US3827189A

US3827189A - Sheet glass seaming machine

Info

Publication number: US3827189A
Application number: US00349741A
Authority: US
Inventors: C Highberg; G Roesch
Original assignee: Engelhard Minerals and Chemicals Corp
Current assignee: Unicorn Industries Ltd; Engelhard Minerals and Chemicals Corp
Priority date: 1973-04-10
Filing date: 1973-04-10
Publication date: 1974-08-06
Anticipated expiration: 1991-08-06
Also published as: DE2417568A1; BR7402836D0; FR2225252A1; IT1004201B; JPS5012108A

Abstract

A machine for grinding edges of cut-to-size sheets of glass and the like with an abrasive wheel, especially where the edge has an irregular contour such as with convex or concave curved portions. The wheel supporting mechanism and glass sheet are translated relative to one another in the plane of the sheet and the abrasive wheel is supported with its axis perpendicular to the sheet and in a manner to permit its movement toward and away from the edge of the sheet while continuously grinding so as to follow the edge contour. The abrasive wheel is urged selectively in an upstream direction when contacting a downstream facing edge portion (incurve) and in a downstream direction when contacting an upstream facing edge portion (outcurve), to prevent skipping and assure a uniform grinding pressure generally normal to the edge portion engaged by the wheel.

Description

States 1191 ighberg et al.

[ SHEET GLASS SEAMING MACHINE [75] Inventors: Carle W. Highberg, Sylvania; Primary Examuler Donald Kelly George R. Roesch, Sylvania Twp Assistant Exammer-K. J. Ramsey both of Ohio [73] Assignee: Engelhard Minerals & Chemicals [57] ,ABSTRACT Corporation, Murray Hill, A machine for grind ng edges of cut-to-s1ze sheets of glass and the like with an abras1ve wheel, especially Flledi P 1973 where the edge has an irregular contour such as with [21] APP] NO: 349,741 convex or concave curved portions. The wheel supporting mechanism and glass sheet are translated relative to one another in the plane of the sheet and the U.S-

R, 5 abrasive wheel is supported its axis perpendicular [5 Cl. t to the heet and in a manner to its movement Field of Search 51/33 47, 101 R, toward and away from the edge of the sheet while 51/283 continuously grinding so as to follow the edge contour. The abrasive wheel is urged selectively in an up- References Cited stream direction when contacting a downstream facing UNITED STATES PATENTS edge portion (incurve) and in a downstream direction 1,997,386 4/1935 Louviaux 51 33 R when Contacting an upstream facing edge Portion 2.597.180 5/1952 Reaser et a]. 51 /101 R ut u e), to prevent pp g and assure a uniform 2,723,598 1 1/1955 Mann 51/99 X grinding pressure generally normal to the edge portion 2,795,086 6/1957 Clark 51/283 X engaged by the wheel, 2,826,872 3/1958 Robbins 5l/283 X 3,466,807 9/1969 Haas 51/35 x 10 clalms, 9 Drawlng Figures 35 13 a 9 P n PATENTED AUG 61974 SHEEI 2 BF 9 PATENTEDAUB 619M SHEET 3 BF 9 FIG. 3

PATENTED we 51914 SHEEI b BF 9 FIG. 4

PATENTEDAUG 5 I974 SHEEI 5 OF FIG. 5

PATENTED B 61974 SHEU 5 BF 9 ATENTEI] AUG BIEIN 3,827, 189

SHEEY 7 [If 9 PAIENIEBM B 61m SHEET 9 [IF 9 SHEET GLASS SEAMING MACHINE BACKGROUND OF THE INVENTION This invention relates to the grinding of edges of cutto-size sheets of glass and the like with abrasives such as diamond grit. More particularly, the invention relates to the grinding of the edges of cut-to-size glass sheets such as for vehicle Windshields, backlites and door lites.

Cut-to-size sheets of glass or glass-like material usually have razor sharp edges resulting from the cutting tool that travels around a predetermined outline pattern to cut a contoured piece from a rectangular sheet of standard dimensions. Usually the tool is a carbidetipped cutting wheel that leaves razor sharp edges which must be dulled for safety. This grinding operation, which is referred to in the art as seaming, is usually performed with special abrasives. Other similar operations to which the invention also relates, provide different qualities of edge and include what is referred to in the art as pencil edging.

Where the sheet or lite has relatively straight or gradually curved contoured edges to be seamed, the operation may be performed with a grinding wheel turning about an axis perpendicular to the sheet and urged into engagement with the edge while being moved progressively along the edge from one end to the other. The pressure or force urging the wheel against the edge of the glass is, of course, generally normal to the edge and of the desired magnitude to keep the wheel in continuous engagement with the edge to obtain uniform cutting.

Where the glass sheet or lite has an irregular edge contour such as more sharply curved, convex or concave portions, the seaming or other edging is most often done with abrasive belts. Grinding wheels have not been satisfactory in such circumstances since the wheel tends to jump intermittently out of engagement with the glass when contacting an edge portion that faces partly in an upstream or downstream direction relative to the direction of travel. This condition occurs because the forcing urging the wheel against the edge is normal to the direction of relative travel between the tool and the glass and thus not necessarily normal to edge portions that are angularly disposed relative to the direction of travel.

Also, the prior art techniques employed in the edging of glass sheets or lites with irregular edge contours generally require cam plate matching the edge contour of the sheet or lite and a cam follower on the support for the grinding wheel to guide the wheel along the edge.

The apparatus of the present invention has the advantage and capability of seaming or edging automatically with an abrasive wheel the edges of continuously moving, cut-to-size glass sheets or lites having irregular contours with relatively sharp curves.

SUMMARY OF THE INVENTION It is among the objects of the invention to seam or edge cut-to-size glass sheets or lites and the like with an abrasive wheel turning about an axis perpendicular to the sheet and adapted for movement toward and away from the sheet to accommodate an irregular edge contour.

Another object is to provide increased production rates and improved quality in the seaming or edging of glass sheets with irregular edge contours.

These and other objects are accomplished by means of a unique mechanism for supporting the abrasive wheel in a manner permitting its movement toward and away from the sheet to accommodate irregular edge contours while at the same time uniformly urging it against the sheet in a direction generally normal to the edge portion to be ground. The mechanism of the invention comprises a supporting structure and a member operatively connected between the supporting structure and the wheel and adapted to urge the wheel in a direction generally toward the edge and downstream relative to the direction of relative translation of the cut-to-size sheet. A second member is operatively connected between the support and the wheel and is adapted to urge the wheel in a direction generally toward the edge and upstream relative to the direction of relative translation. Apppropriate means are provided for actuating the first and second members selectively so that the wheel is urged by the first member when engaging an upstream facing contoured edge portion (outcurve) of the sheet and by the second member when engaging a downstream facing contoured edge portion (incurve).

In the preferred embodiment the first member is a pivot arm pivotally connected at one end to the support for movement about a fixed axis parallel to the grinding wheel axis, and the second member is another pivot arm pivotally connected at one end to the other end of the first pivot arm for movement about a second axis parallel to the grinding wheel axis. The second pivot arm is adapted to carry the grinding wheel at its outer end.

The first and second axis are located upstream and downstream respectively from the grinding wheel axis relative to the direction of translation. A hydraulic rotary actuator is connected between the support and the first pivot arm for urging the pivot arm toward the edge and a second hydraulic rotary actuator is operatively connected between the first and second pivot arms for urging the grinding wheel toward the sheet. Appropriate hydraulic controls are provided for pressurizing the two actuators selectively so that the grinding wheel is urged by the first actuator when engaging an upstream facing edge portion (outcurve) of the sheet and by the second actuator when engaging a downstream facing edge portion (incurve).

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view illustrating a machine for seaming the edges of cut-to-size glass sheets for automobile backlites wherein the glass sheets are transported by conveyor rolls past a seaming station including a diamond grit-type seaming wheel supported by a mechanism embodying the invention;

FIG. 2 is a fragmentary side elevation of the machine of FIG. 1 illustrating in particular the seaming wheel supporting mechanism embodying the invention;

FIG. 3 is a fragmentary side elevation on an enlarged scale illustrating in greater detail the tool-supporting mechanism of the invention with parts broken away and shown in section for the purpose of illustration;

FIG. 4 is a fragmentary sectional view taken on the line 44 of FIG. 3 and illustrating the motions of the tool-supporting mechanism;

FIG. 5 is a sectional view taken on the line 5-5 of FIG. 3; and

FIGS. 6 through 9 are schematic diagrams illustrating the hydraulic control system for the tool-supporting mechanism embodying the invention and showing the sequence of operation during a portion of the edge seaming cycle.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawings and initially to FIGS. 1 and 2, there is shown a seaming unit A located at a grinding station on one side of a conveyor B for transporting cut-to-size glass sheet past the station. In the embodiment shown the particular glass sheet being transported is a cut-to-size sheet 10 with a predetermined contour for use in forming the backlite of a certain type of automobile. The conveyor B includes a plurality of driven rollers including lower rollers 11 uniformly spaced parallel to one another and upper rollers 12 each cooperating with a respective lower roller to grip and transport therebetween the glass sheet 10. The seaming unit A includes a diamond grit-type seaming wheel 13.

The seaming wheel 13 is adapted to engage and seam the irregularly contoured bottom edge 16 of the sheet 10 as the sheet is transported past the seaming station by the conveyor B. The bottom edge 16 has a recessed portion 21 located between a downstream facing edge portion (incurve) 22 and an upstream facing edge portion (outcurve) 23. It will be noted that a grinding pressure applied perpendicular to the direction of travel of the glass sheet 10 would be normal to the edge 16 only in relatively straight portions but would be in angular relation to the upstream and downstream facing

edge portions

23 and 22 respectively. Thus, a grinding pressure applied only in a lateral direction would be inadequate in that the wheel 13 would tend to jump intermittently out of engagement with the glass during travel in the upstream and downstream facing

edge portions

23 and 22. Accordingly, the seaming unit A is provided with the tool-supporting mechanism of the invention which applies a seaming pressure generally normal to the edge portion engaged by the wheel 13 throughout the irregular contour of the bottom edge 16.

The unit A comprises a supporting structure 30, a pivot arm 31 connected to the supporting structure 30 and another pivot arm 32 connected to the pivot arm 31. The pivot arm 32 carries the seaming wheel 13 at one end thereof and a drive motor 33 at the other end. The motor 33 has a drive pulley 34 on its output shaft that drives a pulley 35 keyed to the shaft of the wheel 13, through a V-belt 36. The arm 31 is urged in a clockwise direction as viewed in FIG. 1 by means of a hydraulic rotary actuator 38 while the arm 32 is urged in a counterclockwise direction by another hydraulic rotary actuator 39 of smaller capacity.

The

actuators

38 and 39 may be, for example, the type manufactured and sold by Ohio Oscillator Co., Inc. of 1116 Ridge Avenue, Pittsburgh, Pennsylvania, the type referred to under the designation II-Series being illustrated herein.

Referring next to FIGS. 3, 4 and 5 which best illustrate the seaming unit A, it will be seen that the supporting structure 30 has a pair of vertically spaced

brackets

41 and 42 which support bearing

units

43 and 44 respectively. A vertical pivot pin 45 is joumaled in the

bearing units

43 and 44 and is fixed to the pivot arm 31 by set screws. The upper end of the pin 45 is coupled to an extension shaft 46. The shaft 46 has a pinion 47 that is engaged and turned by a rack 48 on the rod of a double-headed piston 49 in the rotary actuator 38.

The body of the rotary actuator 38 is connected to the supporting structure 30 so that movement of the piston 49 in the actuator 38 in response to hydraulic pressure urges the pivot arm 31 about its vertical axis defined by the pivot pin 45. The pivot pin 31, as best indicated in FIGS. 1 and 4, has a generally L-shaped configuration and the pivot pin 45 is located generally upstream of the seaming wheel 13 with respect to the direction of movement of the sheet 10.

The opposite end of the pivot arm 31 is located generally downstream of the seaming wheel 13 relative to the direction of movement of the glass sheet 10, and has a vertical bearing tube 51. The ends of the bearing tube 51 are joumaled in upper and

lower bearing units

52 and 53 bolted to the inner end of the pivot arm 32 so that the pivot arm 32 is pivotable about the vertical axis through the bearing tube 51.

A pivot pin 54 extends upwardly from the top of the pivot arm 32 coaxially with the bearing tube 51 and is coupled to a shaft with a pinion 55 that is engaged and turned by a rack 56 on the rod of a double-headed piston 57 in the rotary actuator 39. The body of the actuator 39 is attached to the pivot arm 31 by means of an upright post 58 mounted at the angled bend in the pivot arm 31 (FIG. 5). A plate 59 supporting the rotary actuator 39 is bolted to a flange welded to the top of the post 58. Accordingly, movement of the piston 57 in response to hydraulic pressure urges the pivot arm 32 about the vertical pivot axis defined by the pin 54.

The seaming wheel 13 is mounted at the right-hand end of the pivot arm 32 and is positioned vertically at the same level as the edge of the glass sheet 10. Also it is located well below the

pivot arms

31 and 32 so that there is no interference during its movement toward and away from the bottom edge 16 of the glass sheet, to follow the edge contour. In the embodiment shown the drive motor for the wheel 13 is located at the opposite end of the pivot arm 32 for the purpose of better balance. However, it will be apparent that a drive motor could be connected directly to the shaft of the grinding wheel 13 if desired.

It will be apparent that the

pivot arms

31 and 32 are articulated in such a way that one or the other may be used, depending on the pressure condition in the respective rotary actuator, as the primary means for urging the seaming wheel 13 against the edge of the glass sheet. The hydraulic control system for the

rotary actuators

38 and 39 is best illustrated in FIGS. 6 through 9 wherein one exemplary portion of a seaming cycle is illustrated sequentially during the seaming of the bottom edge 16 of the glass sheet 10.

The hydraulic control system comprises in addition to the

rotary actuators

38 and 39, a constant pressure hydraulic pump 60, a reservoir 61, a four-way solenoid valve 62 that controls the pressure supply to the actuator 39, a low-pressure regulator 63 and a high-pressure regulator 64 in the supply circuit to the actuator 38, a two-way solenoid valve 65 that selects high or low fluid pressure from one or the other of the two

regulators

63 and 64, a four-way solenoid valve 66 that controls the supply of fluid pressure to the actuator 38, and a fluid shock cushioning circuit 67 consisting of two parallel branches in the line to the right-hand end of the actuator 38 as viewed in FIG. 6. The cushioning circuit includes a left-hand branch 68 that includes a one-way ball check and a restricter and a right-hand parallel branch 69 that includes a one-way reverse ball check and a restricter.

OPERATION The operation of the seaming unit A and the hydraulic control system associated therewith will be described sequentially with reference to FIGS. 6 through 9. Referring initially to FIG. 6, it will be seen that the glass sheet 10 is being transported from right to left past the seaming unit A and that the seaming wheel 13 is engaging a straight portion of the bottom edge 16. In this condition the two-way solenoid valve 65 is moved to the left by solenoid 65a as illustrated in FIG. 6 to feed pressure from the high-pressure regulator 64 to the four-way solenoid valve 66. The valve 66 is moved to its right-hand normal operating position by solenoid 66a so that the high fluid pressure is directed through the check valve and restricter in the branch 69 to the right-hand end of the rotary hydraulic actuator 38. The resulting pressure urges the piston 48 to the left with a high magnitude of force to turn the pinion 46 in a clockwise direction. This urges the pivot arm 31 to the position shown and against an adjustable stop in the actuator that prevents further clockwise movement. The stop is a standard feature associated with the rotary hydraulic actuator 38.

Hydraulic pressure from the pump 60 is also supplied through the valve 62 which at this point in the cycle is moved to its right-hand position by the solenoid 62a to supply hydraulic fluid at pump pressure to the bottom end of the rotary hydraulic actuator 39. This pressure is regulated by direct adjustment of the pump pressure. The resulting pressure against the piston urges the pinion in a counterclockwise direction. This urges the pivot arm 32 in a counterclockwise direction carrying the seaming wheel 13 into engagement with the edge 16. The grinding pressure is thus provided by the actuator 39.

Referring next to FIG. 7, it will be seen that the downstream facing portion 22 of the recess 21 has been moved to the left to a position adjacent the grinding wheel and the pivot arm 32 has pivoted in a counterclockwise direction to move the wheel as needed to continue the grinding engagement. The necessary grinding pressure continues to be supplied through the actuator 39 to urge the pivot arm 32 in a counterclockwise direction. It will be seen that in this condition due to the unique geometry of the unit A, the grinding pressure is applied in a direction generally normal to the edge of the glass sheet throughout the upstream facing edge portion 22 of the recess 21. The rotary actuator 38 remains in a stop position with high pressure being supplied through the high pressure regulator 64.

Referring next to FIG. 8, it will be seen that the glass sheet 10 has been transported to the left to a position where the upstream facing portion 23 of the recess 21 is adjacent the seaming wheel 13 and the wheel is engaging the edge in that portion. Just before the glass sheet reached this position the leading edge thereof has tripped a microswitch that operates the solenoid 65a to move the valve 65 to the right wherein the pressure line from the high-pressure regulator 64 is closed and the line from the low-pressure regulator valve 63 is open to provide a lower pressure through the four-way solenoid valve 66, to the right-hand end of the rotary actuator 38. This pressure is normally not sufficient to keep the pivot arm 31 in its stop position and as the seaming wheel 13 moves through the upstream facing edge portion 23 the arm 31 pivots about its axis in a counterclockwise direction to permit the wheel to follow the contour of the edge portion 23. In this condition the relationship between the force urging the pivot arm 32 and the force urging the pivot arm 31 is such that the arm 32 and rotary actuator 39 move to a stop position and the necessary retraction movement is accommodated by the pivot arm 31. The result of this force relationship is that the pressure urging the grinding wheel 13 into engagement with the upstream facing edge portion 23 is applied in a direction generally normal to the edge being engaged.

The counterclockwise pivotal movement of the pivot arm 31 continues until the grinding wheel 13 reaches the position illustrated in FIG. 9. At this point the pivot arm 32 remains in a stop position and the pivot arm 31 has been pivoted counterclockwise to a position wherein the seaming wheel 13 is engaging a relatively straight edge portion of the glass sheet 10. The hydraulic pressure supplied to the rotary actuator 39 keeps the pivot arm 32 in a stop position while low fluid pressure continues to be supplied to the actuator 38 to provide the necessary grinding pressure.

This condition continues until another microswitch is tripped by the leading edge of the glass sheet. This energizes solenoid 62a and deenergizes solenoid 65a to move the four-way valve 62 to the left or to its reversing position and to move the two-way valve 65 to the left as illustrated in FIG. 6 to again supply high pressure to the rotary actuator 38. In this condition the seaming wheel 13 will normally have reached the end of the bottom edge 16 of the sheet 10 and the pivot arm 32 will pivot in a clockwise direction to a retracted position while the pivot arm 31 pivots in a clockwise direction back to its initial stop position. After a brief pause the solenoid 62a will be energized to move the four-way valve 62 back to the left to the position illustrated in FIG. 6. This occurs as the seaming wheel 13 engages the advancing edge of another glass sheet being transported across the conveyor B.

A principal advantage of the invention thus described is that the seaming wheel 13 guides itself automatically along the contour of the edge to be seamed. The grinding pressure remains generally uniform and normal to the glass edge throughout the cycle. The only setup of equipment required for a particular size and shape of cut-to-size glass sheet is the positioning of microswitches for energizing and deenergizing the various solenoids for the control valves of the hydraulic control system. Timers can be used to replace the microswitches if desired.

In some applications 'it may be more advantageous to hold a glass sheet in a fixed position and transport the tool-supporting mechanism of the invention along the sheet to accomplish the edge seaming or other grinding. Also other types of force-applying mechanisms than the rotary

hydraulic actuator

38 and 39 may be used to control the

pivot arms

31 and 32.

While the invention has been shown and described intended for the purpose of illustration rather than limitation and other variations and modifications of the specific construction herein shown and described will be apparent to those skilled in the art all within the intended spirit and scope of the invention. Accordingly, the patent is not to be limited to the specific embodiment herein shown and described nor in any way that is inconsistent with the extent to which the progress in the art has been advanced by the invention.

We claim: 1. In a machine for grinding contoured edges of sheets of glass and glass-like material with an abrasive wheel turning about an axis perpendicular to the sheet while relative translation occurs between the wheel and the edge during engagement therebetween, the improvement which comprises:

a support,

first means operatively connected between said support and said wheel for urging said wheel in a direction generally toward said edge and downstream relative to said direction of relative translation of said sheet,

second means operatively connected between said support and said wheel for urging said wheel in a direction generally toward said edge and upstream relative to said direction of relative translation of said sheet, and

means for actuating said first and second means selectively whereby said wheel is urged by said first means when engaging an upstream facing contoured edge portion of said sheet and by said second means when engaging a downstream facing contoured edge portion of said sheet.

2. A machine as defined in claim 1 wherein said sup- 3. A machine as defined in claim 2 wherein said glass sheet is transported in a horizontal position between driven rollers and wherein said grinding wheel has a vertical axis of rotation.

4. A machine as defined in claim 1 wherein said first means comprises a first pivot arm pivotally connected at one end to said support at a pivot axis located parallel to and downstream from said grinding wheel axis and fluid pressure means operatively connected between said support and said first pivot arm and wherein said second means comprises a second pivot arm pivotally connected at one end to the other end of said first pivot arm at a pivot axis located parallel to and upstream from said grinding wheel axis and fluid pressure means operatively connected between said first pivot arm and said second pivot arm.

5. A machine defined in claim 4 wherein said fluid pressure means for said first pivot arm and said fluid pressure means for said second pivot arm are both rotary hydraulic actuators.

6. In a machine for grinding contoured edges of sheets of glass and glass-like material with an abrasive wheel turning about an axis perpendicular to the sheet while relative translation occurs between the wheel and the edge during engagement therebetween, the improvement which comprises:

a support,

a first pivot arm pivotally connected at one end to said support for movement about a first axis parallel to said grinding wheel axis,

a second pivot arm pivotally connected at one end to the other end of said first pivot arm for movement about a second axis parallel to said grinding wheel axis, and being adapted to carry said grinding wheel at the other end thereof,

said first and second axes being located upstream and downstream respectively from said wheel axis relative to the direction of said translation, first means operatively connected to said support and said first pivot arm for urging said first pivot arm toward said edge, second means operatively connected to said first pivot arm and said second pivot arm for urging said grinding wheel toward said sheet, and means for actuating first and second means selectively whereby said wheel is urged by said first means when engaging an upstream facing edge portion of said sheet and by said second means when engaging a downstream facing edge portion of said sheet.

7. A machine as defined in claim 6 wherein said first means and said second means are fluid pressure devices.

8. A machine as defined in claim 7 wherein said fluid pressure devices are hydraulic rotary actuators.

9. A machine as defined in claim 8 wherein said rotary actuators have an adjustable stop associated therewith to define limit positions for said pivot arms in their respective pivotal movements toward said glass sheet.

10. A machine as defined in claim 9 wherein said means for actuating said rotary actuators comprises valve means for directing relatively high fluid pressure to said rotary actuator of said first means to move said first pivot arm to said limit position, and alternatively to direct relatively low fluid pressure to said rotary actuator of said first means whereby said second pivot arm moves to its limit position and said first pivot arm moves to accommodate edge-contour-following movement of said grinding wheel.

Claims

1. In a machine for grinding contoured edges of sheets of glass and glass-like material with an abrasive wheel turning about an axis perpendicular to the sheet while relative translation occurs between the wheel and the edge during engagement therebetween, the improvement which comprises: a support, first means operatively connected between said support and said wheel for urging said wheel in a direction generally toward said edge and downstream relative to said direction of relative translation of said sheet, second means operatively connected between said support and said wheel for urging said wheel in a direction generally toward said edge and upstream relative to said direction of relative translation of said sheet, and means for actuating said first and second means selectively whereby said wheel is urged by said first means when engaging an upstream facing contoured edge portion of said sheet and by said second means when engaging a downstream facing contoured edge portion of said sheet.

2. A machine as defined in claim 1 wherein said support is located in a fixed position and said glass sheet is transported linearly past said support continuously during the grinding operation.

3. A machine as defined in claim 2 wherein said glass sheet is transported in a horizontal position between driven rollers and wherein said grinding wheel has a vertical axis of rotation.

6. In a machine for grinding contoured edges of sheets of glass and glass-like material with an abrasive wheel turning about an axis perpendicular to the sheet while relative translation occurs between the wheel and the edge during engagement therebetween, the improvement which comprises: a support, a first pivot arm pivotally connected at one end to said support for movement about a first axis parallel to said grinding wheel axis, a second pivot arm pivotally connected at one end to the other end of said first pivot arm for movement about a second axis parallel to said grinding wheel axis, and being adapted to carry said grinding wheel at the other end thereof, said first and second axes being located upstream and downstream respectively from said wheel axis relative to the direction of said translation, first means operatively connected to said support and said first pivot arm for urging said first pivot arm toward said edge, second means operatively connected to said first pivot arm and said second pivot arm for urging said grinding wheel toward said sheet, and means for actuating first and second means selectively whereby said wheel is urged by said first means when engaging an upstream facing edge portion of said sheet and by said second means when engaging a downstream facing edge portion of said sheet.