Maximize laser tube cutting accuracy

Today’s fab shop faces significant pressures to reduce costs, improve quality, and optimize output. Laser tube cutting operations require a vastly different mindset from more common plate cutting operations. Not only do fabricators need to contend with these basic shop standards, but tube profile shapes, sizes, part intricacies, hole geometry, special cuts, and tighter tolerances are pushing fabricators to rethink their laser tube cutting processes to help maximize quality and efficiency. To ensure the highest accuracy, shops have a few things to consider.

Raw Materials

A number of widely recognized organizations set standards to specify the material, chemical, mechanical, and metallurgical properties of metals. These bodies include the Aluminum Association (AA), Steel Founders of America (ACI), American Iron and Steel Institute (AISI), SAE Aerospace Material Specifications (AMS),

American Society of Mechanical Engineers (ASME), ASTM International (ASTM), American Welding Society (AWS), Federal Specification (QQ), Military Specification Numbers (MIL-S), and Society of Automotive Engineers (SAE). The associated standard is applied to the full names of common metal alloys. However, when it comes to laser tube cutting, just because the tube complies with these standards doesn’t mean they are always the best option.

“There are a lot more tolerances to consider with tubing than when compared to flat sheet or plate,” said Robert Adelman, North American laser product manager, BLM Group USA, Novi, Mich. “In general, they follow the ASTM standard, however with that we know the tubing isn’t exactly the 2-in. square it is listed as, it is slightly larger or smaller and possibly not a square but a trapezoid with either convex or concave sides. Also, it has some degree of bow as well as twist. Lastly, we know that 3 of the radii will be consistent in size where one will be considerably different due to the welded side.”

Right off the bat, the raw material will play a significant role in the overall accuracy of the finished part. And that’s based on the part meeting the appropriate standards, but there are also some other issues to contend with.

When cutting with lasers, material thickness matters. For example, when cutting a 2-in., 0.120-in. wall or 11-ga. material, there is the potential to hit a section of the tube that is thicker than specified, and the machine must be able to compensate for this.

“Accuracy will really depend on what you are starting with,” said Tyler Van Wyhe, applications engineer, Mazak Optonics Corp., Elgin, Ill. “If the material is oversized or undersized, that’s one thing, but bowing and twisting will also be a significant factor. If the raw material cannot hold the tolerance – say ±0.03 in. – then you can't expect to make a part better than that out of the material. Getting a good-quality part out the door starts with high-quality tube. Depending on accuracy and tolerance required, it might be worth investigating where to best purchase the material.”

With supply chain issues popping up all across the manufacturing sector, accessing high-quality tube can be a challenge. But it’s important to note that not all tube mills are created equal, and finding one that meets tolerance requirements might require further discussions and inquiries outside of traditional options.

“There is an important distinction between tube mills in Europe and North America, for example,” said Adi Buerkler, TruLaser Tube product manager, TRUMPF, Farmington, Conn. “When I speak with my European counterparts, the quality of the tube from mills in North America is just OK in relation to the quality of tube we see in Europe. In my opinion, they have less twist, which is one of the biggest factors or challenges fabricators have to deal with. When you talk to a mill, make sure you request the best quality you can afford, something in the A, B, or C grade. I typically recommend B grade, but if you can afford to spend a little more money, A grade is best.”

Material Handling and Processing

While proper material handling and storage is necessary for any production process, it plays a significant role in tube processing operations.

“Material storing is a huge factor for shops to consider,” said Buerkler. “Without proper handling and storage, there is the potential for distortion. We always recommend a tube storage system. It not only helps ensure the quality of the raw material, but it can be paired with the tube laser machine to alleviate any material handling issues.”

Beyond storing and raw material handling, machine loading/unloading needs to be addressed. Unlike plate, tube is loaded as a bundle that requires each piece to be singled out and supported at the beginning, through the cutting process, and during unloading.

“Each step of this process requires a type of support,” said Adelman. “First is the need to support the tube on the raw material side, while loading and processing, as well the part during and processing and unloading. Whether the square tube is placed flat or is rotated on the diagonal, finding a machine that can address any supporting issues is a good starting point. Some machines have universal supports that can handle the tube vertically and laterally so that you're producing the best tolerances in your part.” Universal supports work well for closed sections, however rollers that move up and down with the section work best for open profiles like angle or channel.”

When it comes to handling, tube length comes into play. Especially with a long tube – more than 4 ft. – fabricators are more likely to encounter bowing and twisting.

“With long tube, it’s important to find a machine that can handle the entire length of the part,” said Van Wyhe. “For example, a machine with a four-chuck design is ideal for processing structural materials such as I- or H- beam, C-channel, angle iron, and HSS. Keeping full control of the materials helps with inconsistencies. Self-centring helps keep the material in line before cutting, which helps to make very precise parts. In a two-chuck system, not having the chucks on the back side makes dealing with inaccuracies even more challenging.”

Measurement

Depending on the diameter and wall thickness of the tube and how well it is supported throughout the machine during processing, there still can be added sagging, flexing, and bowing of the material. This will no doubt widen the existing tolerance range of the tube, which may already be at an unfavourable limit.

Many of today’s laser tube cutting machines have compensation cycles built into the cutting process to help ensure both positional and dimensional accuracy, especially for parts with holes and slots requiring tight tolerances.

Probes and Sensors. Touch probes and laser sensors are common methods of measurement that will help ensure dimensional accuracy during the laser cutting process.

“Some machines have built-in measurement cycles, where a capacitive height sensor measures how wide the tube is and the true position of the tube,” said Buerkler. “This measurement cycle is usually done once the machine pulls the tube in and clamps it. If the part requires a certain tolerance from the outside wall of the tube to a hole or slot, we have a measurement cycle for that as well. Using the data collected, the machine is able to calculate the offset to hold the tolerance needed.”

Van Wyhe added that machine options like touch probe and laser sensor aid in correcting material issues. The sensors can find the current location and orientation of the material and use that data to shift the program or turn the material to correct.

Cameras and Vision Systems. While probing is the most common form of measurement on laser tube cutting systems, cameras and vision systems offer another method of data capture and accuracy assurance.

“For example, a set of laser blades and cameras is placed right in front of the cutting head where it can scan at the point of cutting,” said Adelman. “Within 0.6 seconds the system can determine the amount of bow and twist that is in that area of the tube. The machine then uses this information to compensate and place the hole in the center of the tube, because we've captured say a 1-degree twist and it's bowed 0.01-in. to the left. Not only is this method faster but also has a higher accuracy than traditional probing as the tube can remain stationary during the compensation.”

Software. When most programmers begin to program a part in the CAM software, they don’t necessarily know the status or quality of the material they are receiving for it. One thing that they do know is the tolerance of the part.

Adelman gives the example of a customer with a 12-ft.-long tube part with many drain holes along with three pinholes that are 24 in. apart. The pinholes have a tolerance of ±0.004 in. to be centred in a tube.

“Within the CAD/CAM system, the programmer can choose those three pinholes with high tolerance levels, and they can set them to high accuracy,” he said. “The operator just turns on compensations in the machine, and it will not spend any time checking the drain holes. However, once the machine gets to that pinhole, right before it cuts the pinhole, it will identify that it’s a high-accuracy hole. It will then scan it and compensate, ensuring that high accuracy hole is correct. Having the ability to discriminate between a high-tolerance and a high-accuracy hole is very important, but the operator still has flexibility to control when and how they're using the compensation system.”

Machine Features

Many features are available on today’s laser tube cutting machines, and whether a feature is required is essentially application-based. But there are a few common considerations for the machine itself that fabricators should assess before purchasing.

For example, as previously mentioned in relation to long tubes, chuck design is important to overall quality.

“Fabricators may also want to consider a design of the infeed/feed-through chuck that allows the processing of tubes across the entire clamping range without any jaw changes,” said Buerkler. “This unique clamping method by mechanically connecting jaws centres the tube automatically without the need of any individual jaw calibration. Understanding the clamping method will help determine if the machine fits a shop’s needs.”

There are various options, like pneumatic and hydraulic jaws, so getting a better sense of how the clamping mechanisms work is a good starting point.

“There may be times or jobs that come in where a weld seam is required to be on or at a certain location,” said Van Wyhe. “This is where weld seam sensors and similar features may be a good investment.”

A flat sheet fabricator may not understand just how much the weld seam can affect laser cutting. The surface of this part feature is thicker and requires the laser cutter to compensate for that. And it’s not always easy to detect the weld seam with bare eyes. Tubes often have oil on them, are marked up, or have a little bit of accumulated rust, making seams difficult to find.

“A camera weld seam detection system can inspect internally where the tube is typically cleaner and has not been scratched up from shipping,” said Adelman. “The system can detect the weld seam from the images captured and then store it for the full bar nesting. Traditionally we relied on the operator to compensate with power or speed as a work around to ensure the weld seam cut consistently. Today’s systems are capable of adapting the parameters automatically where geometries are located on the weld seam by adjusting the power or speed of the machine. This means the machine is staying at full speed but then slowing down or increasing power on its own when it approaches a geometry located on the weld seam.

Source：Canadian Fabricating & Welding