Implementation of high speed machining in thin-walled aircraft integral elements

Abstract

High speed milling (HSM) is currently one of the most important technologies used in the aviation industry, especially concerning aluminium alloys. The difference between HSM and other milling techniques is the ability to select cutting parameters – depth of the cut layer, feed rate, and cutting speed, in order to simultaneously ensure high quality, precision of the machined surface, and high machining efficiency, all of which shorten the manufacturing process of the integral components. By implementing the HSM technology, it is possible to manufacture very complex integral thin-walled aerial parts from the full quantity of the raw material. At present, aircraft structures are designed to mainly consist of integral elements which have been produced by welding or riveting of component parts in technologies utilized earlier in the production process. Parts such as ribs, longitudinals, girders, frames, coverages of fuselage and wings can all be categorized as integral elements. These parts are assembled into larger assemblies after milling. The main aim of the utilized treatments, besides ensuring the functional criterion, is obtaining the best ratio of strength to construction weight. Using high milling speeds enables economical manufacturing of integral components by reducing machining time, but it also improves the quality of the machined surface. It is caused by the fact that cutting forces are significantly lower for high cutting speeds than for standard machining techniques.

A Comparison of the Geometrical Accuracy of Thin-Walled Elements Made of Different Aluminum Alloys

“…Depending on the ratio of wall height to its thickness, one of the thin-wall machining methods can be used. They are based on different configurations of the alternating machining of both sides of the wall at successive levels, but these strategies do not have universal character [ 17 , 23 , 24 , 25 ]. However, the application of these strategies has some disadvantages: Repeated tool transitions introduce additional residual stresses, which can play a significant role in causing deformations in the machined walls; Making successive transitions along previous machining traces can cause a regenerative effect, i.e., a loss of stability in the machined wall or tool; During the machining of successive layers, the tool edge “rubs” against the previously machined surface, which accelerates tool wear and downgrades the quality of the machined surface.…”

Techniques for Thin-Walled Element Milling with Respect to Minimising Post-Machining Deformations

“…The enhancement of dimensional and shape accuracy of manufactured thin-walled elements is obtained by the application of the following machining error minimisation methods [ 10 , 16 , 17 , 47 , 48 ]: correct milling strategy selection; increase in cutting speed v c (HSC); rationalisation of cutting parameters (in particular: feed per tooth f z and milling width a e ) aimed at decreasing the component of cutting force perpendicular to a wall being machined. …”

“…The choice of an optimum thin-walled element machining strategy mostly depends on the ratio between the machined wall height and its thickness. Due to the above, three cases can be identified [ 17 , 48 , 49 ]: low height to thickness ratio < 15:1—where separate milling of each wall side in non-overlapping passes is recommended; moderate height to thickness ratio < 30:1: milling on a constant level—alternate machining at a constant depth of cut a p of both side walls, also in non-overlapping passes; milling at a difference of levels—alternate milling of both side walls with non-overlapping levels between consecutive passes; the depth of cut a p at the first pass should be a p /2; high height to thickness ratio > 30:1—Where it is recommended to change the sides and apply the “christmas tree” routine in order to achieve the wall thickness setpoint in stages. …”

“…The enhancement of dimensional and shape accuracy of manufactured thin-walled elements is obtained by the application of the following machining error minimisation methods [10,16,17,47,48]:…”

“…The choice of an optimum thin-walled element machining strategy mostly depends on the ratio between the machined wall height and its thickness. Due to the above, three cases can be identified [17,48,49]:…”