Patent GB-1206049-A (GB)
Title: Methods of machining perforate electrodes from pyrolytic graphite
IPC: H01J19/00
Filed: 1968-03-01
Abstract: 1,206,049. Blasting. COMPAGNIE FRANCAISE THOMSON HOUSTON-HOTCHKISS BRANDT. 1 March, 1968 [7 March, 1967], No. 10234/68. Heading B3D. [Also in Divisions Cl and H1] A method of making a perforate electrode for an electron discharge device comprises: (i) Forming a blank of pyrolytic graphite having its maximum conductivity directions parallel to major surfaces of the blank; (ii) discharging a jet of abrading material and directing the jet to impinge on a said major surface;  (iii) producing relative movement between the jet and blank; (iv) restricting the areas of impact of said jet on said major surface to a predetermined pattern during said movement; and (v) continuing the discharge and relative movement until the blank has been completely perforated in accordance with said pattern. A pyrolytic graphite blank 10, Fig. 1, in the form of a tubular shell closed at one end is secured around a support 11 having a shank 11b held in a lathe chuck 11a. A mask 13 in the form of a tube of thin sheet steel or other material having circumferentially spaced longitudinal slots is mounted with a sliding fit on the blank 10. A blasting unit 14c has a nozzle 14 discharging abrasive powder, e.g. silicon powder, in a stream of pressure fluid, e.g. air at 4-7 atmospheres; the spacing of the nozzle tip from the mask may be 0À5-5 mm. The unit 14c is reciprocated by a lead-screw 16 controlled by limit switches 17, 18. Means may be provided for reversing the rotation of the spindle 11b to correct for lack of perpendicularity between the spindle and the nozzle. For manufacturing lattice-shaped grid electrodes, the mask is of wire, e.g. tungsten, molybednum, nickel, tantalum or the like, and has a diamond mesh-type lattice. In a modification, Figs. 3, 4, the mask is in two parts 102, 104, each in the form of a sheet metal tube having helical slots, oppositely inclined in the two parts; one part is inserted in the other part as a close fit, and the parts may be soldered in assembled position. In a further modification, Fig. 7, a wire 22 is wound helically around a tubular sheet metal member having longitudinal slots. Masks may be made from metal or plastic sheet and may be provided with a protective coating, e.g. of nickel, chromium &c. The mask may have a metal body provided with a readily renewable coating, e.g. plastic. If wires are used, they may have a coating of varnish or nitrocellulose. It is found that at an early stage of blasting the hole formed through the blank is strongly convergent; as blasting continues the angle of convergence diminishes. Just before the hole becomes cylindrical the outer end of the hole becomes rounded. This may be undesirable and in such cases blasting is stopped just before the hole becomes truly cylindrical, and the residual taper in the hole substantially compensated for by a short blasting step with the jet directed at the inner side of the blank. After the perforating operation, the jet may be directed at predetermined angles and for predetermined lengths of time at the outer and inner surfaces of the blank to produce the desired cross-sectional contours to the holes and intervening bars or strips. To produce a flat, e.g. disc-shaped, grid for use in an electron tube embodying flat electrodes, a pyrolytic graphite blank 31, Fig. 15, covered by a square-mesh wire mask 32, is mounted on an arm 33 moved up and down in a guideway 34 by a cam 35. The nozzle 14 is reciprocated by a lead-screw 16. In a modification, a number of blanks covered by wire-mesh masks are arranged in a circular array round a supporting disc mounted for rapid rotation; the nozzle is reciprocated. In another arrangement, Fig. 17, a partspherical pyrolytic graphite blank 38 covered by a wire mesh mask 39 is mounted at its centre on an arm 40 extending along a radius of the sphere and swivelled on a universal pivot 41 at the centre of the sphere; the arm 40 has imparted thereto a two-directional scanning motion as indicated by the arrows 42, 43; the nozzle 14 is stationary. The mask, instead of being formed separately from the blank, may be formed directly on the surface of the blank. E.g. the surface may be coated with a photosensitive varnish which is allowed to dry in the dark. The coated blank is then exposed to radiations, e.g. ultra-violet or visible light, through a mask similar in pattern to that of the mask to be formed. The blank is then dipped in solvent, e.g. trichloroethylene, to dissolve away the unexposed varnish. The blank is then passed through an electroplating bath to coat the areas laid bare with a metal coating, e.g. copper, nickel, chromium &e. After blasting, the metal coating constituting the mask is removed by dipping in an acid etching bath. In a modification, the blank is first copper-plated and the mask pattern etched thereon by photo-engraving. In another modification, the mask pattern is formed with a polymerizable resin, by a silkscreen process or otherwise, and the resin polymerized in situ. In a modification, Fig. 18, the rate of rotation of the spindle 11 is so correlated with the rate of traverse of the nozzle 14 that the jet 15 cuts into the blank 10 along a helical path 44, whereby the pattern cut out in the blank will be a mesh-type pattern generally similar to that obtained when using a mask, as shown in Fig. 7. The position of nozzle 14 may be displaced by a predetermined axial amount at each of a series of passes to form more than one intertwined helical cuts 44. Alternatively, the mask may be one or more helical strips wound round the blank or a plurality of circumferential strips, e.g. elastic bands, surrounding the blank, while the movements of spindle 11 and screw 16 are correlated to impart to jet 15 a selected path of relative movement with respect to the blank. Said path may be a helix of reverse pitch from that of the mask helix; said path may be a circumferential set of equispacial axial lines, in which case the spindle 11 is indexed instead of continuously rotated. In a modification, Fig. 19, no mask is applied to the blank, but a rotary obturator disc 45 is used, having a circumferential series of slots therein. For making a coaxial cylindrical pair of grids for a tetrode, a single mask 51, e.g. of the wire mesh type, is used having a wall thickness corresponding to the radial inter-grid spacing in the finished tube. The inner, control grid is produced by blasting a blank 50, Fig. 21, surrounded by the mask 51. To form the outer, screen grid, the mask 51 is inserted into a blank 50a, Fig. 22, which is blasted by a nozzle 52. The two electrodes are thus formed with accurately corresponding and registerable mesh patterns. In a modification, Fig. 23, inner and outer blanks 56, 55 are blasted at the same time, using a mask 57, e.g. of molybdenum.  In a modification, Fig. 24, outer and inner tubular blanks 60, 61 have substantially different diameters. Blank 61 is mounted on circular base 66 secured on rotatable shaft 68. A tubular mask 62 is mounted as a close sliding fit within blank 61 for bodily rotation therewith, the mask having a main mesh section 64 and solid end sections 63. Blank 60 is mounted on an annular base 67 secured on a tubular shaft 81 rotatable in eccentric relation with respect to shaft 68. Shaft 81 is coupled for rotation with inner shaft 68 by gears 69, 70, 71, 82. A nozzle 65 is reciprocated across the height of the mesh section 64 of the mask, with its outlet directed radially towards the common tangential area of blanks 60, 61. In all embodiments blasting may be monitored by means such as thickness gauges, transparency inspection using a light beam and a photo-electric cell, capacitance indicating means with a pair of electrodes positioned on opposite sides of the blank, optical contourdetecting means &c.