US2815611A - Machine and method for grinding turbine blades and the like - Google Patents

Description

Dec. 10, 1957 Filed Sept. 25. 1952 H. E. BALSIGER MACHINE AND METHOD FOR GRINDING TURBINE BLADES AND THE LIKE '7 Sheets-Sheet 1 INVENTOR Meow EBHLS/GEA BY g ATTORNEY Dec. 10, 1957 H. E. BALSIGER MACHINE AND METHOD FOR GRINDING TURBINE BLADES AND THE LIKE 'I Sheets-Sheet 2 Filed Sept. 25, 1952 w I Pull a mu INVENTOR Mama .2 E. 5445/6519 Dec. 10, 1957 H. E. BALSIGER- 2,315,611

MACHINE AND METHOD FOR GRINDING TURBINE BLADES AND THE LIKE Filed Sept. 25, 1952 7 Sheets-sheaf. 5

INVENTOR l-lmow gum/s51? ATTORNEY BY/yZ M Dean 10, 195.7 H. E. BALSIGER MACHINE AND METHOD FOR GRINDING TURBINE BLADES AND THE LIKE 7 Sheets-Sheet 4 Filed Sept. 25. 1952 INVENTOR fi/fifOLZBALS/GER BY Z ATTORNEY Dec. 10, 1957 H. E. BALSIGER MACHINE AND METHOD FOR GRINDING TURBINE BLADES AND THE LIKE 7 Sheets-Sheet 5 Filed Sept.- 25. 1952 INVENTOR ,hfiaw sans/s52 TTORNEY Dec. 10, 1957 H. E. BALSIGER 2,315,611

MACHINE AND METHOD FOR GRINDING TURBINE BLADES AND THE LIKE Filed Sept. 25. 1952 7 Sheets-Sheet 6 FEE lllllll-lh INVENTOR memo EBIILSIGER Dec. 10, 1951 H. E. BALSlGER 2,815,611

MAGHINE AND METHOD FOR GRINDING TURBINE BLADES AND THE LIKE Filed Sept. 25, 1952 '7 Sheets-Sheet 7 iNVENTO R h ARaLa 5.2415165 ATTORNEY United States Patent AND METHOD FOR GRINDING TURBINE BLADES AND THE LIKE Harold E. Balsiger, Waynesboro, Pa., assignor to Landis Tool Company, Waynesboro, Pa.

Application September 25, 1952, Serial No. 311,510 2 Claims. (Cl. 51-1 01) This invention relates to apparatus and method for grinding the buckets, vanes, or blades used in steam and gas; turbines.

In this: application the term blade will be used to include all three types. These blades vary in contour from acrescent shape having athick leading. edge and a thintrailing edge to: a substantially flat blade. These blades all have one feature in common: the leading and trailing edges are high points on the contour when the blade is rotated: about its longitudinal axis-. This means that the cradle rocks twice for each revolution of the work; present these bladesare ground by' one or the other of twogeneral methods.- The first method consists ini grindingthe entire contour of the blade including the concave side. This method requires the use of a very small grinding wheel-1 having a radius corresponding to the minimum radius of. the concave surface of the blade. The disadvantage ofthis method is obvious. The other method utilizes a full-size wheel having a face equal to the length of the blade for grinding the convex surface only of the blade by'plunge-cut operation. This method isilirnitedto the grinding. ofblades of. such shape thatthey can be plunge ground. by a wheel equali inwidth to the length of the=blade. Its disadvantageliesinthe fact,.first, that. it is limited to this one type ofblade and, second, that even in grinding this. type of blade the axis. of-the blade. must. be manipulated in order that there will always. be continuous contact betweenthe surface of" the blade: and. the grinding wheel.

The grinding of turbine blades is basically a cam grinding; operation. However, itpresents a somewhat different problem thanthegrinding of automotive and other cam shapedpieces. From. the cross section oh the blade i't is obvious. that. if the blade is mounted. for rotationabout its ownaxis, twosharp lifts-occur for each revolution ofl the workpiece. Furthermore, itis desirable togrindthe con.- tour notionly, on the convex surface, of the blade but alsoon the leading and trailing edges, andi the shoulder and radiusbetween theblade and the-rootl Another problemisencountered. in providing. a master cam. 'Ifheusualipractice, in producing. a master cam is to use a model substantially identical" with the product as apattern for, the master,.the.master being. ground. about a hase circle of: a, diameter siX or, more times that of the modeler product cams. This procedure provides amaster cam whichoperatesmore smoothly with its follower at the conventional working; speed than if the master were of. the same dimension as themodeh- Because of the double lift, required in grindingihe blade, it is essential thattlie' master cam be made as large as possible, so-tliat abrupt changes'in contourof the workpiece are miiiimized in the rotation of the master'cam'againstfthefol lower. However, eventhis'does not'solve the entire problem. Becauseof the fact that one side of the blade is hollow'or concave"; a master cam ground from an exact modeli of? the? blade i Willi have i one side hollow or fiat n'o mattenhowrlarge.theibasewirclensedt With su'ch disaster it would be impossible to operate smoothly in contact with the follower at the required speed. I To overcome this problem a model is provided in which the normally concave side is filled out as much as possible without interfering' with the contour of the leading and trailing edges. The master ground from this model has no flat surfaces, and will rotate smoothly with its follower at speeds never before used in this typeof grinding. p

Iii conventional Cam grinding the master cam is ro nd with a; grinding" wheel much smaller than that used in grinding the product cam, and in grindin the roduct earn the master cam followeris' the same diameter as the wheel in which the master cam is ground; In" production of the master cam for grindin turbine blades, the ma ter is ground with a fullsize grinding wheel, and ii the grinding of the product hlade,the master cam follower is of substantially the same diameter as the" grinding wheel which grindstheblade. v A v Another feature which does not enter directly into the process but is; nevertheless essential to its successful operation is a fly wheel mounted on the end of the work drive motor shaft. Additional fly wheels may be used in' the form of one' or more of" the work drive pulleys. The inertia of the fly wheel controls any tendency to a momentary lag or advance in the rotation of the master camand work piece due to the double lift for each revolution of the work, and thusprovides the uniform rotation necessary to the successful grinding of a peripheral contour.

it is therefore an' object of this invention to provide a blade grinding machine and method in which the leading and trailing edges and the convex surface may b ground in asingle operation using a full si z'e grinding wheel.

A further object is to: provide a' machine capable of rihdin 'alr t pes of blades. I p

A further object is to provide a simplified method of producing a master c'am forg'rin'ding' turbine blades;

A further object isto provide" means for eiiecdhginht form rotation of the master" car and work piece.

A further ohje'et is" to provide means: for grindin a dirhihe blade to size in one pa s;

A further object is to' pr vide means for grinding a turbine brads while rotating, it" about its own center.

Fi ure" 1 is a plan-viewof a machine setup to grind a master cam. v v

Fi ure 2' is end elevation; closeup, of. the same setup.

Figure 3 i's a larrriew of the' samemachine, set up to rind a turbine Blade. p I

Fi ure 4 is an end elevation, closeup, of'the" setup show'n iii Figure 3. v

Figure 5 is a arasipihri elevation showing the shape ofthe g'rin'dihg wheel and thenature of the grindin operation. t a I Figure 6'- is a hydraulic and electric diagram I Figures 7" to 1 0- inhlusive show the relative positions or the master cam and associate'd parts with-the'worh'piece in each or fear sin ularly spaced positions.

Figure 11 is a section showing means for actuating the feed control valve:

The rn'deh-ih'e 01 this ihvenridri consists e'ss'e'riti'ally or a bed 10, a work support on cradle 11 mounted forr'oel i'n'g" movement transversely of itsaxis, a footsto'cli" f2 for the master card rinding setup of Figure 1 and a fontsto'ck 1 3* for the blade- 7 "ding setup of Fi ure" said woYK-sflppdrt is Iiilititd for oscillation in the conventi'ori'al manner 011 trimriihn's ofieofwhich I4? is shown on bed 102 The headstock identified hereafter by its baseportioh 20; includes a drivih' 'mdter 21 a flywheel zz aiid rooved pulley" 2s th'e'roh'; Pulley 23 is" connected through belt 2,507,330 granted May 9, 1950.

A grinding wheel base 40 is slidably mounted on carn'age 41 for transverse and longitudinal movement on bed relative to the work support 11. A grinding wheel 42 between said center 55 and center 57 on footstock 13.

Master cam 50 is rotated in engagement with a roller follower 60, rotatably mounted in bearings 61 on bracket 62 attached to carriage 41. Follower 60 has a diameter equal to the mean diameter of grinding wheel 42 that is, halfway between the diameter of a new wheel and that of a wheel worn to minimum diameter. Said master cam 50 is held in engagement with follower 60 by means of a spring 65 suitably mounted for action between bed 10 and cradle 11.

In order to grind a master cam, the machine is set up as shown in Figures 1 and 2. Bearings 51 and 52 are shifted to a position midway on the cradle 11 and a master cam blank 50 rotatably mounted therebetween on a spindle 53. A model 70 is mounted in work driver 54 and rotatably supported between headstock center 55 and center 71 of a special footstock 12. Said model differs from the product blade 56 in that the portion A corresponding angularly to the concave portion A of blade 56 and not including the leading and trailing edges 1 and 2 respectively is convex in contour. The master cam 50 produced from model 70 could be used to grind a work piece identical with model 70. Portion B of work piece 56 is identical with portion B of model 70. The master cam 50produced from model 70 will control the rocking movement of piece 56 as though it was the same shape as model 70. For this reason, there is no contact between work piece 56 and the grinding wheel during the time when portion A is adjacent the wheel surface. This effect is due to the fact that the master cam corresponds to the model 70 rather than piece 56 as far as portion A is concerned. The modification of the workpiece found in model 70 is a filling in of the concave portion of a blade to produce a curved surface tangent with the leading and trailing edges at a point which will produce a master cam free of contour changes which would prevent smooth operation, and at the same time to leave a substantial portion of said edges uncovered so that they will be transferred to the master cam and then to a workpiece. Drive shaft 72 connects the work drive with themaster cam spindle. Model 70 is rotated at a relatively slow speed in engagement with the curved face of a follower above 73 which is mounted for longitudinal adjustment in bracket 74. Bracket 74 is mounted on carriage 41 in front of wheel base 40. The radius of follower shoe 73 is the mean radius of wheel 42, the same as that of follower roller 60.

Since the master 50 and model 70 are driven at a relatively slow speed, it is not necessary to apply a force such as spring 65 to cradle 11. Instead a counter balancing arm 80 is attached to said cradle to extend in a direction to hold model 70 in contact with shoe 73. Weights 81 may be added to said arm to exert the desired force on said cradle.

In order to grind blade 56 most eificiently and to combine what would normally be four operations into one, grinding wheel 42 is formed so that a side face grinds 'is rotatably mounted thereon and driven through a suitthe shoulder portion on the root 91 of blade 56. The left corner of wheel 42 is formed to a radius 92 for grinding the radius between blade 56 and its root 91. The peripheral face of wheel 42 is dressed to a radius 93 to which radius 92 is added. The purpose of radius 93 is to adapt said wheel to any longitudinal curvature in blade 56 and also to facilitate grinding the work to size in one pass after plunge grinding to that size as shown in Figure 5 in which the solid line represents the unground work piece and the broken line, the extent of the grinding operation.

Operation To start the machine the operator presses the machine start switch connecting L1 through line 101 to wheel start pushbutton 102. From line 101 current is also directed through line 103 and 104 through normally closed machine stop pushbutton 105 and line 106 to relay coil OP for starting the oil pump motor 107. Relay OP is disclosed as having only a single contact 108 which completes the circuit from L1 through line 103, line 104, pushbutton 105 and line 106 to serve as a holding circuit for relay coil OP, the other side of said coil being connected to L2. Starting oil pump motor 107 makes fluid under pressure available throughout the hydraulic system. The operator then depresses wheel start pushbutton 102 to complete a circuit from line 101 through line 109 to the coil of the wheel motor relay W. This relay, like the oil pump motor coil OP, shows only a holding circuit contact 110. However, it can be understood that the contacts of both of these relays which have not been shown serve to start the oil pump motor and the wheel motor respectively. The same is true of the coolant pump motor, the coil 111 of which is energized from L1 through oil pump contact 108, wheel motor contact 110, line 113, line 109, line 114, and switch 115. The opposite side of coil 111 is connected directly to L2.

The machine described herein may be operated in accordance with any one of three distinct cycles. The first of these will be referred to as the shear cut cycle. This consists in first feeding the grinding wheel into the work to the full depth of cut, that is, to a predetermined size and thereafter initiating a relatively slow traverse movement to perform a grinding operation which is known as a shear cut. At the end of one traverse movement, the work and wheel are sepaarted and the traverse movement is reversed to return the work and wheel rapidly to starting position.

Another cycle is referred to as patch work grinding. This cycle consists in feeding the grinding wheel into the work to a predetermined depth, but not to finished size. Thereafter, traverse movement is effected at a rate considerably greater than that used at the above described shear cut. At the end of the first pass, a counting mechanism is actuated and the work and wheel are reciprocated axially relative to one another for a number of passes which is determined by the counter. At each reversal the grinding wheel is fed a predetermined amount so that on the final pass across the work it has been fed to the desired size.

The third cycle is simply a plunge grinding operation which consists in using a wheel having a width of face equal to the length of the surface to be ground. The wheel is fed as in any plunge grinding operation until it reaches the desired size after which the wheel and work are separated. This cycle may be used only on blades which have straight line contact or which can be manipulated during a grinding operation to provide such contact with the grinding wheel. I The shear cut cycle is {preferred to the other two and will be discussed in greater detail than the others. To begin with, a traverse operated reverse is not necessary for this cycle and therefore there will be no reference to elements necessary to such a mechanism such as reversing valves and a reversing lever.

To start a grinding operation, the operator shifts lever 2100 to the left connecting ports 20.1 and 202 in valve 203. Fluid under pressure in line 204 is directed by said valve to line 205 to the right hand end of reversing valve 206 shifting said valve to the left. In this position of valve 206, fluid under pressure in line 207 is directed through line 208 and check valve 209, lines 210, 211, and 212 to one end of rapid feed cylinder 213 tov drive piston 214. and feed screw downwardly in Figure 6. Feed screw 15 acts through vertical shaft 227 in wheel base to move said wheel base and grinding wheel 42 mounted thereon into operative relation with awork piece 56. This movement is retarded by dashpot 219 to grind shoulder of the work piece. At the same time fluid passing through line 211 is conducted through a branch line 220 to the left end of cylinder 221 to shift piston 222 therein to the right. This movement of piston 222 actuates switchto the position opposite to that shown at the bottom. of the diagram. In this position of switch 120 current from L1 is directed through line 121 to the normally closed contact 122 of relay HR. Contact 122 completes. a circuit through line 123 to the coil of relay HF and through line 124 to the coil of relay CR4.

Energizing relay CR4, closes contacts 125 and 126, contact 125 connects L1 to line 127 and. normally closed contact 128 of CR1 and line 129 to normally opened tappet switch solenoid 130. Contact 126 closes a circuit from L2 through line 131 and normally closed contact 132 of CR1 through lines 129 and 133 to the other side of, solenoid 130. Solenoid thus energized closes contact 134.

Energizing relay HF starts work drive motor 21 through relay contacts (not shown). Energizing relay HF also closes a contact 135 which completes a circuit through line 136, tappet switch contact 134, line 137, contacts 138 and 139 of jog switch 190', line 141 through coolant switch 115 to energize relay coil 111 for starting the coolant motor (not shown).

When piston 214 and wheel base 40 reach theendfof the rapid positioning movement, feed screw 215serves to throttle the escape of fluid under pressure from. line 216, thus causing pressure to be exerted to shift valve 230 against spring 231 to direct fluid under pressure from line 232 through line 233 to rapid feed pressure switch 234. In the shear cut cycle switch 234performs no function until the end of the slow feed.

Fluid under pressure is supplied from a pump 107 through line 240 to one end of a pressure operatedrrelief valve 241 of a well-known type disclosed in Serial No. 18,044, filed March 30, 1948, now Patent 2,663,995, granted December 29, 1953. Fluid under pressure passes through said valve to a line 242 to a throttle valve 243 which in turn is connected through a line 244 and line 245 to the left end of relief valve 241. Fluid from line 245 also enters the left end of valve 241, is conducted through lines 218 and 207 to various parts of the system which will be described later.

When valve 203 is shifted in a counter-clockwise direc tion as described above and fluid under pressure directed through line 205 to shift valve 206 to the left, the discharge of fluid under pressure through 253 is blocked because said line is connected through valve 206 to pressure line 207. Fluid from the left end of valve 241 is thereafter directed to line 245, valve 246, line 247, throttle valve 248, and line 249 to the head end of cylinder 250 driving piston 260 to the left, to provide a grinding feed for grinding wheel 42 and wheel base 40. As said piston approaches the end of its movement to the left, the piston rod 261 having rack teeth thereon, engaging pinion 262 on said screw 215, it throttles the fluid from pressure line 263 causing pressure to build up therein and shift valve 264 to the left against spring 265. Said valve in this position directs fluid from pressure line 263 through line 266 to actuate slow feed pressure switch 267 closing contact 263.

Closing contact 268 of slow feed pressure switch 26 at the.- end of the slow feed movement completes a circuit from L1 and rapid feed pressure switch 235 through line 160, line 170., and traverse selector switch 171, line 172, contact T R1, lines 173, 174', to the coil of relay CR2.

Energizing CR2; closes: contacts 175 and 176 connecting L1 and L2" respectively with traverse solenoid 177 to shift traverse. valve 270 to. the left against spring: 271 com necting pressure line 2.72 with line 273 leading through sections 2 and 3 of traverse. selector valve 2.74. From section; 2. of said valve, fluid under pressure passes through section 4 and line 275 to the small bore. 276 at the left end of valve 277 to shift said valve to the rights Fluid is; supplied to valve 277 from pressure line 272, through section 3' of traverse selector valve 274 and line 278. With valve 277 in the. righthand position fluid under pres sure from line 278 isdirected. through line 279 tothe left end of traverse cylinder. 280 to. shift piston 281 and wheel carriage. 41 to the right. During this movement follower 60 moves from leftto right on master cam 50 while grinding. wheel: 42 moves. in. the same direction on work piece 56. At: the same time. fluid from line. 279 also passes through; line. 282' to actuate pressure operated traverse direction switch 283. to. open. contact 180 of. said switch.

1 The function performed by the opening of this.- switch is part of another cycle and will be described later;

As; carriage 41 reaches. the end; of" its movement to the right; a cam 145 thereon: engages; wheel base retraction limit switch 44; completing; a circuit fromLl' through. line; 1185, position selector. switch 186:and line 187 to energize; the coil. of: relay CR3. Energizing CR3 closes contacts 188i and 189- to: connect lines: L1 and. L2 through lines- 191 and 192respectively toenergize wheel base retraction solenoid. 1931 Said; solenoid shifts valve 285? against spring 286 to connect pressure line 287 with. line: 288 leadingto. the right end ofpiston 289; Saidpiston moves to.the left engaging a notched sleeve 217 on.valve 203 to shiftsaidivalve 203* ina clockwise direction to the position shown in Figure6. In this position, of valve: 203 fluid: from pressureiline 204' is directed through line 290 to the' lefhend. of: valve; 206 shifting saidvalve; to the right as shovm in. Figure 6. In this1position' of valve=206 fluid from pressure line 207 passesthrouglr check valve 295: andlline- 296 to chamber 297. adjacent rapidtfeed'icylinden 213: Fluid under pressure. introducedlin'to thischamber; returnsrapidfeedipistona214 to starting position. As soon. assaid piston:has.moved:enough. to uncover linef298 fluid: isrdirected=therethrough to; slow fecd cylinder'250 tomove: piston: 260: to, thearight: and: to reset the: feed mechanism for the next operation. At this time fluid is also directed from line 298 through line 299 to the right end of cylinder 221 to shift piston 223 to the left and switch 120 to the position shown in Figure 6.

With switch 120 in the position shown in Figure 6, a circuit from L1 through line 121 normally closed contact 122 of HR and line 123 to HP and CR4 is open. However, HF and CR4 remain energized from L1 through a holding circuit normally open contact 135 of HF, line 136 tappet switch 134, line 137, contact 138 of jog switch line 145 to 121. In this position of switch 120 a circuit is closed to brake 25 in which switch 320 may be opened by rotation of arm 321 upon reverse movement of motor 21. From switch 320 the circuit continues through line 300, contact 301 of HF, line 302 and 303 to contact 304 of CR3, line 305 to the coil of CR1. Energizing CR1 coil opens contact 128 and 132 connected through lines 129 and 1.33 to tappet solenoid 130 deenergizing said solenoid. De-energizing said solenoid releases contact 134 which, however, is held closed by cam 310 on the headstock spindle until said headstock turns to a predetermined position at which said cam will release contact 134 opening a circuit through line 137, contact 138 of jog switch 190, line 145 and line 121 and normally closed contact 122 of HR and line 123 to the coil of HF. De-energizing HF, closes a circuit through normally closed contact 315 energizing CR1 from L1 thru switch 120, plugging switch 320, line 322 contact 323 of CR1 line 302, contact 315 of HF to HR. Another line 303 connects line 302 thru normally closed contact 304 of CR3 and line 305 to hold CR1. Energizing HR opens contact 122 and closes contacts (not shown) to reverse the connection to motor 21. Reversing motor 21 releases arm 321 which is withdrawn to open plugging contact 320 thus cutting 011 all power to said motor 21. Opening contact 320 breaks the holding circuit thru contact 323 of said relay and contact 304 of CR3 to energize the coil of CR1.

When rapid feed piston 214 and slow feed piston 260 are returned to starting position, pressure is removed from pressure switches 234 and 267, CR2 is de-energized, opening the circuit to traverse solenoid 177 and permitting spring 271 to move traverse valve 270 to the right. In this position, fluid under pressure from line 272 passes through said valve to line 400 section 1 of traverse valve 274, line 401 to the small bore 402 at the right end of valve 277 shifting said valve to the left. In this position of the valve 277, fluid under pressure from line 278 is directed through line 403 to the right end of cylinder 280 to return piston 281 and carriage 41 to the left hand or starting position.

By the use of the various selector switches disclosed in Figure 6, the other operating cycles may be utilized. If it is desired to reciprocate the carriage 41 for several successive passes, reversing valve 224 may be utilized. Said valve may be actuated by dogs (not shown) on carriage 41 or manually by lever 225. The left hand end of said reversing valve is a throttle valve 226 which is connected to provide an adjustable tarry during manual or automatic operation of the reversing mechanism.

I claim:

1. In a machine for grinding workpiecs of non-cylindrical contour, a bed, a work support and a grinding wheel support mounted thereon, a grinding wheel rotatably mounted on said grinding wheel support, a cradle on said work support, means for rotatably supporting a workpiece and a pattern in axial alignment thereon, means for rotating said workpiece and pattern, means for effecting longitudinal movement of said wheel support including a carriage slidably mounted on said bed, said wheel support being slidably mounted on said carriage for feeding said grinding wheel transversely toward and from said work support for grinding a workpiece, said gn'nding wheel being subject to change in diameter within predetermined limits due to wear and dressing, a follower on said carriage in operative engagement with said pattern and having a diameter equal to that of a half worn wheel, for causing said workpiece to move toward and from said grinding wheel in accordance with the contour of said pattern, whereby to minimize the eflect of wheel wear on the relation between wheel and follower.

2. A method for grinding workpieces having peripherally spaced convex and concave surfaces joined by leading and trailing edges of relatively small radius which consists in providing a model of the workpiece with the concave portion filled in to provide a convex curved surface joining the leading and trailing edges so as to expose to a grinding wheel, an arc of about on each of said edges adjacent said convex surface, using said model to produce a master cam by mounting said model and master cam on a common axis and rotating them with the model engaging a follower and the master cam engaging a grinding wheel, replacing said model with said master cam and the master cam with an unground workpiece and subjecting said workpiece to the action of a grinding wheel having a radius substantially greater than the concave portion of the workpiece while said master cam engages said follower so that only the convex portion and a portion of the leading and trailing edges of the workpiece are acted on by said grinding wheel.

References Cited in the file of this patent UNITED STATES PATENTS 927,130 Dorsey July 6, 1909 954,447 Lyons Apr. 12, 1910 1,156,323 Shearer Oct. 12, 1915 1,798,832 Buckingham Mar. 31, 1931 2,071,180 Shaw Feb. 16, 1937 2,118,806 Brown May 31, 1938 2,191,898 Nelson Feb. 27, 1940 2,243,410 Balsiger May 27, 1941 2,507,330 Balsiger May 9, 1950 2,529,026 Kestell Nov. 7, 1950 2,535,130 Green Dec. 26, 1950 2,599,453 Hook June 23, 1952 2,645,967 Zelewsky July 21, 1953 2,656,651 Seyferth Oct. 27, 1953 2,693,125 Zelewsky Nov. 2, 1954 FOREIGN PATENTS 544,203 Great Britain Apr. 1, 1942 616,642 Germany Aug. 5, 1935 625,816 Great Britain July 5, 1949

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