Patent GB-699452-A (GB)
Title: Improvements in or relating to contour forming machine including tracer control mechanism
IPC: B23Q35/129
Filed: 1949-11-29
Abstract: 699,452. Electric follow-up systems. CAMPBELL, P. J. Nov. 29, 1949 [Dec. 13, 1948], No. 30562/49. Class 40 (i) [Also in Groups XXII and XXIV] In a copying device for forming an article from a profiled pattern such as a drawing, model or other representation, the pattern is rotated by a carrier and co-operates with a follower connected in an electric circuit including a spark gap at the edge of the follower, the follower being driven by electrical means responsive to the length of the spark gap so as to maintain this length substantially constant and thus cause the follower to follow the pattern profile. The invention is described for (1) making a turbine blade template from enlarged sectional drawings thereof; (2) making an actual blade from the template; and (3) comparing the manufactured blade with the sectional drawings. A large drawing is first made of the desired sectional shape of a blade at a plurality of sections, e.g. the four sections 102, 104, 106, 108, Fig. 2, spaced lengthwise of the blade, and showing also the angular relationship of the four sections. From this drawing a template 112, Fig. 9, consisting of four elements 114, 116, 118, 120 is made. The four elements correspond in shape, but on a reduced scale, to the respective drawing sections 102 ... 106. They are mounted on a common axis 128 at a distance apart corresponding to the sections they represent and are disposed angularly relative to each other similarly to the four drawing sections. From this template the actual blade is copied, the blade shape between the template sections being effected by interpolation. In a modification, however, the template may be de-twisted, i.e. the four elements 114', 116', 118', 120', Fig. 12, are mounted with their longitudinal axes parallel, this " de-twisting " being compensated for in manufacturing the actual blade. Cutting the template. The sectional drawing 100 is mounted on a table 130, Figs. 2, 8, with the spanwise axis 110 of the drawing sections 102 ... coincident with the table arbor 128 which also carries the corresponding template blanks 114 ... 120. A thin transparent insulating sheet 131 is then secured over the drawing and one of the curves (102 say) traced thereon with electrically conductive ink as at 144. The table is then motor driven about the arbor 128 at a slow speed. A tracer head 148 mounted on an arm 160 of a pantograph 107, 109, 134, 136, includes an arcshaped line electrode 156 which engages the sheet 131 over the whole of its length. An electric spark is produced between the trace and the electrode, which are connected in the control circuit of an electric motor 170, Fig. 10, which through gearing drives the pantograph about its pivot points 138, 140 to maintain the spark at constant length. The tracer head 148 is thereby constrained to follow the trace 144 as it rotates with the table. The movements of the tracer head 148 are mechanically repeated on the appropriately reduced scale by the independently rotated cutter 124, Fig. 8, mounted on shaft 127 in the pantograph arm 134, which thus cuts the corresponding template element 114 as it rotates with the table. After the first template element 114 has been cut from a blank in this manner, the cutter is raised on its spindle 126 in line with the next template element blank 116. A fresh insulated sheet 131 is then placed upon the table and the second drawing section 104 traced thereon, this trace being used to control the cutting of the second template element 116. In like manner the remaining template elements 118, 120 are cut under control of traces of the corresponding drawing sections 106, 108. Template cutting control circuit; Fig. 10. The spark between the tracer head electrode 156 and the trace 144 is produced by an 800-volt D.C. supply applied over resistors 213, 166, so that with a spark of the correct length a predetermined potential is applied to the grid of a triode 217. Variations of this potential, caused by variations in the spark length, are amplified in the triode and applied via the cathode load 219 to the next triode 225. A differentiating network 233, 229 also applies the first derivative of the signal to the input of triode 225 to prevent hunting of the motor 170. The output of the amplifier 225 is applied in antiphase to the grids of power amplifiers 247, 255, the anodes of which feed the saturating windings of transformers 249, 257 fed over line 263. Normally, with the spark of the correct length, the tubes 247, 255 conduct equally so that no current flows in the series-opposed similar secondaries 267, 269, the transformers, and hence in the control winding 271 of the two-phase motor 170 which is therefore stationary. Variations in the spark length cause corresponding differential variations in the conduction of the tubes 247, 255. The motor control winding is thereby energized and drives the motor in a sense to bring the spark back to its normal length, this being achieved as stated above by driving the pantograph so that the tracer head follows the trace 144. Contouring machine. For cutting an actual blade from a twisted template, as shown in Fig. 9, the template is mounted on a shaft 314, Fig. 13, which is mounted co-axially with the blade blank 300' in the headstock 344, and rotated synchronously therewith at slow speed. The cutter 302, which is independently rotated by a motor 362 through a flexible shaft 366, is carried by the cross-slide 320 mounted on the carriage 322 slidable along the ways 340. Biased to engage the respective template elements are slides 306, 308, 310, 312 of equal length, the outer ends of which are magnetised to hold a thin flexible magnetic strip 304 so that the contour of the strip corresponds to that of the four points of the template elements engaged by the slides, and varies according to the operative template element points as the template rotates. The contour strip co-operates with a follower electrode 316 carried by an arm 511 rigid with the cross-slide. In operation, with the template and blank rotating together, and the cutter rotated independently, the carrier 322 is moved towards the blank by clutching it to the usual lead screw as for a lathe, and the cross-slide is moved laterally in the appropriate direction by energizing one or the other of clutches 324, 326 under the control of the contour strip 304 and the associated follower electrode. This involves a control circuit for keeping a spark between the contour-strip 304 and the follower electrode of constant length, which is effected by driving the cross-slide laterally, so that as the carriage is advanced the cutter fashions the blank 300' in correspondence with the shape of the contour strip and hence according to the template. The contour strip thus functions to interpolate the shapes of the imaginary templet elements lying between the four elements 114, 116 ... 120. When a "de-twisted" templet, Fig. 12, is used, the templet and blank 300' are connected through a differential mechanism, Fig. 23 (not shown). A further flexible contour strip 334, Fig. 13, is adjusted by slides 372, 374, 376, 378 corresponding to the correct angular displacement between the associated template elements, and co-operates with the follower electrode 619 carried by a micrometer in a tracer head 641 mounted on an arm 597 of the carriage 322. A spark of constant length is maintained between the contour strip 334 and the electrode 619, this being effected by a motor 336, which, as the carriage advances, rotates the micrometer to advance or retract the electrode 619, Fig. 21 (not shown), according to the contour of the strip 334. The motor 336 at the same time drives through a flexible shaft 587 the planet member of the differential to vary the angular displacement between the templet and the blank so that the appropriate " twist is applied to the blank relative to the templet as it is being cut. Contouring machine control circuit, Fig. 25. This circuit is similar to that of Fig. 10. Variations in the spark length between the contour strip 304 and associated electrode 316 produce a corresponding potential variation on the grid of a triode amplifier 217', the output of which is fed through a further amplifier 701 (left side) and line 711 to the inner control grid of a pentode amplifier 719. A phase inverted portion of the output of the left hand triode 701 is doubly differentiated at 233, 743, 703 and fed through the phase inverting right hand triode 701 to the second grid in the pentode where the signal and its second derivative are added algebraically and the amplified output applied in push-pull to beam power amplifier tubes 737, 739, which respectively energize the coils of the clutches 324, 326. Normally, both clutch coils are energized equally, but any variation in the spark length causes a differential variation in the energizations of the two clutches so that the lead screw of the cross-slide is driven in the correct sense to bring the spark length back to normal. The spark length between the contour strip 334 and its associated electrode similarly controls the grid potential of a triode 755, the output of which is series connected with marginally operated solenoids 757, 759. Normally, solenoid 759 is energized, variations of the anode current as the spark length varies from normal, either releasing solenoid 759 causing the D.C. motor 336 to run in one direction, or energizing the solenoid 757 in addition, to drive the motor in the reverse direction. The motor runs, as above described, until the spark length has been brought back to normal. Comparison mechanism, Figs. 28 to 31 (not shown). For comparing a manufactured blade with the sectional drawings, a pantograph arrangement similar to that of Fig. 2 is used. In this case, however, a tracer head, controlled by a constant length spark control circuit, as shown in Fig. 10, is arranged to follow the blade contour as it is rotated, the fol