in other cases, it will be impossible to maintain the 90-degree angle without a collision with another section of the model. Second, maintaining the cutter at 90 degrees to the surface of the part means that only a small part of the cutting surface of the tool is ever used. As a result, tool life is much shorter. Also, the tool tip travels a shorter distance at a given rotational speed than the full width of the cutter so the material removal rate will be lower when using ball-nose cutters. Alternative approaches (which Delcam uses) generate five-axis tool paths based on machining through a point or to a point, or to or from a line. These approaches let the software incorporate automatic adjustment of the tool's "lead angle" and "lag angle"-measured in the cutting direction-and "lean angle"-measured at right angles to the cutting direction. Control of these tilt angles can give better cutting conditions and avoid collisions. For example, a technique Delcam's PowerMill uses for finish machining with a five-axis machine involves the use of a reference surface onto which a pattern representing the contact point of the cutter can be projected. The software then generates the corresponding tool path, with tool tilt angles set such that collisions and gouges are avoided. http://www.mmsonline.com/articles/100106.html |
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