US3496046A

US3496046A - Engraving devices for printing cylinders

Info

Publication number: US3496046A
Application number: US513273A
Authority: US
Inventors: Joseph Charles Cornil Lootvoet
Original assignee: NATIONALE D EDITIONS ARTISTIQU
Current assignee: NATIONALE D EDITIONS ARTISTIQU; Nationale D'editions Artistiques Nea soc
Priority date: 1964-12-14
Filing date: 1965-12-13
Publication date: 1970-02-17
Anticipated expiration: 1987-02-17
Also published as: NL6516071A; FR86998E; BE673002A; GB1129675A

Description

I Feb. 17, 1970 J. c. c. LOOTV'OET 3,496,046

ENGRAVING DEVICES FOR PRINTING CYLINDERS Filed Dec. 13, 1965 2 Sheets-Sheet 1:

19 70 J.IC.I C. LOOTV-OET 3, 5

ENGRAVING DEVICES FOR PRINTING CYLINDERS Filed Dec. 13, 1965 2 sheets-sheet 2 United States Patent 6) 3,496,046 ENGRAVING DEVICES FOR PRINTING CYLINDERS Joseph Charles Cornille Lootvoet, Issy-les-Moulineaux,

France, assignor t Societe Nationale dEditions Artistiques N.E.A., Lille, France, a society of France Filed Dec. 13, 1965, Ser. No. 513,273 Claims priority, application France, Dec. 14, 1964, 998,505 Int. Cl. C23f 1/08 US. Cl. 156345 12 Claims ABSTRACT OF THE DHSCLOSURE The present invention relates to improvements in devices for the chemical engraving of cylindrical surfaces intended to be used for printing purposes, this engraving being performed by gradually attacking said surfaces with a liquid mordant, the composition of which varies during the engraving operation, through especially prepared sheets partly permeable to said mordant.

The invention is more especially concerned with engraving devices including: a tank containing the liquid mordant; means for supporting the cylindrical surface to be engraved, the axis of which is horizontal and lined with its partly permeable sheet, above said tank; means for rotating said surface about its axis; a pump for circulating the liquid mordant; a suction pipe filled with said mordant and connecting the tank with the input of the pump; a discharge pipe filled with said mordant and connecting the output of the pump with the tank; and means for gradually modifying one of the factors capable of varying the action of the mordant during engraving, in particular by gradually introducing into the above mentioned suction pipe an additional liquid of predetermined flow rate and composition.

The chief object of the present invention i to facilitate the determination, at any desired time, of the optimum modification to be brought at this time to the factors which control the action of the mordant, and preferably automatically to perform this optimum modification.

e invention consists chiefly in further providing the devices of the kind in question with programming means capable of generating, at least at a plurality of successive times during the engraving operation, an indication concerning the assigned degree of engraving that should be attained at these times; means for detecting the actual degree of engraving at said times; means for comparing, at any of these times, these two degrees of engraving, to wit the assigned one and the actual one respectively; and means responsive to the results of this comparison for automatically correcting at said times at least one of the factors capable of influencing engraving, in particular the degree of dilution of the mordant, in a manner to tend to make the actual degree of advancement of the engraving operation at successive times identical to the assigned degree of advancement of this operation at the same times, respectively.

The invention is based upon the fact that the color of the metallic surface and hence of every area (consisting of a metallic surface element lined with a portion of a partly permeable sheet) varies considerably within a 3,496,046 Patented Feb. 17, 1970 relatively short time (from 10 to 30 seconds) at the time the engraving of this area starts. In a preferred case, where said metallic surface is of copper, where the semipermeable sheet is a sheet of tanned bichromate gelatin provided with a network and where the mordant is an aqueous solution of iron perchloride, this color shifts from light red to coal grey at said time.

The detecting means preferably comprise: in the partly permeable sheet, a series of areas of decreasing permeabilities, this series of areas extending along one of the generatrices of the partly permeable sheet applied on the cylindrical surface to be engraved, said areas being intended to produce, in said surface to be engraved, a series of recesses of respective depth, themselves intended to produce prints of lighter and lighter shades; an aiming instrument adapted to permit the illumination of the area located in the aiming field thereof to be appreciated; means for mounting this instrument in such a manner that it is given a translatory displacement parallel to the series of areas so that it is directed successively on said areas; and means, either manual or automatic, for placing at every time the aiming instrument in a position where it is directed toward that of said areas where there is produced at this time the variation of illumination which shows that it is beginning to be engraved.

According to this preferred embodiment of the present invention, comparison of the actual and assigned degrees of engraving corresponds to the determination of a magnitude indicative of the difference between the position actually occupied by the aiming instrument and the assigned position it should occupy.

It would also be possible, according to a second embodiment of the invention, to make use of detective means of the type above described but wherein the aiming instrument, instead of following the actual progression of engraving, would be driven in accordance with the assigned program, the desired comparison at a given time being between the actual illumination of the area toward which the aiming instrument is directed at this time and the illumination of an area the engraving of which i at least started.

But the first embodiment is to be preferred because the corrective term generated from the determination of the above mentioned dilference may have several different values and permits to be taken into account the actual degree of error by ordering a correction the greater as this degree is higher.

On the contrary with the second embodiment of the invention, the corrective term that is generated gives a mere indication of the direction in which the error has taken place, without making it possible to appreciate its importance, whereby regulation then depends upon the frequency of the comparisons.

In either embodiment, two cases may be considered for appreciating and following the actual evolution of engraving along the series of areas, according as this appreciation is performed by the operator or on the contrary ensured automatically.

In the first case, the aiming instrument is a magnifying instrument permitting visual observation, such as a microscope, and this instrument is advantageously mounted upon a sliding structure driven, parallelly to the axis of the cylindrical surface, according to a predetermined assigned program in such a manner that, if the degree of advancement of engraving is correct at a given time, the aiming axis of the instrument is directed at this time toward the area of the series for which engraving has just started; the mounting of said instrument upon said structure and is however such that, in case of incorrect engraving (that is to say too fast or too slow engraving),

the operator can easily displace this instrument into a position such that he is looking at the area where engraving is starting and thus determine the degree by which engraving goes too fast or too slow, means being provided for making use of these displacements of the instrument with respect to the sliding structure to control the desired correction, According to a preferred embodiment, these displacements automatically control variable rate pumps ghiich permit the composition of the mordant to be modi- In the second case above referred to, the aiming instrument is an illumination detecting system such as a photo-electric cell and it is advantageous to proceed as follows in order to avoid the necessity of temporarily stopping the cylindrical surface to detect a misadjustment. The detecting system is associated with a locking device which permits such an illumination detecting system to be made operative only during the short times for which one of the areas of the above mentioned series passes in front of it.

In this second case of the first above mentioned embodiment, said detecting system is further associated with means for generating, at every time it is operative, an electric control signal if the illumination of the area that is then being aimed at drops below a predetermined limit value as a consequence of the beginning of engraving, this signai serving to displace the aiming instrument in the direction of the areas of lighter color.

Preferred embodiments of the present invention will be hereinafter described with reference to the appended drawings, given merely by way of example, and in which:

FIG. 1 diagrammatically shows an engraving device according to the present invention corresponding to the first case of the first embodiment above referred to;

FIG, 2 diagrammatically shows another engraving device made according to the second case of said first embodiment;

FIGS. 3 and 4 diagrammatically show two modifications of a detail of the device of FIG. 2;

FIGS. 5 and 6 diagrammatically show two engraving devices according to the invention corresponding to the second case of the above mentioned first embodiment.

Use is made of a cylinder 1 lined on its external surface with an especally prepared partly permeable sheet 2 (preferably obtained by photography through both a network and a transparent positive image of the design to be engraved). Cylinder 1 is rotatable about its horizontal axis, above a tank 3 containing the mordant. A lower point of this tank is connected through a suction pipe 6 with the input 7 of a pump 8 the output 9 of which is connected with said tank through a delivery pipe 10.

Two pumps (or groups of pumps) the outputs of which open into pipe 6, are diagrammatically shown at and 12. Pump 25 supplies a liquid capable of reducing the speed of passage of the mordant through sheet 2. Pump 12 supplies a liquid capable of accelerating this passage. The delaying liquid may be for instance concentrated iron perchloride and the accelerating liquid may be water in the preferred case where the mordant is an aqueous solution of iron perchloride.

In known installations, in order to control the variations of composition of the mordant, the operator had to stop the rotation of cylinder 1 and to examine through a magnifying glass the state of advancement of engraving. He then decided, according to what he had observed, whether it was necessary to add an accelerating or delaying liquid to the mordant. In view of the essentially subjective character of the appreciation of said state of advancement it was difficult to obtain reproducible results, and this was susceptible of very detrimental consequences, for instance in the case of polychrome printing obtained by superposing several monochrome prints. In this case, too strong an engraving of one of the printing cylinders produces too great an intensity of the corresponding color in the color print, or again in the case of the mere printing of a picture, even a monochrome one, different inten- 4 sity tints, in which case a modification in the original distribution of the tints may lead to too sharp, or too flat a reproduction.

The invention permits of obviating this drawback by reducing the difficulty of this appreciation by the operator or even by making it unnecessary.

For this purpose, there is prepared either in sheet 2 or along it, a series of areas 26 of decreasing permeability for the mordant (this decrease being either continuous or discontinuous), this juxtaposition of areas extending along one of the generatrices of sheet 2 mounted on cylinder 1. A program of engraving is prepared by a preliminary study of a plurality of complete cycles of engraving, this program determining the assigned variations of composition of the mordant as a function of time for given conditions of engraving (such as the nature of the mordant, the temperature and the hygrometry of the room where the device is located).

To this assigned program must correspond a well determined law of advancement of engraving along the succession of areas 26, this law representing the evolution of the engraving as a function of time. But, in practice, this is not always the case because of variable factors which cannot be controlled such as the sensitivity of the photosensitive layer to the action of light or the nature and thickness of this iayer.

It can easily be checked whether this law of advancement is correctly complied with due to the fact that, for every area (or portion of area)--and whatever be the depth of the recess that corersponds theretothe beginning of engraving is disclosed by a very noticeable change of color, which passes from light red to coal grey within some seconds (for instance 20 seconds) in the preferred case of a copper cylinder attacked by means of iron perchloride.

Some devices according to the invention will now be described, these devices permitting of checking whether the actual evolution of engraving takes place according to the assigned program and of producing the necessary corrections if this is not the case.

The first device (FIG. 1) comprises a microscope 27 for successively observing the line of areas 26, this microscope being mounted on a carriage 28. Carriage 28 is itself movable in a direction parallel to the axis of cylinder 1 along slideways 28 belonging to a fixed frame 30. These displacements are controlled, according to the assigned theoretical law above referred to, by the rotations of a screw 31 cooperating with a nut 32 fixed with respect to the carriage, these rotations being produced by a motor 33 through the intermediary of a speed reducing gear 34. The speed of revolution of this motor 33 varies automatically as a function of the position of carriage 28 and, for this purpose, said carriage 28 is provided with a finger 35 adapted to push successively push-buttons 36 mounted on frame 30 and disposed along the path of travel of said finger 35. These push-buttons control an electrical programming system 37 made according to the above mentioned law and therefore capable of adjusting said speed of rotation of motor 33. Finally suitable means 38 constantly control pump 12 in such a manner as to send at the desired times through pipe 6 the volumes of accelerating liquid defined in the above mentioned assigned program.

The operation of this device is as follows:

At suitabie time intervals, the operator stops cylinder 1 in such manner as to have the line of areas 26 located along the aiming axis of microscope 27.

If all things work according to the theoretical program, engraving is started in the area 26 which is then aimed at by the microscope. In this case the engraving cycle may be pursued without any particular action of the operator,

If, on the contrary, engraving is already performed in the area that the operator is observing through the microscope, or if it has not yet started, a correction must take place.

For this purpose, microscope 27 is pivotally mounted on carriage 28, so that the axis of said microscope can move along the series of areas 26. Therefore the operator can bring the microscope into the position where its axis intersects the area where engraving is actually being started. In the embodiment of FIG. 1, engraving is lagging by two areas with respect to its assigned evolution and dot-and-dash lines indicate the position of the microscope, which can be read on a graduation 39.

The operator can then manually control the modification of composition of the mordant necessary for restoring the conditions of the program. It sufiices to act upon one of the

pumps

12 and 25 in accordance with the magnitude of the difference between the actual position of the microscope and the position it should occupy.

According to a preferred embodiment, the olfsetting of the microscope may be used through automatic means for producing the desired correction. For instance this offsetting, which can easily be converted into a potential difference by means of a potentiometer, serves to control pumps having a variable output and driven by direct current motors as indicated by arrows 40.

In the construction illustrated by FIG. 2 the possible variations of illumination of areas 26 are directly recorded by a photo-electric cell 41, which eliminates any necessity of visual observation.

If the photo-electric cell is not very sensitive, it is difficult to use it in satisfactory conditions by making it responsive merely to absolute value of illumination of the areas since, for a given state of engraving of said areas, said values may vary within substantially wide limits, in accordance for instance with the local thickness or transparency of the partly permeable sheet and with the color of the mordant. This is why it may be advantageous to use the photo-electric cell not to detect absolute values but relative values of illumination by comparing the illumination of a portion of an area 26 subjected to engraving with that of another portion of the same area subjected to the same local conditions of illumination as said first portion but which cannot be attacked by the mordant.

Such an arrangement permits of wholly eliminating the influence of the thickness of the partly permeable layer and that of the color of the mordant on detection, the illuminations of the two portions of areas that are compared being similarly influenced by these two factors.

According to a preferred embodiment, there is applied, upOn the set of areas 26, at least one narrow and thin strip 42 (FIGS. 3 and 4) of a varnish which is transparent to light and impermeable to the mordant. In the case of FIG. 3, two such strips, located on either side of a narrow strip subjected to engraving, have been provided. In the case of FIG. 4, only one strip of varnish 42 is disposed in the middle part of the width of areas 26, so that two strips responsive to engraving are located on either side thereof.

In order to pass in front of photo-electric cell 41 the area portions thus prepared, it suffices to rotate cylinder 1 about its axis. In order to avoid that, during these rotations the cell delivers undesirable signals corresponding to the surface portions located on the outside of areas 26, said cell is made to operate only at the times when said areas pass in front of it. For instance, there is interposed, in the feed circuit of this cell (which circuit comprises a stabilized voltage source 43) a switch 44 closed only at said times through rotating means 45 driven by cylinder 1.

The arrangement diagrammatically illustrated by FIG. 2 further comprises a system 46 capable of driving photoelectric cell 41 in a direction parallel to the axis of cylinder 1, an amplifier 47 correcting and amplifying the signals generated by said cell, an oscilloscope 48 capable of reproducing the signals 49 thus amplified and an electronic control device 50 fed on the one hand with said amplified signals, and on the other hand from the signals coming from a programming device 51.

The operation of this device is as follows:

Initially, photo-electric cell 41 is adjusted so that its beam 52 (FIGS. 3 and 4), which is of smaller Width than that of the areas 26 to be scanned, cooperates with the end of the series of areas 26 where the permeability is maximum. Cylinder 1 is rotated and wetted with a mordant of suitable composition (for instance an aqueous solution of iron perchloride of a strength corresponding to 43 Baum degrees) and programming device 51 is started.

The three area portions 26 scanned by the photo-electric cell on every revolution of cylinder 1 then have the same color and the same coefi'icient of reflection since the three of them are moistened with a thin film of mordant, and the cell transmits no signal.

After some minutes, the coloration of the portion (FIG. 3) or portions (FIG. 4) of the scanned area 26 that is, or are, subjected to the action of the mordant changes, within some seconds, creating a sharp contrast with the portions (FIG. 3) or portion (FIG. 4) of said area 26 that are, or is, not sensitive to this attack, which produces an electric pulse at the output of the cell.

This pulse, visible at 49 in the oscilloscope after amplification is either negative or positive according as use is made either of the arrangement of FIG. 3 or that of FIG. 4.

When the intensity of this pulse exceeds a given limit value, this means that engraving has been started for the area that is scanned. System 50 then makes use of this pulse on the one hand for controlling, through channel 53, feed system 46 until cell 41 scans an area 26 that has not yet been attacked by the engraving liquid (which cancels said pulse) and on the other hand for energizing, according to the instructions then received from programming device 51, one or the other (or none) of two time relays 54 and 55 which respectively control the operation of

pumps

12 and 25 during predetermined periods.

The instructions received from programming device 51 may be in the form of voltage for releasing relays 54 and 55, said voltages being generated at any time by comparison between the actual position of cell 41 at this time (retransmitted through channel 56) and the assigned position that it should then occupy according to the preestablished program).

In each of the embodiments of FIGS. 5 and 6 the aiming instrument is a photo-electric cell 41 sensitive merely to the absolute values of illumination (in these embodiments the series of areas 26 is not divided into adjacent longitudinal strips). In this case also, this cell -41 is associated with suitable illuminating means and locking mean which neutralize the indications thereof as long as the strip 26 of the rotating cylinder is not located opposite said cell. Photo-electric cell 41 generates electric control signals as soon as the area of said strip 26 that is being scanned changes color (Which indicates that its engraving has started).

The electric signals thus generated in an electronic system are used differently in the respective embodiments of FIGS. 5 and 6.

In the example of FIG. 5, the mounting of cell 41 is an improvement of the mounting of the microscope 27 of FIG. 1. This embodiment includes carriage 28, slideways 29, frame 30, screw 31, nut 32, motor 33, speed reducing gear 34, finger 35, push-buttons 36, programming means 37 and control means 38. The photo-electric cell is carried by a lever 57 pivoted about an axis 58 fixed with respect to carriage 28 and perpendicular to the direction in which said carriage is moving. The angular displacements of lever 57 about axis 58 are used to correct the engraving conditions through a potentiometer 59 the sliding element of which is actuated by said lever 57 and through a calculator 60.

But whereas, in the construction of FIG. 1, it was necessary manually to adjust the microscope, after stopping 7 the cylinder, in the embodiment of FIG. 5, the setting of cell 41 is automatic.

Frame 30 supports a speed reducing motor unit 61 capable of rotating a screw 62 parallel to screw 31 and which cooperates with a nut 63 mounted on lever 57. The mounting of nut 63 on lever 57 and the member 64 for connection of screw 62 with the output shaft of unit 61 are such that the displacements of lever 57 about its axis 58 take place without wedging.

The electric control signals generated in unit 65 are applied to unit 61 in such a manner that every time such a signal is applied it causes screw 62 to turn for a short time (for instance four seconds) in the direction for which cell 41 moves toward the lighter colored areas of strip 26.

It follows that, every time the area located opposite cell 41 shifts in color (which indicates that it begins being engraved), this cell is jerked toward the light colored areas, that is to say toward the right in FIG. 5, lever 57 being then driven at 63 in this direction. Then, as the point 63 of lever 57 is again immobilized whereas the point 58 of said lever keeps moving toward the right according to the program, cell 41 gradually moves toward the left until it is located opposite a sufficiently dark area of 26, which causes it to jerk once more toward the right, and so on.

Cell 41 thus automatically follows the progression of engraving along the strip of areas 26 with small oscillations on either side of this progression.

In order to avoid having recourse to a potentiometer 59 in the form of a circular arc having its center on axis 58, it is advantageous to make use of a rectilinear potentiometer over or through which passes the sliding element of lever 57 and flexibly mounted so as to be able to undergo the slight displacements imposed by this sliding element during the angular displacements of the lever. Alternatively said sliding element itself may be flexibly mounted on the lever and the potentiometer may be fixed.

The locking means which neutralize photo-electric cell 41 (or rather the information issuing from this cell) as long as the strip of areas 26 is not located opposite said cell advantageously consist of another photo-electric cell 66 which is illuminated by a light source 67, through a screen '68 provided with a hole and rigid with cylinder 1, only at the times corresponding to the useful angular positions of said strip.

In the example of FIG. 6, the arrangement of cell '41 is of the same kind as in FIG. 2. This cell 41 is carried by an arm 69 rigid with a nut 70 movable along a screw 71 parallel to the axis of cylinder 1 and this screw 71 is driven in rotation by a motor 72 every time a control electric signal generated at '65 is applied thereto.

In order to compare the actual engraving state with its assigned state, an optical comparison is made between two illuminations corresponding to said two states, respectively.

In order to produce a first illumination in accordance with the actual degree of engraving, it is not possible directly to make use of strip 26, because, due to the rotation of the cylinder, said strip can give only impulse indications and the brightness at each of its points is practically not a function of the position of this point on the strip, but of the time.

This is why there is driven, together with cell 41, another strip 73 which is movable in front of a fixed photoelectric cell 74. Strip 73 is defined as follows. It is parallel to the axis of cylinder 1. Its length is equal to that of strip 26 and its brightness increases gradually from one end thereof to the other according to the same law of variation as that of the permeability along strip 26. This last mentioned condition is preferably complied with by using as strip 73 a photographic wedge identical to that through which strip 26 has been formed by insolation.

Fixed photo-electric cell 74 is arranged in such a manner as to cooperate, at the beginning of engraving, with the end of strip 73 corresponding to the rear end of strip 26 (the left hand end in FIG. 6).

As the position of cell 41 along strip 26 corresponds at any time to the true degree of advancement of engraving, it may be considered that the illumination of the area of strip 73 located opposite cell 74 at the same time is a direct function of this true degree of engraving.

In order to produce a second illumination depending upon the assigned degree of advancement of engraving at any time, there is provided another strip 75 the brightness of which varies gradually from one end to the other according to preestablished law corresponding to the assigned engraving program and this last mentioned strip is made to move opposite another fixed photo-electric cell 76, preferably with a movement of rotation with a constant and slow speed (for instance of one revolution in twenty minutes) determined by a motor 77.

In an advantageous embodiment, strip 75 (and/ or strip 73) consists of an opaque plate of cardboard, metal or any other suitable material, provided with a slot of gradually increasing width, through which passes light from a source disposed, with respect to the plate, on the side opposed to that on which is located the cell.

It may be considered that the illumination of the area of strip 75 which is located opposite cell 76 at a given time depends directly upon the assigned degree of advancement of engraving at this time.

It suffices to compare the two above stated illuminations, by means of a Wheatstone bridge 78 the Zero of which is preferably adjustable by means of a potentiometer 79. When the two compared illuminations are equal, the galvanometer 80 of this bridge indicates zero and no correction is performed. On the contrary, when one of these illuminations differs from the other, the needle 81 of said galvanometer 80 moves away from zero in a direction which depends upon the direction of the difference between the illuminations that are compared with each other and the more so as said difference is greater. This variation is used to ensure the desired correction through the intermediate of calculator 60.

This correction may be continuous, but it may also be discontinuous. In this last case, for instance, needle 81 may actuate, according to its angular position, one or the other of four microcurrent-breaking devices adapted to control, respectively, a slow dilution, a fast dilution, a slow concentration and a fast concentration of the mordant.

Of course all steps are taken for preventing cells 74 and 76 from being responsive to other light fluxes than those coming from the areas to be studied. As a matter of fact these cells may work with specially treated light sources (for instance periodically cut off) and be associated with filter means adapted to permit other parasitic fluxes to be eliminated.

The optical comparison which has just been discussed involves some advantages and in particular the following ones.

It permits the weight of the movable structure rigid with cell 41 to be reduced and the number of delicate elements disposed close to the tank to be limited. The only sensitive element of the comparator unit disposed close to the tank is cell 74 which is fixed and independent of the movable structure and which may be housed in a suitable protective casing, whereas all the other parts 75 to 81 may be disposed at a distance.

The arrangement in question further permits the law of progression of engraving to be readily modified merely by replacing a strip 75 by another one, for instance in order more especially to stress one of the color components of colored images to be printed, or to modify the intensity of some contrasts in said images.

It permits also of imposing a given program (defined der by gradually attacking the cylinder with a mordant through a sheet partly permeable to the mordant, WhlCh device comprises in combination:

der by gradually attacking the cylinder with a mordant through a sheet partly permeable to the mordant, which device comprises in combination:

by a given strip 75) on cylinders the partly permeable sheet 2 of which carry different strips of areas 26.

It should be noted that strips 73 and 75 may be identical, although this is not necessary.

In order to improve the faithfulness and accuracy of 5 the optical comparison in question, it may be advantageous automatically and simultaneously to vary the luminous intensities of the two light sources corresponding respectively to cells 74 and 76 in such a manner that this intensity is relatively high for the illumination of the dark areas of

strips

73 and 75 and on the contrary relatively low for the illumination of the light colored areas.

Instead of acting upon the composition or the nature In a general manner, while the above description discloses what are deemed to be practical and efficient embodiments of the present invention, said invention is not limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention.

What I claim is: 1. A chemical engraving device for engraving a cylinthe cylinder to be engraved, said cylinder having a horizontal axis and being journalled about said axis;

an elongated tank for the mordant, located under said cylinder, so that said cylinder dips partly in said tank;

a partly permeable sheet on the surface of said cylinder, the permeability of said sheet to the mordant varying according to the depth to which the cylinder is to be engraved at the corresponding region,

means for rotating said cylinder about its axis;

means for circulating a liquid mordant through said tank;

means for adjusting the strength of said mordant;

programming means for indicating the assigned degree of engraving of said cylinder at different times during the engraving operation;

detecting means for indicating the actual degree of engraving at said times;

means for comparing at said times the actual degree of engraving with the assigned degree of engraving;

and means operative by said comparing means for controlling said mordant strength adjusting means to restore equality between the actual degree of engraving and the assigned degree of engraving.

2. A chemical engraving device for engraving a cylinthe cylinder to be engraved, said cylinder having a horizontal axis and being journalled about said axis;

a partly permeable sheet on the surface of said cylinder, the permeability of said sheet to the mordant varying according to the depth to which the cylinder is to be engraved at the corresponding region;

means for rotating said cylinder about its axis;

means for circulating a liquid mordant through said tank;

means for adjusting the strength of said mordant;

detecting means for indicating the actual degree of engraving at said times;

and means operative by said comparing means for controlling said mordant strength adjusting means to restore equality between the actual degree of engraving and the assigned degree of engraving;

said partly permeable sheet being provided, along a generatrix of said cylinder with a series of areas of respective decreasing permeabilities ror said mordant,

and said detecting means including:

an aiming instrument for appreciating the illumination; of the area located in the field of said instrument; guiding means for movably supporting said instrument with a translatory displacement along a path opposite said series of areas; and means for moving said instrument along said path in accordance with an assigned program; said. comparing means being adapted to compare the actual illumination of an area toward which said instrument is directed with the illumination of an area the engraving of which is at least partly started.

3. A chemical engraving device for engraving a cylinder by gradually attacking the cylinder with a mordant through a sheet partly permeable to the mordant, which device comprises in combination:

the cylinder to be engraved, said cylinder having a horizontal axis and being journalled about said axis;

an elongated tank for the mordant, located under said cylilpder, so that said cylinder dips partly in said tan a partly permeable sheet on the surface of said cylinder, the permeability of said sheet to the mordant varying according to the depth to which the cylinder is to be engraved at the corresponding region, said partly permeable sheet being provided, along a generatrix of said cylinder, with at least one series of areas of respective decreasing permeabilities for said mordant;

means for rotating said cylinder about its axis;

metani for circulating a liquid mordant through said an aiming instrument for appreciating the illumination of the area located in the field of said instrument;

programming means for moving said aiming instrument along a line parallel to the axis of said cylinder, according to a program;

actual means for causing at any desired times said aiming instrument to cooperate with that of said areas the color of which is changing at the beginning of its engraving;

means for comparing at said times the actual position given by said actual means with the assigned position to be given by said programming means;

means for adjusting the strength of said mordant;

4. A device according to claim 3 wherein said aiming instrument is a magnifying optical instrument.

5. A device according to claim 3 wherein said programming means comprise a carriage movable parallelly to the axis of said cylinder and a lever carrying said aiming instrument and pivoted to said carriage about an axis perpendicular to said cylinder axis.

-6. A device according to claim 5 comprising a potentiometer to control said mordant strength, the movable element of said potentiometer being carried by said lever.

7. A device according to claim 3 wherein said aiming instrument is'a photo-electric cell, further including:

locking means for making said actual means operative only during the time intervals for which one of said areas passes opposite said aiming instrument; means for producing, during each of said time intervals, a control electric signal when the illumination is below a predetermined value corresponding to the beginning of engraving;

and means responsive to said electrical signal for moving said aiming instrument toward those of said areas which are of lighter color.

8. A device according to claim 7 wherein, on either side of the series of areas of respective decreasing'permeabilities, there are two strips or" areas of corresponding dimensions which are impermeable to said mordant.

9. A'device according to claim 7 comprising two parallel series of areas of respective decreasing permeabilities, and, between them, a strip of areas of corresponding dimensions which is impermeable to said mordant.

10. A device according to claim 7 wherein said programming means comprise a carriage movable parallelly to the axis of said cylinder and a lever carrying said aiming instrument and pivoted to said carriage about an axis, perpendicular to said cylinder axis, further comprising:

a fixed Screw parallel to the direction of displacement 'of said carriage; a nut mounted on said lever and cooperating with said screw; 7 t and a fixed motor unit for temporarily driving said screw in rotation about its axis, in response to the occurrence of said control electric signal, in the direction causing said aiming instrument to move to- V ward those areas which are of lighter color. 11. A device according to claim 7 wherein said programming means comprise fa carriage movable parallelly to the axis of said cylinder and a motor unit for driving said carriage, said motor unit being energized by said control electric signal, said aiming instrument being directly supported by said carriage, further comprising:

a continuous second series of areas the brightness of which decrease gradually in the same manner as the permeability of said first mentioned series of areas decreases and the length of which is the same as that of said first series, said second series being carried by said carriage; V r means forming'a continuous third series of areas of a brightness decreasing gradually in accordance with the assigned engraving program; means for slowly and continuously driving said third series of areas; i V two fixed photo-electric cells disposed so that said second and third series of areas move in front of them, respectively; and comparator means for comparing the indication of said two last mentioned photo-electric cells and delivering a correcting term. t 12. A device according to claim 11 wherein the movement of said third series of areas is a rotary movement.

References Cited UNITEB STATES PATENTS 2,933,094 4/1960 Cunnifi et a1. 1 3457 X 3,032,753 5/1962 Knapp et al 156345 3,074,277 1/1963 Hill Q 13457 X 3,300,362 1/19677 Crosfield 156345 HAROLD ANSHER, Primary Examiner JOSEPH C. GIL, Assistant Examiner US. Cl. X.R.