Modern Machine Shop

JUN 2014

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30 MMS June 2014 mmsonline.com rapid traverse Machining Technology in Brief Typically, conventional CNC toolpath programs are based on bounded geometry. These programs have the tool enter the material and then set off in one direction until it encounters a wall or some other obstacle, whereupon it changes direction. The tool zigs and zags until it covers all of the area delineated by the part model, cutting whatever is in its path—whether material or air. Dynamic tool paths make the tool act differently than expected in a conventional program. The motion of the tool is not defined strictly by bound- aries of the area to be machined. Instead, the tool paths are governed by a highly engineered set of rules that takes into consideration not only the area from which metal is to be removed, but also the changing condition of the material throughout the various stages of machining. The process looks ahead to see what is coming up to modify feeds, stepovers and cutting motions in response to ever-changing material conditions as the part is being cut. The objective is to remove material more ef ficiently by controlling lateral forces to avoid excesses that generate heat. Other rules that govern Dynamic Motion include: • Minimal stepovers to avoid heat buildup and excessive lateral force. • Smooth motions that alleviate stresses on tools and the machine (trachoids are one example). • High spindle speeds, if available. • Maximum flute engagement for cuts that remove the most material. • Continual material engagement (climb mill- ing) to minimize air cutting. • Dynamic adjustments of stepovers to keep tool load constant. • Entry strategies that present the tool to the material at the safest angle. • Material "awareness" that keeps the tool in a consistent, safe cutting condition regardless of the pocket or contour geometry. It modifies the path so that chips will be the same size. • Micro-lifts that raise the cutter away from the part's floor or away from walls so that heat does not build up when cutting speeds are being adjusted during repositioning. Dynamic toolpath processing has the ability to consider various alternatives about what to do next, and then make an intelligent selection based on an analysis of what impact the various options will have on the final outcome. For example, from a number of schemes for entering the part, the program selects the one that enables it to best utilize the algorithm for machining. Let's assume it takes the tool into the more open side of the pocket. Now the tool spirals its way out—always having constant contact by climb milling to avoid the conventional back and forth—until it runs into a wall. The program has the intelligence to figure out what the next move should be for the most efficiency. Say the tool goes into the corner of the pocket. In most cases, when it runs into a wall, it will reposition itself outside of the material, because it knows where the material has been removed as well as where it remains, and it re-enters appropriately. The only time the tool is cutting air is when it is repositioning. Likewise, whereas some programs will retract the tool to a clearance plane or a retract plane, One benefit of machin- ing with dynamic tool paths is the possibility of using one smaller carbide tool to do roughing, rest machining and finishing in a combined operation. 0614_MMS_rapid.indd 30 5/14/2014 10:00:45 AM

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