Improving the strip grinding of gas-turbine blades

Abstract

1273 The aviation industry faces the perpetual challenge of improving the performance, reliability, and durabil� ity of airplane components. One basic approach is to impose stricter requirements on finishing operations, especially in the strip grinding of curvilinear surfaces in the components of gasturbine engines. This machining method does not often yield the required geometry of the gasturbine blades, on account of the nonuniform rigidity of the blank, the fluctuations of the margin, and the tool wear. Thus, such strip grinding is of limited value (1-3). In prac� tice, we need to develop a grinding system capable of correcting the error of previous machining operations and to obtain a machined surface with specified sur� face error Ra, without any traces of machining (resid� ual ridging). Deterioration in the cutting properties of abrasive strip in machining large gasturbine blades creates problems. With slow removal of the metal, the required precision cannot be obtained in a single pass. The discrepancy of the machined surface steadily increases. The need to remove large margins in several passes with supply in the same direction (transverse to the blade axis) increases this discrepancy—that is, the geometric error Δ of the machined surface as a result of the wear of the abrasive strip (Fig. 1a). The error Δ may be eliminated by introducing one or more successive passes with supply that is transverse to the blade axis and in the opposite direction to the supply in the first pass (Fig. 1b). In that case, the dis� crepancy is compensated on account of decrease in the metal removal with opposing supply. By calculation, we may establish the grinding con� ditions with the least error Δ. Compensation of Δ is possible by means of additional opposing passes and the removal of smaller margin. To maintain the machining productivity, the row spacing hc must be increased in that case. The error Δ is determined as the difference between the margin in machining and the cutting depth at the given point of the machined surface Δ ij = δ(u, p) - t 1 (u, p, V 1ij ) - t 2 (u, p, V 2ij ), …, t k (u, p, V kij ), where δ(u, p) is the margin in machining; u, p are curvilinear coordinates specifying the profile of the blade's flow section; V1, V2, …, Vkij are the volumes of metal removed in the corresponding cross sections; i, j, k are the numbers of the passes; and t1(u, p, V1ij), t2(u, p, V2ij), tk(u, p, Vkij) are the cutting depths in layerbylayer margin removal. The cutting depth in pass k is specified by the func� tion obtained as a result of interpolation of the smoothed experimental data t ik = f t (v stk , v bk , P k , h ck , V k ), where v stk and vbk are the speeds of the abrasive strip and the blade; Pk is the force applied to the strip by the roller. The smallest total error over the whole blade profile is obtained on minimizing the square of the mean error min, where I and J are