Technology advances make 'lights-out' operations a reality
May 18, 2002
This article discusses the history of lasers and material handling equipment with relation to unattended operation. It specifically examines material load/unload devices, sheet separation and detection devices, the auto-focus laser lens, raw material storage and retrieval systems, automatic part sorting systems, problem notification systems, cut control devices, and nozzle cleaning equipment.
During the 1980s the term lights-out as it applied to CO2laser processing equipment had a different meaning than it does today. In those days a lights-out environment meant completing the sheets that were left on the cutting table by the previous shift. Sheets that were already loaded onto the cut table or shuttle table before the operator left for the evening could be processed in a lights-out condition. Only cutting programs that had been tested and materials that were considered safe for unattended processing met the lights-out criteria.
In those days mechanical contact heads were prevalent and prone to collisions because of their large footprint and the probability of part tip-up after cutting. Material handling systems were limited to manually controlled suction cup loading; automated unloading did not exist.
In the early 1990s manufacturers of CO2laser systems began to address some of the problems that were impeding the progress of a true lights-out production environment. The initial focus was on achieving a lights-out operation capability that would last for an eight-hour shift.
The first steps were small but significant. The large footprints of mechanical contact cutting heads were replaced with the smaller profiles of the noncontact, capacitive-style cutting heads. These heads were less prone to collisions with adjacent cut parts and were not affected by the molten material that accumulated on the plate surface during the piercing process.
The ability to program microtabs on the contours that were prone to tip-ups further enhanced reliable operation in a lights-out environment.
The adaptive mirror allows automatic focus position control, depending on material type and thickness.
The biggest technological enhancement for lights-out production was an automated material load and unload device. This system automatically loaded the material from a single stack of raw material and unloaded the cut material onto an unload table.
This was the single most important technological advancement toward achieving a lights-out environment for an eight-hour shift.
Early on it became obvious that fabricators needed a way to ensure that sheets were loaded one at a time. When sheets were stacked, the oil or plastic coating on them created a vacuum seal between the sheets.
With ferrous materials, a set of powerful magnets could fan the edges of the sheets and thereby break the vacuum between them. The real challenge was separating nonferrous materials. Magnets were useless, so fabricators needed a way to simulate the fanning effect of the magnets.
A mechanical sheet separation technique was developed whereby one suction cup on a loading device peeled the corner of the sheet by lifting itself repetitively higher than the rest of the suction cups that held the sheet. A jet of air from a prepositioned air nozzle further enhanced the separation by forcing air between the sheets.
To verify that two sheets were not loaded together, a device measured the thickness of the sheet after the separation process. This device detected the presence of two sheets by measuring and comparing the thickness of the material being loaded with the required thickness. If the double-sheet detector found that multiple sheets had been picked up, it automatically re-executed the separation process and then measured again until it found only one sheet being loaded.
The combination of a programmable material storage system and a fully automated laser cutting system capable of loading, cutting, and unloading became known as a flexible manufacturing system cell.
In the laser industry, the term autofocus initially described the ability of the material-sensing device to maintain a constant distance between the nozzle tip and the material. This was achieved either by a mechanical contact or by noncontact, capacitive means. The ability to maintain a constant nozzle height regardless of material height deviations meant that the focus position was automatically maintained, because the lens was located within the cutting head.
One of the first autofocus systems had a motorized focal lens position to allow the adjustment of focal positions for different material processes. In other systems, focal position was adjusted by electromechanical or hydraulic distortion of a beam delivery mirror (see Figure 1).
These mirrors are specially ground to be concave in their normal state, but they can be changed to flat or convex when force is applied to the back of the mirror. The shape of the mirror at the time the beam makes contact determines the focal point projection within the material being cut. This process is computer numerically controlled, and each material type and thickness can have its own automatic preset focal adjustment.
The ability to adjust the focus automatically became necessary to process different materials in a lights-out environment. Without an autofocus feature, production was limited to those materials whose focal position was the same. Autofocus became one of the first features of a flexible manufacturing system (FMS).
With the advent of autofocus, manufacturers had to provide users with a way to store multiple material types and thicknesses. The first raw material storage units were single towers with multiple shelves. The shelves could be controlled to move into the load position.The first systems were considered "dumb" towers because they did not know which materials were on each shelf. They functioned sequentially from shelf to shelf as each preceding shelf was emptied. Users had to sequence their programs' material requirements with the order of the raw materials on each successive shelf.
Programmable "smart" towers later were developed. Storage management software kept track of the material type and thickness on each shelf, including the quantity. This combination of a programmable material storage system and a fully automated laser cutting system capable of loading, cutting, and unloading became known as an FMS cell (see Figure 2).
A few companies have introduced part sorting systems that automatically sort and stack cut parts. Parts are sorted either by a robot with suction cups or a rotary unit with suction cups attached to a movable gantry.
Some manufacturers have implemented a method of detecting a fault in the cut. With these systems, if the material begins to emit a plasma signature higher than what is expected, the CNC automatically detects the problem and attempts to correct the improper cut conditions.
After the parts have been sorted from each sheet, the sheet skeleton is disposed of, and the machine is readied for the next sheet.
With lights-out operation, the machine must keep working uninterrupted. Some systems have the option of a paging or call-back system that notifies users when the laser system stops for any reason. Then they can return to the facility to correct the fault and return the system to automatic operation.
Some manufacturers have implemented a method of detecting a fault in the cut. A photoelectric device constantly measures the intensity of the plasma light generated within the cut kerf (see Figure 3). If the material begins to emit a plasma signature higher than what is expected, then the material is not being cut properly and a loss of cut is imminent.
These systems automatically attempt to correct the cut by varying the feed rate momentarily or re-executing the cut from the point of interruption.
A clean nozzle tip is critical to maintaining a consistent nozzle distance and, subsequently, a consistent cut quality. Accumulation of molten material or residues on the nozzle tip surface affect the nozzle height and, therefore, the focal position.
A CNC can execute a cleaning cycle after a certain number of pierces or at the end of each plate.
As manufacturers make technological advances in creating completely automated laser cells, the benefits of these systems continue to grow.
Less labor is required because the raw material is already inventoried in a tower and retrieved automatically for processing. After being cut, parts are stored and sorted automatically as well.
The ability of these machines to operate unattended during off-shifts also has tightened delivery schedules for just-in-time (JIT) production.