GB733037A - Apparatus for the production of printing plates - Google Patents

Abstract

733,037. Copying-telegraphy. HELL, R. May 8, 1953 [May 9, 1952; Dec. 5, 1952; Dec. 27, 1952], No. 12888/53. Class 40 (3). A device for the production of printing plates of the type wherein the picture original is scanned line-by-line photoelectrically while an engraving tool controlled by the photo-electric current is guided line-by-line across the printing plate blank in correspondence with the scanning means, comprises a slide which is guided positively in relation to the scanning means and carries the picture original on one side and the printing plate blank on the other side. Several embodiments are described but each entails a large C-shaped member which embraces the slide and carries the scanning means at the end of one arm and the engraving means at the end of the other, the slide being reciprocated to produce a line scan while the C-shaped member carries out a feed movement perpendicularly to the line-scanning direction during each return movement of the slide. As the scanning is normally required to be at an angle to the edge of the picture the useful line scan varies in length as the perpendicular feed proceeds and various automatic means for limiting the stroke to the useful scan are disclosed. The machine shown in Fig. 1 comprises a slide 3 traversable on runners 1 and 2 by a motor 13 which drives a threaded shaft 20 through gears 22, 23 or 15, 16, 17 according to the position of reversing lever 38. The gear ratios are such that the slide 3 is driven faster in its return travel than when travelling forward in the direction of arrow 12. The slide 3 has an aperture 5 containing a glass plate onto which the picture original is placed facing downwards, and above which the printing plate blank is suitably supported. A C-shaped member 8, carrying engraving means 6 and photo-electric scanning means 7 arranged directly opposite each other, is guided on a rail 9 and spindle 41, rotation of the latter producing a feed movement in the direction of the arrow 11. Rigidly depending from the slide 3 is a frame 4 corresponding in position and shape to the aperture 5. Carried on the lower part of the C-shaped member 8 is a slidable strip 28 having stops 25, 26 rigidly depending therefrom and projecting into the frame 4 such that the edges of the frame 4 impinge on one or other of the stops respectively at the limits of the useful line-scanning travel of slide 3. The strip 28 is thus moved at these limits and a fork 27 fixed thereto tilts a bar 31 about fulcra 29, 30; this causes lever 32 to move the link 33 and by means of spring- loaded roller 35 imparts a snap-action changeover to reversing lever 38. The link 33 in its passage to the left momentarily releases a detent 43 from notched wheel 42 and allows spindle 41 to revolve (under drive from motor 13 via a slip clutch in gearbox 40) until detent 43 engages the next notch; this produces line feed of member 8 in the direction of arrow 11. In the reverse movement the linkage articulates at pivot 37 and does not affect detent 43. If the picture original is smaller than the aperture 5 the stops 25, 26 may be moved apart by adjusting screw 39. A lever 49 pivoted at 48 is connected to the strip 28 such that a shutter 50 is moved into the path of light in the scanning means 7 during the return movement of the slide 3. In the embodiment of Figs. 2 and 3 (not show-n) the slide 3 has a rotatable insert to carry the picture original and printing plate blank so that the direction of scan may be varied as necessary for the production of colour separation plates; the insert is coupled by a flexible shaft to a rotary colour filter holder such that the appropriate filter is brought into the path of the scanning light beam. To produce a half-tone dot pattern the light beam is interrupted by a rotating perforated disc synchronized with the drive to slide 3 by means of a sonic wheel. Reversal of the scanning drive to slide 3 is effected by an electromagnetic clutch mechanism energized via contacts operated respectively by feelers engaging the alternate edges of the insert in the slide 3. The line feed of member 8 is controlled by a pawl temporarily released by a relay energized by the appropriate reversal contacts. The top arm of the member 8 is pivoted and is maintained at the correct spacing from the printing plate blank by an adjustable supporting foot which rides on the blank in close proximity to the engraving stylus. This feature also allows a new stylus to be readily set to the correct " zero " position. Instead of mechanical or electro-mechanical control of the scanning drive reversals, the photo-electric system may be utilised to signal when the edge of the picture has been reached by backing the picture with a plate-glass mirror as indicated in Fig. 4 (not shown). The large output thus obtained when the edge of the diffusely reflecting picture is passed operates a relay (for example) that does not respond to the relatively weak picture signals. Alternatively the plate-glass may be omitted and the backing plate given a light-reflecting surface, but in this case the optical axis of the light beam must be inclined to allow specularly reflected light to impinge on the photo-cell or the reflecting surface must be corrugated. Instead of producing a half-tone dot pattern by means of intermittent illumination, the picture original may be covered by a screen which need only comprise a raster of parallel lines since the cross scanning will produce the requisite dotfrequency. The spacing of the scanning lines, the' lines on the screen and the angle therebetween will be interdependent for optimum results, as indicated in Fig. 6 (not shown). The modification of Fig. 5 (not shown) provides for the production of printing plates of a size differing from that of the picture original. The original is arranged on an illuminated table carried on an arm depending from the slide 3 which also carries an optical projection system to throw an image onto a screen in the position occupied by the original in the former embodiments. Scanning is carried out by elemental examination of the screen using a photo-cell and lens system on the C-shaped member 8 while slide 3 together with the projection assembly is reciprocated as before. Setting-up plates (for backing printing plates in the printing process so as to relieve pressure on the light portions) may be produced in a similar manner as above but with a larger scanning spot of light, a wider engraving tool or milling cutter and a greater rate of feed. Alternatively the printing plate itself may be engraved on its reverse side for setting-up purposes. The engraved printing plate itself may be used as the original to be scanned, either polished or inked with black or white ink (the control current being relatively reversed for the black or white inked plates). A half-tone dot pattern is not required for this relief engraving but a pulsating current is more readily amplified so the scanning light beam or the photo-cell output is modulated at a high arbitrary frequency and rectified after amplification. Such an arrangement using an electro-magnetically controlled rotary drill engraving tool is illustrated in Fig. 7 (not shown). This process may be greatly simplified by inverting the scanner and engraver and keeping them fixed while the plate is moved by hand, face upwards, over an aperture in a table. The operator can thus see the printing plate and select those light areas requiring back-engraving and omit remaining parts of the plates, thereby eliminating the mechanical drive and reducing the working time.