Method and Apparatus for Subdividing a Gas Filled Glass Tube

This invention relates to the manufacture of tubular radioactive light sources, and more particularly, is concerned with a method and apparatus for subdividing an elongated tube filled with a radioactive gas into a plurality of radioactive light sources.

Self-illuminating devices are known which utilize a glass tube coated on the inside with a phosphor and filled with a radioactive gas such as tritium or krypton 85. In the manufacture of very small light sources of this self-illuminating type, it has been the practice to provide an elongated tube which is coated inside with a luminophor, evacuated, filled with the radioactive gas, and sealed at the ends. The tube is then subdivided by cutting the tube into sections by melting the glass, fusing the glass in the process to seal the ends of the sections and prevent escape of the radioactive gas. Laser beams have been used to heat the glass tube in the subdividing process. U.S. Pat. Nos. 3,706,543 and 3,817,733 describe arrangements incorporating laser beams for subdividing radioactive gas-filled tubes.

Various problems have been encountered in applying the techniques described in the prior art. In the prior art, the practice has been to hold the tube in two spaced chucks which rotate the tube in the laser beam so that the beam effectively scans the whole perimeter of the tube during the melting process. In practice, it has been found difficult to rotate the two chucks exactly in unison while the cut is completed. Any slight deviation from perfectly uniform rotation tends to twist the tube slightly or bend it, which may cause fractures. The larger the cross-sectional dimensions of the tube the more difficulty is experienced in maintaining perfect alignment and rotation of the tube during the cutting process.

Another problem encountered is that the tube is not easily cut at more than one point at a time, since each segment must be held and rotated in exact alignment with all the other segments during the cutting process so that no stress is placed on the tube. As a result, it has been the practice to only cut one segment at a time, advancing the tube between cuts to provide successive cuts along the length of the tube. As each cut takes place, the total volume of the radioactive gas is reduced and therefore the pressure is increased. As each section is cut off, the pressure in the remaining portion of the tube increases by an incremental amount. Thus there is a difference in gas pressure between the first segment cut off the tube and the last segment cut from the tube. This results in a nonuniformity of brightness in the successive tube segments. This increase in pressure also requires an adjustment of the external pressure with each successive cut to maintain the pressure differential necessary to achieve proper collapse and fusion of the tube ends as each new segment is cut.

The present invention provides an improved method and apparatus for subdividing a radioactive gas-filled glass tube into a plurality of separate segments. The tube is held stationary and the laser beam is repeatedly scanned back and forth across the tube, thus making it easy to mount the tube rigidly without torque or lateral forces being applied which can cause fracturing.

Beam splitters, in the preferred embodiment, permit the beam to be divided and directed to a plurality of cutting positions simultaneously. The simultaneous cuts produce no change in internal gas pressure from one section to another, so that the resulting light sources cut from a single tube have uniform brightness. Furthermore there is no need to adjust the external pressure to maintain optimum pressure differentials during the cutting operation.

These and other advantages of the present invention are achieved by providing apparatus for subdividing a gas-filled glass tube into individual sealed sections comprising clamping means for supporting the tube in fixed position. A laser light beam source is directed by an optical system as two focused beams directed at opposite sides of the tube. The focused beams are scanned back and forth across the width of the tube in a direction transverse to the longitudinal axis of the tube to heat the walls of the tube to a temperature at which the glass softens. The tube collapses from a higher ambient pressure maintained outside the tube than inside the tube.

For a more complete understanding of the invention, reference should be made to the accompanying drawings, wherein:

FIG. 1 is a plan view of a device incorporating the features of the present invention;

FIG. 2 is an elevational view partially in section of the invention; and

FIGS. 3, 4, and 5 are a series of waveforms used in describing the features of the invention.

Referring to the drawings in detail, the numeral 10 indicates generally a supporting base. Mounted on the base 10 is a cover 12 which is sealed by suitable means to the base 10 to provide a fully enclosed space into which gas under pressure can be admitted through a control valve 14. Mounted within the enclosed space is a glass tube radioactive light source 16. The glass tube light source is a hollow glass tube coated on the inside with a luminophor material and filled with a radioactive gas such as tritium or krypton 85. It has been the practice to manufacture the tubes in substantial lengths which are then subdivided into much shorter lengths to provide a plurality of individual self-illuminated light sources which can be grouped or arranged in patterns to form alphanumeric characters or other symbols having visual significance. The elongated tube 16 preferably has an elongated cross section with two wide side surfaces and two narrow side surfaces; it is preferably rectangular in cross-section to provide flat light emitting surfaces, but such tubes may be made in a variety of cross-sectional shapes.

The glass tube 16 is clamped in a fixed vertical position within the pressure chamber by means of a suitable clamping unit 18. The clamping unit 18 includes a vertical support 20 mounted on the base 10. A first rigid clamping member 22 is secured to the support 20 and is bifurcated to form a plurality of spaced clamping fingers 24 which extend on one side of the tube 16. A second clamping member 26 is pivotally supported at 28 to the first clamping member 24 and includes clamping fingers 30 which engage the opposite side of the tube from the clamping fingers 24. A suitable spring 32 produces a clamping pressure between the respective clamping fingers 24 and 30 which acts to rigidly and securely clamp and support the glass tube 16. The clamping fingers 24 and 30 provide intermediate spaces in which the walls of the tube are exposed on either side for cutting by laser beams.

The light beams for cutting and subdividing the tube 16 into segments is derived from a laser source 34 which is preferably a CO.sub.2 laser operated in the TEM.sub.0,0 mode at a wave length of the order of 10.6 microns in the infrared region. It is collimated and directed through a beam bending optical unit 36. The position of the beam immerging at right angles from the bender 36 may be laterally adjusted by a micrometer adjustment 38 so as to control the cross-sectional area of the beam which impinges on a beam splitting mirror 40. The mirror 40 is arranged to direct substantially half the beam along a first beam 42, the undiverted portion of the beam being reflected by a mirror 44 along a second beam 46. The beams 42 and 46, respectively, impinge on 45.degree. mirror surfaces 48 and 50 which direct the beams towards opposite faces of the tube 16. The lenses 52 and 54 focus the respective beams on the two wide side surfaces of tube 16 as shown in FIG 1.

Scanning of the beams across the full width of the wide side surface of the tube is accomplished by mounting the mirrors 48 and 50 and lenses 52 and 54 on a carriage indicated generally at 56. The carriage is supported on the surface of the base plate 10 by suitable rollers 58 or other guide means which guide the carriage to move relative to the base in the direction of the arrows in FIGS. 1 and 2. The carriage is preferably in the form of an open rectangular frame including a top plate 60, bottom plate 62, and side plates 64 and 66. The mirrors 48 and 50 and lenses 52 and 54 are supported on the side plates, the side plates having openings through which the beam passes.

An oscillating motion is imparted to the carriage 56 to cause the beams to be scanned back and forth across the width of the tube 16 to distribute the heating of the glass over the full perimeter of the tube. To this end, a motor 70 drives a fly wheel 72 through a suitable variable speed reducer 74 and belt drive 76. The flywheel 72 is journaled on a shaft 78 projecting from a support bracket 80 mounted on the base 10. An eccentric drive pin 82 projects from the flywheel 72. A link 84 extends between the pin 82 and the carriage 56. The radial distance of the pin 82 relative to the center of the flywheel 72 is preferably made adjustable so as to vary the length of throw to change the distance over which the carriage 56 and associated focused light beams are scanned. Speed reducer 74 may be made adjustable so that the frequency of the oscillation of the carriage 56 can be varied. The frequency of scan is preferably of the order of 10 - 20 hz.

More than one cut of the tube 16 can be accommodated by splitting the beams 46 and 42 into two parallel beams one above the other, as indicated at 46 and 46' in FIG. 2, by suitable beam splitters 86 and 88. Two sets of mirrors 48 and 50 and lenses 52 and 54 are arranged one above the other to focus the respective pairs of beams at two vertically spaced locations along the length of the glass tube 16. It will be appreciated that additional beams may be formed at other vertically spaced positions along the length of the tube 16 to provide any desired number of simultaneous cuts of the tube 16. The clamping fixture 18 should provide sufficient bifurcated clamping fingers to insure that the subdivided tube sections are securely held in place on both sides of each cut.

The laser source 34 may be controlled to vary the power of the output beam by the amount of current applied to the plasma tube of the laser source in a well known manner. An automatic control circuit 90 may be provided to control the output of the laser source 34 as a function of time. The control circuit includes means for generating selectable output voltages of different waveforms. FIG. 3 shows a typical modulation waveform showing voltage as a function of time. The beam power of the laser varies directly with the change of modulation voltage from the control circuit. Thus for a simple rectangular shaped tube, for example, the laser beam, varied by the voltage of FIG. 3, has a gradual rise in power. This is desirable to form an effective sealed cut of the tube. It may also be desirable to vary the intensity of the beam as a function of the position of the beam relative to the tube. To this end, a disc 94 having a serrated edge on a plurality of holes may be provided on the flywheel 72 for chopping light detected by a photo detector 96 mounted on the support 80. This provides a periodic signal of the wave shape shown in FIG. 4 consisting of a series of sharp pulses with a single long pulse occuring once each revolution, corresponding to the start of a scan cycle. This signal may be applied to the automatic control circuit 90 to provide a periodic modulation of the beam from the laser source, synchronized with the scanning of the beam across the tube. This periodic signal may be superimposed on the time function signal of FIG. 3 to provide a modulated laser beam having the waveform shown in FIG. 5. For example, the beam intensity may be modulated during each scan cycle so that the beam intensity increases at the edges of the tube where the thickness of glass is the greatest and reduced at the center of the tube where the thickness of glass is the least.

From the above description it will be seen that an arrangement is provided for achieving simultaneous multiple subdivisions of a glass tube radioactive light source in which the tube itself does not have to be rotated or moved during the cutting operation. By providing simultaneous rather than sequential cuts, the pressure in the subdivided sections can be made more uniform. While the mirrors 48 and 50 are shown as being moved linearly by the carriage, other means of scanning the focal point may be used, such as oscillating the plane of the mirrors about a vertical axis to vary the angle of reflection.