Patent GB-734089-A (GB)
Title: Improvements in or relating to the machining of turbine or axial compressor blades
Inventor: LEGG GEORGE CHARLES JAMES
IPC: B24B17/02
Filed: 1952-06-26
Abstract: 734,089. Grinding turbine blades. NAPIER & SON, Ltd. D. Sept. 28, 1953 [June 26, 1952], No. 16056/52. Class 60. A method of machining a turbine blade having an inclined root platform D, Fig. 22, by means of a rotary cutting tool 16 of wheel type with an integral cutting surface comprises machining the major portion of the blade in a first half-cycle during which the blade is reciprocated longitudinally parallel to its axis and given a rotary feed motion, and machining the portion of the blade adjacent the platform in a second half-cycle during which the blade is rotated continuously and reciprocated longitudinally in phase with the rotation while a rocking movement is imparted to the tool about an axis perpendicular to the axis of the blade and passing through the rotary axis of the tool and point of contact between the tool and blade so that the tool then follows the contour of the sloping platform, the blade being moved transversely in a direction normal to its axis in both half-cycles under the control of a follower and former. The machine is a development of that described in Specification 691,937 and employs, as shown in Figs. 1 and 2, a former 12 which rotates in synchronism with the blade 13 and co-operates with a follower 15 to determine the transverse movement of the blade in relation to a grinding-wheel 16. During the first half-cycle when the blade is machined from the tip B, Fig. 22, up to a vertical plane C the blade is reciprocated over this distance and given an oscillating twist about its axis corresponding to the twist of the blade, combined with slow rotation, the rate of which is relatively fast at the edges, slower on the outside convex face and slower still on the inner face ; during the second half-cycle the grinding-wheel and follower are both rocked about transverse axes while the blade is rotated relatively fast (but still with varying rate), reciprocated over the distance G, Fig. 22, and given a slow longitudinal feed from a plane E (parallel to the platform) up to the platform. The former 12 and blade 13, Figs. 1 and 2, are mounted coaxially in a casing 7 on a table 6 which is movable transversely of the machine on a carriage 4 arranged to move longitudinally on the bed 1 ; the rotation of the former and blade is effected by a shaft 9 driven from a shaft 61, Fig. 11, through gearing and shafting permitting the necessary movements of the casing 7. The table 6 is guided transversely on rails 5, Figs. 1 and 14, on the carriage 4 and is controlled for parallel movement by the engagement in vertical slots 208, 209 in its front edge of the upper ends 207 of two arms 205, 206 upstanding from a torque tube 200 rotatably mounted beneath the carriage. The tube is rotated to move the table by a hydraulic ram movable in the tube and operative through internal cams (Fig. 15, not shown). In one position of the tube, the table is positioned forwardly to hold the blade and former away from operative position and in the other the table is free to move transversely as determined by the engagement of the follower with the table, such engagement being ensured by springs. Longitudinal reciprocation of the carriage 4 during the first half-cycle is caused by a double-acting hydraulic ram 240, Fig. 14, having its piston-rod 241 connected to the carriage, automatic reversal of the latter at each end of its stroke being effected by two valves connected to a reversing shaft 242, Fig. 1, arranged to be actuated by stops 244, 245 on the carriage. Over-running is prevented by two stops 249, 250 co-operating with an abutment on the carriage. At the end of the first half-cycle, a hydraulic ram 260, Fig. 14, attached beneath the carriage slides a cam plate 267 longitudinally so as to withdraw the abutment from co-operation with the stops 249, 250 and lower a pin 259 carried from the cam plate into a position for coaction with a link 257, Fig. 11, in a hydraulic follow-up servo-mechanism associated with a cam 126 on the shaft 61 so as to reciprocate the blade longitudinally over the distance G, Fig. 22 ; the longitudinal movement of a bar 252 in the machine bed by contact with the cam 126 causes the link 257 to pivot about the pin 259 and thereby act upon a valve 246 controlling the ram 240 whereby the carriage 4, together with the pin 259, moves until a state of equilibrium is reached. The pin 259 is also moved slowly to the left relatively to the carriage by the continued action of the ram 260 so as to superimpose upon the reciprocation of the carriage a slow longitudinal movement to the right until a stop on the cam plate 267 actuates a switch to stop the ram. The grinding-wheel 16, Figs. 1 and 2, together with a motor 17 for rotating it through a belt-andpulley drive 21 ... 24, is mounted on a platform 25 pivoted in bearings 18, 19 for rocking about a horizontal axis passing through the point of contact of the grinding-wheel with the blade. Rocking of the platform (which occurs only during the second half-cycle) is controlled by a cam 124, Fig. 11, on the shaft 61 operating through a vertical rod 29, Fig. 2, engaging underneath the platform against a bar 26 movable in a slot in the latter ; during the first half-cycle, the bar is urged against a fixed part of the bed by a locking shaft 28 so as to lock the platform against movement. The follower assembly 14, Fig. 2, which is positioned at the rear of the bed, comprises a follower wheel 15, Fig. 6, mounted at the end of a shaft 44 rotatable in a block 43 movable transversely of the machine between guides 42 in a frame 41 which is movable longitudinally on a housing 40. Progressively-increasing cuts are provided during the first half-cycle by moving the follower forwardly a step after each revolution of the blade by means of a series of stops 46, Fig. 2, against which the rear end of the shaft 44 is urged by a spring plunger 47 acting upon the block 43. These stops are circularly arranged around a wheel 70 adapted to be indexed for the distance of one stop after each revolution of the blade by ratchet mechanism (Fig. 8, not shown) operated by a cam 67, Fig. 6, on the shaft 61. At the end of the first half-cycle, the follower remains in its forward position for the second half-cycle which is completed without further advance of the follower ; the locking shaft 28, Fig. 2, then automatically acts through a lever 76 to re-set the ratchet mechanism and return the follower in readiness for a new cycle. A micrometer gauge associated with this mechanism gives a constant reading of the position of the follower. Rocking of the follower about the axis of the shaft 44 (which occurs only during the second half-cycle) is effected by a cam 49, Fig. 6, on the shaft 61 acting through a rod 48 upon a forked arm splined on the shaft 44; during the first half-cycle, the locking shaft 28, Fig. 2, causes a rod 99 to disengage the forked arm from the rod 48 and lock it instead to a fixed pin. To compensate for the fact that the grinding-wheel and follower are of different diameters and so would give rise to errors when these members are rocked, the follower frame 41 is moved longitudinally during the second half-cycle against the action of a spring plunger 56, Fig. 6, by a cam 54 on the shaft 61 effective through a rocker 55. In order to deal with blades of different lengths, the housing 40 is adjustable longitudinally on the bed and the cams 49, 54, 57 are slidable as a unit on the shaft 61, being movable with the housing by arms 105 depending therefrom. The shaft 61, Fig. 11, for driving the cams and shaft 9 is rotated by a motor 180 through a chain-andsprocket drive 162 to differential gearing 125 (Fig. 18, not shown) on the shaft. To provide the desired twisting motion of the blade in phase with its longitudinal reciprocation during the fust half-cycle, an oscillatory movement is superimposed upon the drive to the shaft by means of a slotted cam 151, Fig. 11, which is mounted on the underside of the carriage 4 for angular adjustment in a horizontal plane in accordance with the twist required and by engagement with a pin 156 on a transverselyslidable rod 153 actuates through linkage a hydraulic follow-up servo-mechanism to oscillate the reaction member 168 of the differential; the servo-mechanism comprises a double-acting ram 160, the operating member 159 of which is connected to the member 168. To prevent damage to the machine should the member 151 be set for excessive twist, the rod 153 is formed in two telescopic spring-pressed parts yieldable under excessive resistance. A two-speed gear box on the motor 180 under the control of the locking shaft 28 provides for the drive of the shaft 61 at a faster speed during the second half-cycle than during the first half-cycle. The varying speeds of rotation of the blade when grinding the edges and inner and outer faces of the blade are obtained by using switches 130, Fig. 11, actuated by cams on the shaft 61 to operate valves in the fiuid supply to the motor 180. The complete machining operation is entirely automatic, switches being arranged in the machine for actuation by the various parts to control the operation. At the start, the torque tube 200, Fig. 14, moves the table 6 rearwardly to bring the blade and former into engagement with the grinding-wheel and follower for the first half-cycle when the ram 240 and motor 180 reciprocate the carriage 4 and rotate the blade and former with the desired twist. After the prescribed number of revolutions of the shaft 61 (set by the switches 130 and corresponding to the number of cuts), the tube 200 withdraws the table 6 and the motor 180 is stopped, as is also the ram 240, when the carriage next reaches its left-hand position (i.e. with the tool at the plane C, Fig. 22). The locking shaft 28 is then rotated to free the follower and grinding-wheel assemblies for rocking by their respective cams 49, 124 and the ram 260 is operated, followed by operation of the ram 240