Electric heating – Metal heating – By arc
Reexamination Certificate
2002-06-04
2004-08-17
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121700
Reexamination Certificate
active
06777641
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to machine tools, more particularly relates to laser-equipped machine tools for cutting metal sheet and plate and specifically relates to controlling a laser apparatus to pierce and cut metal sheet and plate of various types and thicknesses.
BACKGROUND OF THE INVENTION
A heavy-duty laser equipped Computer Numerical Controlled (CNC) machine tool has been developed for cutting steel, stainless steel, aluminum and other metal sheets and plates within the range of approximately 0.040 to 1.25 inches thick. The machine has been adapted with a 6 kW laser to cut metal plate, and with linear motors to quickly cut thin metal sheet. A flexible means of controlling a piercing cycle and transitioning to a cut is needed for such a machine.
It is advantageous that the piercing, transition and cutting parameters are associated with a particular machine tool rather than being dependant upon a separate programming means. All of a plurality of identical laser equipped machine tools may not cut exactly the same. Differences in age, run time, and time between service intervals can cause laser cutting machines to have different cutting characteristics. When piercing, transition and cutting parameters are associated with or comprise part of a unique machine tool, it is possible to move a part program to any of a plurality of identical machine tools, run it there without change and achieve good results.
It is also advantageous to improve the process of piercing a plate to reduce the piercing time and to minimize the residual heat left in the plate by the piercing process. Reducing the piercing time improves productivity of the machine tool. Minimizing the residual heat left in the plate by the piercing cycle improves the cutting characteristics of the workpiece.
When a laser beam is applied to cut a workpiece, a piercing operation must be performed at the start of the cut. Piercing thinner metal sheets with a laser is a well-established and conventional process. The piercing operation is typically performed with the output of the laser maintained constant, that is, with the continuous wave power level or pulsed beam power level, pulse frequency, and duty cycle maintained constant from start to end of piercing. Piercing thicker metals is difficult and slow with this process, particularly when piercing carbon steel plate with oxygen assist gas. The piercing time for a one-inch thick carbon steel plate utilizing this process, a 6 kW laser, and a cutting head, in which the focal lens position is fixed relative to the cutting nozzle during automatic operation, may be over one minute.
Nakata et al. U.S. Pat. No. 5,434,383 discloses an improved piercing process in which the pierce is started with an initial pulse frequency and an initial pulse duty ratio, and the pulse frequency and the pulse duty ratio are increased by predetermined increments over predetermined time periods. Column 1, lines 12-20, describes piercing thick carbon steel as extremely difficult. The disclosed method of programming a pulse frequency increment and a pulse duty ratio increment fixes the rate of change of the frequency and the duty ratio such that it is linear over the total time period. One of the inventors of the present invention observed operation of a machine tool equipped with a 6 kW laser. The machine tool was equipped with a cutting head in which the focal lens position was fixed relative to the cutting nozzle during automatic operation. The time of piercing a one-inch thick carbon steel plate using the technique disclosed in U.S. Pat. No. 5,434,383 ranged approximately from 20 to 60 seconds. Variations in chemistry and/or surface characteristics of the material cause the great variation in pierce time.
A defocused pierce process is another method for reducing the piercing time of a metal plate. The process is described in Kanaoka et al. U.S. Pat. No. 5,770,833 (though not called therein “defocused pierce process”) at column 2, lines 6-20, with reference to FIG.
15
. Column 2, lines 21-54 describe problems associated with the process.
FIGS. 16A
,
16
B and
16
C of the patent are examples of the upper surface of 12 mm thick mild steel pierced with the process. Columns 2 and 3, lines 55-7 describe the process using a double nozzle.
FIGS. 18A and 18B
show the upper and lower surfaces of a 12 mm mild steel plate pierced with a double nozzle and the defocused pierce process. The diameter of the pierced hole produced by the double nozzle is approximately twice that of the holes pierced with a conventional or single nozzle.
Carbon steel plate in the range of 0.75 to 1 inch thick can be pierced in 3 to 5 seconds with the defocused pierce process and a 6 kW laser. However, the process is problematic in this range of thickness. The pierced hole is approximately 0.25 to 0.375 inches in diameter. So much material is blown upward that the nozzle and the focal lens are often damaged. The pierced plate is often left very hot impeding the following cutting process. When employing the defocused pierce process on a plate that has many pierced holes, it is advisable to pierce all the holes first then stop and sweep the surface of the plate clean before continuing with cutting the plate. Such requires operator intervention and thus impedes automatic and unattended operation of the machine tool.
Kanaoka et al. U.S. Pat. No. 5,770,833 discloses several improvements to the defocused piercing process. The improvements include the steps of: (1) locating a processing head at a piercing start position such that the laser beam is focused at a point spaced vertically from the surface of the workpiece and offset horizontally from the intended piercing point; and (2) moving, while irradiating with a laser beam and jetting an assist gas, the processing head simultaneously in directions parallel and perpendicular to the workpiece surface, to the piercing point. Further improvements involve the type and/or control of the assist gas.
Topkaya et al. U.S. Pat. No. 5,332,881 discloses a laser cutting head with an automatically adjustable optical focusing system. Column 2, lines 41-43 mentions drilling a workpiece by means of a laser beam. Lines 43-54 describe continuous displacement of the focal point in the direction of the beam during drilling as advantageous for thick workpieces.
BRIEF SUMMARY OF THE INVENTION
The inventors have developed a flexible method and apparatus for controlling laser piercing of metal sheet and plate and transitioning to a cut. A controller is configured such that pierce and cut control parameters are associated with and comprise part of a unique laser equipped machine tool. A piercing cycle is subdivided into a number of sequential increments, and the time duration of each increment is individually selectable, as are the machine operating parameters. The controller is flexible such that a piercing cycle can be optimized for type of material, i.e. mild steel, alloy steel, stainless steel, aluminum, etc., and for material thickness. Further, the controller is adapted such that parameters controlling laser mode, power, pulsing characteristics, focal position, and assist gas pressure can be changed and auxiliary functions can be selectively engaged and disengaged within the pierce cycle.
Preferably the controller is resident in either the CNC of a laser equipped machine tool or another computer communicating with the CNC. The term “computer system” will be used herein as a generic description of the multiple types of computer configurations found in machine tools of this type. The terms “CNC” and “computer system” are inclusive of one or more of such systems connected in a computer network. In a preferred embodiment, the controller includes a Material Parameter Library which is comprised of a plurality of computer readable files. An individual file is herein called a Material Parameter Library file, a MPL file, or a MPL record. The Material Parameter Library can be considered a database and may be comprised as such. The Material Parameter Library resides in a directo
Cole, III Ira E.
Long William L.
Elve M. Alexandra
Leydig , Voit & Mayer, Ltd.
W.A. Whitney Co.
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