Why Balance?

A shrink-fit toolholder and tool are balanced as an assembly. Photo courtesy of Haimer USA.

Ask anyone in the metal cutting industry about tooling balance and the response is anything from a groan to a laugh accompanied by the statement, “that’s opening up a can of worms.” But, the first reaction is quickly followed by an assertion that having balance in the cutting process is a necessity, not an option.

The can of worms is, in part, the inordinate number of variables involved in trying to achieve balance—variables inherent in the tools, the toolholders, the spindles, and the RPMs. For example, in the toolholders alone, retention knobs, collet nuts, collets, and some of the mechanical features move and can negate balance. Then there’s the number of separate parts that can comprise the tool; for example, the head and body of a face mill can be assembled and attached to an arbor with a threaded bolt and the arbor can have a retention knob. All pieces that can move.

Then there are varying opinions as to exactly what is needed to achieve the balance for a job and how much balance may be too much.

What makes balance a necessity? The constant drive to produce more quality parts at a lower cost to satisfy customers and make more money.

Balance helps this happen by

• Increasing accuracy.
• Ensuring a better surface finish.
• Extending tool life.
• Reducing the wear on the machine tool bearings.
• Enabling higher metal removal rate.

And, although balanced control becomes more critical at higher speeds, it is still important at any RPM. Monitor the performance and insert life of a face mill or boring bar after balancing to see the improvement.

Gain Speed, Lose Problems

Drew Strauchen, vice president of marketing and business development at Haimer USA, said that on average companies will gain 15 to 25 per cent in additional productivity and tool life when they begin balancing their tooling assemblies and even more if they started with a poor balance situation. Balance won’t solve every problem with a shop’s cutting process, but he estimates that it can solve least a third of a shop’s machining issues, especially the ones that never seem to go away.

Balancing machines like the Haimer Tool Dynamic use sensors to take a series of measurements to determine the balance or amount of unbalance in a tooling assembly. If the assembly is unbalanced, a series of options tell the operator how to bring it into balance. Photo courtesy of Haimer USA.

“The good thing is that industry has moved the needle on the balance scale and is offering tooling and toolholders with improved balance,” said Strauchen. “It’s certainly a good start, but when you assemble all the components, they can be well out of the industry standard of G2.5. To achieve that balance repeatedly, everything needs to be balanced as an assembly.”

Balance is always needed whether you are cutting with high-speed spindles at 44,000 RPM or at 3,000 RPM. “One misnomer that is prevalent in the industry is that balance is only critical at higher speeds. I think of it like driving your car with unbalanced tires. If you’re driving at 30 MPH, you won’t feel the effects like when you drive 60 MPH. At the faster highway speeds, the unbalance becomes noticeable because it has a bigger impact on the rotors and the steering wheel begins to shake. But the negative impact is still there at the slower speeds and ultimately shows up via uneven tire tread wear.”

Extend Machine Tool Life

Terry Beckwith, an applications engineer at Elliott Matsuura Canada, talked about the effect of balance on a machine: “When a tool is not balanced, it puts a lot of wear and tear on the spindle. At first you might not see it or hear it but running unbalanced over a short period of time will significantly reduce the life of the spindle.

“When somebody buys a high-end piece of equipment, they have to look at everything to get the maximum performance from it, including software, fixturing, and tooling. If they are not going to invest in the proper tooling, they will never get the full benefit of the machine. Balancing is part of that package.

“Some OEMs build software features into their machines to measure the balance during cutting. As long as the spindle is rotating, you can press a function key and it will display the balance of the tool. It’s a nice feature because it can save you from going out and purchasing another spindle plus save you any loss due to unexpected downtime caused by the need to replace the spindle. Wear is more pronounced in the high-RPM spindles like 32k, 42k, and 50k.”

Reduction of vibration, the goal of balancing, also increases tool life. “A balanced tool will actually hold an edge longer than an unbalanced tool because it doesn’t have the harmonics that occur with unbalance. You may not hear the harmonics during the cutting process, but you will see the results in a lesser-quality part finish and your tooling edge will deplete faster.”

Monitor Balance

A second area of consensus was that balanced shrink-fit or hydraulic toolholders assembled with high-quality, balanced tooling is the first step to achieving the best possible balance of the assembly. But there can be other areas of concern.

The yellow areas indicate where compensation can be made for differences of depth of driving slots on a prebalanced, or balanced-by-design, Coromant Capto toolholder. The red shows where compensation can be made for differences in the magazine orientation slot. Illustration courtesy of Sandvik Coromant.

“Something that is rarely taken into account is the actual cutting force itself. If you balance the tool and the toolholder and you put it in a spindle that is not perfect, that is going to cause runout and alignment issues of the tool and spindle,” said Brian MacNeil, milling products and application specialist at Sandvik Coromant.

“One thing that is contradictory with balancing is that you don’t know how much cutting force is happening at the tool edge and it is a force in itself. You may have a perfectly balanced tool spinning in the spindle at a very high RPM, but there is an unbalancing effect as soon as you engage the tool in the component.

“There are a lot of forces in play, including the weight and length of the tool, so use the lightest-weight tool holder possible and the shortest possible tool. And make sure the tools are in the best possible condition in terms of wear fatigue on the spindle tapers and the contact faces of the machine itself.”

Bob Riberdy, product specialist--deep hole machining applications and special products at Sandvik Coromant, said, “Today everybody wants to reduce the time to do a job, and that can mean reducing the number of tool changes needed. There was a time when shops used a roughing tool, a semi-finishing tool, and a finishing tool to complete some parts. They want to get rid of the roughing and semi and just do the job with one tool, which can cause balance issues. Then when you get into some special tools, one tool might be designed to drill, counterbore, and chamfer. With these you’ve got different pockets in different positions and you need more attention to balance to achieve quality specifications.

“Tooling balance-by-design is one of the ways we help achieve balance today, and the tools are pretty close to being balanced. But a lot of times you have to put the toolholder and the cutting tool together to get balance as a unit once they are balanced, they should stay together as a unit.

“Even for ourselves,” he added, “we want balanced cutting tools to create tooling because they make the insert pockets more accurate.

“Machines today have gotten better in stability, plus they are cutting at higher and higher RPMs, so imbalance shows up a lot faster,” said Riberdy. “Why balance? Everything comes down to being profitable and what makes it cheaper for companies to make components to their customers’ specifications. Balancing is one of those things.”

Associate Editor Sue Roberts can be reached at sroberts@canadianmetalworking.com.

Elliott-Matsuura Canada, 905-829-2211, www.elliottmachinery.com

Balanced tool assemblies enable faster cutting without losing tool life. Photo courtesy of Haimer USA.

Haimer USA, 630-833-1500, www.haimer-usa.com

Sandvik Coromant Canada, 905-826-8900, www.sandvik.coromant.com