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Fig. 2.30. ThinWall Ribs: High Speedvs. Conventional Machining Source: The Boeing Company

the exact same part configuration was successfully cut from a two-inch thick plate of 7050-T7451 using high speed machining techniques. The two-inch high stiffeners ranged from 0.029 to 0.035 in. in thickness. Another example of thin wall machining capability is shown in the space structure in Fig. 2.31. This rather remarkable large part was machined to a wall thickness of only 0.015 in. and floor thickness of 0.020 in. The wall heights are almost 5 in.

High speed machining of aluminum was originally implemented on the F/A-18E/F fighter to save weight; 150 high speed machined "assemblies" saved approximately 80 lb. of weight. It soon became apparent that the higher metal removal rates could also save costs by eliminating multiple parts and assembly wA T * *

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Fig. 2.31. High Speed Machined Thin Wall Space Structure Source: AUT Inc.

costs. An example of a high speed machining replacing an existing composite part is shown in Fig. 2.32. Although the machined part is slightly heavier, the cost savings were so significant that the change was justified. Another example is the large bulkhead shown in Fig. 2.33. Note that the holding fixture is actually the periphery of the plate, eliminating the need for holding fixtures. Finally, in Fig. 2.34 is a one piece unitized substructure that replaced multiple machined spars and ribs assembled with mechanical fasteners.

In conventional machining, roughing is usually followed by finishing using lighter depths of cut at much slower speeds. In high speed machining, it is often possible to combine the roughing and finishing cuts in the same operation. An enabler for high speed machining has been the development of porosity-free thick plate in the 7XXX series of alloys. For example, 7050-T7351 is available in plate stock up to 8 in. thick.

Solid two flute carbide cutters are normally used for high speed machining. Carbide tools reduce the number of tool changes required, due to reduced wear. However, more importantly, they also provide much greater cutter stiffness than high speed steel. Since a stiff cutter can take a much larger depth of cut than a flexible cutter, cutters with small length-to-diameter ratios are also used to maintain maximum cutter stiffness. Smaller diameter cutters may be required for finishing if tight corner radii are required. To provide further stiffness, the shortest possible tool holder is recommended. Since high speed machining is frequently used to produce thin walls, runout should be held to 0.001 in. Since high speeds are used, both the tool and holder need to be balanced. Due to the high rpms involved, inserted tools should not be used for safety reasons.

Vibration is a natural concern when machining at high speeds. Two types of vibration can occur: cutter vibration (chatter) and workpiece, or part, vibration.

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