Tool Balancing: When and Why

Aug 07, 2023

While getting jobs up and running quickly is critical to profitability, spending a few minutes beforehand to properly balance one's tools can pay big dividends.

Machine tools aren't cheap to operate—so using them efficiently by balancing the cutting tools used in them to optimize performance and prevent premature tool wear should be an easy choice. That's not necessarily so, according to industry experts. More tool balancers are indeed finding their way into job shops and OEMs, but widespread usage still lags. And that can be a costly mistake, causing shops to run machines slower or stopping them entirely to correct issues arising from out-of-balance cutting tools and toolholding.

And to be clear, buying pre-balanced tool assemblies isn't a guarantee that you’ll be running a balanced tool; balancing machines employed as quality assurance tools have often discovered imbalances in pre-built assemblies.The upfront cost of a balancing machine is easily made up with improved longevity for tools and spindles, increased material removal rates and better parts, according to tool balancer manufacturers. Yet adoption by small to mid-sized shops remains fairly low, some industry insiders note.

For an investment of roughly $25,000 to $35,000 for entry-level tool balancing machines, the everyday benefits of tool balancing can pay off fairly quickly, they say.

Ultimately, shops that want their machines, spindles and cutting tools to last longer, and want to push their machines harder, achieve better surface finishes and make more parts per hour, can make use of easy-to-use tool balancing machines to help them realize plenty of untapped potential.

Tool balancing can be achieved in one of three ways:

--Adding weight by inserting screws into predrilled spots in the toolholder.

--Balancing rings, which are weighted rings that are put over the toolholder to correct imbalance.

--Destructive means, which entail milling or drilling small amounts of material out of the toolholder.

Today's tool balancing machines give operators options—and it only takes about a minute to measure a tool on a balancer and another two to three minutes to correct the unbalance.

"Our machine gives you the flexibility to use any of those methods," explained Michael Colyer, key accounts manager for Zoller Inc., Ann Arbor, Mich. "You put your tool assembly in the machine, clamp it, hit go, and it will spin that tool at a simulated speed and give you a laser-marked line of exactly where and what your imbalance is. It will tell you that you need to add or remove, for instance, 2.5 grams at a certain spot."

Haimer USA LLC, Villa Park, Illinois, which has been producing balancing machines for more than 20 years, also emphasizes the ease of use of its operator interface when adding material, removing material or using balancing rings. But Haimer adds something else to the equation by providing verified balanced toolholders.

"Balance is becoming more important because machine tools are getting more accurate," asserted Steven Baier, Haimer's vice president of sales. "Many end users unfortunately pay top dollar for the best possible machine tools—capable of the highest metal removal rates—and then put the cheapest toolholders they can find in the spindle."

That invites unbalanced assemblies, gripping torque, runout and other issues. "That forces engineers or operators to slow the machine tool down to make a good part. And when you start slowing that machine tool down, you’re running it like a 1980s machine rather than a 2020s machine. So all the productivity gains that end users invest in the newest technology to achieve are lost because of their tooling choices."

If manufacturers pin their hopes on prebalanced toolholders, they should be aware that not all of them are created equal, Baier continued. "Haimer's toolholders, off the shelf, are balanced to G2.5 at 25,000 rpms. That's what our shrink-fit holders are balanced to when they come from the factory. Not only are they balanced, they’re verified; we 100 percent inspect all our holders to balance them to G2.5 at 25,000 rpms." Still, he cautioned, "that does not guarantee that when you put together a toolholder assembly that your assembly is balanced."

For longer tools, balancing in two planes is essential, Zoller's Colyer added."The heavier and longer the tool, the more important it is to balance it in multiple planes," he explained, noting that Zoller machines can balance tools up to 700 mm long. "If you have a tool that's 500 mm and you’re out of balance at the bottom of that tool, say by 4 grams, it's going to be 20 grams at the top of that tool. As your tools get longer, you increase the amount of balance exponentially."

Using tool balancers to monitor tool unbalance "is an effective way to find consistent angular unbalance that could be reduced upstream in the manufacturing process to reduce balance time and improve parts for the end user, noted Dawn Hines, CEO of Hines Industries Inc., Ann Arbor, Mich.

"Generally, all Hines balancers work the same way," she said. "The part being balanced is placed on the machine. Frequently there is tooling to help accomplish this. The force from the unbalance is measured in either a static or dynamic manner and is displayed on the computer. At that point, the balance can be corrected by adding or removing weight. Toolholders are typically corrected by drilling or adding set screws."

Tool balancing machines come in many flavors, from entry level to advanced. Operation is enhanced by easy-to-use interfaces and prompts. Generally, users are prompted to enter G rating, run speed, assembly weight and the preferred method of balancing (i.e., inserting screws or removing material).For tabletop operation, Haimer offers its TD 1002 for simple tooholders, drills and end mills "if you’re running CAT 40 or CAT 50 and most of your assemblies are under 6" [152.4 mm] in gauge length."

While the TD 1002 balances in a single plane, the more popular Tool Dynamic Comfort balances in tool methods and uses multiple correction methods. The Comfort features proprietary PC-based software and an arm mounted to the side. The TD 800 is designed for balancing grinding wheels.

"Our number one application is end users balancing toolholder assemblies," Baier noted. "Our number two application is cutting tool manufacturers balancing their grinding wheel packs."

Haimer launched virtual Application Center 360 tours in February so users can view these balancers in action with self-guided or expert-guided tours of the company's 25,000 ft2 (2,322.6 m2) technical center at its headquarters in Igenhausen, Germany. Meanwhile, the Haimer U.S. technical center in Villa Park demos about 30 powered machines.

Likewise, Zoller has an entry-level machine, the Tool Balancer Economic, with a built-in computer. While that machine is often used to check pre-balanced tools, Colyer noted, the next-level-up Tool Balancer Comfort is for manufacturers that balance their own tools. The Comfort indicates more imbalance points and can balance in two planes; it also provides inspection reports on each tool that can be attached to a shop's process sheets.

At Hines Manufacturing, "Our most popular machine is the Hines HVR," said Chelsea Gibbons, marketing and sales specialist. The Hines Vertical Balancing Machine is a dynamic balancing machine that can be used for both single and two-plane balancing. "The majority of our customers require custom-engineered solutions, and for our automotive sector customers we are providing high levels of automation." She noted that the company's continuous improvement program has produced improvements in cycle times, unbalance measure sensitivity, tolerances, automation, correction speeds, and robot integration.

Hines recommends inspection, calibration and preventive maintenance for its machines once per year to ensure longevity and repeatability and to reduce down time, Gibbons added. "When designing a machine, our application engineers will review all part specs and unique requirements to build a balancing machine that meets all criteria. Hines service techs review all operating procedures and recommend day-to-day operational processes."

Automakers in particular have demonstrated the value of tool balancing machines. For instance, Haimer's Baier noted that one of the "Big 3" U.S. automakers was running 14 machine tools 24 hours a day. "We found out that 57 percent of the time, one of those machines had an unforeseen stop," he said. "The cutting tool wasn't lasting as long as they expected. For some reason, one of the machines was stopped and there was an operator changing out a tooling assembly at a point in time that was not expected."

The automaker, which was ordering pre-built and balanced assemblies, invested in a Haimer balancing machine for quality control to test those assemblies for imbalance as they came in from the supplier—and returned imbalanced assemblies for rebalancing. The measure not only saved the automaker about $200,000 in six months, but the unexpected stoppages plunged from 57 percent to 7 percent.

Zoller's Colyer also recalls a large automotive company purchasing a balancer three years ago to simply check the balance of complete tool assemblies it purchased from an integrator. "They found that 70 percent of their tools were out of spec coming in," which helped them diagnose multiple process issues.

"I have another customer who got into high-speed machining," Colyer added, "and right along with his machine he purchased a balancer. He basically has been running that machine lights out 24/7 for the past two-and-a-half years." That customer tested the spindle a few months ago and it "is still almost dead perfect." If potential customers do the math on replacing a spindle that has lost 40 percent of its operational life—"let's say they’re running 20 of those machines—a $40,000 balancer makes sense."

Tool balancing has proved beneficial across multiple industries, noted Hines’ Gibbons, and Hines balancing equipment is used at major automotive plants and in many other industries. "We have seen an increase in demand for machines to balance diamond tool cutters and toolholders within the defense and agriculture industries," she said. "Through new software development, we have reduced part imbalance measuring times significantly, which improves cycle time."

In his 18 years at Iscar USA, Arlington, Texas, Chief Technical Officer Thomas Raun has seen first-hand the increase in cutting tool complexity, coupled with faster-running machines, yet from his observation only 20 percent of shops that typically run machines past 8,000 rpms are balancing tools.

While noting that shops might view the cost of balancing tools as somewhat prohibitive, "if you are running CAT 40 and HSK 63 spindles that have more than 10,000 rpm capabilities, I’m guessing that it wouldn't take too long to realize the break-even point of that investment."

As machine tools continue to add high-speed capabilities, "maybe they’re less robust in terms of rigidity of the overall machine," he added. "The spindles seem to be getting faster and faster, so the machining approach—especially from the milling perspective—seems to be changing from the heavy depths of cut and widths of cut for roughing phase operations to running the spindle really fast and taking a lighter width of cut."

In terms of an end mill, "in the past you might take a half-inch end mill, go a half-inch deep, and maybe go 3/8s of an inch or even up to full slot on that half-inch end mill. Now the idea would be to take the half-inch end mill, switch it from a four-flute design to maybe a seven-flute design and amp up the rpm to take light cuts and go faster." While the ‘old style’ approach to roughing might be more productive in terms of material removal rates, high-speed machining "seems to be the fad right now: people see the machine spinning fast and the feed rate moving fast, and they like that."

That said, Raun continued, "the old-school rule of thumb that I was always told by the machine tool companies was that if you start to get up over 8,000 rpm, you should be thinking about having balanced toolholder assemblies. More recently, I’ve seen information from other companies that tout balancing machines that you should be balancing at a much lower rpm than that and that there is value in doing that."

And yet, usually, it's the "high-level" Iscar customers in high-production environments that are balancing milling tool assemblies, he said. "An example would be in aerospace, using a Makino Mag machine or some other type of super high-speed machine center at 30,000-plus rpms. They are concerned with balance. I see plenty of customers out there using the traditional CAT 40 milling machine spindle with 15,000 rpm and they’re putting tool assemblies in those spindles and they’re not worried about balancing them. I would balance them if it were me."

In those high-speed applications, particularly aerospace and medical, Iscar provides guidelines to preserve tool life and optimize material removal, "especially for the tools designed specifically for those types of applications—like the aluminum applications for example—that we know are going to be running at very high surface speeds. We’re providing a cutter body that's balanced up to typically somewhere around 30,000 to 33,000 rpms. And there are other toolholders that are balanced or can run at rpms rated much, much higher than that. We provide toolholders or integral tools—that is, a one-piece cutting tool assembly—and they’re balanced to a particular spec, usually G2.5 at 33,000 rpm."

For shops seeking a competitive advantage, Raun concluded that "unknowingly, they benefit from having a balanced tool; they might not even know it's balanced. They look at an end mill and say, ‘It's the diameter and length that I need,’ and they don't really concern themselves with how the tool was produced. If it is a tool that was produced with balance in mind, they probably benefited from that because when they put the tool in their assembly and put it in their machine, it was probably a balanced situation. Typically if it's a balanced tool, the cutting tool suppliers are going to show you that in the information that's provided in the electronic catalog."

Understanding the terminology of tool balancing starts with understanding the G rating in a balancing guidance, such as G2.5 at 30,000 rpms. How well a part needs to be balanced depends on part weight, speed of rotation, and application. The G rating is used in an equation that calculates the permissible residual unbalance, expressed in g-mm, based on operating speed and weight of the rotor. Details can be found in International Standards Organization (ISO) IS0-1940 (for those so inclined). The lower the G rating, the better the balancing grade, so G2.5 is better than G6.3.

A convenient graphical table and equations are contained in the ISO standard. "ISO has issued guidelines with regard to a number of different kinds of devices," according to Applications Engineer Larry Ketola of Hines Industries. "The ISO standards contain detailed methods of calculating different static and couple unbalance tolerances that are dependent on the ratio of the part's diameter to its length."

The Hines website has an ISO calculator where operators can find the recommended ISO grade for their specific part: https://hinesindustries.com/isotol/IsoTolerance.html

The smaller the balancing grade number, the higher the speed of the application or operation. G2.5, for instance, is recommended for:

--computer drives,

--electric motors and generators,

--compressors,

--gas/steam turbines, and

--machine tool drives.

"In most cases, the part unbalance does not change with the rpm," Ketola explained. "Only the force created by the unbalance changes. Machines with larger grades are much larger and slower, with less need for tight balance; they can withstand more unbalance and still function correctly. The smaller number corresponds with smaller machines where accuracy and tighter balance requirements are necessary." Even a small amount of unbalance can affect operations with small machines, he said.

Earlier, after Starrett DataSure® Wireless Data Collection Technology was developed, Starrett conducted a controlled, 100% inspection test to measure the impact of DataSure® on throughput and quality assurance. Starrett made three measurements per part and recorded the data on 500 parts.

Method 1: Measure, Handwrite Results, Enter Data Remotely--37 time/ motion elements:--28.9 seconds per part--62 entry errors. Factors affecting accuracyand throughput:

--Measurement stops so operator can write results

--Illegible handwritten numbers, mistakes noted but not corrected, data written in shorthand and misread by the transcriber

--Value can change when the gage is released

--Data entry errors at the PC

Method 2: Measure and Enter Results to PC

--20 time/ motion elements: 15.3 seconds per part

--4 data entry errors. Factors affecting accuracyand throughput

--Alternating measuring and data entry caused errorsGage not seated correctly when released to key-in data

--Missed data entry, incorrect keystrokes, data entered intowrong cell

Method 3: Measure and Enter Results Directly with a Starrett DataSure® Wireless DCS

--17 time/ motion elements: 6.6 seconds per part--0 entry errors--Fast and Direct -- 5x faster than Method 1

Factors affecting accuracy and throughput:--Measurement technique

--No interpretation or memory errors--Immediate, direct data entry eliminates errors is maintained

Starrett has recently introduced a new version of DataSure® called DataSure® 4.0, the industry's most complete, scalable, secure and robust wireless measurement data acquisition solution for Industry 4.0. See user article in this issue on pages 38-41.

Learn more at:www.starrett.com/datasure4

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