US3891409A

US3891409A - Polishing apparatus

Info

Publication number: US3891409A
Application number: US501694A
Authority: US
Inventors: Jr Marvin W Keith
Original assignee: Buehler Ltd
Current assignee: Buehler Ltd
Priority date: 1974-08-29
Filing date: 1974-08-29
Publication date: 1975-06-24
Anticipated expiration: 1992-06-24
Also published as: JPS5128292A; JPS5322316B2

Abstract

Polishing apparatus of the type having at least one roll of abrasive paper which is drawn over an elongated platen to provide a flat abrasive surface suitable for the manual polishing of specimens for purposes of microstructural analysis, including hydraulic means for supplying water to wet the abrasive surface and utilizing the same water source for simultaneously clamping the length of abrasive paper in fixed position on the upper surface of the platen during a polishing operation.

Description

United States Patent 1191 Keith, Jr. June 24, 1975 [54] POLISHING APPARATUS 2,472,899 6/1949 Hein 51/358 [75] Inventor: Marvin W. Keith, Jr., Evanston, II]. P E Om n M S r1mary xammere lmpson [73] Asslgneer Buehler Ltd., Evanston, Ill. Amy-hey Agent (jh pi J [22] Filed: Aug. 29, 1974 1211 Appl. No.: 501,694 1571 ABSTRACT Polishing apparatus of the type having at least one roll 52 u.s.c1. 51/361- 51 /1s0- 51/267 of abrasive Paper which is draw 1511 lm. c1 1524a 17/00- 13 241; 55/02 Pmvid a abrasive Surface suitable [58] Field of Search 51/356, 358-361, the manual Polishing f n y n r 51/373 267 16 I50 I49 crostructural a al s1s including hydrauhc mea s to supplying water to wet the abrasive surface and utiliz- [56] References Cited ing the same water source for simultaneously clamping the length of abrasive paper in fixed position on the I 908 048 A1 t [PATENTS SW3 upper surface of the platen during a polishing operaayer c a ti 1338,6344 l/l944 lllncr et all. 7 2.425369 8/1947 Fleming 5l/358 10 Claims, 7 Drawing Figures PATENTED JUN 24 I975 SHEET SHEET PATENTEI] JUN 24 I975 POLISHING APPARATUS BRIEF SUMMARY OF THE INVENTION The present invention relates to the polishing of specimens for microstructural analysis such as in the fields of metallurgy, petrography and mineralogy, and in particular to the manual polishing of such specimens utilizing one or more selected grades of abrasive paper which is wetted with water or other suitable liquid to facilitate the polishing operation. A polishing apparatus of the type to which the present invention relates may be referred to as a roll grinder in the sense that it utilizes rolls of abrasive paper which are usually relatively coarse and ofa type suitable for grinding of specimens, i.e., a relatively coarse stage of polishing carried out prior to a final polishing operation.

It is known in the art to provide apparatus for the manual polishing of specimens including at least one roll of abrasive paper which is drawn out over an elongated platen and clamped in place on the platen to provide a flat polishing surface. The polishing surface is preferably wetted with water or other suitable liquid, and the polishing is performed manually by simply moving a specimen back and forth over the wetted abrasive surface.

In polishing apparatus of the foregoing type, it is necessary to provide releasable clamping means. Thus, with the clamping means in its released position, a length of used abrasive paper may be pulled out and discarded and fresh paper from a supply roll drawn over the upper surface of the elongated platen, and when the fresh supply of abrasive paper is in covering relation to the platen, the clamping means may then be actuated to clamp the paper on the surface of the platen. A supply of water or other suitable liquid is then turned on to direct a controlled amount of water to the abrasive surface to wet the same, after which a manual polishing operation may be conducted by moving a specimen back and forth over the wetted polishing surface.

A conventional polishing apparatus of the foregoing type usually includes a plurality of supply rolls of abrasive paper and a corresponding number of platens arranged in parallel, side-by-side relation. The advantage of such apparatus is that it permits use of different grades of abrasive paper each of which may be drawn out over a corresponding flat platen thereby providing several polishing surfaces each offering a somewhat different grade of paper whereby a specimen may be polished successively on different grades of abrasive paper to achieve a desired surface for purposes of microstructural analysis.

Polishing apparatus of the type described hereinabove usually includes mechanical mechanism for re leasably clamping a length of abrasive paper to the upper surface of a corresponding elongated platen. For example, cam mechanism is commonly used in combination with a clamping member, whereby actuation of the cam mechanism will move a clamping member between a clamping position and a release position. Due to the difficulty of providing a single cam mechanism to operate a plurality of clamping members, the usual practice is to provide separate mechanical actuators or cam mechanisms for each of several platens in apparatus of the type having multiple platens. It will thus be understood that after drawing out a length of fresh paper over each platen, the mechanical clamping mechanisms corresponding with each such platen must be actuated to clamp the several lengths of paper in place, and then the water supply must be turned on to wet each of the papers, after which manual polishing of a specimen may proceed.

It is a general object of the present invention to provide improved releasable clamping means which automatically effects clamping of several lengths of abrasive paper on a plurality of corresponding platens of a polishing apparatus whenever the supply of water or other suitable liquid is turned on, and which automatically releases the several papers when the liquid supply is turned off.

It is a more specific object of the invention to provide hydraulic clamping means for each of the several platens of a polishing apparatus, such clamping means being arranged in combination with water supply means for wetting of the abrasive papers so as to operate in conjunction with the latter.

The foregoing and other objects and advantages of my invention will be apparent from the following description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings:

DESCRIPTION OF THE DRAWINGS FIG. I is a top plan view of polishing apparatus constructed in accordance with the present invention and having four supply rolls of abrasive paper and four elongated platens, one platen being aligned with each of the supply rolls;

FIG. 2 is a longitudinal sectional view, taken substantially along the line 22 of FIG. 1, showing a length of abrasive paper drawn out from a supply roll along the length of an elongated platen, clamping means for the paper being shown in released position;

FIG. 3 is an enlarged, fragmentary longitudinal sectional view similar to FIG. 2 showing the clamping means in operative position to clamp the length of abrasive paper in position on the upper surface of the corresponding platen;

FIG. 4 is a longitudinal sectional view, partly broken away, taken substantially along the line 4-4 of FIG. 2;

FIG. 5 is a transverse sectional view taken substantially along the line 55 of FIG. 3;

FIG. 6 is a transverse sectional view, taken substantially along the line 6-6 of FIG. 2, showing a single platen assembly with the clamping means in released position to permit drawing of a length of paper from a supply roll over the platen; and

FIG. 7 is a transverse sectional view, taken substantially along the line 7-7 of FIG. 3, showing a single platen assembly with the clamping means in operative clamping position to clamp a length of paper on the top surface of the platen.

Now, in order to acquaint those skilled in the art with the manner of making and using my invention, I shall describe, in conjunction with the accompanying drawings, a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, there is provided a large rectangular pan 10 having upturned walls l2, 14, I6 and 18 and a bottom 20. The pan 10 has mounted therein four

platen assemblies

22, 24, 26 and 28 (see FIG. I) which include elongated

flat platens

30, 32, 34 and 36. The pan 10 is mounted in a housing indicated generally at 38 which as shown in FIG. 2 includes a bottom wall 40, an upright front wall 42 and an upright rear wall 44. The pan is fixedly mounted in the housing 38 by a plurality of screw fasteners 46 which secure a rear end of the pan to a mounting bracket 48, and a plurality of screw fasteners 50 which secure a front end of the pan to the housing front wall 42. When in its fixed position in the housing 38, the pan 10 is inclined downwardly toward the rear of the housing as best shown in FIGS. 2 and 3. At the rear of the housing 38 there is provided a roll shaft 52 supported between a pair of

mounting plates

54 and 56 for rotatably supporting a roll R of abrasive paper. The roll R, is aligned with the platen 36 and is approximately the same width as the latter so that a length of paper may be drawn from the roll R over the top of the platen 36. In a similar manner, a shaft 58 is supported between

mounting plates

60 and 62 to carry roll R a shaft 64 is supported between

mounting plates

66 and 68 to carry roll R and a shaft 70 is supported between

mounting plates

72 and 74 to carry roll R each of the rolls being aligned with a corresponding one of the

platens

30, 32, 34 and 36. A cover plate 76 having a top portion 78 and a front portion 80 extends across the width of the housing and is pivotally mounted on a hinge rod 82. FIG. 2 shows the cover plate 76 in its closed position in covering relation to the several rolls R of abrasive paper, and it will be understood that the plate 76 may be swung upwardly about the hinge rod 82 in order to gain access to the compartment in which the rolls are stored.

The four platen assemblies 22, 24, 26 and 28 are substantially identical to one another and thus by way of example the platen assembly 28 will now be described in detail. The assembly 28 comprises a generally U- shaped paper clamp bracket 84 (see FIGS. 6 and 7) having a bottom wall 86 and

side walls

88 and 90, the clamp bracket being open at its two ends. Each of the

side walls

88 and 90 is bent inwardly at its upper end to form longitudinal

edge clamp portions

92 and 94 which serve to clamp the side edges ofa length of paper drawn over the platen 36 from the roll R as will be described more fully hereinbelow.

An expander bag 96 is positioned on top of the bottom wall 86 of the clamp bracket 84 and extends longitudinally along substantially the full length of the latter. A fiat plate 98 is positioned on top of the expander bag 96 so as to lie substantially parallel to the bottom wall 86 of the clamp bracket, with the expander bag located between the bottom wall 86 and the plate 98. The flat plate 98 is bent upwardly along its sides to form

upright guide portions

100 and 102, and it is similarly bent upwardly at its ends to form upright front and rear edge portions 104 and 106 (see FIGS. 2 and 3).

The flat plate 98 and the bottom wall 86 of clamp bracket 84 are each provided with four aligned apertures located generally in the corners thereof to receive four

post members

108, 110, 112 and 114. Each post member has a cylindrical shank portion 116 and an enlarged head 118. Thus, referring by way of example to FIG. 7, the post 108 is positioned with its head 118 overlying the top of the flat plate 98 while the shank portion 116 projects through aligned apertures in the plate 98 and bottom wall 86 of clamp bracket 84 to abut against the bottom wall of the pan 10. A screw fastener 120 extends upwardly through the wall 20 and is threaded into the shank 116 to firmly secure the post 108 to the pan bottom wall 20. In a similar manner,

each of the other three posts 110, I12 and 114 is positioned with its enlarged head in overlying relation to the plate 98 while its shank projects downwardly through the plate 98 and wall 86 and abuts against the pan bottom wall 20 to which it is secured by a screw fastener 120.

As described above, the four

posts

108, 110, 112 and 114 are fixedly secured to the bottom wall 20 of the pan l0, and it will be seen that the platen 36 rests on the tops of the heads 118 of the four posts. At least one of the posts in each of the platen assemblies, such as the post 108 in FIG. 6, is provided with an aperture extending through the entire length of the post, and an elongated screw 120 is utilized which extends into a hole in the underside of the platen 36 to locate the platen and prevent the same from sliding in the clamp bracket 84. It will further be noted from FIG. 6 that the shank portions 116 of each of the

posts

108, 110, 112 and 114 are sufficiently long to permit relative vertical movement between the bottom wall 86 of the clamp bracket 84 and the flat plate 96, such movement being effected by expansion and collapse of the expander bag 96.

When a length of paper P is drawn from the roll R the paper passes under a guide 124 (see FIGS. 2 and 3) carried on the housing wall 80, and then passes over the top of the platen 36 so that the side edges of the paper underlie the

edge clamp portions

92 and 94 of the clamp bracket 84. At the forward end of the platen 36 the paper is passed between a rod 126 and a guide plate 128 and out between the latter and a trim strip 130 as shown in FIG. 2. If desired the end of the paper strip may be pulled upwardly utilizing trim strip 130 to sever the end of the paper to form a neat edge. During the pulling of the paper along the platen 36 to advance a section of fresh paper over the platen for a polishing operation, the expander bag 96 is in its collapsed condition as shown in FIGS. 2 and 6, and under such conditions the

edge portions

92 and 94 rest freely on the side edges of the paper and do not impede the advancing movement of the paper. However, after the paper has been advanced to a desired position, expansion of the expander bag 96 will spread apart the plate 98 and the bottom wall 86 of the clamp bracket 84 as shown in FIG. 7 thereby causing the

side edges

92 and 94 of the clamp bracket to bear down firmly on the side edge portions of the length of paper P and clamp the latter against the platen 36. In the foregoing manner, the length of abrasive paper is clamped in a fixed position for purposes of a polishing operation, as will be more fully described hereinafter. While only the platen assembly 28 has been described in detail herein, it will be understood that the other platen assemblies 22, 24 and 26 are substantially identical in structure.

A water supply system for the polishing apparatus of the present invention will now be described. A water supply tube made of nylon or the like is shown at 132 (see FIGS. 2 and 4) leading to a valve 134 controlled manually by a knob 135 mounted on the front of the assembly for access by an operator. Water flowing past valve 134 is conducted by a fitting 136 to a pair of

water lines

138 and 140 best shown in FIG. 4. The supply line 140 has a fitting 142 which connects with the expander bag 96, and a fitting 144 which connects with an adjacent expander bag 960. In a similar manner, water line 138 is utilized to supply water to expander bags (not shown) associated with the

platen assemblies

22 and 24.

A water pipe 146 (see FIG. I) extends the full width of the polishing apparatus and has

plugs

148 and 150 in its opposite ends. As shown in FIG. 3, the water pipe 146 is positioned under the trailing edge of the trim strip 130 so as to lie above and immediately forwardly of the forward ends of the

platens

30, 32, 34 and 36. The water pipe 146 is provided with four groups of small spray openings 146' along its length, and each of the spray openings is directed rearwardly toward a corresponding one of the platen members. By way of example, approximately four or five small spray openings are provided adjacent to each of the four platens for the purpose of directing a spray of water onto the top of the abrasive paper P disposed on each platen. In the foregoing manner, the water pipe 146 serves to maintain each of the lengths of abrasive paper P in a wetted condition suitable for polishing, and the water sprayed on the forward ends of the lengths of abrasive paper will tend to flow rearwardly along the lengths thereof due to the downward inclination of the various platens.

FIG. 5 shows a fitting 152 and a connecting tube 154 whereby water is supplied from the supply line I40 to the water tube 146. In an identical manner, the supply hose 138 is preferably connected to the water tube 146 toward the opposite end of the latter. It will now be understood that the valve control knob [35 regulates the amount of water flowing to the four expander bags 96 and to the water pipe 146. The supply of water from the supply tube 132 flows past the valve 134 to the two

supply tubes

138 and 140. Each of the

supply tubes

138 and 140 supplies water to a corresponding pair of expander bags 96. Moreover, each

such supply tube

138 and 140 has a connection with the water pipe 146 so that the latter receives a supply of water from both of the

supply tubes

138 and 140 at separate locations along the length of the pipe 146.

It will further be understood that the only outlet for water passing the valve 134 comprises the small spray openings 146' in the spray pipe 146, since the two ends of the pipe are plugged, and the expander bags 96 provide a closed system. Accordingly, the pressure developed in the expander bags 96 is a function of the setting of the valve control knob 135, and the size and number of the spray openings I46 in the water pipe. Accordingly, in addition to the control afforded by the valve 134, it will be observed that a decrease in the size or number of the spray openings 146' in the water pipe 146 will tend to increase the pressure in the water system thereby increasing the clamping force developed by the various expander bags 96. v

The operation of the polishing apparatus described herein will now be briefly reviewed. In order to prepare the apparatus for a polishing operation using fresh abrasive paper, the various lengths of paper P are manually pulled forwardly so as to discard used paper and draw fresh abrasive paper from the rolls R R R and R After the used paper has been drawn beyond the trim strip 130, the paper may be pulled upwardly to sever the same along the forward edge of the trim strip. The foregoing operation of drawing out fresh paper over each of the four

platens

30, 32, 34 and 36 is performed with the water valve control knob 135 turned to its off position, it being understood that as long as the water valve I34 is turned off", the various expander bags 96 will be in collapsed condition as shown in FIG. 6, and thus the

edge portions

92 and 94 of the clamp bracket 84 will rest easily on the edges of the strips of paper P and permit free movement of the paper strips in an advancing direction along the corresponding platen members.

After a fresh length of paper has been drawn over each of the

platens

30, 32, 34 and 36, the water valve control knob is turned on". thereby admitting water to the

supply tubes

138 and 140. The opening of the water valve I34 causes water to be sprayed through the small spray openings [46 in the water pipe 146 onto the forward ends of each of the lengths of abrasive paper P so as to wet the latter and condition the same for the polishing of specimens. Moreover, opening of the water valve 134 automatically causes water under pressure to be supplied to each of the four expander bags 96 thereby producing in each instance a downward force on the clamp bracket 84 causing the

edge portions

92 and 94 of the latter to clamp down firmly on the side edges of the length of paper P disposed on a platen member beneath such edge portions. The manual polishing of specimens on the strips of abrasive paper may then proceed, it being observed that the wetting and clamping of each of the several lengths of abrasive paper P is effected by operation of the single valve control knob 135. Similarly, at the completion of a polishing operation, the closing of the valve control knob [35 cuts off the spray of water from the water pipe 146 and also permits the collapse of the several expander bags 96 thereby releasing the clamping pressure on the lengths of abrasive paper P.

It should be understood that the term water as used herein and in the appended claims is intended to cover normal tap water or any other suitable liquid or liquid compound desired for the wetting of the abrasive paper and the present invention is not limited to any specific liquid.

[ claim:

1. In a polishing apparatus of the type where a length of abrasive paper is drawn over a platen to provide a polishing surface for the manual polishing of specimens, the improvement comprising, in combination, clamp means positioned under said platen and having clamp members which overlie edge portions of a length of paper drawn over said platen, expander water bag means disposed under said platen in engagement with said clamp means so that when in its expanded condition said water bag means applies a downward clamping force to said clamp means, water pipe means having at least one spray opening directed at said length of abrasive paper for wetting the same, common water supply means connected with said water pipe means and with said expander water bag means, and control valve means connected with said water supply means, whereby when said control valve means is open said water pipe means will spray water on said abrasive paper and said expander water bag means will be expanded causing said clamp members to clamp said length of paper down against said platen, and when said control valve means is closed siad water pipe means will discontinue spraying and said expander water bag means will collapse relieving the clamping pressure on said length of paper to permit advancement thereof.

2. A polishing apparatus as defined in claim 1 where said expander water bag means extends longitudinally under said platen along the major portion of the length of said platen.

3. A polishing apparatus as defined in claim 1 where said clamp means comprises a generally U-shaped member which extends longitudinally along the major portion of the length of said platen in encompassing relation thereto, said clamp means having edge clamp portions which overlie edge portions of a length of paper drawn over said platen, and said expander water bag means is disposed between said platen and said U- shaped member so that when in its expanded condition it exerts a downward force on said U-shaped member.

4. A polishing apparatus as defined in claim I including a plurality of platens and associated clamp means, and a plurality of expander water bag means, one associated with each of said clamp means, said common water supply means being connected with each of said plurality of expander water bag means, whereby operation of said control valve means effects substantially simultaneous expansion and collapse of each of said expander water bag means.

5. A polishing apparatus as defined in claim 1 where said expander water bag means comprises an elongated bag which lies substantially flat when in its collapsed condition.

6. A polishing apparatus as defined in claim 1 including a pan, a plurality of post members fixed to a bottom wall of said pan and extending upwardly therefrom, said post members having shank portions with enlarged heads at their upper ends, said platen being supported on the upper surfaces of said enlarged heads, said shank portions extending through apertures in said clamp means to guide said clamp means during vertical movement thereof between its clamping and released positions, and said expander water bag means extending longitudinally between said post means.

7. A polishing apparatus as defined in claim 6 including plate means disposed within said clamp means in overlying relation thereto, the shank portions of said post members extending through said plate means and said clamp means while permitting relative vertical movement of said plate and clamp means, and said expander water bag means being disposed between said plate means and said clamp means to effect such movement upon expansion and collapse of said water bag means.

8. In a polishing apparatus of the type where lengths of abrasive paper are drawn over corresponding ones ofa plurality of platens arranged in side-by-side parallel relation to provide a plurality of polishing surfaces for the manual polishing of specimens, the improvement comprising, in combination, a plurality of clamp means, one positioned under each of said platens and each clamp means having clamp members which overlie edge portions of a length of paper drawn over a corresponding platen, a plurality of expander water bag means, one disposed under each of said platens in engagement with the corresponding clamp means so that when in its expanded condition each water bag means applies a downward clamping force to its corresponding clamp means, each of said water bag means comprising an elongated bag which lies substantially flat when in its collapsed condition and extends longitudinally under a corresponding platen along the major portion ofthe length of said platen, common water pipe means extending transversely relative to said plurality of platens and having a plurality of spray openings directed at said lengths of abrasive paper for wetting the same, common water supply means connected with said water pipe means and with each of said plurality of expander water bag means, and control valve means connected with said common water supply means, whereby when said control valve means is open said water pipe means will spray water on each of said lengths of abrasive paper, and each of said plurality of expander water bag means will be expanded causing said plurality of clamp members to clamp the corresponding lengths of paper down against a corresponding platen, and when said control valve means is closed said water pipe means will discontinue spraying and each of said plurality of expander water bag means will collapse relieving the clamping pressure on said lengths of paper to permit advancement thereof.

9. A polishing apparatus as defined in claim 8 where each of said clamp means comprises a generally U- shaped member which extends longitudinally along the major portion of the length of a corresponding platen in encompassing relation thereto, said clamp means having edge clamp portions which overlie edge portions of a length of paper drawn over said platen, and each expander water bag means being disposed between said corresponding platen and U-shaped member so that when in its expanded condition it exerts a downward force on said U-shaped member.

10. In a polishing apparatus of the type where a length of abrasive paper is drawn over a platen to provide a polishing surface for the manual polishing of specimens, including a water pipe with spray openings directed at said length of abrasive paper for wetting the same, a water supply connected with said water pipe, and a control valve for said water supply, the improvement comprising, in combination, clamp means positioned under said platen and having clamp members which overlie edge portions of a length of paper drawn over said platen, and expander water bag means disposed under said platen in engagement with said clamp means so that when in its expanded condition said water bag means applies a downward clamping force to said clamp means, said expander water bag means being connected with the water supply for said water pipe, whereby when said control valve is open the water pipe will spray water on said abrasive paper and said water bag means will be expanded causing said clamp members to clamp said length of paper down against said platen, and when said control valve is closed the water pipe will discontinue spraying and said water bag means will collapse relieving the clamping pressure on said length of paper to permit advancement thereof.

* h i l

Claims

3. A polishing apparatus as defined in claim 1 where said clamp means comprises a generally U-shaped member which extends longitudinally along the major portion of the length of said platen in encompassing relation thereto, said clamp means having edge clamp portions which overlie edge portions of a length of paper drawn over said platen, and said expander water bag means is disposed between said platen and said U-shaped member so that when in its expanded condition it exerts a downward force on said U-shaped member.

4. A polishing apparatus as defined in claim 1 including a plurality of platens and associated clamp means, and a plurality of expander water bag means, one associated with each of said clamp means, said common water supply means being connected with each of said plurality of expander water bag means, whereby operation of said control valve means effects substantially simultaneous expansion and collapse of each of said expander water bag means.

8. In a polishing apparatus of the type where lengths of abrasive paper are Drawn over corresponding ones of a plurality of platens arranged in side-by-side parallel relation to provide a plurality of polishing surfaces for the manual polishing of specimens, the improvement comprising, in combination, a plurality of clamp means, one positioned under each of said platens and each clamp means having clamp members which overlie edge portions of a length of paper drawn over a corresponding platen, a plurality of expander water bag means, one disposed under each of said platens in engagement with the corresponding clamp means so that when in its expanded condition each water bag means applies a downward clamping force to its corresponding clamp means, each of said water bag means comprising an elongated bag which lies substantially flat when in its collapsed condition and extends longitudinally under a corresponding platen along the major portion of the length of said platen, common water pipe means extending transversely relative to said plurality of platens and having a plurality of spray openings directed at said lengths of abrasive paper for wetting the same, common water supply means connected with said water pipe means and with each of said plurality of expander water bag means, and control valve means connected with said common water supply means, whereby when said control valve means is open said water pipe means will spray water on each of said lengths of abrasive paper, and each of said plurality of expander water bag means will be expanded causing said plurality of clamp members to clamp the corresponding lengths of paper down against a corresponding platen, and when said control valve means is closed said water pipe means will discontinue spraying and each of said plurality of expander water bag means will collapse relieving the clamping pressure on said lengths of paper to permit advancement thereof.

9. A polishing apparatus as defined in claim 8 where each of said clamp means comprises a generally U-shaped member which extends longitudinally along the major portion of the length of a corresponding platen in encompassing relation thereto, said clamp means having edge clamp portions which overlie edge portions of a length of paper drawn over said platen, and each expander water bag means being disposed between said corresponding platen and U-shaped member so that when in its expanded condition it exerts a downward force on said U-shaped member.