US3818640A

US3818640A - Work carrier drive for double disc grinder with reversible drive and automatic stop means

Info

Publication number: US3818640A
Application number: US00365585A
Authority: US
Inventors: E Dunn
Original assignee: Litton Industries Inc
Current assignee: Litton Industries Inc
Priority date: 1973-05-31
Filing date: 1973-05-31
Publication date: 1974-06-25
Anticipated expiration: 1991-06-25
Also published as: GB1469658A; GB1469660A; FR2231474B3; JPS5021392A; IT1013248B; DE2425786C2; DE2425786A1; GB1469659A; JPS5726905B2; FR2231474A1

Abstract

A grinding machine comprising a pair of coaxial abrasive discs, means for rotating the abrasive discs about the axes thereof, a work paddle, means for mounting a workpiece for rotational displacement on the work paddle, means for displacing the work paddle from a first retracted position whereat the workpiece is remote from the abrasive discs to a second advanced position whereat the workpiece is located in the grinding zone intermediate the rotating abrasive discs, means for rotating the workpiece in a first direction when the workpiece is located within the grinding zone, means for rotating the workpiece in a second opposite direction when the workpiece is located within the grinding zone, and means for periodically reversing the direction of rotation of the workpiece.

Description

United States Patent [191 Dunn [ 3,818,640 June 25, 1974 WORK CARRIER DRIVE FOR DOUBLE DISC GRINDER WITH REVERSIBLE DRIVE AND AUTOMATIC STOP MEANS Inventor: Elman R. Dunn, Roscoe, 111.

Litton Industries, Inc., Beverly Hills, Calif.

Filed: May 31, 1973 Appl. No.: 365,585

Assignee:

References Cited UNITED STATES PATENTS 3,513,600 5/1970 Dunn 51/111 R Primary ExaminerAl Lawrence Smith Assistant Examiner-Nicholas P. Godici Attorney, Agent, or FirmSpencer T. Smith 5 7] ABSTRACT A grinding machine comprising a pair of coaxial abrasive discs, means for rotating the abrasive discs about the axes thereof, a work paddle, means for mounting a workpiece for rotational displacement on the work paddle, means for displacing the work paddle from a first retracted position whereat the workpiece is remote from the abrasive discs to a second advanced position whereat the workpiece is located in the grinding zone intermediate the rotating abrasive discs, means for rotating the workpiece in a first direction when the workpiece is located within the grinding zone, means for rotating the workpiece in a second opposite direction when the workpiece is located within the grinding zone, and means for periodically reversing the direction of rotation of the workpiece.

9 Claims, 6 Drawing Figures PATENTEDJUNZSEUH SHEET 2 OF 3 WORK CARRIER DRIVE FOR DOUBLE DISC GRINDER WITH REVERSIBLE DRIVE AND AUTOMATIC STOP MEANS SPECIFICATION The present invention relates to disc grinding machines and more particularly to double disc grinders wherein workpieces are inserted between a pair of parallel abrasive discs to effect controlled stock removal from the parallel faces of the workpiece.

In conventional prior art double disc grinding machines as disclosed in U.S. Pat. No. 2,899,779, a work carrier is rotatably supported and reciprocated between the opposing faces of the abrasive discs. The work carrier is rotated in a single direction by means of a motor driven power transmission chain which drives a work carrier sprocket on the inner surfaces of the chain. The rotation of the work carrier is brought to a halt at the conclusion of a grinding cycle with the work carrier at a predetermined orientation in order that workpieces may be automatically loaded into and unloaded from the work carrier.

It has been discovered by applicant that periodic reversal of the direction of rotation of the work carrier, and hence, the workpiece results in a substantial increase in the period of time between required dressings of the abrasive wheels and a concomitant increase in abrasive Wheel life.

Accordingly, it is an object of the present invention to periodically reverse the direction of rotation of the work carrier.

It is another object of the present invention to provide a grinding machine wherein the workpiece which is mounted on the work carrier will have a predetermined orientation at the termination of a grinding cycle regardless of the direction of rotation of the workpiece during the grinding cycle.

It is a further object of the present invention to provide a drive system for the work carrier which will distribute the drive and gravity forces completely around the periphery of the work carrier and which will thereby minimize wear due to friction between the work carrier and its supporting structure.

Other objects and advantages of the present invention will become apparent from the following portion of this specification and from the accompanying drawings which illustrate in accordance with the mandate of the patent statutes, a presently preferred embodiment incorporating the principles of the invention.

Referring to the drawings:

FIG. 1 is a front oblique view of a horizontal double disc grinder constructed in accordance with the teachings of the present invention;

FIG. 2 is an enlarged view partially broken away of the workholding fixture or paddle illustrated in FIG. 1 with a work carrier carrying a workpiece mounted on the paddle;

FIG. 3 is a view of the paddle illustrated in FIG. 2 taken along lines 3-3 thereof with portions of the opposing sides of the workholding paddle also being shown;

FIG. 4 is a view of the workholding paddle illustrated in FIG. 2 taken along lines 4-4 thereof with portions of the opposing sides of the workholding paddle also being shown;

FIG. 5 is a schematic diagram of the hydraulic circuit for the workpiece carrier drive system of the double disc grinder illustrated in FIG. 1; and

FIG. 6 is a schematic representation illustrating the electronic components of the work carrier drive system illustrated in FIGS. 2 and 5.

The basic components of a horizontal double disc grinding machine are illustrated in FIGS. 1 and 2. The double disc grinder includes left-hand (not shown) and right-hand l0 abrasive discs which preferably include central axially extending bores. These discs are conventionally rotatably mounted on a pair of slides 12 which are longitudinally displaceable for facecut infeed along a machine bed 14. Conventional feed mechanisms 16 such as are disclosed in US. Pat. No. 3,513,600 selectively advance these abrasive discs in unison towards the workpiece W which as disclosed may be a Wankel engine cylinder, to effect stock removal from the parallel faces of the workpiece. A workholding paddle or fixture 20, which 'is slidably mounted on a horizontal monorail type support 22 such as is illustrated in detail in US. Pat. No. 3,503,155 supports a workpiece carrier 24. The paddle is reciprocated in a conventional manner between the abrasive discs 10 by a reversible fluid motor or cylinder 28 which is mounted on the monorail support 22 and extends through a pair of bridge plates 30 to fixedly engage with the workpiece paddle 20.

The work carrier 24 is of two piece construction including an annular rotary carrier 25 which includes a row of sprocket teeth 32 extending around the outer periphery thereof intermediate a pair of annular track surfaces 33 (FIGS. 3 and 4) and a workpiece supporting plate 26 which is suitably secured to the annular rotary carrier 25. The work carrier 24 is rotatively supported on five rollers which are secured to the opposing side walls 34 of the paddle at spaced intervals about the periphery of the work carrier.

Two of these rollers are chain or idler rollers 40 (FIG. 3) which are secured to the paddle by pins 42 or the like. A power transmission chain 36 which is positioned as illustrated in FIG. 2, passes partially around each of the chain rollers and along the sprocket portion intermediate the chain rollers. The exterior surface of the power transmission chain engages with this sprocket portion to drive the work carrier.

To permit this arrangement, an annular groove 44 is defined in the periphery of the chain rollers which will accommodate not only the sprocket teeth 32 but the power transmission chain 36 as well. Rolling engagement of the chain rollers with the annular tracks 33 of the rotary carrier 25 accordingly takes place at the outer peripheral or bearing surfaces 46 on the upstanding side portions 48 of the roller.

Since the power transmission chain drives the rotary carrier on the exterior surface of the chain, a portion of the tensional forces of the chain are absorbed by the chain rollers thereby decreasing the tensional forces applied to the work carrier whereby frictional wearing of the rotary carrier 25 can be reduced.

Three carrier support rollers 50 are illustrated in FIG. 4. These rollers are substantially similar to the chain rollers except that the annular peripherial groove 52 of the carrier support roller is only wide enough to permit free passage of the sprocket teeth 32 therethrough. Accordingly, the bearing surfaces 54 of the carrier support rollers are wider providing larger bearing surfaces. Since the bottom, horizontally adjacent,

carrier support rollers additionally support the weight of the work carrier 24 and the workpiece which is sup ported thereon, these additional bearing surface areas tend to equalize the pressures exerted on the carrier support and chain rollers.

The hydraulic circuit for the carrier drive system is illustrated in FIG. 5. This circuit includes a source of pressurized liquid 64, a pilot operated normally closed two-way valve 66 operated by a first solenoid 68, a normally closed two-way valve 70 operated by a second solenoid 72, first 74 and second 76 variable constant volumetric flow control valves, a normally closed pilot operated three position valve 78 including a selfcentering spool and third 80 and fourth 82 solenoids which are selectively energizable to define first and second flow paths through the valve 78 and the hydraulic car rier drive motor 38.

Energization of the first 68 and second 72 solenoids as well as energization of third solenoid 80 allows pressurized liquid from the source 64 to flow through both of the constant volumetric

flow control valves

74, 76 with the combined flow being directed through one of the through-flow paths of the three position valve 78 to the hydraulic carrier drive motor 38. The hydraulic drive motor, and hence, the work carrier will therefore rotate in a first (clockwise) direction at grind speed. Deenergization of the third solenoid 80 and concurrent energization of the fourth solenoid 82 will result in the carrier drive motor 38 being rotated at the grind speed in an opposite (counterclockwise) direction. When the second solenoid 72 is deenergized, flow through the second constant flow control valve 76 will be blocked, and accordingly, only the pressurized constant volumetric flow through the first flow control valve 74 will drive the carrier drive motor in either the clockwise or counterclockwise direction, and such will be at a reduced or slow rate. When both of the first and

second solenoids

68,72 are deenergized, all flow to the work carrier drive motor will be blocked and rotation of the work carrier will accordingly be tenninated.

Once the grinding cycle has been initiated by depressing a pair of start buttons (not shown), the paddle will be advanced by the paddle drive motor 28 at a rapid rate from the load-unload position illustrated in HO. 1. When the paddle has been advanced to a position where the workpiece W lies approximately midway between the inner and outer peripheries of the abrasive discs, a suitable trip dog carried by the paddle actuates a carrier drive limit switch LS1 (FIGS. 1 and 6) mounted on the monorail 22. Solenoids 68, 72 and 82 will be energized and the carrier drive motor 28, and hence, the work carrier 24 will be rotated in a first direction counterclockwise at grind speed.

The paddle is further advanced at the rapid rate until the paddle reaches the fully advanced position where the workpiece partially overlaps the central bore (inner periphery) of the abrasive discs and a suitable trip dog carried by the paddle 20 actuates the fully advanced limit switch LS2 which is mounted on the monorail 22.

The paddle is then repetitively retracted and advanced at a rapid rate between the fully advanced position and the retracted position where the workpiece partially overlaps the outer periphery of the abrasive discs and where the retracted limit switch LS3 is actuated. When the workpiece has been ground to size, the paddle is retracted past the retracted position, past a carrier drive motor slow-stop position where a trip dog carried by the paddle actuates the carrier drive slow-stop limit switch LS4, to a fully retracted position where a monorail mounted cycle stop limit switch LS5 is actuated to bring the paddle drive motor 38, and hence, the work carrier 24 to a halt so that the ground workpiece W can be unloaded and a workpiece to be ground can be loaded into the work carrier.

As can be seen from FIG. 6, the two three

position valve solenoids

80, 82 are alternately energized with each complete cycle. Accordingly, the direction of rotation of the carrier drive motor, and hence, the work carrier reverses every cycle. It has been found that such work carrier rotation reversal substantially increases the life of the abrasive discs with a concomitant reduction in the number of abrasive disc dressings which are required.

It is desirable to stop the rotation of the work carrier, which may as above described be rotating in eitherof two directions, at a predetermined orientation to facilitate automatic loading and unloading of the workpieces. To this end, first PS1 and second PS2 inductive (radio-frequency) proximity switches are mounted within the workholding paddle side walls 34 (FIGS. 1 and 6). The sensing head of each sensor detects electrically conductive materials up to a predetennined distance away and the sensing heads are selectively located so that only the peaks of the carrier periphery teeth will be sensed.

Actuation of the carrier slow-stop limit switch LS4, after a workpiece has been ground to size, enables the first inductive proximity switch PS1 to detect the presence or absence of carrier teeth. To prevent the inductive proximity switches from successively triggering the second 72 and first 68 solenoids with the passage of successive sprocket teeth, a pair of timers are associated with the corresponding proximity switches PS1 and PS2. With the grinding speed of the carrier drive motor being selectively determinable to be within a predetermined range, the maximum time which would be required for the work carrier to rotate one full tooth can be ascertained and the timer associated with the first proximity switch PS1 is accordingly set to at least that length of time. When the sensor senses the peak of a sprocket tooth the timer will be started, and if another peak is sensed within the set timer period, the timer will be reset and again started. At least one sprocket tooth is cut off (preferably to the pitch line) and the number of cut off teeth is predetermined to assure that the timer will time out and thereby energize the second solenoid 72 and concurrently enable proximity switch PS2 to sense the absence of teeth on the carrier periphery. As above described, energization of the second solenoid 72 results in the speed of the carrier being reduced from grind to slow speed.

The second inductive proximity switch PS2 is similarly connected to timer 2 which will time-out after a predetermined period of time (which, if the second proximity switch is actuated prior to the blank to its location with the blank, must be sufficient to prevent premature actuation of the first solenoid as excessive teeth pass therepast) when the toothless section of the work carrier periphery passes the sensor with the carrier rotating at slow speed. When this timer times-out, the first solenoid 68 will be deenergized and the rotational movement of the work carrier will, accordingly, be terminated, perferably dead center within the peripheral zone of the work carrier where teeth have been removed or cut off.

When the work carrier is rotating in the counterclockwise direction and the carrier slow-stop limit switch LS4 is actuated, the first proximity switch PS1 will be actuated to slow the speed of the work carrier and the second proximity switch will be then actuated to stop the rotation of the work carrier.

When the grinding cycle is repeated with the next workpiece, actuation of the carrier drive limit switch LS1 energizes the fourth solenoid 82 and deenergizes the third solenoid whereby the direction of rotation of the carrier will be reversed.

In order that the carrier rotation may be stopped with the workpiece at the predetermined orientation, the mode of the proximity switches PS1, PS2 is also reversed each cycle. With the work carrier rotating in the clockwise direction, actuation of the carrier slow-stop limit switch LS4 enables the second proximity switch to deenergize the second solenoid 72 to slow the work carrier drive motor and enables the first proximity switch PS1 to subsequently deenergize the first solenoid 68 to stop the rotation of the work carrier. The proximity switches may be radially and tangentially adjusted as desired.

if desired, the timers may have different settings by providing suitable timer alternators so that one time will always be associated with the first proximity switch to be actuated regardless of carrier direction, and the second timer will always be associated with the second proximity switch to be actuated regardless of carrier direction.

By selectively locating the proximity switches, by cutting off a predetermined number of sprocket teeth, by adjusting the timers which are associated with the proximity switches and by reversing the mode of the proximity switches every grinding cycle, the rotation of the work carrier (clockwise or counterclockwise) can be brought to a halt with the workpiece at a predetermined orientation.

Having thus described my invention, what is claimed l. A grinding machine comprising a pair of coaxial abrasive discs,

means for rotating said abrasive discs about the axes thereof,

a work paddle,

means for mounting a workpiece for rotational displacement on said work paddle,

means for displacing said work paddle from a first retracted position whereat the workpiece is remote from said abrasive discs to a second advanced position whereat the workpiece is located in the grinding zone intermediate said rotating abrasive discs,

first means for rotating the workpiece in a first direction when the workpiece is located within the grinding zone,

second means for rotating the workpiece in a second opposite direction when the workpiece is located within the grinding zone, and

means for periodically reversing the direction of rotation of the workpiece.

2. A grinding machine according to claim 1, further comprising means for stopping the rotation of the workpiece when the workpiece is rotating in either said first or second direction with the workpiece at a predetermined orientation. 3. A grinding machine according to claim 1, wherein a workpiece is ground to size each grinding cycle and 5 said periodically reversing means comprises means for reversing the direction of rotation of the workpiece every grinding cycle.

4. A grinding machine according to claim 2 wherein said mounting means comprises a work carrier having a metallic sprocket wheel having a least a portion of at least one successive tooth cut thereoff defining a blank sprocket wheel portion,

said first and second rotating means comprises a power transmission chain engaging with a portion of said sprocket wheel,

a rotative motor means for driving said power transmission chain in either of said first or second directions at a grind speed, and

means for reducing the speed of said motor means to reduce the power transmission chain speed to a slow speed, and

said stopping means comprises first and second selectively enabled inductive proximity switches selectively located to sense the presence of only those sprocket teeth of said sprocket wheel which do not have said portion cut therefrom,

means for preventing the actuation of said switches as successive sprocket teeth pass therepast, said preventing means being adapted to allow the actuation of said switches as said blank sprocket wheel portion passes past said switches means for terminating the rotation of said motor means,

means for operating said grinding machine in a first mode when said work carrier is rotating in said first direction wherein said first and second switches will sequentially actuate said speed reducing means and said terminating means, and

means for operating said grinding machine in a second mode when said work carrier is rotating in said second direction wherein said second and first switches will sequentially actuate said speed reducing means and said terminating means,

said first and second switches being selectively separated, said blank portion having a selected length, and said preventing means being selectively adjusted so that said work carrier will stop rotating, regardless of rotative direction, at a predetermined orientation,

and means for switching from one mode to the other when the direction of rotation of said work carrier changes from said one direction to said other direction.

5. A grindline machine according to claim 4 further comprising means for conjointly actuating said speed reducing means and enabling said second proximity switch when said grinding machine is operating in said first mode, and

means for conjointly actuating said speed reducing means and enabling said first proximity switch when said grinding machine is operating in said second mode.

6. A grinding machine according to claim 4 wherein said preventing means comprises first and second timers.

7. A machine tool comprising a rotatable member having a metallic sprocket wheel having at least a portion of at least one successive tooth cut thereof defining a blank sprocket wheel portion,

means for rotating said rotatable member including power transmission chain means engaging with a portion of said sprocket wheel, rotative motor means for driving said power transmission chain in first or second directions, and means for stopping said rotatable member, whether said rotatable member is rotating in said first or second directions, at a predetermined orientation including, first and second selectively enabled inductive proximity switches selectively located to sense the presence of only those sprocket teeth of said sprocket wheel which do not have said portion cut therefrom, means for preventing the actuation of said switches as successive sprocket teeth pass therepast, said preventing means being adapted to allow the actuation of said switches as said blank sprocket wheel portion passes past said switches, means for terminating the rotation of said motor means, means for operating said machine tool in a first mode when said rotatable member is rotating in said first direction wherein said first and second switches will sequentially actuate to energize said terminating means, means for operating said machine tool in a second mode when said rotatable member is rotating in said second direction wherein said second and first switches will sequentially actuate to energize said terminating means, and means for switching from one mode to the other mode when the direction of rotation of said rotatable member changes from one of said directions to the other one of said directions.

8. A grinding machine according to claim 7 further comprising means for preventing actuation of said second switch prior to said first switch when said machine tool is operating in said first mode and means for preventing actuation of said first switch prior to said second switch when machine tool is operating in said second mode.

9. A grinding machine comprising a pair of coaxial abrasive discs,

means for rotating said abrasive discs about the axes thereof,

a work paddle,

means for mounting a workpiece for rotational displacement on said workpaddle, said mounting means including a work carrier including an annular row of sprocket teeth extending peripherally thereabout intermediate adjacent peripherally extending track surfaces,

a power transmission chain engaging, on the exterior surface thereof, with a portion of said sprocket teeth,

rotative motor means for driving said power transmission chain,

a pair of chain rollers mounted on said work paddle and including a pair of peripheral track surfaces for rolling engagement with said work carrier track surfaces and an annular groove having a width substantially equal to the width of said power transmission chain intermediate said roller track surfaces for permitting passage of said power transmission chain between said chain rollers and said work carrier, and

a plurality of work carrier support rollers mounted on said work paddle remote from said sprocket teeth portion and including a pair of peripheral track surfaces for rolling engagement with said work carrier track surfaces and an annular groove substantially narrower than the width of said chain roller grooves intermediate said track surfaces for permitting passage of said sprocket teeth between said work carrier and said work support rollers.

Claims

1. A grinding machine comprising a pair of coaxial abrasive discs, means for rotating said abrasive discs about the axes thereof, a work paddle, means for mounting a workpiece for rotational displacement on said work paddle, means for displacing said work paddle from a first retracted position whereat the workpiece is remote from said abrasive discs to a second advanced position whereat the workpiece is located in the grinding zone intermediate said rotating abrasive discs, first means for rotating the workpiece in a first direction when the workpiece is located within the grinding zone, second means for rotating the workpiece in a second opposite direction when the workpiece is located within the grinding zone, and means for periodically reversing the direction of rotation of the workpiece.

2. A grinding machine according to claim 1, further comprising means for stopping the rotation of the workpiece when the workpiece is rotating in either said first or second direction with the workpiece at a predetermined orientation.

3. A grinding machine according to claim 1, wherein a workpiece is ground to size each grinding cycle and said periodically reversing means comprises means for reversing the direction of rotation of the workpiece every grinding cycle.

4. A grinding machine according to claim 2 wherein said mounting means comprises a work carrier having a metallic sprocket wheel having a least a portion of at least one successive tooth cut thereoff defining a blank sprocket wheel portion, said first and second rotating means comprises a power transmission chain engaging with a portion of said sprocket wheel, a rotative motor means for driving said power transmission chain in either of said first or second directions at a grind speed, and means for reducing the speed of said motor means to reduce the power transmission chain speed to a slow speed, and said stopping means comprises first and second selectively enabled inductive proximity switches selectively located to sense the presence of only those sprocket teeth of said sprocket wheel which do not have said portion cut therefrom, means for preventing the actuation of said switches as successive sprocket teeth pass therepast, said preventing means being adapted to allow the actuation of said switches as said blank sprocket wheel portion passes past said switches means for terminating the rotation of said motor means, means for operating said grinding machine in a first mode when said work carrier is rotating in said first direction wherein said first and second switches will sequentially actuate said speed reducing means and said terminating means, and means for operating said grinding machine in a second mode when said work carrier is rotating in said second direction wherein said second and first switches will sequentially actuate said speed reducing means and said terminating means, said first and second switches being selectively separated, said blank portion having a selected length, and said preventing means being selectively adjusted so that said work carrier will stop rotating, regardless of rotative direction, at a predetermined orientation, and means for switching from one mode to the other when the direction of rotation of said work carrier changes from said one direction to said other direction.

5. A grindline machine according to claim 4 further comprising means for conjointly actuating said speed reducing means and enabling said second proximity switch when said grinding machine is operating in said first mode, and means for conjointly actuating said speed reducing means and enabling said first proximity switch when said grinding machine is operating in said second mode.

7. A machine tool comprising a rotatable member having a metallic sprocket wheel having at least a portion of at least one successive tooth cut thereof defining a blank sprocket wheel portion, means for rotating said rotatable member including power transmission chain means engaging with a portion of said sprocket wheel, rotative motor means for driving said power transmission chain in first or second directions, and means for stopping said rotatable member, whether said rotatable member is rotating in said first or second directions, at a predetermined orientation including, first and second selectively enabled inductive proximity switches selectively located to sense the presence of only those sprocket teeth of said sprocket wheel which do not have said portion cut therefrom, means for preventing the actuation of said switches as successive sprocket teeth pass therepast, said preventing means being adapted to allow the actuation of said switches as said blank sprocket wheel portion passes past said switches, means for terminating the rotation of said motor means, means for operating said machine tool in a first mode when said rotatable member is rotating in said first direction wherein said first and second switches will sequentially actuate to energize said terminating means, means for operating said machine tool in a second mode when said rotatable member is rotating in said second direction wherein said second and first switches will sequentially actuate to energize said terminating means, and means for switching from one mode to the other mode when the direction of rotation of said rotatable member changes from one of said directions to the other one of said directions.

9. A grinding machine comprising a pair of coaxial abrasive discs, means for rotating said abrasive discs about the axes thereof, a work paddle, means for mounting a workpiece for rotational displacement on said workpaddle, said mounting means including a work carrier including an annular row of sprocket teeth extending peripherally thereabout intermediate adjacent peripherally extending track surfaces, a power transmission chain engaging, on the exterior surface thereof, with a portion of said sprocket teeth, rotative motor means for driving said power transmission chain, a pair of chain rollers mounted on said work paddle and including a pair of peripheral track surfaces for rolling engagement with said work carrier track surfaces and an annular groove having a width substantially equal to the width of said power transmission chain intermediate said roller track surfaces for permitting passage of said power traNsmission chain between said chain rollers and said work carrier, and a plurality of work carrier support rollers mounted on said work paddle remote from said sprocket teeth portion and including a pair of peripheral track surfaces for rolling engagement with said work carrier track surfaces and an annular groove substantially narrower than the width of said chain roller grooves intermediate said track surfaces for permitting passage of said sprocket teeth between said work carrier and said work support rollers.