Electric heating – Metal heating – By arc
Reexamination Certificate
2000-08-10
2002-07-16
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121830, C219S121630, C219S121640, C219S121620
Reexamination Certificate
active
06420674
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, or focus. The laser beam is directed to position the focal point above, at 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 the cutting process. 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. The present invention is directed to a machine having those capabilities and, in some instances, 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 and cut thereby forming holes and shapes in the material, and then to cut the part from the material.
In configuring a versatile machine tool capable of cutting heavy plate, it is highly advantageous to provide for the use of focusing optics with different focal lengths. An optic with one focal length can be used for cutting thick plate, and another with a different focal length can be used for cutting thinner materials. The ability to change focal lengths is an important feature in a heavy-duty plate machine adapted to also cut lighter materials.
The focal length of the optic contributes to the diameter of the focal spot and thus the energy density, Watts per unit area, at the focal spot. Shorter focal length optics create smaller focal spots having higher energy densities. The focal length of the optic also contributes to depth of focus of the focal spot with longer focal lengths having greater depth of focus. Shorter focal length optics are advantageous for cutting thinner materials while longer focal length optics are advantageous for cutting thicker material. The focal length of the optic and the power level of the laser contribute to the energy density remaining in the laser beam at distances beyond the workpiece during various stages of the cutting process.
Adapting high power lasers to cut thicker materials leads to using focusing lenses with longer focal lengths. Below the focal point, a laser beam expands at approximately the same rate that it was focused. For example, if a 35 mm diameter laser beam is focused by a lens with a 10″ focal length, then, 10″ below the focal point, unless absorbed by the material cut, the beam would be approximately 35 mm in diameter again. Twenty inches below the focal point the beam would be roughly 70 mm in diameter. This remnant diverging beam from high power lasers has considerable capability to cause damage. For example in certain tests, a 0.125″ thick aluminum plate was scuffed with steel slag, then a 38 mm diameter 5500 Watt beam was directed at the surface. The aluminum was cut through in 40 seconds. Similar tests were made with 0.25″ inch thick stainless steel and carbon steel. Both were cut through in well under a minute. These tests indicated that a scrap collection bed underlying the cutting area of a high power laser system, with long focal length optics in use, would be at considerable risk of being damaged by the remnant laser beam.
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, of a typically diverging beam, increases at a given focal optic, the focus spot diameter decreases due to a decrease in diffraction. In addition, as said 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
Cole, III Ira E.
Scott William B.
Elve M. Alexandra
Leydig , Voit & Mayer, Ltd.
W. A. Whitney Co.
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