It’s said that holemaking is the most commonly performed of all machining operations. And it stands to reason that most of those holes must be reamed or most likely bored after drilling. For as long as machinists have been boring holes, however, they’ve struggled with a variety of problems. Chip evacuation, coolant starvation, straightness and surface finish—never mind not knowing just what the heck is going on down there—are only a few of the challenges faced by anyone who’s operated a CNC lathe or machining center.
Yet boring life has become a bit easier over the years. Cutting tool and tooling suppliers have addressed these and other deep-hole boring difficulties with technology that makes this often difficult machining operation much less troublesome. Anti-vibration boring bars eliminate chatter. Coolant-fed tools break up bird nests and flush chips out of the hole. Fine-boring tools make the days of “sneaking up on the bore” a shop floor anachronism. Modular systems simplify changeover and reduce downtime. And Industry 4.0 is finally answering the mysteries of the unseen events down yonder.
Let’s start with the most basic and common boring problem: vibration. The maximum overhang for a steel-shank boring bar is around four times its diameter (4xD), maybe a bit more depending on the workpiece material, depth of cut, and how securely the bar is being held. Carbide can easily go 6xD, and up to 10xD if everything is just right. Get much beyond this ratio and vibration leading to chatter begins to rear its ugly head. Maintaining an acceptable surface finish becomes problematic, as does tool life and part tolerance.
Fortunately, machinists now have better options than the solid-steel or carbide bars of yesteryear. Most cutting tool manufacturers provide some form of anti-vibration or damped boring bar, some capable of depth-to-diameter ratios of 14xD or more. For instance, Walter USA LLC, Waukesha, Wis. offers its Accure.tec system, and as Product Manager Sarang Garud explained, it delivers benefits far beyond chatter reduction.
“Our new Accure.tec tool contains a heavy metal damper, a patented technology that acts both radially and axially thanks to the elasticity of its pre-tuned damping element,” Garud said. “Whether it’s a finishing or roughing operation, the operator can simply place the bar in a CNC lathe or machining center and start boring. Additionally, our QuadFit quick-change cutting head interface has a short overhang, low weight, and is positioned as close as possible to the damping mechanism, all of which is important for optimal boring performance.”
Garud listed a range of suitable applications for deep boring tools, including aircraft landing gear, pump housings and valve bodies for the oil and gas industry, automobile differential cases, gas turbine components, plastic injection molds, and indeed anywhere a deep reach is required together with high accuracy and surface finish requirements. “The typical length-to-diameter ratios for such applications are 6xD up to 12xD, although deeper applications can also be tackled,” he said.
Ashok Guruswamy has a similar solution. The product manager for Grip/Turn/Thread at Iscar Tools Inc., the Canadian division of Iscar Ltd., Tefen, Israel, Guruswamy noted that Iscar engineers designed WHISPERLINE anti-vibration tools for operations in which large overhangs are required. These tools include Dynamic Vibration Absorber (DVA) systems to increase damping and therefore stability during machining. The DVA, he explained, is a heavy tungsten mass supported by elastomeric components and located in an internal cavity as close to the machining area as possible.
“During machining, a vibrating tool generates undulations on a workpiece surface,” he said. “In a subsequent tool pass, the cutting edge would then be machining the previously generated wavy surface, leaving behind a newly-generated wavy pattern. Because of this, the chip thickness and therefore the cutting forces vary with time, a phenomenon that can greatly amplify vibrations and develop chatter. Chatter vibrations are detrimental to the safety and quality of machining operations. They cause a rough surface finish, increase cutting forces, reduce tool and machine life, decrease productivity, and create irritatingly high noises. Our WHISPERLINE eliminates all this.”
Both cutting tool experts offered comparable usage recommendations: Always clamp the shank of any boring bar in a split sleeve or bushing. Set the insert edge exactly on centerline. Apply feeds and speeds comparable to that of a shallower bore, but reduce the depth of cut slightly depending on the workpiece material. If chatter does occur, reduce the spindle speed first, not the feed rate. Use a bar that’s perhaps 20 percent of the drilled hole diameter to allow for chip evacuation, and, if necessary, apply a “stairstep” approach, roughing the hole in shorter sections until the final depth is reached.
Walter, Iscar, and other tooling providers also offer a range of fine-boring heads for use on machining centers and other rotary applications. They are capable of boring diameters smaller than a spaghetti noodle, larger than a truck tire, and everything in between. Sandvik Coromant, Fair Lawn, N.J., for example, manufactures the CoroBore line of boring heads, the smallest able to bore holes down to 0.04" (1 mm), while Iscar’s TCH AL system tackles holes as large as 47.24" (120 cm) in diameter, 1,200 times that size.
As the name implies, all provide fine adjustment capabilities—some in increments of just 0.00008" (0.002 mm). Many have digital readouts and are balanced (or auto-balancing) for high spindle speeds. For example, BIG Kaiser Precision Tooling Inc., Hoffman Estates, Illinois, boasts a head able to spin at 30,000 rpm. Coolant through the tool is also common, eliminating problems with chip evacuation. Simply put, boring holes with tight tolerances and extremely fine surface finishes—even very deep holes—is much easier than it once was.
Thanks to their modular construction, these boring systems are easier to use as well. A variety of shank lengths, diameters, and materials are available, including heavy metal, carbide, steel, and the damped bars just discussed. This means a machinist or tool crib attendant can quickly replace one boring head with another, or use the same shank for a solid-carbide milling cutter, indexable drill or end mill, chamfering tool, and so on, greatly reducing setup time. And in the case of those systems that accommodate special multi-step boring heads and form cutters (which many do), cycle times are also reduced.
At least one company aims to take its fine-boring technology to the next level. Matt Tegelman, applications and product manager for BIG Kaiser, said the company has developed an automated boring system that eliminates the need to stop the machining cycle for manual adjustments. It uses a Bluetooth-capable, battery-powered, EWA-series boring head that communicates with BIG Kaiser’s proprietary boring software. In coordination with either an inline probing system or using external measurements, the PC can send appropriate offset values to the boring head, which then adjusts the head accordingly via an internal servo motor.
Tegelman is quick to point out that the system is still under development, and that attendees to IMTS 2020 can expect to see an integrated, closed-loop version. Nor does the automated boring head work in conjunction with BIG Kaiser’s Dynamic Damping boring bars, although that too is coming. “We displayed the second generation of our automated boring system at EMO, and the next iteration should be released soon,” he said. “Ideally, we will also incorporate features like real-time vibration analysis, temperature sensing, cutting time, and possibly even connect the system wirelessly to our offline presetters or third-party tool management software.”
BIG Kaiser’s not alone in its smart boring tool venture. Jeff Rizzie, director of digital machining for the Americas at Sandvik Coromant, explained that the company’s Silent Tools family of damped boring bars and milling extensions became quite a bit smarter with the introduction several years ago of an Industry 4.0-capable system for internal turning (i.e., boring) operations.
“Silent Tools Plus monitors heat and vibration, indicates whether the tool tip is on centerline, and lets operators know if the insert is in the cut or not, something not always easy to determine with a damped boring tool,” he said. “All of this and more can be displayed on a tablet, a smartphone, a separate PC, or even the machine control, provided it has Bluetooth capabilities and can run the software application. Based on this information, the operator can make better decisions about tool life and machining parameters, and because these systems have quick-change interfaces, setups are also much faster.”
These types of smart tooling systems clearly represent the future for boring and other machining operations, but their capabilities will remain limited without support from machine builders. Attend any machine tool show and you’ll quickly learn that Mazak, Okuma, DMG Mori, and others are all moving into smart tool adoption territory, although Linz, Austria-based WFL Millturn Technologies GmbH & Co. KG recently suggested it is the leader in this area.
In a recent press release, WFL Millturn named the Sandvik Coromant and BIG Kaiser systems just mentioned, noting that “sensor technology in tools offers additional protection for workpiece, tool and machine, operators are relieved and there is a higher process reliability during machining. Overload and vibration during the machining process is detected early and the boring bar works fully automatic, because of an interactive adaptation of the cutting parameters by the machine control.”
Aside from its partnerships with third-party tooling providers, WFL Millturn has developed its own smart tool technology. One example is a CNC contour boring bar equipped with hydraulically-actuated U-axis “paddles” for the turning of internal contours such as bottle shapes and seat pockets. Other applications include “special tools used to machine complex workpieces” and “long special tools for hard to reach areas,” all with prismatic mounting interfaces (HSK or Capto) and sensors for process optimization.
According to the release, the integration of these sensors with the machine tool enables detailed tool information and machining states to be displayed on the controller, a tablet, or a PC. Signals are transmitted via Bluetooth so that the machine can respond interactively to a defined trigger event, and the process can be visualized and recorded for documentation purposes.
To support these technologies, the machine builder has also developed a process monitoring tool called WFL iControl, which can monitor and record up to 16 user-configurable process signals and is designed to protect the machine if a collision or overload condition is detected, “even during autonomous production throughout the night.”
Trouble-free boring is important (and thankfully, becoming much easier to achieve), but how do you know what the inside of any deep bore actually looks like? Some might opt to sacrifice a first article by milling the workpiece in half lengthwise, but destructive testing like this is not only wasteful, it only provides a limited snapshot of the bore’s interior—the first piece might be beautiful, but who’s to say what the next part will look like?
Doug Kindred, president of Gradient Lens Corp., Rochester, N.Y., has a better idea. It’s called a Hawkeye precision borescope, and it gives manufacturers a way to peer into the innards of practically any aircraft engine, fuel system, cylinder block, hydraulic manifold, or other object with an access hole at least 0.02" (0.5 mm) across. “Every M16 rifle made was inspected with one of our borescopes,” he said.
Three types of borescope are available—rigid, fiberoptic, and video—each with different models and capabilities. The most suitable for machine shop applications is a rigid borescope, Kindred explained, which provides the best image quality at the lowest price. They range in size from the 0.035" (0.9 mm) diameter Hawkeye Pro MicroFlex Semi-Rigid Borescope to the 0.283" (7.20 mm) Hawkeye Pro Super Hardy Borescope. Tube lengths of 6" (150 mm) to 37" (940) mm are offered, although special orders are not a problem and Kindred said his team once built a steerable, fiber-optic scope 19' (5.8-m) long for inspecting the bore in a crankshaft used in heavy machinery.
Borescopes are typically used for visual inspection only. Surface finishes, burrs, scratches, porosity, and similar defects are clearly visible via the borescopes eyepiece or displayed on the optional video monitor. For those who want to measure distances between intersecting holes, for example, or the width of a groove, a micrometer-style stage is also available.
As a former consultant and expert in optical systems, Kindred is happy to discuss resolution and magnification and depths of field until the cows come home. For those short on time, however, he offers one piece of advice, followed by his company’s value proposition. “First, always buy the biggest scope possible for your application,” he said. “The image will be brighter and higher resolution, and the scope more durable. Second, we make great scopes at half the price of those coming from Europe, they’re made in the United States, and we have them on the shelf. And if you can’t find what you need on our website, call me.”
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