US 3463476 A
Description (OCR text may contain errors)
Aug.26, 1969 E MAR; EITAL 3,463,476
7 VACUUM CHUCKv Original Filed m. 27. 1964 2 f' f FIG. 4
a I V/// //////5 Vacuum 5O 52 Switch I Vocuum Pump W M f;
ATTORNEYS FIG. 3 jg L '89 United States Patent 3,463,476 VACUUM CHUCK John De Maria, Providence, R.I., and Harlan A.
Bentzinger, Rehoboth, Mass., assignors to Chemical Products Corporation, East Providence, R.I., a corporation of Rhode Island Continuation of application Ser. No. 347,748, Feb. 27, 1964. This application Apr. 18, 1966, Ser. No. 554,921 Int. Cl. BZSb 11/00; B4112 27/00 US. Cl. 269-21 4 Claims ABSTRACT OF THE DISCLOSURE A vacuum base for holding a sheet on a printing press has a surface divided by a continuous groove into a smooth marginal portion and a roughened interior portion. Holes within the groove and throughout the interior surface portion are evacuated to obtain a tight seal about the edges of the sheet that overlie the margin, while the roughened interior surface portion frictionally resists creeping of the sheet relative to the base.
This is a continuation of our copending application Ser. No. 347,748, filed Feb. 27, 1964, now abandoned.
Our invention relates generally to an improved vacuum base or chuck. In particular, it concerns an improved vacuum base for use with a printing press, whereby a mounting sheet for a printing plate is held more securely and effectively to the vacuum base during the printing operation.
A principal object of this invention is to provide increased resistance to sliding or creep of the sheet on the vacuum base due to forces acting upon the sheet during press operation.
Vacuum bases or chucks are employed with various types of printing presses, particularly with flatbed printing presses, to hold a mounting sheet bearing one or more printing plates securely to the reciprocating press bed during the printing operation. One form of conventional vacuum base comprises a closed rectangular box havin a flat upper surface, the interior of which forms an air compartment in communication with a vacuum source, usually through a flexible hose conduit extending from one end of the base. A flat holding surface on the base has a series of drilled and countersunk holes of uniform size and distribution communicating with the base interior, whereby upon application of vacuum the mounting sheet is held in place on the base surface. A conventional system may also include a vacuum-sensing switch whereby, upon a predetermined substantial loss of vacuum such as occurs when the mounting sheet is stripped from the vacuum base, the vacuum-sensing switch deenergizes the press drive means. This safety measure prevents or minimizes damage to the press or to the plates when the mounting sheet is picked up off the vacuum base by the rotating impression cylinder, thus preventing malfunctioning of the printing press.
This conventional vacuum base has many disadvantages, particularly where the mounting sheet is in the form of a thin, flexible sheet bearing one or more printing plates. For example, the spaced holes provide maximum holding power in the center of the mounting sheet where the leak rate about the holes is the lowest, but relatively poor holding force along and near the edges of the mounting sheet. The result is a tendency of the mounting sheet to creep during press operation. The high leak rate near the edges of the sheet also reduces the holding force on these edges and often permits the mounting sheet edges to curl, to be stripped from the base, and to wrap around the impression cylinder of the press.
3,463,476 Patented Aug. 26, 1969 Another disadvantage of this conventional vacuum base is the slow rate of reduction of the vacuum when the mounting sheet is unintentionally stripped from the vacuum base. Rapid activation of the vacuum-sensing switch and stopping of the printing operation are most desirable under these circumstances in order to minimize damage to the press, the mounting sheet and the printing plates. 0n the other hand, the rapidity with which the vacuum is lost and the vacuum-sensing switch is activated depend on the open area exposed, i.e. on the number and size of holes exposed. The employment of very large holes in the base is not a wholly satisfactory solution to this problem, since the leak rate is correspondingly higher. Larger holes also may produce local depressions when using flexible plastic printing plates and plastic mounting sheets, which adversely affect the printing quality of the plates. Additionally, the vacuum base requires a relatively large and expensive mechanical pump to maintain effective vacuum and holding force on the mounting sheet, in View of the above considerations.
Another conventional approach toward overcoming these problems is to provide a gasket between the mounting sheet, near its edges, and the vacuum base. This helps to reduce the leak rate near the edges and to assist in preventing the edges from lifting away from the base. However, the gasket is subject to damage and its use is inconvenient.
It is, therefore, an object of our invention to provide an improved vacuum base for holding sheet material in a flat plane, which base has a low leak rate and better holding ability particularly at the edges.
Another object is to provide an improved vacuum base whereby a rapid and large loss of vacuum occurs upon a relatively minor displacement of the mounting sheet from the surface of the vacuum base, thereby preventing or minimizing damage to the printing press and the mounting sheet.
A further object is to provide an improved vacuum base which operates effectively at a lower vacuum level than that required by a conventional base, and which requires a simpler and less expensive vacuum source than the type required by present vacuum bases.
Another object is to provide an improved vacuum base that significantly reduces the make-ready time for printing press operation.
With the foregoing and other objects hereinafter appearing in view, the features of this invention include a novel vacuum base having a groove that defines a smooth sealing margin and a roughened interior surface, the groove being connected to a source of vacuum. The groove holds the vacuum very efliciently near the edges of the mounting sheet where the leakage between the sealing margin and the mounting sheet occurs. We have discovered that with such a construction, if the margin is smooth and close-fitting with the mounting sheet, the interior surface is held by a strong normal force very firmly against the mounting sheet. Consequently, by roughening the interior surface to a certain degree of roughness a very large frictional resistance can be created against creeping forces acting on the mounting sheet and tending to shift it over the surface of the vacuum base, even when the vacuum pump is substantially smaller than those now employed in conventional systems. We have also found that this can be accomplished without causing local depressions or distortions in the plates.
Further features include the achievement of better edge holding power with this smaller vacuum pump, the retention of the quick vacuum release feature, and the elimination of any need for a gasket between the mounting sheet and the vacuum base.
Other features reside in certain details of the vacuum base construction and in modes of operation that will be understood from the following description of a preferred embodiment of the invention, having reference to the appended drawings in which:
FIG. 1 is a fragmentary plan view of our vacuum base;
FIG. 2 is a fragmentary bottom view of our vacuum base without the bottom cover plate, including a fragmentary view of a portion of the base with the bottom cover plate;
FIG. 3 is an enlarged cross sectional view of a portion of our vacuum base along the lines 3-3 of FIGS. 1 and 2 and including the bottom cover plate and a mounting sheet with printing plates, mounted on the vacuum base surface; and
FIG. 4 is a schematic diagram of our vacuum base employed in flat bed printing press.
FIGS. 1 and 2 show our vacuum chuck including a rigid rectangular, relatively thick, base 12 which is smaller in size and shape than the bed of the printing press upon which the vacuum base is to be securely mounted. The base 12 has a flat upper interior surface 14 containing a series of relatively small holes 16 drilled therein and uniformly distributed throughout the surface, for example on 4-inch centers. The holes communicate with a series of parallel longitudinal channels or grooves 18, 18a, etc., in the bottom of the base 12, which channels are directly below the holes and run across the base 12. The channels 18, etc. are interconnected at both ends and communicate with a channel 26 and a larger main channel 20 which run along the ends of the base 12. A pipe 24 is tapped into a flange 22 on the base 12 and provides communication by means of a flexible vacuum hose 25 (FIG. 4) between the vacuum source (not shown) and the main channel 20. In the embodiment shown, channels 18 and 18a extending across the center of the base 12 are somewhat larger than the other channels.
The particular size of the channels is determined by the vacuum holding force which is desired in the system. In the embodiment illustrated in FIGS. 1 and 2, channels are grooved in the bottom side of the base 12 in a parallel arrangement and each channel communicates at each end with an end channel of the base 12. The main channel 20 of greater size is employed to permit a rapid removal of air from the interconnected channels communicating with the holes on the surface of the base 10. Additional other larger channels such as the center channels 18 and 18a may be employed as desired.
The base 12 further has on its upper surface a recessed circumferential gutter or groove 28 which is spaced inwardly a short distance from the outer edges. Equally spaced and uniformly distributed about and within the base of the groove 28, are a series of relatively large diameter holes 30, for example on 2-inch centers, preferably with additional holes at each corner of the base 12. The holes 30 are drilled in the bottom of the gutter 28 and are of larger size than the holes 16 located within the interior surface area 14 outlined by the gutter 28, since the latter holes must be sulficiently small to prevent the formation of depressions in the plates when a vacuum is drawn. The holes 30 are in communication with the peripheral channels 20, 18/1, 26, etc. on the undersurface of the base.
Between the gutter 28 and the outer edge of the base 12 is a narrow, flat, highly polished margin 32 about the entire circumference of the upper surface of the frame 12. This margin is ground, polished, stoned or otherwise finished to a highly smooth, flat finished surface. We have found it particularly satisfactory to stone the margin 32 by lengthwise stoning until the finished grinding marks have disappeared. The purpose of the margin 32 is to provide a surface which easily forms a good sealing relationship with the edge surface of a mounting sheet placed on the vacuum base.
Within the outlines of the gutters 28 is the surface 14- which has a high co-efficient of friction with the mounting sheet material. Depending upon the degree of roughness desired in this surface area, it may be the unfinished or unpolished surface of the original base material, such as base aluminum alloy; but preferably the surface is roughened by sandpapering, grinding or other means to produce an uneven, rough surface finish through out its area, as shown in exaggerated form in FIG. 3. The roughness of the interior surface 14 should be suflicient to prevent or inhibit the slippage or skidding of the mounting sheet on the surface due to any differences in the relative speeds between the printing cylinder and the vacuum base in the printing operation. We have found that a roughened surface in this area with a very high co-efficient of friction requires less vacuum holding force to retain the mounting sheet on the vacuum base. For example, we have found that a satisfactory surface may be prepared for use with polyester or plastic mounting sheets by grinding it with a Blanchard grinding machine to produce a surface finish between A thin, flat cover plate 34 is secured over the entire bottom of the base 12 to seal the channels. An air tight seal is provided by employing a sealing or gasketing material such as an epoxy resin about the periphery of the base 12 and the plate 34. Prior to attaching the bottom plate 34, all vacuum channels should be free of dirt, burrs, slivers and loose particles.
If desired, the thin cover plate 34 may be omitted and a gasketing material such as rubber or a closed cell vinyl plastisol gasketing strip may be attached between the lower circumferential edge of the base 12 outside of the periph eral channels of the base and a flat continuous surface of the press bed. In this manner the channels are sealed and air tight when the vacuum base is placed on such surface.
FIG. 3 shows a fragmentary cross section of our vacuum base taken along the lines 33 of FIGS. 1 and 2. FIG. 3 shows a mounting sheet assembly 38 comprising a thin, flexible mounting sheet 40 such as a plastic sheet or some other sheet material like paper, metal, etc., which is air-impermeable or has a low air-permeability rate, and a series of printing plates 42 having character matrices on the surfaces thereof, securely fastened to the sheet 40. Although we have described the use of our vacuum base with a flexible mounting sheet with plastic printing plates it will be recognized that our vacuum base may be advantageously employed either with other flexible plates or with rigid printing plates and mounting sheets such 'as stereotypes, electrotypes, thermosetting plates, photoengravings, etc. The mounting sheet 40 is initially cut to size so that its outer peripheral edges extend slightly beyond the gutters 28 and are placed in a tight, flat, air-sealing relationship with the polished margin 32. In this manner a continuous air seal with a very low leak rate is provided around the entire edge of the sheet 40. Our margin 32 permits a uniform and constant holding force against the edge of the sheet 40 and inhibits 0r prevents the edges from being released or curled. The polished margin 32, by considerably reducing the leak rate, permits the employment of much smaller vacuum sources. For example, with our vacuum base a mechanical pump is not absolutely required, and a less expensive rotary vane-type pump with a capacity of 7 /2 cubic feet per minutes is entirely satisfactory. Conventional vacuum bases often require a vacuum of 20 inches or more, and usually more than 27 inches, to be maintained in order to provide a suificient holding force for the mounting sheet and to overcome the greater air leaks. Our vacuum base with the roughened surface 14, the polished margin 32 and the gutter 28 is satisfactory for most purposes with a lower vacuum level of from 10 to 15 inches.
Additionally, the gutter 28 permits any air which leaks between the edge surface of the sheet 40 and the margin 32 to be rapidly and efliciently removed at or near the place where the leakage occurs, to prevent its affecting a greater area. The holes 30 in the gutters 28 are of a size sufficiently large to permit a rapid withdrawal of any air leaking into the gutter 28. We have found that a polished margin about one-half inch in width and a gutter 28 one-eigth inch in width and one-quarter inch deep, with holes 30 of about one-sixteenth of an inch, are quite satisfactory. As illustrated in FIGS. 1 and 2, we prefer that the gutter holes 30 be spaced equally distant on two-inch centers. We prefer also that a gutter hole 30 be present in each corner of our base 12, to provide additional holding force in these areas. Vacuum channels have been found to be satisfactory in the operations described herein when they are approximately 0.375 inch wide and 0.25 inch deep with the larger main and center channels, such as the channel 20, being about one-inch wide and one-quarter inch deep.
In operation, the mounting sheet assembly 38 having the base sheet 40 is placed on our vacuum base 10 and smoothed out so that the peripheral edges fall on the polished margin 32 as previously described. The printing plates 42 mounted on the base sheet 40 are then located within the roughened interior surface area 14. The roughened surface prevents slippage or skidding of the mounting sheet during the printing operation, which slippage is due to the difference in relative speeds of the printing cylinder and the vacuum base 10. The surface 14, having a high coeflicient of friction, prevents move ment of the mounting sheet assembly 38 during the printing operation, and thereby markedly reduces the required vacuum holding force.
As noted above, the holes 16 drilled in the roughened surface 14 are relatively small holes of up to about oneeighth inch, typically & of an inch. These small holes provide a vacuum holding force sufiicient to hold the mounting sheet to the roughened surface 14, and yet are not large enough to affect printing quality by causing depressions in the plastic printing sheet about the outer edges of the holes. The holes 16 are of smaller size than the holes 30, but of sufficient size to permit removal of small amounts of air trapped between small areas of the sheet and the vacuum base surface 14 when the sheet 38 is placed on the base.
FIG. 4 is a schematic diagram of our vacuum base 10 mounted on the bed 44 of a fiat bed printing press having a rotating printing cylinder 46. A printing plate mounting sheet assembly 38 is shown on the upper surface of the vacuum base in position for the printing operation. The vacuum base communicates through the flexible conduit 25 with a vacuum pump 48. A vacuum-sensing switch 50 and a vacuum gage 52 are connected to the conduit. The vacuum-sensing switch may be electrically connected with the driving means of the printing press (not shown) so that upon its actuation by the rapid and sudden predetermined loss of vacuum in the vacuum base 10, the printing press drive mechanisms or the printing press operation will be immediately stopped. Our v'acuum base 10 containing the gutter 28 has the additional safety feature which permits a very rapid drop of vacuum to occur when the mounting sheet 38 is beginning to be stripped or removed from the vacuum base 10. Unlike previous vacuum bases, there is no progressive loss of vacuum upon the gradual stripping of the sheet 38. Rather, if and when either the mounting sheet edges are stripped from the vacuum base 10, or there is any movement from the proper sheet position, there is a rapid drop of Vacuum due to the exposure of at least a portion of the gutter 28 and the relatively large diameter holes 30. This rapid drop in vacuum quickly activates the vacuum switch and denergizes the printing press operations.
A further feature of our vacuum base is that the sizes of the holes in the gutters are so selected as to permit a vacuum drop sufficient to activate the switch 50, but insufiicient to lose completely all vacuum holding force on the mounting sheet assembly 38 unless all or a substantial part of the mounting sheet is exposed. For example, if a corner of the mounting sheet is lifted from the vacuum base 10 exposing three or four holes 30, the drop in vacuum would be sutficient to actuate the vacuum switch 50; however, suflicient vacuum holding force would still be retained along the remaining outer edges and the holes 16 on the surface 14 of the vacuum base 10; that is, a residual pressure force would be retained on the remaining part of the mounting sheet.
We have thus described our improved vacuum base which may be employed to retain sheet materials, and which is particularly useful in printing operations. Although our description has been directed to the use of our vacuum base in a flat bed printing press, it will be recognized that its particular distinguishing structural features may be suitably employed in a rotary press or in any operation which requires a sheet material to be securely held to a base. Our vacuum base avoids many of the difiiculties associated with prior vacuum bases employed in the printing art, and in addition it has superior safety and efiiciency.
While the invention has been disclosed above in a preferred embodiment, it will be evident to those skilled in the art that various modifications and adaptations of the parts can be employed without departing from the spirit or scope thereof.
Having thus described our invention, we claim:
1. A vacuum base for holding a sheet including, in combination:
a base plate having in a first side thereof a continuous groove defining a continuous circumferential margin surface and an interior surface, the margin surface being polished and the interior surface being roughened sufficiently to produce a high coefficient of friction with said sheet, said plate having a plurality of holes in the groove and a plurality of holes distributed over the interior surface, the holes passing through the base plate, and
means cooperating with the opposite side of the base plate for connecting said holes with a common source of vacuum.
2. The combination according to claim 1, in which the finish of the interior surface is between 32 R.M.S. and 63 R.M.S.
3. The combination according to claim 1, in which the finish of the interior surface is between 32 R.M.S. and 63 R.M.S. and the holes in the groove are larger in diameter than the holes in the interior surface.
4. The combination according to claim 1, in which the last mentioned means comprise a second plate sealed to the second side of the base plate.
References Cited UNITED STATES PATENTS 687,677 11/1901 Du Brul 26921 2,590,044 3/ 1952 Schaller. 2,782,574 2/1957 Copold 269-21 X 2,936,139 5/1960 Lindstrom 269-2l X ROBERT C. RIORDON, Primary Examiner D. R. MELTON, Assistant Examiner U.S. Cl. X.R. 101-382