An aircraft component may lose up to 80 percent of its weight during machining, which results in very long cycle times. This is why time is such a critical parameter to focus on. The roughing operation mainly aims at a maximum metal-removal rate. Using large cutter diameters and indexable inserts while deliberately leaving some stock left for a rest and finish milling operation is a common solution. As these operations make up a substantial share of the overall machining time, great cycle time reductions can be achieved by optimizing them.
The remaining material is machined with solid carbide end mills. The main requirements – good tool life, machining time, surface quality and security – can be achieved by keeping the radial engagement and cutting width low while increasing the number of teeth, cutting speed and cutting depth. Recommended rest milling techniques are slicing in the corner and trochoidal milling in slots.
Airframe components often have thin walls to save weight. These are milled using a zigzag path, first with a reduced cutting depth on one side and then at the normal depth, alternating between the wall sides. Solid carbide end mills require a rigid machine and a CNC control system with excellent look-ahead capabilities. Accurate holders with a low run-out are also necessary (a 100 percent increase in the run-out reduces the tool life by 50 percent).
Coolant must be used with titanium an super alloys (HRSA) to improve chip control and limit the cutting temperature, preferably applying an internal high-pressure supply for better performance.
A recommendable solid carbide end mill has a 50° helix angle, a flute design optimized according to the number of flutes, sharp edges with positive rake angles, a small support face, a corner radius rather than a chamfer and high-performance coating. Neck-type cutters may be used for better accessibility with controlled material contact to reduce vibrations.
Originally published in Metalworking World 1.2011, a business magazine published by Sandvik Coromant.
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