Abstract:
A method of calibrating a gravure engraving machine includes the steps of providing an engraving signal to the engraving machine to cause production of a cell, measuring the volume of the cell, comparing the measured cell value to a predetermined cell volume and adjusting the engraving cell in accordance with the comparison.

Description:
TECHNICAL FIELD 
   The present invention relates generally to a method of calibrating an engraving machine and more particularly to an engraver calibration method using a non-contact optical profiler. 
   BACKGROUND ART 
   Gravure printing is an intaglio process employing one or more engraved gravure printing cylinders. Image areas of each cylinder are engraved by an engraving head of an engraving machine to create cells. The cells vary in volume corresponding to the tonal values in the images. 
   The quality of the final printed product depends upon engraving the correct cell sizes on the cylinder. The shape and volume of each cell dictates how much ink that cell will hold and correspondingly, how an ink dot will appear in print. Even small variations in cell size can produce changes in dot size noticeable to the human eye. It has been shown in testing that the actual cell volume is more representative of the actual print density than the surface area of the cell. Therefore, it is necessary to calibrate the engraver so that accurate and repeatable cell volumes can be produced. 
   Past attempts to accurately calibrate an engraver have included the use of precision optical instruments to measure various spatial parameters of the cells in order to estimate actual cell volume. This technique is detailed in Wouch et. al. U.S. Pat. No. 5,293,426 assigned to the assignee of the present application. As detailed in such patent, an optical microscope is used to measure the length and width of the surface of a plurality of test cells. From such measurements, the depth, face area, and volume per unit area of each test cell may be estimated. Using statistical analysis, the average cell width, length, depth, face area, and volume per unit area are calculated. The average cell width, area, or volume per unit area is then compared with a predetermined standard value to compute any variance and to adjust the engraving head accordingly. 
   Non-contact optical profilers, such as the WYKO Rollscope, a vertical scanning interference microscope, have previously been used to characterize the surface roughness of such products as rubber, paper, ceramics, textured steel and aluminum, adhesives, films, and others. The WYKO Rollscope has also been used to characterize cells formed in anilox rolls used in flexographic reproduction. Each of these applications is characterized by relatively uniform depth, shape, volume and density of surface deformations. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a method of calibrating a gravure engraving machine which engraves images on a printed member such as a gravure printing cylinder includes the steps of providing an engraving signal of a predetermined waveshape to the engraving machine to cause the engraving machine to produce a gravure cell having a volume and measuring the value of the gravure cell using a non-contact optical profiler. The measured volume is compared with a predetermined cell volume to obtain a variance indication and the engraving machine is adjusted according to the variance indication. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become more apparent from a detailed consideration of the invention taken in conjunction with the drawings in which: 
       FIG. 1  is a block diagram of a printing cylinder engraver system utilizing the method of the present invention; 
       FIG. 2  is a flow chart of steps undertaken to produce printed copy; and 
       FIG. 3  is a flow chart of steps taken in accordance with the calibration step illustrated in  FIG. 2  according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention relates to electromechanical engraving of gravure printing cylinders, and more specifically, to a method of calibrating an engraving machine used to engrave gravure cylinders or other intaglio printing cylinders. Referring to  FIG. 1 , a gravure engraving system  10  includes a computer  12 , which processes image densities or other optical parameters recorded by one or more reading heads  13  or stored in a data file, and an engraving machine  14 , which receives electrical signals developed by the computer  12 . The engraving machine  14  includes one or more engraving heads  15  having a diamond engraving stylus that engraves gravure cells in a copper cylinder in accordance with the electrical signals. The gravure cells are typically engraved at a speed of 3600 to 8000 cells per second. The engraving machine  14  further includes one or more resistance potentiometers  17  which control the penetration depth of the engraving heads  15 , and thus the cell volume, by varying the electrical waveshape supplied to the engraving heads  15 . 
   Prior to performing a production engraving process, the engraving system  10  must be calibrated. To accomplish this calibration, one or more test cells  18  are engraved in a cylinder using gravure engraving machine  14 . A well-known gravure engraving machine  14  is a Helio-Klischograph engraving machine manufactured by Dr. Ing. Rudolf Hell GmbH. 
   As noted in greater detail hereinafter, the gravure engraving machine  14  is operated such that the diamond stylus cuts into the copper surface of the gravure printing cylinder  16  to form one or more test cells  18  in the general shape of an inverted pyramid. Although the test cell  18  is depicted as having an inverted diamond shape, those skilled in the art will appreciate that the shape of the test cell  18  will vary depending on a number of factors, including, for example, diamond stylus wear, gravure printing cylinder  16  rotation speed, and the electrical waveshape supplied to the engraving head(s)  15 . The inverted pyramid shape, however, ensures consistent and excellent ink release even when printing on smooth and non-porous surfaces. 
   It is known that two gravure cells with the same spatial measurements of surface width and length, but cut with different diamond styli, can have different cell volumes. Testing has shown that the volume of a cell primarily dictates how the human eye perceives the optical density of the color printed by the cell. Even small variations in cell volume can produce large changes in the perceived color. Therefore, to effectively calibrate a gravure engraving machine  14 , it is necessary to accurately correlate the actual volume of the test cells  18  produced by the machine  14  to the waveshape(s) used to produce the test cells  18 . 
   One embodiment utilizes a non-contact optical profiler  20 , such as the WYKO® RollScope (a vertical scanning interference microscope manufactured by Veeco Instruments, Inc., Plainview, N.Y.), to provide a fast, highly accurate measurement of gravure cell volume in order to permit calibration of an engraving machine. 
   A method of producing one or more gravure print cylinder(s)  16  for printing is set forth in the flow diagram illustrated in  FIG. 2 . Upon starting the production printing process, the gravure engraving machine  14  is calibrated at a step  200 , described in greater detail in  FIG. 3  below. After calibrating the gravure engraving machine  14 , a gravure cylinder  16  is placed in the gravure engraving machine  14  and is engraved at a step  202 . The engraving process  202  includes the steps of providing data representing a desired image to the computer  12 , causing the computer  12  to convert the image to electrical waveforms of predetermined amplitude and pulse width, and transmitting the electrical waveforms to the gravure engraving machine  14 . It should be noted that although the computer  12  is depicted as a single element in  FIG. 1 , it may, in practice, consist of multiple computers or central processing units which are interconnected together, for example, by a network. Also, a driver circuit which develops waveforms of appropriate magnitude and waveshape may be coupled between the computer  12  and the engraving machine  14 . 
   Continuing with  FIG. 2 , once the engraving process  202  begins, a step  204  periodically determines whether the engraving process  202  is complete. If the engraving process  202  is not complete, a step  206  determines if recalibration of the gravure engraving machine  14  is necessary. The step  206  may use such factors as the last time the gravure engraving machine  14  was calibrated, the potential wear on the diamond engraving styli, the desired image quality, and other factors. If the step  206  determines that recalibration is not necessary, the engraving process  202  continues. If, however, the step  206  determines that recalibration is necessary, the calibration step  200  is reinitiated. 
   Returning to the step  204 , if it is determined that the engraving process  202  is complete, a step  208  determines whether there are more cylinders to be engraved. If so, a new gravure cylinder  16  is loaded into the engraving machine and the process returns to the step  206 . On the other hand, if there are no more cylinders to be engraved, the completed gravure cylinder(s)  16  are utilized at a step  210  to print copy. 
   Turning to  FIG. 3 , there is shown in detail the method of calibration utilized by the step  200  according to the present embodiment. The process begins at a step  300  by determining the ink color the gravure cylinder  16  will print (for example, cyan, yellow, magenta, or black), and by determining the type of engraving stylus used by the gravure engraving machine  14  to engrave the gravure cylinder  16  (for example, a 140°, 130°, or 110° stylus). Following the step  300 , at least one predetermined cell volume is established at a step  302  based upon the ink color and the stylus type determined at the step  300 . It should be noted that the establishment of the predetermined cell volume need not be based on the variables of ink color and stylus type, but may be based on one or more other factors, including paper quality, paper roughness, batch variations in ink and papers, and stylus wear. Once the predetermined volume is established at the step  302 , the gravure engraving machine  14  is operated at a step  304  to produce at least one gravure test cell associated with each predetermined cell volume. The step  304  is undertaken by causing the computer  12  to provide a waveform for each test cell having an amplitude and/or pulse width that nominally should produce a cell having the predetermined cell volume. 
   In the present embodiment, three linear rows of gravure test cells  18  are preferably (although not necessarily) produced as a test array in the gravure cylinder  16  at the step  304 . Also preferably, each row includes approximately 70 test cells  18 . The test cells  18  are produced with a cell volume which corresponds to a midtone tonal value (i.e., an optical density of approximately 0.48). It will be appreciated by those skilled in the art, however, that the tonal value chosen may range anywhere from a light highlight tone to a shadow tone, and may not be limited to a single tonal value. 
   After the production of the gravure test cells  18 , the volumes of the gravure test cells are measured at a step  308 . It has been determined, however, that a more accurate measurement at the step  308  may be obtained by optionally cleaning the gravure test cells  18  first, as noted at a step  306 . In the preferred embodiment, the gravure test cells  18  are cleaned at the step  306  by the application of an antiperspirant sold under the trademark Degree® by Helene Curtis, Inc., of Chicago, Ill. (which includes the constituent aluminum sesquichlorohydrate). While it is not completely understood why the application of this cleaning agent has an effect upon the measurement of the gravure cell volume, two hypotheses have been considered. The first hypothesis is that the engraving process creates debris within the test cells  18  and the cleaning agent applied to the cells  18  washes away the debris. The second hypothesis is that the cleaning agent creates a more reflective surface that is more conducive to measurement. 
   After the gravure test cells  18  have been cleaned at the step  306 , or directly following the engraving step  304 , the gravure test cell volumes are measured at the step  308  by the optical profiler  20 . The optical profiler  20  is capable of being equipped with various objective lenses which determine the number of test cells  18  measured at the step  308 . For example, a 20 obj. lens will typically measure between three and five test cells, a 40 obj. lens two or three test cells, while a 50 obj. lens will typically measure one cell. 
   In the present embodiment, a test cell location is selected from the second or third rows of the three linear rows of test cells  18 . Typically the fifth test cell from the leftmost test cell of the second or third row from the top of the test cell array is chosen. The optical profiler  20  is fitted with a 20 obj. lens and is adjusted to target a subset of the test cells including the chosen test cell. The optical profiler  20  displays a visual depiction of the targeted test cells  18  and is adjusted so that as many complete test cells  18  as possible are within the scanning region of the optical profiler  20 . For example, the scanning region of a 20 obj. lens targeting the fifth test cell  18  in the second linear row may contain the fifth and sixth cells of both the second and third linear rows (i.e., four cells in a two by two pattern). 
   Regardless of the objective lens chosen, the optical profiler  20  scans the test cells  18  and calculates various measurements, each of which is a highly accurate, non-contact, three-dimensional volumetric measurement of each test cell  18 . The optical profiler  20  provides measurements of the X, Y, and Z (length, width and depth) spatial dimensions, as well as the volume of each test cells  18 . The optical profiler  20  also provides a statistical average of each measurement (i.e., length, width, depth, and volume) over the multiple cells, as well as maximum and minimum values for each dimensional measurement for all of the cells taken as a group. 
   Once the measurements are obtained, the maximum and minimum volumetric measurements for the target cells are first analyzed for a variance at a step  310 . A step  312  then determines whether the variance between the maximum and minimum volumes of the test cells is greater than a desired threshold, for example 2 μm 3 . If the variance is greater than the desired threshold, the measurement step  308  is repeated, using either with the same target cells or other target cells in the existing test cell array. Alternatively, after an appropriate number of failed attempts (for example, three) at obtaining a suitable volumetric variance, the step  304  may be repeated to obtain a new test cell array, which is thereafter analyzed at the steps  308 ,  310  and  312  as noted above. 
   It will be further appreciated by those skilled in the art that if the optical profiler  20  is fitted with an objective lens that measures only one test cell  18 , the steps  310  and  312  will not be necessary and can be omitted. 
   Once an acceptable maximum and minimum volumetric variance is obtained, a comparison is performed at a step  314  between the measured average test cell volume determined at the step  308  and the predetermined cell volume from the step  302 . A step  316  then determines if the variance between the average volume of the test cells  18  and the predetermined volume is less than a desired threshold value (i.e., the step  316  determines whether the variance is within acceptable limits), for example 1 μm 3 . If the step  316  determines the gravure engraving machine  14  is calibrated within acceptable limits, the calibration step  200  is terminated, otherwise, the gravure engraving machine  14  may be adjusted at a step  318 . 
   To adjust the gravure engraving machine  14 , the resistance potentiometer(s)  17  is (are) manipulated to change the electrical waveshape supplied to the engraving head(s)  15  in accordance with the average volume variance calculated at the step  314 . Upon completion of the step  318 , the calibration may preferably return to the step  304  to produce new gravure test cells  18  for calibration verification, or alternatively, the calibration step  200  may end. 
   While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions or deletions may be made to the disclosed embodiments without departing from the spirit and scope of the invention.