Electric heating – Metal heating – By arc
Reexamination Certificate
1999-09-15
2002-05-21
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121630, C219S121640, C219S121620
Reexamination Certificate
active
06392192
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to laser-equipped machine tools, and more particularly relates to real time control of laser beam characteristics for improved machine tool performance.
BACKGROUND OF THE INVENTION
Laser-equipped machine tools are often used to cut parts from sheet metal and relatively thin plate. They are also used to weld together cut and machined parts. In such machine tools, a laser beam is employed to process the material. A laser beam, also simply referred to as a beam, 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 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 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 focal 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.
Laser-equipped machine tools are typically Computer Numerically Controlled (CNC) and are manufactured in many configurations and sizes and with lasers of various types and power. Generally speaking, there are two beam delivery configurations utilized: those with a fixed length between the laser output coupler and the processing head and those with a variable path length between the laser output coupler and the processing head.
In one cutting machine configuration, typically called “flying optics,” the cutting head is adapted for movement along one axis, such as the Y-axis, which is mounted on a bridge adapted for movement in 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 the material, to form holes and shapes in the material and to cut the part from the material. Such machines can be configured with either a fixed length or a variable length beam path.
In a cutting machine configured with flying optics, a fixed length beam path is typically created in one of two ways. In one method, the beam path between the output coupler and the processing head consists of sections of tubular arms. The arm sections are connected via pivotable joints containing preloaded bearings with mirrors at the entrance and exit to steer the beam. As the process head moves, the tubular sections translate and pivot about the joints to follow the motion. While the fixed beam path length of such a system eliminates divergence problems due to path length, there remain concerns about the ability of the system to withstand high acceleration forces. Such a system also poses some difficulty with regard to adequately supporting the arms.
Another fixed length beam path approach is to provide an additional axis within the beam path and coordinate its movement to compensate for the positioning of the cutting head such that the length of the beam path does not change. One control means for such a system is disclosed in Fanuc Ltd. U.S. Pat. No. 5,406,048. Other methods are also in use.
On some machines, such as a “gantry” cutting machine, in which the laser is carried, this fixed length concept is relatively easy to implement. The machine consists of floor-mounted rails or ways about two parallel sides of a fixed table which supports the work. The rails carry a platform on which the laser is mounted. The rails also carry a gantry or bridging section over the work. Typically the laser-mounting platform is located
Cole, III Ira E.
Scott William B.
Elve M. Alexandra
W. A. Whitney Co.
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