Getting Better at Boring
Flexibility, quality, and speed of today's boring tools can boost shop productivity
By James R. Koelsch
Today's new boring tools squeeze setup time from the process and help users to produce their products in small lots quickly, making boring even more at home in factories and shops practicing agile and just-in-time manufacturing. At the high end, one such development has been the emergence of tools that adjust themselves automatically to correct for wear, compensate for error, or produce shapes.
An example is KomTronic Tools, the line of servodriven boring tools from Komet of America Inc. (Schaumburg, IL). A servomotor drives a slide inside the boring head. "It counts the pulses and moves the boring bar to the larger diameter or backs it up to the smaller diameter," says Mike Herman, a specialist in boring at the company. The mechanism increases accuracy and eliminates the need to adjust a screw manually to move the bar off-center to a larger diam.
Komet's engineers have also placed slides inside the bar to move a tapered rod axially, using a servomotor to pivot a boring insert. "It acts like a rocker that allows us to pivot the insert out to a larger diameter or down to a smaller one," says Herman. "So depending on how we engineer a tool, it can compensate in two planes automatically through a closed-loop system." The amount of travel varies with the head; the M042 can adjust within one micron in a -1.0, +1.5-mm range of travel and the U axis can move as much as ±25 mm off-center to cut with ±10-µm accuracy.
Although this kind of automation is expensive, it can pay big dividends, especially in mass production. In the automobile industry, for example, an M042 boring head is adjusting itself for each cut in connecting rods based on measurements from a gage. In another application, a U-axis boring system gives a machining center the ability to behave like a lathe in the sense that it can cut a recess or chamfer as it's boring, effectively adding a machining axis. Consequently, a machining center can bore a part that's difficult to hold in a chuck at high speed. Because the tool turns, not the workpiece, a machining center can produce the required surface finishes in much less time with this tool.
Makino Inc. (Mason, OH) has taken a different tack with its line of Smart Tools. Rather than using motors for actuation, it makes use of the cutting fluid as it travels through the tool, forcing the fluid to perform another task before it flows into the cut to lubricate and cool the cutting zone and assist in chip evacuation. A two-insert design called the coolant-adjustable boring bar, or CABB, contains an internal nylon bladder. As pressure increases, the bladder swells, pushing the two leaves containing the insert cartridges outward, thereby increasing the tool's diam.
The limitation, however, was that the tool needed to be at least 2" (51 mm) in diam to accommodate the bladder. To shrink the mechanism so it would fit in tools as small as 1" (25-mm) diam, the design team developed what it calls a Simple Seal. "We basically put a third piece of steel between the two leaves," explains Billy Grobe, process development manager. The center member remains stationary, and fluid pushes the outer leaves apart as pressure builds.
Although Makino has designed and produced bars that will expand as much as 0.020" (0.51 mm), it designs most of its CABB tools to expand only 0.010" (0.25 mm) or so. The reason is that accuracy begins to degenerate excessively for most jobs once the tool expands much more than this amount. For most applications, however, the 0.010" limit is of no importance. Most inserts wear only 0.005" (0.127 mm) or less before needing to be replaced. Consequently, there is still room for the tool to expand for a second or even third pass.
This ability can eliminate a lot of unproductive time that would otherwise be spent adjusting offsets, changing tools, and retracting through the bore. "We can machine on the way in, expand in the clearance--usually at the bottom of every bore--and machine on the way out," says Grobe. Software in the CNC makes the adjustments automatically based on pressure readings obtained at the back of the spindle. For high accuracy, an air gage can measure the bore and provide the necessary feedback for closed-loop control.
Makino has built a working prototype of another hydraulic boring tool that can move the insert in and out radially while it's cutting, allowing production of slightly oblong shapes in holes that are 2" and larger in diam. As the CNC varies the pressure of the cutting fluid, a leaf spring behind the insert either extends to push the insert outward or collapses to draw it inward. The maximum amount of movement is about 0.005", which means that the CNC can vary the depth of cut by as much as 0.005" per side. This small amount of movement is enough to shape bores to correct cylindricity problems in engine production.
Another reason modern boring tools can offer greater flexibility at high accuracy is that manufacturers of modular tools, like everyone else, have invested in better manufacturing processes to exploit the capabilities of modern machine tools. Consequently, modular boring bars are now much more accurate. "Ten years ago, it was acceptable to put together components with a repeatability of 5 to 7 tenths [0.0127 - 0.0178 mm]," says Mike Butler, boring line manager, Ingersoll Cutting Tools Co. (Rockford, IL). "That's no longer the case." He notes that his company's modular tools regularly hold tolerances in the 2 to 4-µm range.
This is not to say that Butler believes the one-piece, solid boring bar is going away any time soon. Ingersoll has also invested in a line of solid boring bars that use guide pads to get them started. "Then, we could have a blade, maybe PCD-edged for a nonferrous application or CBN for cast iron," says Butler. "By adjusting this blade in both the front and the back, you would be able to control your bore diameter incredibly close."
Rather than following the drilled hole, these kinds of bars are capable of making their own bore, he notes. "Sometimes we produce these types of bars on what we call a steerable arbor," he explains, "so that we can compensate for any misalignment between the spindle and the workpiece, thereby achieving nearly perfect results." These arbors have setscrews to compensate for tilt in the X and Y axes, allowing an operator to tram the tool relative to the workpiece.
One use of this kind of tool is in boring valve seats for automobiles. "By producing a specific tool for one application on a steerable arbor, the customer can perhaps eliminate subsequent grinding operations on a block or a head, which is a tremendous savings," says Butler.
Another evolution in boring technology has resulted in better ways of holding the tool. For boring on a lathe, for example, Sandvik Coromant Co. (Fair Lawn, NJ) developed what it calls an EasyFix sleeve that simplifies the task of setting the tool dead on centerline. This capability can save a lot of time when you're installing smaller-diam bars, which are difficult to handle and usually employed in applications where small deviations in the centerline are disastrous.
For rotating applications, Sandvik's engineers concentrated on stability. "In the past, people held the tool in an end-mill holder that has straight shanks and a couple of screws," explains Andrew Pitsker, senior product specialist, tooling systems. He says that a conventional holding mechanism using Allen screws and a bore grip only about 7 - 10% of the affected diam. The small amount of contact is unstable and allows vibration, especially for long overhangs and at the higher cutting speeds that are increasingly popular today. Consequently, many cutting tool manufacturers have gone back to the drawing board to develop coupling mechanisms that increase stability.
In Sandvik's case, the engineers used the company's Coromant Capto coupling, which is based on a polygon system that makes contact with the boring bar all the way around its circumference and along the portion of its axis in the coupling. The activating screw in the back of the holder pulls the pieces together on a locking taper with about 8 tons (35.6 N) of axial force. "You have not only face-to-face contact, but also peripheral clamping around the tool," says Pitsker. "Our coupling clamp also distributes the cutting forces all around the polygon." Runout is 0.0002" (0.0051 mm) per coupling.
The stability and rigidity created by the large surface contact and high axial forces, as well as smooth faces hardened to RC63, give modular tools an advantage over solid bars. The joints between the various elements not only do not vibrate, but also break the bar into segments, thus altering the harmonics to more favorable frequencies. Pitsker reports that modular tools using this concept are actually more stable than solid bars.
Despite the importance of more stable designs and better manufacturing techniques, imbalance is inherent to the process and is a problem that becomes acute at the high spindle speeds commonplace today. "Finish boring tools typically feature a cutting tool that is mounted eccentrically to the center of the spindle," says Dennis King, director of engineering, Command Tooling Systems LLC (Ramsey, MN). "Diameter corrections are made by moving this tool in a direction perpendicular to the axis of rotation, which usually creates a larger imbalance." So holding tolerances and producing fine finishes became increasingly more difficult as spindle speeds rose over the years.
Consequently, boring tool manufacturers have for decades built counterweights for their heads. Using the first generation of these products, however, required skill and time. Either the operator set the weights manually while adjusting the diam, or a technician measured the tool in a balancing machine, and adjusted the counterweights until the tool was balanced. In today's world, however, where shops are squeezing every second possible from their setups to make smaller lots pay, these methods simply take too long.
For this reason, cutting tool manufacturers like BIG Kaiser Precision Tooling Inc. (Elk Grove Village, IL) have developed heads that compensate for balance automatically when the user turns a dial. "While the insert moves, the counterweight is continuously moving in the opposite direction to compensate," says Matt Tegelman, product manager, Kaiser Tooling Systems. "So you only need to make one adjustment." This approach saves time and reduces the chances of introducing error.
Command also has a built-in mechanism that simplifies the balancing of its Urma MicroMax boring head for the particular diameter being cut. Its design allows operators to balance the heads accurately without having to put them on balancing machines. "This allows for higher boring speeds than are attainable with conventional tools," says Command's King. "Boring in the balanced state also improves roundness and surface finish." Under certain conditions, the head can offer these advantages while rotating as fast as 20,000 rpm.
Improvements in stability are not limited to innovations in boring heads. Cutting tool manufacturers also have been working hard to counteract the vibration of their boring bars by installing some sort of damping mechanism inside them. These mechanisms can extend the practical length:diameter aspect ratios, or overhangs, to 15 X diam. "In some cases, we can go to 20 X the diam of the bar, if applied correctly with the right harmonics and insert geometries," says Pitsker at Sandvik Coromant.
For conventional boring bars without these devices, cutting tool suppliers tend to agree that 3 - 4 X diam of the bar or coupling, whichever is smaller, is normally the maximum aspect ratio, or overhang. They normally put the limit at 6 - 8 X diam for conventional carbide bars.
Although damping devices increase the cost of boring bars, Sumitomo Electric Carbide Inc. (Mount Prospect, IL) believes that its new Anti Vibration X-Bar will change the economics of this technology. The steel bar contains a simple mechanical damping device that reportedly controls chatter well enough to create fine finishes and extend tool life at 6 X diam.
Better balance and greater rigidity in tool systems have made it possible to deploy some insert technology that is a bit unconventional for boring. Sandvik Coromant, for example, has added inserts with wipers to its line of finishing tools for boring. Because the land is longer on wiper geometry, the wipers both reinforce the nose radius and cause the cutting edge to remain in the cut longer. "So wiper geometries not only help us to increase feed rates, but also to improve finish, virtually eliminating grinding," says Pitsker. "Because they also allow very sharp cutting edges, it's not uncommon to hold a half a thousandth tolerance on the bore."
Of course, greater contact in the cut creates more friction. Consequently, Sandvik has engineered specific grades and geometries for its wiper inserts, both to make the cutting action as free as possible to limit heat generation, and to withstand the extra heat that can be generated. It also recommends using through-the-tool cutting fluids to help keep the cutting zone cool and hold good tolerances.
To boost productivity in roughing, Sandvik has introduced a three-edge modular boring bar that it calls CoroBore 820. The company's engineers developed a system of modular slides for this rotating tool, replacing the conventional cartridges that fit into fixed pockets on two-edge boring bars. The extra pocket increases productivity by 50% over two cutting-head versions, according to Pitsker.
Another important ramification of the better balance and stability of modern boring tools is that they can use inserts made from advanced materials, such as CBN and PCD-tipped-or-faced inserts. "There have been many tests done to prove the effectiveness of balance for higher-rpm boring, particularly for nonferrous materials such as aluminum," says Jay Verellen, product manager, Seco Carboloy Inc. (Warren, MI).
Consider the ten 12-mm-diam holes that Carboloy bored in a 25-mm thick test block of aircraft aluminum with and without a balanced head. Technicians cut the holes at 4000 rpm with diamond-tipped inserts on an axially mounted steel bar. Surface finish improved to 8 µin. Ra (2.0 µm) on the balanced head from 20 µin. (5.1 µm) Ra, and roundness and straightness were twice as good.
The bottom line is that modern heads have improved both tolerance and geometry--that is, roundness and straightness--at both low and high speeds. Because these heads also streamline setup and increase tool life, they boost the flexibility of the tools, make holding tight tolerances easier, and cut the overall cost of boring. "The initial investment would likely be slightly higher, but in even moderate production runs, the quality of bore tolerance and repeatability will greatly reduce machine time and also work to reduce the overall scrap rate," says Verellen. It's an evolution that means better boring.
Is It Boring or Reaming?
Engineers at Iscar Metals Inc. (Arlington, TX) have designed a tool that can be called a cross between a reamer and a boring tool. Although it carries the name Bayo T-Ream reamer, "you could call it a multitooth, indexable, disposable-insert boring system, or a multiflute reamer," says Iscar's Craig Segerlin.
The reason is its geometry. Not only is the space between its teeth asymmetrical to break up harmonics and prevent vibration, but its edges also have a cylindrical land that acts like a burnishing or piloting device. "It acts like a double margin to guide the tool straight and burnishes the surface to give you a better finish," says Segerlin. By cutting straight and not wandering in the cut following the path of least resistance, the tool behaves more like a boring tool and less like a reamer.
This stability in the cut allows the tool to cut more than 30 X faster than a conventional reamer and produce surfaces finer than 2 µm Ra. "I've seen it produce finishes to 0.6 µm Ra," says Segerlin. A rigid toolholder accurate within 0.0002" (0.005 mm) TIR is crucial, however.
On a test cut that Segerline performed in 12L44 aluminum a few days before talking to Manufacturing Engineering, he produced a 4-µ/m Ra finish while cutting a 0.6308" (15.3-mm) diam by 0.75" (19-mm) deep hole at 1500 rpm and 36 ipm (914 mm/min). He reports that the measurements with a bore gage were exactly 0.6308" for each check at the top, middle, and bottom every 45º.
This article was first published in the November 2005 edition of Manufacturing Engineering magazine.