Fiber laser cutting heads detect slugs

Process could eliminate need for post-cutting detection equipment

TOM KUGLER

The use of fiber-delivered lasers and robots is a natural combination for flexible, low-cost, three-dimensional (3D) cutting for production applications in many industries, with automotive systems leading this trend. In automotive manufacturing, production volumes requiring flexible tooling, along with the use of very strong and hard materials such as boron steels and high-strength steels, make lasers a natural choice for cutting solutions. For these reasons, robotic laser cutting applications have grown since the early 1990s. Using better trepanning robots, modern solid-state lasers, and more complex fixtures and control systems, as well as new robotic cutting heads, have all aided this growth.

The challenge

One process issue that requires attention is the challenge of "slug" detection. A slug is the material inside a cut feature that normally drops away during the process, such as the disk that falls away when a circular hole is laser-cut. Retained slugs can be related to many issues, including a bad cutting process, an incomplete cut because of thickness changes, a contaminated surface, poor gas flow, damaged optics, or poor focus position. Other contributors to the problem include material thickness changes from lot to lot, unintended changes to the cutting program relating to speed or power, and that thicker materials sometimes have a cut slug that rotates and becomes jammed in the hole.

Detection of a slug can help eliminate poor processing parameters, but it is also vital because a retained slug can cause problems down the line in quality control, manufacturing, or even life of the component or end product.

Slugs that fall during the laser cutting process are handled as recycled scrap and there are specific steps in the laser cutting process when slugs are removed. For flat sheet metal components, the slugs fall free during cutting to a scrap conveyor or receptacle. However, with tubing or other closed parts, the slugs are removed by strategically tilting the component at specific orientations above the scrap handling hardware to fall out by gravity.

In other instances, slugs may be lightly retained, easily falling out during further assembly or when the required component is placed into the hole. However, if they are solidly retained, that assembly step will fail and can cause damage to components. In the case where slugs fall out during later assembly steps, they now become loose debris that could interfere with other processes or become a nuisance rattle when trapped inside the closed structure of a completed component.

Recently, a Tier One automotive supplier of automated systems contacted Laser Mechanisms (Novi, MI) for a slug detection solution that would have minimum cost and cycle-time implications.

The tool

Robots provide a special environment for laser cutting heads. In comparison to two-dimensional systems, the robotic cutting head needs to be lightweight but also very robust to deal with the occasional collision with the part, or tooling when errors are made in programming or operation. Often, the cutting head employs a crash-protection link between the robot and the cutting head to deal with collisions with the part or tooling, or the hazard of snagging the cutting head umbilical (including the fiber-optic cable), and damaging the fiber or other conduits. Robotic heads must also be compact—not just for weight savings, but also because they are manipulated near tooling and the other surfaces of the 3D parts they process. Just like other cutting heads, robotic cutting heads must allow for different collimator and focus lens combinations for the material being cut, 20 bar coaxial gas pressure, and have all the alignment capability for tip centering and focus position adjustment.

FIGURE 1 depicts straight and right-angle robotic cutting heads. Straight versions are easy to integrate with systems that process both left- and right-hand parts on automobiles, but the straight versions have more access issues. While the head is not that long, the fiber connector and ~100–200mm minimum bend radius of the fiber itself requires a larger work envelope of 600–700mm above the surface. By employing a right-angle design, the fiber-optic cable and the rest of the umbilical connections can easily be directed down the robot arm, minimizing the working envelope to no more than ~ 275mm.

A working clearance envelope comparison of straight and right-angle robotic heads
FIGURE 1. A working clearance envelope comparison of straight and right-angle robotic heads.

Laser Mechanisms' FiberCut line of robotic cutting heads incorporate all of these features and more to optimize the robotic cutting process. Specific to the head is its low-moving-mass, self-contained z-axis for capacitive height sensing of surface contours. If the entire head moved on a z-axis, then this action would shake the robot and create inaccurate cuts. FiberCut only moves the lower section, containing the focus lens and gas nozzle, which results in a moving mass of only about 15% of the head's total weight and moves that mass at over 1g acceleration rates without shaking the robot's arm. With a laser power rating exceeding 4kW, these processing heads can handle all robotic applications in materials up to and over 6mm, and from mild steel to aluminum alloys.

The method

Dedicated slug detection stations add cost and complexity, use valuable floor space, and have slow total cycle times. Off-line detection tools also require that the part be loaded into a different fixture in an area where the hole cannot be re-cut if a retained slug is found.

Laser Mechanisms' solution to this issue uses the robotic cutting head itself to test for slug retention. If found, it can immediately re-cut the feature while still fixtured in the robotic cutting cell.

The patent-pending method employs special optical detection hardware installed into the robotic cutting head and software added to the head controller. It was found that if a slug is still in position, a short sensing pulse of light sent onto the area where the slug is suspected will return different signals if the slug is there or not. If the slug is detected, then the system can log the data and re-cut that feature immediately.

The low level of the short optical signal at the end of the cut indicates that no slug is detected
FIGURE 2. The low level of the short optical signal at the end of the cut indicates that no slug is detected.

FIGURES 2 and 3 show the signals returned from the sensors, with and without the slug dropping out of the feature. Note the variation at the end of the signal. The long duration signal is the laser cutting process, while the short signal just after is the slug detection signal. The high spike indicates the presence of a slug, and the low-intensity signal shows that there is no slug present.

The high level of the short optical signal indicates that a slug is present
FIGURE 3. The high level of the short optical signal indicates that a slug is present.

Most slug retention issues are related to the slug hanging up at the cut's start/stop point. In this instance, the check can be performed very quickly after the robot has completed the cut path by moving directly onto the area where the slug might be and performing the quick test. It is estimated that this check can be accomplished in only a few hundred milliseconds and isolated to areas on the part with known problem features that the end customer wants to inspect.

The slug detection hardware is safely installed into the robotic cutting head and uses signals in the existing control cable, so no extra wires are required. Because the optical path inside the cutting head must be clear for processing, it is also open for slug detection without interference from other processes or the dirty environment of a laser cutting cell.

Available as an optional feature, these slug-detection robotic heads can monitor system performance and part quality, and potentially eliminate the need for post-cutting detection equipment.

ACKNOWLEDGEMENT

FiberCut is a registered trademark of Laser Mechanisms.


TOM KUGLER (tkugler@lasermech.com) is fiber systems manager at Laser Mechanisms, Novi, MI; www.lasermech.com.

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