Electric heating – Metal heating – By arc
Patent
1991-04-10
1993-03-16
Albritton, C. L.
Electric heating
Metal heating
By arc
21912175, B23K 2600
Patent
active
051947110
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present application relates to a method of making a cut using high energy radiation 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 especially 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 diamonds is that usually only a small percentage of the radiation may be absorbed, so it is important that the 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 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. high numerical aperture). The machined V-section has then to be wider, otherwide the laser energy focussed into the cut is apertured (vignetted) at the entrance to the cut as the depth increases.
THE INVENTION
According to the invention, the axis of the incident radiation relative to the cut is moved repeatedly from a position in which it is directed towards a first side of the cut and is inclined towards the other, second side of the cut to a position in which it is directed towards the second side of the cut and is inclined towards the first side of the cut, and back again.
The most intense part of the beam can be centered over the cut opening, reducing the effect of vignetting upon energy entering the cut by oscillating the beam.
In addition, the angle between the incident radiation and the side wall of the cut is increased, thereby reducing the size of the focal spot projected onto the side wall, and consequently increasing the energy density.
The technique is applicable for most laser types; although currently the preferred system for diamonds uses infra-red radiation of 1.064 nm from a Q-switched, neodymium-doped YAG laser, it is possible that frequency doubled YAG (532 nm), frequently quadrupled YAG (266 nm) and even shorter wavelength lasers such as ArF and KrF excimer lasers (193 nm and 248 nm wavelengths respectively) can be used. The technique is theoretically most beneficial with a Gaussian beam; in practice an advantage may be achieved using a near-Gaussian beam. One suitable system has a 10 mm expanded beam diameter and a focussing lens with a focal length of 50 mm.
Significant advantages can be obtained if the invention described herein is combined with the invention described in a patent application filed in August 1989 and entitled "Making an elongate cut using high energy radiation" and claiming priority from GB 88 19351.
THE DRAWINGS
The invention will be further described, by way of example, with reference to the accompanying drawings, in which:
FIGS. 1 to 3 are three schematic views along the cut, illustrating the principle, and showing the beam below the lens respectively centered, cutting the right hand wall and cutting the left hand wall;
FIG. 4 is a schematic diagram of a practical embodiment;
FIGS. 5a, 5b and 5c illustrate an advantage of the invention; and
FIGS. 6a and 6b illustrate another advantage of the inventio
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Albritton C. L.
Anstalt Gersan
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