Making an elongate cut using high energy radiation

Details Making an elongate cut using high energy radiation Making an elongate cut using high energy radiation

1991-04-04

1993-09-28

Albritton, C. L.

Electric heating

Metal heating

By arc

21912172, 21912175, B23K 2600

Patent

active

052488772

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to making an elongate cut using high energy radiation focussed to a focal spot in the cut, the radiation usually being a laser beam. A blind groove or hole may be formed in the workpiece, or the workpiece may be cut or pierced right through. The invention has general applicability and in general terms, unnecessary material removal must be avoided as this slows down the cutting process; however, the invention is particularly applicable to gemstones, where it is specially important to avoid unnecessary material removal, and it is also necessary to avoid applying excessive thermal stress; in addition, a particular problem with gemstones such as diamond is that usually only a small percentage of the radiation may be absorbed, so it is important that coupling of the energy to the workpiece be as effective as possible.
In gemstones, the laser cutting can be referred to as forming a kerf (a groove, either for cleaving or for later sawing), or sawing (cutting right through). In other applications using jewels or gemstones, a blind hole or a through-hole may be formed in order to employ the stone as a bearing.
When sawing gemstones, normal practice is to form a V-section, which should be as narrow as possible to reduce weight loss. There is automatic machinery (work handling) for benching out the V-shape (i.e. scanning the area formed by the length and width of the cut) and for refocussing the beam as the cut depth increases.
A fundamental problem is that the energy density in the focal spot should be as great as possible, but a smaller focal spot (and thus higher energy density) can only be obtained by increasing the cone angle of the focussed beam (i.e. higher numerical aperture). The machined V-section has then to be wider, otherwise the laser energy focussed into the cut is apertured (vignetted) at the entrance to the cut as the depth increases.


THE INVENTION

In accordance with the invention, a system is used which converges the radiation at a substantially greater angle of convergence in the plane in which the cut lies than in the plane transverse to the cut.
Using the invention, the energy density of the focal spot can be increased without widening the machined V-section, as the uninterrupted access provided longitudinally of the cut is utilised. Consequently the angle of the V-section may be reduced and rate of propagation of the cut increased.
Using the invention, the amount of laser energy entering the cut can be increased by cylindrically compressing a spherically expanded beam in the transverse direction. This allows the use of higher spherical beam expansion ratios than is possible without use of the invention because the effect of vignetting by the cut opening is reduced.
The numerical aperture of the optical system is higher in the longitudinal direction than in the transverse direction. The focii must be at the same depth in each plane (transverse and longitudinal). The focal spot (as seen on the flat bottom of the cut) will be an ellipse with its major axis transverse of the longitudinal direction of the cut. The size of the focal spot in the major axis (transverse) direction will be larger than that without using the invention. The full advantage of the invention cannot be observed unless the spherical beam expansion coefficient is increased, so that the focal spot area is reduced without a proportional reduction in the energy entering the cut. The beam can be expanded and then focused in both planes in order to achieve a sufficiently high focal spot energy density.
If the numerical aperture were to be increased by the same factor in both planes (transverse and longitudinal), as would be the case with a conventional beam expander, any increase in energy density resulting from a reduction in focal spot area, will be at least partially offset by a reduction in energy entering the cut, due to vignetting at the opening, as cutting progresses.
In practice, the ratio of convergence angles (included angle) of the transverse to longitudinal dire

REFERENCES:
patent: 3534642 (1970-10-01), Cruickshank et al.
patent: 3941973 (1976-03-01), Luck et al.
patent: 4401876 (1983-08-01), Cooper
patent: 4546231 (1985-10-01), Gresser et al.
Electro-Optics/Laser 80 Conference and Exposition, Boston, Mass., Nov. 19-21, 1980, pp. 223, 226.
IBM Technical Disclosure Bulletin, vol. 14, No. 9, Feb. 1972, pp. 2641 and 2642.

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