It's Time to Get Serious About Tool Balancing
I bet you balance your tires. And most of your friends probably do so as well. So why do so many shops launch production without balancing their cutting tools and grinding wheels?
The reasoning shouldn’t be because they are running at low speeds. After all, when cruising down the highway, the tires on your car are only spinning at 700-900 rpm. More likely, it's because too many manufacturing professionals underestimate the potential benefits from balancing their tools. And, in some cases, improvements make balancing a no-brainer.
So why balance? And how?
All types of machining benefit from balanced tooling because that’s the only way you can optimize your surface finish and spindle life, according to Brendt Holden, president of Haimer USA, in Villa Park, Illinois. Conversely, he observed, operators often slow a machine to solve a surface finish or accuracy problem, when the real difficulty is an unbalanced tool.
In fact, Holden said some of Haimer’s biggest successes are with shops running boring heads at relatively low speeds. He recalled one such example in which the customer slowed to 300 rpm, because otherwise movement in the boring head resulted in oversized holes. By simply balancing the tool, the company was able to get the required accuracy and “run the tool at 1,200 rpm with four times the feed rate,” Holden added.
While cases are application dependent, there are situations and factors that demand balancing, noted Dieter Marx, founder and managing director of MPM Micro Präzision Marx GmbH, Erlangen, Germany. For example, he asserted, absolutely every tool running over 4,000 rpm needs to be balanced, as should every polycrystalline-diamond (PCD) tool. Single flute and single point tools (including many boring heads), meanwhile, are inherently unbalanced, so they are almost always a “must” too, according to Marx.
Complex porting tools and tools with multiple inserts at odd angles are also obvious candidates for balancing.
Asymmetrical tools also require balancing, at least to run at higher speeds, added Simon Manns, vice president of the tool grinding division at United Grinding North America, Miamisburg, Ohio. This includes tools with variable helices and unequal indexing (features that themselves have been added to combat harmonics that occur when running at higher speeds).
Manns pointed out that tools with an odd number of flutes are not symmetrical, even if equally indexed. “It’s the gashing. If it’s a three-flute tool you’d typically have one long and two short teeth. It’s gashed more heavily on the long tooth than on the short teeth. And this creates unbalance on the end of the tool.”
Size and speed matter, Manns continued. “When you get up to a half-inch, five-eighths, three-quarter-inch tool, there’s quite a bit of difference in the material that you have removed on the end for the gashing.”
Curiously, Holden said he’s run into cases of seemingly symmetrical, large diameter two-flute, solid-carbide end mills that were significantly out of balance. He attributed this to the toolmaker focusing on achieving a perfect cutting edge while neglecting to grind identical flutes.
Balancing also may be the solution to a seemingly unrelated problem. Holden recounted the case of an automotive company that bought strictly pre-balanced toolholder assemblies for engine production. “Their print said that the tool assembly had to come to them balanced to a quality grade of G2.5 at 15,000 rpm. But they had no way to check.”
The company had no machines running more than 8,000 rpm, yet its unplanned tool change rate was 57 percent during production.
The automaker then bought a Haimer balancer to check incoming tools and rejected any tool assembly that didn’t meet spec. As a result, unplanned tool change times fell to just 7 percent, Holden said, noting that the company saved $250,000 during a six-month-trial period.
“They bought the balancer as a go/no-go gauge to hold their cutting tool suppliers accountable, and it solved internal problems that they didn’t realize they had,” he added.
Good cutting tool manufacturers consider balancing when making the tool. For example, Sandvik Coromant US, Westminster, S.C., employs a “balanced by design” approach, according to Design Manager Ranulfo Vieiro. The company created its indexable tooling in 3D with Siemens NX software, which allows Sandvik to analyze a tool’s imbalance and adjust the design accordingly.
This improves the design, but it may not be perfect.
Once a tool body has been built, Vieiro continued, it is then balanced on a Haimer machine. About 90 percent of the time, some small material removal is required to bring the body into balance to meet G2.5 balance quality levels. Sandvik will check a tool body without inserts, and the completed assembly with master inserts used just for that purpose, Vieiro said.
With regard to solid-carbide cutters, there seems to be less attention paid to ensuring that the tool is balanced. United Grinding’s Walter brand and ANCA USA, based in Wixom, Mich., both offer software to do just that. These systems evaluate the 3D-tool geometry to predict the imbalance, then offer options for moving the center of gravity to bring the tool into balance.
One common balancing method is to slightly lengthen a flute. Often times this can be barely noticeable, perhaps just 0.5 mm, Manns said.
These techniques are useful, though not perfect. They are also limited to tools of uniform density—they will not do nearly as well on a brazed-PCD tool.
Balanced cutting tools give manufacturers the freedom to replace tools in their holders without having to rebalance the assembly. But it’s the complete assembly that really counts, as it doesn’t do any good to have a balanced tool if the chuck is out of whack.
You could even have a balanced tool in a balanced chuck and find that the pull stud throws it out of balance. Holden described an unfortunate scenario in which an operator ground a flat on a perfectly balanced cutting tool because he needed to run it in a Weldon holder. Such a situation screams “rebalance it!” Marx maintains a tool should be rebalanced even after changing a single insert.
The wheel is the tool in grinding operations, and balance is equally important if you’re trying to maximize wheel and spindle life while also getting the best surface finish on a part. On the other hand, as with cutting tools, you can get away without doing it if you’re not too picky about these factors.
As Manns put it, no one ever regrets buying a balancer. “Once you see it, the benefits are obvious.”
But, both Manns and Jack Hooper, senior process engineer at Sandvik Coromant, said the quality of today’s tool-grinding wheels is such that with proper dressing, you can grind good tools without balancing. Not the best tools. But good tools.
There are two cases in five-axis tool grinding that virtually require wheel balancing, according to Manns. The first is in making micro and small tools. “You only need a tiny bit of vibration to destroy a half-millimeter tool,” he explained.
“Your wheels are key. Balancing is very important on smaller tools.” Wheel balancing is also “almost necessary to accurately create a really nice end geometry on those smaller tools.”
The second case is on erosion machines with a double ended-spindle. In that configuration, you typically erode with copper electrodes on one side and grind on the other. Manns said the copper wheels are often “quite out of balance.” That’s not a factor when you’re eroding, because that occurs at very low rpm. But when you switch to grinding, the entire spindle might run at 4,000 rpm or more. So the unbalanced electrodes will harm the grinding operation.
There is one more strong vote for balancing in tool grinding: maximizing throughput. Citing a study done by OSG USA in Bentonville, Illinois, seven years ago, Holden called it “probably the easiest payback we’ve ever seen in any of our products.”
OSG was grinding a million tools per year before adding wheel balancing to its process, providing a statistically significant study base. Balancing wheels to G2.5 at 10,000 rpm decreased a machine’s power consumption by 18 percent, while boosting wheel life 20 percent and spindle life by 30 percent, with an evident improvement in surface finish.
But the biggest benefit was the 18 percent increase in grinding speeds. OSG added nearly 120,000 tools in annual output just by balancing its wheels.
Moving beyond tool grinding into cylindrical and surface grinding, where the wheels are larger and heavier, balancing becomes even more important. As Hooper explained, the runout tolerance for finished cutting tools is on the order of 15 µm, and “it’s best to be well below that for the tool blank.” Because the OD grinding wheels used to produce the blanks are 300 mm in diameter or larger, running at roughly 7,000 rpm, “it is absolutely critical that you balance that wheel in the machine,” Hooper concluded.
The two market leaders in stand-alone balancing machines are Haimer and MPM—the latter is represented in the U.S. by Rollomatic, Rush Machinery, and Toolroom Solutions. While Haimer and MPM claim the same impressive ability to reduce imbalance by 98 percent and achieve quality grades up to G0.4, the companies take different electromechanical approaches.
Haimer uses “hard-bearing technology, with centrifugal force sensors that measure the force being applied against the spindle as the toolholder assembly rotates,” Holden explained. In contrast, MPM features a soft bearing, meaning the spindle can wobble a bit. Acceleration sensors measure the vibration displacement caused by the rotation of an unbalanced rotor, said Marx.
The advantage of Haimer’s approach is twofold, according to Holden. “One, you can calibrate the machine itself. There’s no specific tool calibration required,” he explained, noting both adapters and tool types can be switched without having to recalibrate (unless the machine is moved).
Secondly, the machine is not influenced by vibrations from outside sources. “It’s literally just measuring the rotational forces applied against the spindle,” Holden said. “You’re getting repeatable, consistent results.”
The Haimer machine is a 1,200-lb Meehanite cast base unit, Holden explained, with the base similar to the casting used for a machine tool.
From MPM’s perspective, that’s a disadvantage. The competitor’s system costs more (all things being equal) and must be moved with a forklift, according to Marx, while the MPM can be carried by hand and placed on a sturdy bench next to a machining center. In other words, he said, the MPM unit works well in the presence of vibrations in the shop. While an MPM machine should be calibrated more frequently—such as at the start of a shift or when changing toolholder types—this is a fast and easy process, Marx said.
The other advantage of MPM’s approach is that it doesn’t require a separate balancing machine. MPM offers portable systems that can be attached to any machine spindle and then moved to another machine, as needed. Such systems are used routinely on cylindrical grinders.
At Hooper’s Sandvik-Coromant plant, the Reinecker peel grinders use a competing system from Elaso.
ANCA offers a comparable option for its tool grinders, called iBalance. The process is “very quick” and should take no more than five or six minutes, according to Thomson Mathew, the company’s MX and software product manager.
It makes sense to include the grinder due to the potential for imbalance, Mathew added, citing everything from the grinding spindle and the mechanics of the C-axis to the “whole mechanical aspect of where you mount the wheels to the spindle.
“The total system could have some imbalance that you would not capture by measuring just the wheel set outside the machine,” he continued. Balancing in the machine also gives the ability to check the wheels at specific, critical positions in the grinding process, Mathew said.
Manns countered that it’s more productive to balance wheels offline, leaving the grinder producing tools.
All the balancing systems mentioned include software that tell the operator how to correct the imbalance by adding or removing weight from a specific place. For example, ANCA’s iBalance shows the operator where to add a weighted screw (or several) to the clamping nut on the end of the wheel adapter.
The approach is commonly used on toolholders that have threaded holes around the flange. Holden referred to this as “good for quick, easy adjustments. The only limitation is you don’t have a tremendous amount of weight to displace, so you can’t correct for a big imbalance.” It’s ideal for shrink fit holders, which have no moving parts and are generally built well balanced, but it may not handle other chucks.
For tougher challenges, Haimer and others offer elliptical rings. In other words, the ring itself is unbalanced and the balancer tells you where to rotate it to counteract the imbalance of the entire assembly. Most toolholders present a surface to attach such rings but this isn’t always the case, Holden said. MPM offers a clamping nut with movable weights. Rather than rotate an elliptical ring, users slide the weights around a track to bring the assembly into balance.
As noted earlier, higher degrees of imbalance can be addressed by removing weight from the toolholder, either by drilling or milling. Ideally that’s done when manufacturing the holder, but there are situations where the final tool assembly will have to be balanced in this way. Although different terminology may be used, the software running the Haimer and MPM machines covers the same techniques.
So should you get serious about balancing? Holden said he often tells reluctant prospective customers to call him on their thorniest applications, when they’re “pulling their hair out because they’ve tried everything. They’ve looked at the fixture. They’ve analyzed the spindle, the machine, thought about the feeds and speed, thought about everything, and they’re still not making good parts or making cycle time. Honestly, about 90 percent of the time it is balancing.”
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