US2754637A - Shrouded turbine blade airfoil grinder - Google Patents

Description

y 1956 M. w. SCHIAVONE SHROUDED TURBINE BLADE AIRFOIL GRINDER 2 Sheets-Sheet 1 Filed July 1, 1954 INVENTOR MIEHAEL W. EEHIAVEINE ATTORNEY y 1956 M. w. SCHIAVONE 2,754,637

SHROUDED TURBINE BLADE AIRFOIL GRINDER Filed July 1, 1954 2 SheetsSheei 2 INVENTOR MICHAEL w SEHIAVDNE ATTORNEY United States Patent SHROUDED TURBINE BLADE AIRFOIL GRINDER Michael W. Schiavone, Verona, N. J., assignor to Curtiss- Wright Corporation, a corporation of Delaware Application July 1, 1954, Serial No. 440,641

8 Claims. (Cl. 51-144) the work against the grinding mechanism at the proper O attitude. The work is operatively connected through the fixture, through a series of cams, and through a master follower to a master airfoil. Generally, a compounding cam is incorporated in these machines. This cam controls the reciprocation of the movable portion of said machine at one end point of said movable portions t'raversing feed. This controlled reciprocating motion is particularly of use when the piece of work incorporates a flange which is not perpendicular to the longitudinal axis of said work. This controlled reciprocation permits grinding the blade surface parallel'to said flange without gouging out the flange itself. However, this motion has heretofore been obtainable at only one" end of the traversing stroke of the movable portion. of said machine. Of late, turbine blades having flanges at both ends, that is, shrouded at both root and tip, have come into prominence. The prior procedure inthese cases has been to grind a portion of the blade surface down to one flange, remove the work from the fixture, invert it about its longitudinal axis, replace it in the work fixture, and grind the remaining portion of the bladesurface to the second flange. Unfortunately, it is almost impossible to precisely relocate the blade in the fixture. Consequently,- aclearly discernible ridge appears on the surface of the work at the point Where the firstportionof the grinding meets the second portion of the grinding. This results in a great number of. scrapped blades of the requirement of hand dressing theridge on the blade surface with the attendant inaccuracy of the blade profile at this point.

An object ofthis invention is to incorporate into the conventional grinding machine a system of compoundingcams which will permit the continuous grinding ofthe work regardless of the fact that a blade isshrouded atboth of its ends. Removal of the workfromthe fixture during: the grinding: operation isthereby eliminated as is the necessity for hand-dressing any surface ridge resulting from such. a discontinuity in the grinding. operation.

Further objects and advantages of this invention will be apparent in readingthe detailed description below in which:

Fig. 1 is a simplified drawing. of the machine;

Fig. 2 is-a schematic diagram o'f'the invention incorporated in the grinding machine; I

Fig. 3 is a representative turbine blade mounted'in the special fixture; and

Fig. 4 is the same turbine blade after being rotated 180 degrees from the position illustrated in Fig; 3.

Referring first to Fig; 1; 10'-is amain frame or supporting structure of thesubject grinding machine, 12 is a movable WO'Ik' supporting member mounted in guide rails 14 of the main frame-10: The movable memberlZ has aheadportio'n' I6 hingedly mounted on-bearing's'-18 which "ice permit the head I6 to oscillate in pendulum-like fashion about a pivot axis a--a. The head portion 16 of said movable work supporting member 12 comprises, among other parts, a hydraulic spindle drive motor 20, an upper spindle portion 22, and a lower spindle portion 24. The upper spindle portion 22 carries a pair of cam shafts 25 and 26 driven therefrom, said camshaft 25 carrying a velocity cam 28' and the camshaft 26 carrying a Wave or normalcy 'cam 301-- The lower spindle portion 24 rotatably drives an airfoil master shaft 36 which attaches to an airfoil master 38. Said airfoil master 38' is retained on its other end by a master tailstock 40. The lower spindle portion 24 also rotatably drives a work element shaft 42 carrying a work element 44 which is held on its other end by a work element tail stock 45. The rotational speed of the shafts-36' and 42 m'ay be varied by regulating the flow of fluid fo the hydraulic spindle drive motor 20 which drives saidshafts This can be done through a velocity cam follower 2'9'ridingon said velocity cam 28, said follower 29' through suitable linkage controlling a valve (not shown) which regulates the How of fluid to said hydraulic motor 20'. The axis of rotation bb is common for both of said shafts 36 and 42. An airfoil master follower roller 46 is mountedon a master swivel table 48' operated by a wave cam follower 31 riding on the wave cam 30' through suitable linkage schematically shown as line SI in Fig. I so as to keep said master roller normal to the surface of said airfoil master being contacted. For grindingthe workelement 44, a grinding belt 54 is driven by suitable mechanism supported on a swivel table 52. The swivel table 52" is connected to master swivel tabl'e4'8by suitable linkage schematically indicated at 5 3 inIFig. l-so that the table 52' swings with the table 48 to keep the grinding. belt 54 at all times normal' to the portion of thework element surface contacted thereby as illustratedin Figs. 3 and 4. Mounted on the main frame 10 is-a hydraulic drive mechanism 56 which actua-tes a driverod 58 hereinafter referred to as the power member; Said power member 58 transmits horizontal translational motion to the movable member 12 through means described below.

The aforementioned structuredescribes a conventional grinding beltmachine which isspecifically designed for grinding the surfaces of compressor and turbine blades or the like, which machine operates as follows. The hydraulic drive mechanism 56 imparts horizontal reciprocating translational movement to the power member 58 which in turn transmits such motion to the movable member 12'. The airfoil master 3'8 and the work element to be ground, 44, carried'in the lower spindle portion 24 which is part of said' movabl'e member 12', thereby acquire this reciprocating motion, the length of a stroke of said reciprocatingmotion being previously set according to the lengthof the piece of'work to be ground. Besides this reciprocating. translational movementflhe airfoil master 33' and the-work element 44 are being. rotated in the manner described above. Consequently, the airfoil master follower. roller 46 describes a helical path along the airfoil master 38. In addition, as previously stated, the connection 51: between the camfollower 31 and the swivel table 48 is'eifective= torotatively position: said table so that itsiroller'46 is atall times normal to" the portionof the; masteri airfoilv'surface contacted by said roller.- This composite'relative movement bet-'weeirf the airfoil master and the roller 46 is duplicated exactly between the" grinding belt- 52; and the eleme'nfi 44' through com onents and in the manner describedi The master roller 46; furthermor'e, causes the pivoted head portion 1 6'of the movable'membef 1 2to swing about axis'aaso' thatthe limit te which the grinder s fleanap roach the axis of m tafi'on; =1,*' -bofthe work element 44 is determined bythe relation between the master roll 46 and the axis of rotation bb of the master airfoil 38.

Fig. 2 is a schematic drawing of the grinding machine best illustrating the new features, partially shown in Fig. l, permitting the continuous grinding of a work element having flanges at two opposite ends. Mounted on the power member 58 and integral therewith is a clutch block 70. Attached to the movable member 12 are a pair of pawls, a right pawl 72 and a left pawl 73, pivoted about pivot pin 74 which is carried by said movable member 12. Connected to said pawls at the end farthest removed from the clutch block 70 are a pair of trip latch buttons, a right button 76 and a left button 77, which pass through passageways 78 in said movable member 12. Seated on the movable member 12 and concentric with the trip latch button arms 80 are a pair of washers 82. A pair of springs 84 are disposed in compression between said trip latch buttons 76 and 77 and said movable member 12 for urging said pawls 72 and 73 toward their clutch engaging position. A pair of fixed trips, a right trip 86 and a left trip 87, are attached to the main frame so that one trip will depress its corresponding button when said button is carried by the movable member 12 into a position where said button will slide on said trip.

A compounding cam follower rod 88 is connected to the power member 58 and actuated thereby. A compounding cam follower roller 90 is attached to the end of said rod 88 and is free to rotate about an axis coincident with the longitudinal centerline of said rod 88. The roller 90 is disposed between two compounding cams, a left hand compounding cam 92 and a right hand compounding cam 94. Said compounding cams are mounted on the camshaft 26 which is carried by the movable member 12 and is driven by the hydraulic motor 20. As shown in Fig. 2, the roller 90 rides clear of both compounding cams, except at each cams profile high spot, when the pawls 72 and 73 are engaging the clutch block 70; the compounding earns rotate at all times. The contour of each compounding cam is designed to impart a predetermined movement to the movable head 12 when that particular compounding cam is operative.

A piston-cylinder assembly 96 operating on a suitable fluid is mounted on the movable member 12 and comprises a cylinder 98 with a piston 100 slidably mounted therein. Bosses 102 are provided at either end of the cylinder 98 for limiting the sliding motion of said piston. A yoke 104 is connected to the piston rod 106 and moves in conjunction with said piston 100. Fluid line connections 108 and 110 permit fluid pressure to be applied to either side of said piston 100 thereby actuating said piston. Said fluid connections 108 and 110 are connected by suitable means to fluid valve 112 comprising a casing 114 and a rotatable member 116. The casing 114 contains a fluid inlet port 118, a fluid exhaust port 120, and ports 122 and 124 connected by suitable means to the fluid connections 108 and 110 respectively. Passageways 117 and 119 are located in the rotatable member 116. Connected to said rotatable member is a valve actuating rod 126 which is operatively connected to the power member 58 through a head portion 128 on said rod 126 disposed between a pair of lugs, a right lug 130 and a left lug 131, said lugs being integral with the power member 58. The power member 58 also has an arm 132 disposed between the left leg 134 and the right leg 136 of yoke 104. A pair of springs are disposed for compression between the arm 132 and the legs of said yoke 104, the spring 138 being disposed between said arm and the left leg 134 and the spring 140 being disposed between the arm and right leg 136.

The operation of this grinding machine incorporating the subject invention is as follows. The hydraulic motor 56 moves the power member 58 in Fig. 2 left and right in a reciprocating motion. The words left and right as hereinafter used refer to the apparatus as viewed in Figs. 1 and 2. The power member is positively connected to the movable member 12 when the clutch block 70, mounted on said power member, is held between the pawls 72 and 73 mounted on said movable member. If, as illustrated in Fig. 2, the power member 58 is moving towards the left, the clutch block pushes on the left pawl 73 which in turn pulls along the movable member. The master airfoil and the work element, mounted in the movable member as described previously, are also moved toward the left. At the same time the lugs 130 and 131 are moving to the left with the right hand lug 130 pushing on the valve actuating rod head portion 128 thereby rotating said valve rotatable member 116 in a clockwise direction. Some time prior to the end of the leftward stroke the rotatable member 116 is then in the position illustrated in Fig. 2. With the valve 116 in this position, fluid will flow from the cylinder 98 through passageway 110, into passageway 119, and then out of the exhaust port 120. At the same time fluid will flow from an external fluid pressure supply passage 142, through inlet port 118, into passageway 117, to port 122, thence through passageway 108 into said cylinder thereby moving the piston 100 to the right. The piston will carry the yoke 104 to the right so that leg 134 of said yoke and arm 132 of the power member will compress the spring 138. The ports 118, 120, 122 and 124 are circumferentially elongated, as shown, in order to keep the described relationship between the ports and the rotatable members passageways 117 and 119 for a desired period of time so that spring 138 is compressed before the clutch disengages, as is hereinafter described. Said ports remain in this condition throughout the remainder of the stroke.

Near the end of the stroke towards the left, the left hand trip latch button 77 contacts the left hand trip 87. Said trip depresses the button which rotates the right pawl 72 about the pivot pin 74 in a counterclockwise direction thereby permitting said pawl to override the clutch block. The compressed spring 138 then urges the movable member 12 ahead of the power member 58 towards the left so that the right hand compounding cam 94 contacts the compounding cam follower roller 90. Said spring 1.38 holds the cam 94 against the follower roller whereby, as the cam rotates, the profile of the cam determines the motion of the movable member 12 along its guide means 14.

As previously stated, when the pawls 72 and 73 are engaged the roller 90 rides clear of both earns 92 and 94 except that said roller 90 engages the high spot of each cam. Hence, once the pawl 72 has disengaged the clutch block 70 the pawl 72 can only re-engage said clutch block when the roller 90 contacts the cam high spot. However, while the motion of the movable member 12 is being determined by the cam 94 at the left end of the stroke of said member 12 engagement of the trip 87 and button 77 prevents re-engagement of the pawl 72 with the clutch block 70.

When the power member 58 begins the return stroke towards the right, the follower roller 90 engages the cam 94 to push the movable member 12 to the right. The button 77 is promptly moved to the right beyond the trip 87 so that when the roller 90 next contacts the high point on the cam 94 the spring 84 is effective against the left button 77 to swing the right pawl 72 downward into re-engagement with the clutch block 70. The clutch block 70, pawls 72 and 73 and trip buttons 76 and 77 are now again in the relative positions illustrated in Fig. 2. As the power member 58 moves to the right the lug 131 thereon is effective to rotate the valve element 116 in a counterclockwise direction. Prior to the end of the rightward stroke of the power member 58 the valve element 116 reaches a position in which the port 122 and fluid connection 108 are in communication with the drain port 120 through the valve passage 119 and the port 124 and fluid connection 110 are in communication with the inlet port 118 and its supply passage 142 through the valve passage 117. As a result, the piston travels to the left to its opposite end positiontliereby relieving the spring 138 and compressing: the-spring: I402 Isle'arthe end of the rightward stroke of the power member-8 the trip button 76 is depressed'by-the trip 86. thereby raising the left pawl 73 to disengage it'fro'm the clutch 70. The previously compressed spring 140 is then eifective to hold the movable member cant 92 the: right against the cam follower roller 90 whereupon. themotion of the movable member 1-2Qat' theend: of itsrightward stroke is determined by the: cam 92- jus't', as previously described, the cam 94 determines the motion of the movable member 12 at the end of its leftwardstroke,

With the apparatus operating as described above, when the movable member 12' has moved towards the right and the left pawl 73 is disengaged from the clutch block 70 the compounding cam- 92 will be controlling the translational motion of said movable member. The work element 44 carried by said movable member will then appear relative to the grinding belt 54' (full line position) as illustrated in Figs. 3 and 4". Fig. 4 shows the work element 44 after having. been rotated about axis b-b 180 from the position illustrated in Fig. 3.- The work element 44, as illustrated, has awork element surface 143 disposed between a left flange 144; having a fiange surface 146, and a right flange 148, havinga flange surface 150. The compounding cam 92 controls the translational motion so that the grinding belt is able to grind the work element surface 143 immediately adjacent to the flange 144 during the complete revolution of the work element. The shaded par-t 152 of the surface 143 is the minimum portion of the surface ground while the cam 92 is controlling the translational motion of the work element. If the clutch had remained'engaged while the grinding belt came into contact with that shaded portion 152, the result would have been the undesired removal of material from the flange surface- 146- by said grinding belt, the amount of material removed and the position of the flanged surface 146 aflected increasing as the translational motion continued. The grinding belt 54 shown in dash-dot outline illustrates the relationship of said grinding belt and the flange l'48' 'when the com pounding cam 94 controls the translationakmotion ofthe work element 44. Said compounding cam. 94' is operative when the right pawl 72 disengagesthe clutch block 76, and permits the grinding belt 54' to grind at leastthe shaded portion 154 of the work element1surfa'cer143 without having said grinding belt interfere with. the surface 150 of the flange 148. Thus; the entiresurface; 143 of the work element 44 can be ground in one single operation without removing said work element? from the machine.

While I have described my inventionyindetai'l' in: its present preferred embodiment, it willibe obvious. to those skilled in the art, after understanding/my invention: that various changes and modifications: may be made therein without departing from the spirit or scope'thereof. I aim in the appended claims to cover all? such' modifications.

I claim as my invention:

1. A grinding mechanism comprising power means; a member connected to said power means for reciprocation thereby; a movable member havingrmeansfor'rotatably supporting a work piece; motor means operatively connected to said work piecesupporting. means for rotating said supporting means; means for supporting said movable member for reciprocatingmotion; means for clutching said movable member. to" said power. means connected member; means for disengaging the clutching means near the end of each stroke ofsaid reciprocating motion and for re-engaging said'clut'chi ng means near the commencement of the next stroke of said reciprocating motion; and cam means connected to said motor means for movement thereby for imparting" a predetermined movement to said movable member when said clutching means is disengaged.

2. A grinding'mechanism comprising power means; a member connected to saidpower means for reciprocation thereby; a movable member having means for rotatably supporting a work piece; motor means operatively connected to said work piece supporting means for rotating said supporting means; means for supporting said'movablemember for reciprocating motion; means for clutching said movable member to saidpower means connected member; means for disengaging the clutching means near the end of each stroke of said reciprocating motion and for re-engaging said clutching means near the commencement of the next stroke of said reciprocating motion; and meansfor imparting a predetermined movement'tosaid movable member when said clutching means is disengaged; said last mentioned means including elastic means operatively connected to the movable member, means for straining said elastic means during at least the last portion of each: stroke of said reciprocating ower means so as to urge said movable member relative to said power means in one or the other direction and cam means connected to said motor means for rotation thereby for controlling the extent of movement'of said movable member under the urging of said elastic means.

3. Apparatus for grinding the surface of a work element, said work element having a flange at each of two opposite ends with said flanges being inclined to the axis about which a work element is rotated during the grinding operation; said apparatus comprising a movable member having means for rotatably supporting a work element; motor means mounted on said movable member and connected to said work element supporting'meansfor rotating said supporting means; guide means mounting said Work element supporting movable member for linear movement therealon'g; power means; a member connected to said power means for reciprocation thereby; clutch means engageable for connecting said work element supporting movable member and said power member so that reciprocation of said power member causes reciprocation of said work element supporting movable member along its said guide means; means operative to disengage said clutch means adjacent to the end of each stroke of' said power member and to re-engage said clutch means during the initial portion of the next stroke of said power member; and cam means connected to said motor means for rotation thereby for moving said work element supporting movable member along said guide means in a predetermined manner when said clutch means is disengaged.

4. Apparatus for grinding the surface of a work element, said work element having a flange at each of two opposite ends with said flanges being inclined to the axis about which a work element is rotated during the grinding operation; said apparatus comprising a movable member for supporting a work element; guide means mounting said work element supporting member for linear movement therealong; power means; a member connected to said power means for reciprocation thereby; clutch means engageable for connecting said work element supporting movable member and said power member so that reciprocation of said power member causes reciprocation of said Work element supporting movable member along its said guide means; means operative to disengage said clutch means adjacent to the end of each stroke of said power member and to re-engage said clutch during the initial portion of the next stroke of said power member; and means operative for moving said work element supporting movable member along said guide means in a. predetermined manner when said clutch means is disengaged; said last mentioned means including elastic means operatively connected to said work element supporting movable member, meansfor straining said elastic means during at least the last portion of each stroke of said reciprocating member so as to urge said work element supporting movable member relative: to said power member in. one or the other direction along said-v guide means, a cam follower on said power member, a pair of cams carried by said work element supporting member, means for rotating said cams during operation of said apparatus, said cams and cam follower being so disposed that when the clutch means is engaged they do not afiect the motion of the work element supporting member and when said clutch means is disengaged said spring means is effective to move one or the other of said cams into engagement with said follower, depending on the direction of motion of said power member, for controlling the movement of said work element supporting member along its said guide means.

5. Apparatus for grinding the surface of a blade for turbines, compressors, or the like in which said blade has a flange portion at each end inclined to the longitudinal axis of said blade; said apparatus comprising a movable member having blade supporting means and means for rotating said blade supporting means; blade grinding means; means for oscillating said movable member about a pivot axis to maintain said blade grinding means in contact with the blade to be ground as the blade rotates; guide means for mounting said blade supporting movable member for linear movement along said guide means; power means; a member connected to said power means for reciprocation thereby; clutch means engageable for connecting said movable member to said power member so that reciprocation of said power member causes reciprocation of said blade supporting movable member along its guide means; means for disengaging said clutch means near the end of each stroke of said reciprocating motion and for re-engaging said clutch means during the initial portion of the next stroke of said reciprocating motion; and means for imparting a predetermined movement to said movable member when said clutch means is disengaged, said last mentioned means including elastic means operatively connected to the blade supporting movable member, means for straining said elastic means during at least the last portion of each stroke of said reciprocating member so as to urge said blade supporting movable member relative to said power member in one or the other direction along said guide means, and means for controlling the extent of movement of said blade support movable member along its said guide means under the urging of said elastic means.

6. Apparatus for grinding the surface of a blade for turbines, compressors, or the like in which said blade has a flange portion at each end inclined to the longitudinal axis of said blade; said apparatus comprising a movable member having blade supporting means and means for rotating said blade supporting means; blade grinding means; means for oscillating said movable member about a pivot axis to maintain said blade grinding means in contact with the blade to be ground as the blade rotates, guide means for mounting said blade supporting movable member for linear movement along said guide means; power means; a member connected to said power means for reciprocation thereby; clutch means engageable for connecting said movable member to said power member so that reciprocation of said power member causes reciprocation of said blade supporting movable member along its guide means; means for disengaging said clutch means near the end of each stroke of said reciprocating motion; means for re-engaging said clutch means during the initial portion of the next stroke of said reciprocating motion; and means for imparting a predetermined movement to said movable member when said clutch means is disengaged; said last mentioned means comprising a piston and cylinder assembly mounted on said blade supporting movable member for linear movement therewith; a pair of opposed springs operatively connected between said piston and said power member, means operative for supplying a fluid under pressure to one end of said cylinder for straining one of said pair of springs during at least the last portion of each stroke of said power member in one direction and to the other end of said cylinder for straining the other of said pair of springs during at least the last portion of each stroke of said power member in the opposite direction so as to urge said blade supporting member relative to said power member in one or the other direction along said guide means; and means for controlling the extent of movement of said blade support movable member along its said guide means under the urging of one of said springs.

7. Apparatus for grinding the surface of a blade for turbines, compressors, or the like in which said blade has a fiange portion at each end inclined to the longitudinal axis of said blade; said apparatus comprising a movable member having blade supporting means and means for rotating said blade supporting means; blade grinding means; means for oscillating said movable member about a pivot axis to maintain said blade grinding means in contact with the blade to be ground as the blade rotates; guide means for mounting said blade supporting movable member for linear movement along said guide means; power means; a member connected to said power means for reciprocation thereby; clutch means engageable for connecting said movable member to said power member so that reciprocation of said power member causes reciprocation of said blade supporting movable member along its guide means; means for disengaging said clutch means near the end of each stroke of said reciprocating motion and for re-engaging said clutch means during the initial portion of the next stroke of said reciprocating motion; and means for imparting a predetermined movement to said movable member when said clutch means is disengaged; said last mentioned means comprising a piston and cylinder assembly mounted on said blade supporting movable member for linear movement therewith; a pair of opposed springs operatively connected between said piston and said power member, means operative for supplying a fluid under pressure to one end of said cylinder for straining one of said pair of springs during at least the last portion of each stroke of said power member in one direction and to the other end of said cylinder for straining the other of said pair of springs during at least the last portion of each stroke of said power member in the opposite direction so as to urge said blade supporting movable member relative to said power member in one or the other direction along said guide means, a pair of cams and cam follower means cooperating so that when said clutch is disengaged said spring means is effective to maintain said cam follower means in contact with one or the other of said cams depending upon the direction of movement of said blade support movable member along its guide means.

8. Apparatus for grinding the surface of a blade for turbines, compressors or the like in which said blade has a fiange portion at each end inclined to the longitudinal axis of said blade; said apparatus comprising a movable member having blade supporting means and means for rotating said blade supporting means; blade grinding means; means for oscillating said member about a pivot axis to maintain said blade grinding means in contact with the blade to be ground as said blade rotates; guide means for mounting said blade supporting member for linear movement along said guide means; power means; a member connected to said power means for reciprocation thereby; clutch means engageable for connecting said members so that reciprocation of said power member causes reciprocation of said blade supporting movable member along its said guide means; means operative to disengage said clutch means adjacent to the end of each stroke of said power member and to re-engage said clutch means during the initial portion of the next stroke of said power member; and means operative for moving said blade supporting member along its said guide means in a predetermined manner when said clutch means is disengaged; said last mentioned means comprising a piston and cylinder assembly mounted on said blade supporting member for linear movement therewith with the axis of said piston and cylinder assembly being disposed parallel to the direction of said linear movement, a pair of opposed springs each operatively connected between said piston and said power member, means operative for supplying a fluid under pressure to one end of said cylinder for straining one of said springs during at least the last portion of each stroke of said power member in one direction and to the other end of said cylinder for straining the other of said springs during at least the last portion of each stroke of said power member in the opposite direction so as to urge said blade supporting member relative to said power member in one or the other direction along said guide means, a cam follower on said power member, a pair of cams carried by said blade supporting member, and means for rotating said cams during operation of said apparatus so that when said clutch means is disengaged during one stroke of the power member one of said springs is effective to move said movable member to maintain one of said cams against said follower and when the clutch means is disengaged during the next stroke of the power member the other of said springs is eflective to move said movable member to maintain the other of said cams against said follower, said cam follower being disposed between said cams and the cams being disposed sufiiciently far apart so that when the clutch is engaged they do not afiect the motion of the blade supporting member.

References Cited in the file of this patent UNITED STATES PATENTS 1,155,432 Muller Oct. 5, 1915 1,283,142 Fraser Oct. 29, 1918 1,651,432 Bath Dec. 6, 1927 2,380,286 Bauer July 10, 1945 2,479,622 Johnson Aug. 23, 1949 2,606,406 Mueller Aug. 12, 1952 2,656,651 Seyferth Oct. 27, 1953

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