May 6, 2011
Ace Metal Crafts installs a laser tube cutting system with integrated tapping, a system that has shown its strength when producing structural tube. The machine cuts tube shapes, laser cuts holes, and performs tapping all in one setup.
For decades design engineers have embraced the efficiency and elegance of the structural tube. Equipment of all kinds have frames made of rectangular or square tubes attached to which are all sorts of elements that require mechanical fastening. That means those tubes have plenty of threaded holes, and Ghalib Abbed of Ace Metal Crafts knows all about them.
The Franklin Park, Ill., shop specializes in stainless steel fabrication, and during the past few years it has processed more stainless steel structural tubing for large pieces of equipment. Very large. One recent job called for long, 400-pound tubes with 20 threaded holes in each.
Abbed does not break his back transporting such tube between a band saw and tapping press, though. If he did, the shop couldn't quote such jobs competitively. Abbed is the company's laser specialist overseeing a team who can process those unwieldy tubes with minimal handling—using a 4-kW tube cutting laser with an integrated tapping unit (see Figure 1). The fabricator uses the machine, a Fabri Gear model from Mazak Optonics, to handle workpieces up to 26 feet long.
As Dale Ball, Ace Metal Crafts president, explained, the machine "rotates the tube, cutting it on all sides, and integrates the tapping during the operation."
"It's what we call a 'structural' processing system," said Keith Leuthold, director of inside sales for Mazak Optonics, Elgin, Ill., meaning that it can process a variety of open and closed shapes, including round, square, and rectangular tubing; and open shapes like C-channels, I-beams, and angles.
Within the work envelope, taps hang parallel to the laser cutting head, which is next to a touch probe that measures hole and cut positions. A chucked tube feeds into the work area, where the laser performs the end cut. Then the threaded-hole fabrication begins. For each, a probe descends to ensure that the actual workpiece position matches the program in the controller. Then the laser performs the initial pierce. Seconds later a spinning, lubricated tapping tool descends to cut the threads, after which it recoils and returns to the home position.
During the tapping tool's ascent, a lubricator sprays oil on the spinning tap to remove metal chips and strings remaining and to lubricate the tap for the next hole. For particularly heavy-duty jobs involving thick-walled tube, an operator may brush off the taps manually between jobs. Besides this, workers load and unload material and call up prewritten programs on the controller. Extended laser downtime is minimal.
To prepare a tube job that requires tapping, Abbed imports a 3-D model from the engineering department. With the programming software, he applies a toolpath for the end cuts and other geometries required. "Then, if there are tapped holes, I select which holes need to be tapped and choose the size of tap I want to use." Based on preprogrammed parameters, the software then adjusts the hole diameter to account for the tapping operation.
Performing the tap while the tube remains chucked in the system eliminates the secondary operation and, as Leuthold explained, makes it easier to achieve tighter tolerances. "If you did all your holes in one machine, took all the tube components out for tapping, there would not only be another setup, but it also would be more difficult to achieve the accuracy," he said.
As Ace Metal has found, the touch probe has been instrumental when cutting and tapping rectangular and square tube. The machine fabricates to tolerances as tight as ±0.005 in. As Abbed explained, Ace engineers have worked with their tubing suppliers, many of whom traditionally supply tube produced within only ±0.030 in. or so, as measured every 3 ft. of tubing. "But we must work with the entire length of the tube," he said. "The machine will chuck on one end of the 20-foot tube, and at the other end we're cutting. Over that length you can see the twist and bow, and the touch probe really helps us minimize those effects."
The tube cutting machine's probe-cut-tap sequence is significant. Laser cutting all holes first and then tapping them would complicate the operation, because every move the workpiece makes may affect positioning accuracy. It's a little like reclamping a machined part. Every time a part is unclamped and reclamped in a machining center, positioning accuracy is lost. In the machining arena, the done-in-one concept of multiaxis machines helps mitigate this. An operator clamps the part once, and the machine turns, mills, and drills the workpiece in one setup. Similarly, on the tube laser the probe-cut-tap sequence keeps the tube in one place for each tapped hole.
Abbed brought up another benefit. "I notice that if the laser cuts a small hole, and I allow it to cool, the small tap tool actually breaks," he said, adding that work hardening may be to blame. "This isn't scientifically proven, but I feel that you may get a stronger, more durable thread life out of that hole by tapping it while it's still hot, immediately after laser cutting, and then allowing it to cool after tapping," he said.
He added that the servos driving the taps themselves also help improve tap life and threading repeatability. Without the integrated tapping in the tube laser, operators would bring tubes to a drill press with a clutch, which Ace has used for years to tap holes in flat plate.
With the old system, the press forced the tap into the hole, made the threads, and then released the pressure, which reversed the tapping tool so it would spin up and out of the hole. But as Abbed explained, that clutch recoil action at the bottom "adds an excessive amount of drag and pressure on the tool itself, which is why we feel the tap tool wears a lot quicker." The servo-driven tapping action in the tube cutter, he said, with its position calibrated with the laser cutting head, helps increase tap life.
The shop also uses gold-colored, cobalt-covered taps suited for the shop's stainless steel work. Such tools have shown their strength, especially when tapping small holes. "We'll perform a 6-32 tap, which is a fairly small-diameter hole, working with 3⁄8-in.-thick mild steel angle material. That's an ugly process to do manually." But the machine, with its gold-colored tap tools and with the proper cutting conditions, can thread such holes very smoothly.
"Doing it the manual method, our sheet metal operators said they would be lucky to get 100 holes out of the tap," Abbed said. "Today we've been able to get 700 holes out of one tap."
Jobs sometimes call for tapping in extremely small-diameter tubes, and to do this requires special considerations. Such holes need the tap to descend a certain depth. The smaller the tube ID, the higher the risk of colliding with the bottom inside diameter of the hole. So for these holes the shop uses flat-bottom, blind-hole tapping tools, which can descend deeper into the hole without colliding with the tube's bottom ID (see Figure 2).
"We usually use [the taps with] the sharp points, because it helps clear out the spatter you get from the laser cutting," Abbed said. "But the blind-hole taps allow us to go a bit deeper without crashing or hitting the bottom inside diameter of the tube."
While the laser adds flexibility, it also requires additional programming considerations, especially when cutting a round hole in a round tube. The laser cuts normal, or perpendicular, to the surface, which is desirable when cutting rectangular and square tubes or other shapes with flat surfaces. However, the laser head also can cut normal to the surface of a round tube, which creates a conical shape that's especially noticeable in thick-walled material. Such a shape wouldn't be suitable for the tapping tool either, which descends in Z and is always perpendicular to the X axis.
A tapping tool descending into a conical hole isn't a recipe for precision. This is why programmers take special care when instructing the machine to cut round holes in round tubes, ensuring the laser head always cuts perpendicular to the X axis—not perpendicular to the round-tube surface.
The Timeless Threaded Hole
Ace's system isn't a panacea for all tube cutting challenges, of course. The shop's particular machine model does not offer countersinking, for instance. But overall, Abbed said that the tapping option has been a natural addition to laser tube cutting. Where the laser really shines is in structural tubing for large -equipment manufacturers. Just as it has for hundreds of years, inserting a screw into a threaded hole remains the most efficient way to put together countless assemblies.
And Ace Metal can promise that those threaded holes will be tapped just where they should be—within ±0.005 in.
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