Electric heating – Metal heating – By arc
Reexamination Certificate
2000-09-13
2002-10-08
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121600
Reexamination Certificate
active
06462301
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to laser-equipped machine tools, and more particularly to a heavy-duty laser plate cutting machine.
BACKGROUND AND SUMMARY OF THE INVENTION
In the following paragraphs, background information, and information summarizing the invention will be presented together so as to convey a coherent view of the significance of the invention.
Laser-equipped machine tools are often used to cut parts from sheet metal and plate. In such machine tools a laser beam, concentrated by a focusing lens or mirror to a small diameter spot, is directed to position the focal point above, on or below the surface of the material to be cut. The laser beam is directed by the focusing optic through a nozzle disposed immediately above the workpiece, with a pressurized gas being directed through the nozzle, typically coaxially with the laser beam, to assist making the cut. The pressurized gas interacts with the laser beam and material, facilitating the cutting process, and creates a high velocity stream that carries the melted material away from the cut.
Laser-equipped machine tools are usually Computer Numerically Controlled, and are manufactured in many configurations and sizes and with lasers of various types and power. The present invention relates to heavy-duty plate lasers, such as those which are capable of cutting steel plate on the order of one-inch thick or more at production cutting rates on the order of 24 inches per minute (ipm). The present invention is directed to a machine having those capabilities and sufficient adaptability to also efficiently handle lighter materials, such as sheet metal. In the most preferred embodiment, a “flying optic” configuration is utilized. In that configuration the cutting head is adapted for movement along one axis, such as the Y-axis which is mounted on a bridge adapted for movement along an orthogonal, X-axis. The work is supported on a stationary pallet or table below the bridge. Movement of the cutting head is coordinated with movement of the bridge to define a precise path on the part. The cutting head and laser are controlled to pierce, cut and form holes and shapes in the material, and then to cut the part from the material.
In a laser cutting machine, the laser beam is produced in a laser generator and is directed along a beam path via a beam delivery system. A beam delivery system is a collection of optical elements, such as reflective mirrors and transmissive optics, which may redirect the beam, alter the propagation characteristics of the beam or focus the beam. The beam delivery system is enclosed for safety and for control of the beam path environment within. The laser beam is concentrated by a focusing lens or mirror to a small diameter spot, which is directed to an appropriate position relative to the surface of the material to be processed.
In most implementations, the laser beam exits the laser through an output coupler, a partially transmissive and partially reflective optical element which seals the laser cavity and transmits a portion of the beam out of the laser cavity or resonator. The beam is then directed along a beam path to a focusing optic in a processing head near the work. In most cutting applications, the beam is directed by the focusing optic through a nozzle disposed immediately above the workpiece to be cut. A pressurized gas is also directed through the nozzle, typically coaxial to the beam, to assist the cutting process. The pressurized gas serves to facilitate and/or shield the cutting process, and creates a gas stream which helps remove vaporized and molten material from the cut or kerf. Kerf refers to the zone of material which is acted upon and removed by a cutting process. Kerf width refers to the width of the slot created by the cutting process, such as the width of the slot cut by a laser beam as it moves along a path.
Key factors in laser processing include the diameter of the focus spot and the position of the focus spot relative to the material to be processed. The control of these focal characteristics is critical to maintaining the quality of the process. During processing, unintended deviation in the focus spot size and position may produce a deterioration in process quality and may even cause the process to fail.
The first of two main factors which influence the focus characteristics is the diameter of the laser beam at the focal optic. Due to diffraction, the minimum focal spot diameter, for a given focal length optic, is limited. Diffraction causes light beams to diverge or spread transversely as they propagate. As the input laser beam diameter increases for a given focal optic, the focus spot diameter decreases due to a decrease in diffraction. In addition, as the input laser beam diameter increases for a given focal optic, the focus spot position shifts closer to the focus optic.
The raw laser beam, issuing from the laser resonator, exhibits the characteristic of divergence. The beam diameter will change as a function of the distance from the output coupler. Typically, as the processing head moves over the processing area the distance from the output coupler to the focal optic will change. When a large processing area is required, some method of maintaining the proper beam diameter must be employed in order to avoid significant changes in focus diameter and position.
Additionally, changes in the output power level of the laser will affect the divergence of the output beam. The largest effect on beam divergence comes from the thermal loading of the output coupler which produces thermal lensing. Thermal lensing is distortion of an optical component caused by heat absorbed from the input beam. The absorbed portion of the beam causes expansion of the output coupler such that the curvature of the surface changes. The expansion causes a change in the divergence of the output beam thereby changing the beam size at any given distance from the output coupler. The rate and amount of distortion is dependent upon the power of the beam, optic contamination, thermal conductivity of the optic and its cooling system and the length of time the beam is applied. Upon reaching thermal equilibrium, when absorbed heat is in balance with that removed by the lens cooling system, the shape of the optic surface remains constant. When the beam is turned off, the optic surface gradually relaxes and returns to its original shape. When a high output power laser is required, some method of maintaining the proper beam diameter, in a time dependent response to output power changes, must be employed if significant changes in focus diameter and position are to be avoided.
The second of two main factors which influence the focus characteristics is the distortion of the focus optic due to heat absorption. In a manner similar to that described for the laser output coupler, thermal lensing occurs in the focus optic. The expansion of the focus optic reduces the effective radius of curvature which causes the focal spot to shift closer to the focus optic. When a high output power laser is required, some method of maintaining the proper focal position, in a time dependent response to input laser power changes, must be employed if significant changes in focus position are to be avoided.
Proper focal position is very important in cutting heavy plate. In initiation of a cut, the plate must be pierced, and a preferable piercing technique requires “driving” the beam through the plate. This can be accomplished by altering the position of the focal spot, by actually moving it into the plate as the piercing operation progresses. Furthermore, in cutting different types of materials, it is often useful to alter the focal spot position with respect to the surface of relatively thick materials so as to optimize the quality of the cut.
Turning now to the divergence issue mentioned above, one method employed to reduce the divergence of the laser beam is to expand or magnify it with a collimator. The rate of divergence of a beam is reduced in inverse proportion to the amount it is magnified. If a beam is magnified by
Cole, III Ira E.
Scott William B.
Elve M. Alexandra
Leydig , Voit & Mayer, Ltd.
W. A. Whitney Co.
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