April 28, 2014
To avoid a heat affected zone and improve accuracy and throughput, fabricators can turn to nonthermal beveling options, incluiding milling, nibbling, and automated grinding
Say you need a way to bevel that’s fast, extremely accurate, and, thanks to material property requirements, has little or no heat-affected zone (HAZ). Such application requirements have driven companies to alternative beveling technologies that aim to create an accurate bevel faster, freeing a common constraint.
Options range from shape beveling power tools that fit in the palm of your hand to immense automated machinery that can create extremely large, precise bevels in relatively short order (see Figure 1). For the thickest of workpieces, these machines can shave hours off cycle times. If properly used, they can eliminate the need for postbevel grinding and finishing. Ideally, plates large and small emerge ready for the next operation.
Many power tool beveling options involve some type of milling cutter. Think of a wood router, just for metal. These power tools come in two broad categories: straight and shape bevelers (see Figures 2-4). Straight bevelers often have guide plates to position the tool along a plate edge. Some tools have a fixed bevel angle, while others are adjustable, and operators can make adjustments to change the depth of cut and bevel width.
Shape bevelers, as their name implies, can bevel not only straight sections but also contours. “Shape bevelers are not restricted to straight plate,” said Iwan Antonow, executive vice president at Saar-Hartmetall USA LLC, Covington, Ky. “They can be guided around curved and round edges, and they can be great for countersinking.”
“A shape beveler may not be as effective in straight cutting as machines that are designed for straight beveling,” said Phil Heck, president of Heck Industries, Hartland, Mich. “They may be a little slower. But shape bevelers really shine on contour cuts and holes,” he said. “Some of the smallest shape bevelers can work on holes down to 3⁄8 inch in diameter.”
Basic factors to consider include depth of cut and quantity. As Heck explained, “Are you going to use a hand-type beveler, or look at a power-feed style for higher production?”
Automatic-feed machines fall into two general categories: those for easy-to-lift parts and those for large workpieces in which the machine or driven power tool is brought to the plate. For easy-to-lift parts, you may choose to use stationary machines, which can work with a range of part sizes. On certain machines, a drive wheel or other mechanism guides the plate across a milling cutter that emerges between two guide plates. You set the angle and depth of penetration by adjusting the guide plates and tool position, and set the feed and speed. You then place the workpiece in position and bevel away, producing the bevel in one pass or several, depending on the bevel width. For K bevels, you run the plate through, flip it over, and produce the second cut (see Figure 5).
“These machines can operate in a continuous flow,” said Antonow. “You let the guide wheel push it through, and it emerges burr-free.”
For workpieces that are too large or heavy to lift, some power-feed systems can be brought to and affixed to the plate. And still others have a custom tooling arrangement that bevels a little like an electric can opener, though one with a cutter rotating at a low RPM. “As the cutter turns at a low RPM, it creates the bevel and moves the material at the same time,” Heck said.
If it becomes difficult to get a bevel cut started, that’s a key sign that tools are becoming dull. Milling inserts can be indexed, allowing you to wear three or four working surfaces off of a single insert.
“With today’s newer technology, most inserts now have a coating on them that helps them protect the cutter and produce a better edge quality,” said Mike Marshall, technical sales and customer service manager at CS Unitec, Norwalk, Conn.
Some large, stationary power tools can automatically apply lubricant to the beveled edge, and technicians can manually apply lubricant for use with hand-held power tools. “From just a tool life perspective, it’s always advantageous to use lubricant,” Marshall said, “but most people don’t do it.” Why not? It’s often because of the workpiece cleaning requirements for paint prep or other downstream operations. The time it takes to clean the workpiece properly may cost more than simply living with a shorter tool life on the beveling tool.
If inserts wear prematurely, the depth of cut may be too aggressive; more cutting passes may be necessary to complete the bevel. Some harder grades may require more passes than other grades. Cutting too aggressively on hard material like stainless can cause enough frictional heat to work-harden the surface. Of course, a cut that is too light may require an excessive number of passes to complete the bevel. What you save on better tool life may be lost to lower productivity. Similar thinking applies to feed rate. If the feed is a little slower, tool inserts will probably last longer, but again at a cost of lower productivity.
“The first determining factor is the material grade, the hardness of the material,” Antonow said. “Next, how wide of a bevel are you applying in one pass? And is the operator applying steady, uniform pressure to the machine? Operator input is important. With the [semiautomated] machines, it’s a little bit more predictable, because you’re taking the human factor out of it.”
Marshall added that when you use hand-held power tools, tool life will certainly increase with experience. Someone who hasn’t used a beveling tool before may well apply excessive pressure, or let the tool bounce or chatter. “There’s a learning curve,” he said. “You’re going to go through your first set of cutters very quickly, until you get a feel of the pressure needed and the optimal speed and feed.
“To start, try using less pressure at first,” Marshall added, “and then slowly feed the tool in. As you increase the bevel face width [after several passes], you have less of a sharp edge [of the plate] contacting the sharp edge of the cutter, so it will start to smooth itself out.”
Other tools employ a nibbling operation in which a rectangular punch moves up and down to shave an edge. “Measuring the hypotenuse of the bevel angle, this tool can take about 5⁄8 in. off in one pass, at about 6 feet per minute,” said Tony Mirisola, product group manager, power tools, at Farmington, Conn.-based TRUMPF Inc.
Different nibbling punches are used for different material. Nibblers can bevel around contours as well as straight sections, and they come in manual feeding and self-feeding varieties (see Figure 6).
Nibbling tools can be indexed as well. The rectangular punch has four cutting edges. You can start in the middle of a plate edge and move one direction, to wear one corner of the nibbler’s punch, then move in reverse to wear the other corner. Once those two edges are worn, you can index the tool 180 degrees to expose the other two working edges (see Figure 7). Once all four sides are worn, the tool can be brought to a surface grinder for sharpening.
Mirisola added that a nibbling operation doesn’t lend itself to multiple passes. “Those extra passes will actually wear out the punch sooner,” Mirisola said. “So we suggest you do it in a single pass at the desired cut length.”
A precise bevel can be the difference between success and failure in operations downstream. But many of these jobs involve plates too thick for a typical plate-bevel milling process to be cost-effective. “The thicker your plate is, the more costly the milling option becomes,” Antonow said.
“In Europe, welds in structural components now require non-heat-affected weld preparations for any component with structural strength requirements, such as shipbuilding, railcars, cranes, forklifts, pressure vessels, and large-capacity tanks,” said Dan Dechamps, president of Plainville, Conn.-based MetalFinish LLC.
From all this came the development of automated CNC grinding systems for beveling (see Figure 1). The machine clamps material, and a belt-grinding head contacts the workpiece edge at the appropriate angle to create a bevel, usually in multiple passes (see Figure 8). Despite the sparks, the grinding process remains a cold beveling process, with most of the heat going into the chips being ground off the workpiece.
“The grinding unit positions in the correct angle, height, and horizontal position; turns on; and rapid-traverses to the start of the bevel,” Dechamps said. “Feed rate is reduced to the processing speed, and the unit grinds to the programmed lengths.”
These machines use a specialized ceramic abrasive belt running over a hard-rubber contact wheel. In addition, the grinding unit can oscillate up and down, so that the entire belt width gets utilized, rather than a thin strip.
The parameters for such a grinding process don’t change dramatically between material types. “The grinding process is almost insensitive to the material, be it carbon steel, stainless, or titanium,” Antonow said. “The grinding process and wear of the belt are almost the same.” Consumable life depends, as always, on the application, but Antonow referred to one application in which the operation ran through about two belts a shift.
Some machine models have a belt head that swings up and down, allowing it to approach the workpiece from above as well as below to create K bevels without flipping the plate. They also can create a machined surface on the land between the bevels for fit during fixturing and welding. And like their milling cutter cousins, these automated grinding machines don’t require postprocess polishing or finishing of the beveled edges (see Figure 9). Sources added that these systems are mostly limited to workpieces with straight edges, but on occasion some have used the machine to grind a bevel on an edge with a very slight contour.
Plates can sometimes be bigger than the machine itself. In these cases, some of the plate can extend beyond the work envelope. The belt system bevels a certain length; then the material is pushed forward to complete the rest of the bevel.
Such systems can handle numerous small pieces as well. “You can clamp down 20 of the same pieces that all have an identical setting, and bevel them all in one setup,” Antonow said.
Some of the bigger machines are capable of applying bevels up to 4 in. wide. Such a bevel width at 45 degrees would of course take multiple passes, and the total cycle time would depend on the workpiece length, but according to sources, such machines have been known in certain applications to shorten cycle times by a matter of hours.
As Dechamps explained, in this setup the machine would remove about 12 cubic inches of metal a minute—a significant amount of material.
Sources also report that the parts emerge from the system with tight bevel angle and dimensional tolerances. Tolerances can vary with the wear of the abrasive and application, but according to Dechamps, some applications have a tolerance of ±0.5 degrees on the angle and ±0.012 in. on the bevel surface.
When it comes to exploring the beveling options, it comes down to weighing the part mix, application requirements, part flow, material handling, and consumables. As with so much else in manufacturing, it’s quite the balancing act.But with more beveling technology options also comes more possibilities. No one beveling technique works for every application, but fabricators have plenty of choices beyond manual grinding and torch cutting.
Some power tools and benchtop machines create not only bevels but also smooth edges, either with abrasive or curved cutting tool inserts that provide a radius edge.
“This works well for structural steel that’s going to be painted outdoors,” said Phil Heck, president of Heck Industries, Hartland, Mich. “They want a small radius on that edge, called a paint-chip edge, to prevent the paint from peeling off immediately.” He added that some shops use the radius tools to remove precise amounts of weld metal on a workpiece edge.
Other tools use a segmented abrasive flap disc, placed behind perpendicular guide plates, to deburr and create a smooth edge that can have a bevel or chamfer. Some of these tools can be brought to the workpiece, while in other instances small workpieces can be brought to a stationary system for edge finishing (see figures).
One such system uses “two stationary flap wheels that rotate in opposite directions,” said Mike Marshall, technical sales and customer service manager at CS Unitec, Norwalk, Conn. “This kisses the edge of the material.” The result: A corner that was once sharp is now smooth.
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