3D Contouring Systems For Laser Cutting
Laser processing has become increasingly competitive with conventional methods for cutting three-dimensional metal and non-metal parts. Major productivity increases have been reported by the growing number of users of industrial multiaxis carbon dioxide (CO2) laser systems. For example, in replacing conventional machining, laser cutting has increased throughput in trimming a deep drawn gas turbine engine component from 18 pieces per day to 18 pieces in 30 minutes. For another, laser cutting has replaced hand methods for cutting intersecting tubing used in aerospace applications. With the hand method, one assembly was produced every 1.5 hours. With laser cutting, the rate has been increased to one assembly every minute.
Reports like these are largely due to the laser system features discussed in the following sections.
Performance of a multiaxis system is defined in terms of processing (cutting) rates, dimensional control (accuracy, repeatability), and physical characteristics (edge finish, heat affected zone size) of the laser processed part. With average CO2 laser power of 1kw, cutting rates of several hundred inches per minute are easily attained. Furthermore, laser processing can easily produce dimensional repeatability of 0.002 inch or better in sheet metal thicknesses.
Computer Control, motion system, and beam delivery system design must be carefully considered in high performance, multiaxis systems.
To achieve dimensional tolerances and processing rates consistent with laser processing capabilities, the following must be considered in design of the system.
- The control (CNC) must be capable of data processing rates complementing high laser cutting speeds. At high feedrates, controls designed for conventional machining or sheet cutting may reach computing limits and starve the motion system when processing five or more axes of data.
- Axis feedrates must be sufficiently high (around 500 inches/minute) to allow high velocity at the nozzle tip. Small moves of related linear axes near the part surface can require substantially longer moves of related linear axes. Consequently, moderate surface feedrates may require very high feedrates on axes.
- The motion system must be sufficiently rigid at all processing speeds to promote dimensional accuracy.
- The ability to control laser power from the CNC allows the power to be synchronized with feedrate to, thereby improving accuracy and cut quality.
- Automatic focus control compensates for part variation, spring back and stress relief, again improving accuracy and cut quality.
Motion systems generally consist of laser beam and/or workpiece positioning. For example, applications such as tube cutting of round parts are best performed using a combination of beam and workpiece motion. For trimming of three-dimensional parts, beam motion has the following advantages:
- Being flexible inasmuch as part size and shape do not influence performance of the motion system.
- Since the workpiece remains stationary, parts may be set up in one area while parts are being processed in another.
- Moving-beam systems require less space than the equivalent system in which the workpiece is moved.
- Since the parts are stationary, fixturing is simple. There is no inaccuracy due to the part being rapidly positioned.
- Parts much larger than axis travel limits may be processed:
Design of the gas assist assembly is critical to performance of the system. The gas assist assembly must be designed to provide nearly laminar, coaxial flow and to allow access to tight corners. Access is especially important for trimming parts with small inside radii while maintaining the beam perpendicular to the surface and providing an adequate supply of assist gas to the cut.
Many formed metals and plastics are characterized by high residual stress and dimensional instability and variation. Consequently, parts may vary from one to another and/or change shape significantly during processing. Consequently, this characteristic must be considered to insure repeatable finished dimensions of cut parts. A non-contact automatic focus control will compensate for these effects.
A common solution to this problem is to fixture the workpiece to repeatedly locate a cut line. Sophistication of the fixture depends largely upon the quantity of parts for which it will be used. Cutting fixtures vary from fiberglass structures matching the part to be trimmed to aluminum casting with computer controlled electromagnets. The major requirement is that the fixture be designed to avoid material directly behind the cut line that will interfere with the cutting process.
The effective performance of a system is increased by features that allow the system to perform multiple tasks. For example, software for character generation and serial numbering allows the system to be used for marking while cutting, thereby eliminating an additional operation.
The ease of programming and operation determines the required level of programmer and operator skill required to make the system. The system features that influence this are identified in this section.
NC programs for contour cutting on three-dimensional parts are developed by a combination of the methods mentioned.
Programming features available in many CNCs are also used in programming three dimensional motion paths. For example, linear and circular interpolation software and convenience features such as variable and subroutines significantly reduce programming time for cutting on a primary plane. In addition, plane rotation allows a routine written for one of the primary planes to be rotated as needed.
The ability to control laser conditions is key to these systems. Frequently, laser conditions must be changed many times throughout a part program to optimize quality and throughput. Automatic power control provides automatic control of average laser power as feedrate changes. he benefit of this is significant reduction in programming and set up time.
As the number of axes and part complexity increase, manual programming becomes less viable. For many applications, particularly in trimming irregular metal or plastic shapes, teaching the system the part path is the most effective (or only) means of programming. However, since many systems are used for low volume production of many different parts, programming and setup can account for a significant amount of the system's available time. Consequently, it is important that the teaching time can be minimized. Teach mode features that support this objective are:
- Manual programming using linear and circular interpolation, and programming utilities such as plane rotation, R axis (movement along the beam axis) focal lock and Auto Power Feedrate Programming
- Teaching of laser beam path coordinates
- Program generation routines for special applications
- Interfacing to CAD/CAM systems that support three dimensional part programming
- Splining is a mathematical function used for multiaxis curve smoothing. It is extremely useful for smoothing the motion path in up to five axes between taught points on a contoured surface. With splining, a user-defined number of beam path coordinates is generated from a significantly smaller number of points entered through the Teach mode. This substantially reduces the number of points required to provide a smooth contour, thus shortening the programming time.
Focal lock is a software feature which shortens teach mode programming time. Focal lock allows the programmer to quickly orient the beam axis perpendicular to the part surface by keeping the nozzle at the taught point while the operator manually jogs only the rotary axes.
Editing taught splined three-dimensional programs is often desirable to create a more accurate tool path. Two software aids are helpful in editing. An edit mode allows the operator to exchange taught points to create a more accurate program. Another mode uses an automatic focus control in an auto-correct mode to automatically step through a previously taught program, stopping at each major point to turn on the automatic focus control and corrects the part program for focal distance.
Automatic feedrate programming is another software feature which reduces part 0rogramming. This software examines the tool path and calculates an optimum feedrate for each point along the path depending on path curvature. Automatic feedrate programming is then tied to laser power control such that the laser is pulsed at a frequency proportional to feedrate. The resultant advantages are:
- Teaching time is reduced by 50% typically. This can save four to six hours of programming depending on part complexity.
- Since the feedrate is optimized, cycle times are reduced thereby improving productivity.
- Since laser power is synchronized with feedrate, minimal heat is input to the part. This avoids part distortion and burning of delicate part features.
Program generation routines limit or eliminate the need for the programmers by allowing machine operators unfamiliar with CNC programming to convert information from a drawing or work order into a CNC program. Intersecting Cylinders is one example of program generation software used in fabricating intersecting tubing assemblies for the automotive and aerospace industries. This software provides a "user friendly" method of generating programs for (1) producing the cutout in the primary tube and (2) cutting off the mating end of the secondary tube. Intersecting Cylinders prompts the operator for the following information:
- Primary Tube Diameter
- Secondary tube diameter (for generating path for cutoff of Secondary tube)
- Angle of intersection
- Centerline offset (for cylinders whose centerlines do not intersect)
- Number of points along the cutout Tubing materials (type and thickness)
With this information, a complete CNC program for the particular assembly is generated. Laser cutting conditions for the tubing material type and thickness are automatically inserted into the program from a previously defined materials processing database.
The system's ability to satisfy the two other requirements is of little consequence if it is not reliable. For reliability, the system must, first of all, be made up of reliable components with these integrated for reliable operation together. An example is crash protection designed to prevent damage to the system in the event that it is positioned into the workpiece, tooling or other rigid object. A closed loop water chiller that promotes a reliable laser operation. A closed loop chiller maintains a consistent laser temperature, and therefore, a stable power output.
The system must be designed for reliable operation in the environment in which it will be located. For example, a filtered and pressurized beam delivery system insures reliable operation in an industrial environment by protecting optical components from damage by airborne contaminants such as smoke and dust. This also promotes stability of the laser power delivered to the workpiece.
The system must also be designed to avoid failure due to high temperature alone or in combination with high humidity. Water-cooled electrical and optical components can condense water vapor from the air with the corresponding potential for degradation of system performance or failure. Where the system cannot be located in a controlled environment, the system itself can be outfitted with air conditioners for controlling the local environment of the laser control.
The system should also be designed for minimal or easy maintenance. For example, automatic lubrication devices and sealed bearings should be used eliminating the need for manual lubrication. Mirror mounts should be designed for easy replacement following periodic cleaning. Software should compensate for dimensional changes due to a new focusing tens or and adjustment in the beam delivery optics thus avoiding the need for reprogramming.
While industrial three dimensional laser systems represent high capital investments, productivity improvements can generate short payback periods or other benefits that make them attractive. Numerous successful installations of multiaxis laser cutting systems have demonstrated that systems can provide meaningful solutions to real-world laser cutting problems.
For example, multiaxis moving laser beam systems incorporate mechanical designs and motion control that support laser processing's capabilities for high cutting speeds. Applications software provides operators and interactive method of part program generations to reduce programming time. Integration of the systems with CAD/CAM and automated workpiece handling equipment further automates the system.
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