by Kip Hanson
Improved tool life and machine performance are accessible to everyone. Here’s how.
Spin the toolholder and take a measurement. Install a counterweight. Measure again. Drill a hole here, mill a flat there. Take another measurement. Rotate a balancing ring. Repeat. This is what you might hear when asking someone unfamiliar with the process what steps are needed to balance a toolholder. It’s a bit of an art form, they’ll tell you, one that requires lengthy training and no small amount of talent to master. That, and we don’t really need it anyway, because we don’t do any high-speed machining. Right?
The art of simplicity
Wrong on all counts. Most experts agree that tool balancing is no more difficult than any other task a machine shop performs each day. Given the right equipment, it’s often easier. “There’s no art to it at all,” says Michael Colyer, key accounts manager at presetting and measuring machine manufacturer Zoller Inc. “You could hire a high school kid working at a fast-food restaurant, give them a half-hour tutorial, and put them to work balancing your tools. It’s really that simple.”
Granted, that scenario is a little optimistic. A tool balancing machine operator will likely need to drill holes in strategic locations on toolholders, or perhaps attach and adjust a pair of balancing rings. They should also know how to perform other tool crib functions, such as assembling cutting tools and toolholders, then measuring them on an offline presetting unit. That said, the process itself is quite simple, as Colyer suggests, even if the science behind it is not.
“There are three basic ways to balance a toolholder assembly,” he explains. “You can remove weight by drilling small holes or milling flats, you can add weight by attaching set screws or small counterweights, or you can use a set of balancing rings. For very long toolholders, this might need to be done in two locations, known as two-plane balancing. Whatever approach you use, the machine tells you exactly what to do. The only real skill involved is following the instructions.”
Those instructions come in the form of a laser line indicating where the imbalance lies and software explaining how to correct it. According to Brendt Holden, president of Haimer USA, this task is even easier for shops using his company’s brand of toolholders, most of which come standard with tapped holes for weighted set screws.
“We’re big proponents of this approach because it’s both fast and simple, but we also offer balancing rings, which are also easy to use,” he says. “The former probably makes better financial sense for shops with hundreds of toolholders, though. And for heavy imbalance conditions, a shop can always opt to machine material from the toolholder. Either way, our system supports it.”
Some in industry say that a major imbalance can be eliminated by using pre-balanced toolholders. But such toolholders do not eliminate the need for balancing. “I have some big automotive customers that buy pre-balanced tools, but they also buy a tool balancer,” says Colyer. “They’re not taking any chances that an imbalanced tool assembly will destroy a very expensive CNC machine spindle, or worse, hurt the operator.”
These two experts agree that, for a typical 40-taper machining centre application, tool balancing is essential at spindle speeds of 10,000 rpm and higher. But they’ll also suggest that balancing is a good idea at any rpm. Balanced tools reduce vibration, which not only improves tool life and part quality, but extends bearing life as well.
Excellence in balancing
Most commercial tool balancers spin the toolholder at relatively sedate speeds of up to 1,000 rpm or so during the balancing process. Some of those sold by Hines Industries Inc. don’t spin them at all. Sales engineer Larry Ketola explains that the company offers rotating (two plane) and non-rotating (single plane) balancing machines and has been designing and manufacturing industrial balancing equipment since 1979, much of it used for crankshafts, pumps and impellers, transmission parts, driveshafts, blowers and more with an array of automated solutions.
The company has developed a balancing solution that relies on a load cell instead of the more common velocity-based sensor, he says. “We do make a variety of rotating balancers for other applications, but in the case of a toolholder, success is all about centreing on the holder’s geometric axis. This requires a precision spindle, high-quality tooling, and electronic compensation to remove tooling eccentricity and unbalance that may be present.”
Here again, balancing instructions are displayed on a computer screen, telling the operator how much weight to add or remove, and where. Single and two-plane balancing is possible, Ketola notes, and the measurement process itself is quite fast, requiring “just a few seconds.”
He admits, however, that the target audience for this product line is primarily manufacturers rather than job shops.
“We’re happy to sell to smaller shops, but our equipment is intended more for large specialty houses,” he says. “For example, we just built a balancer for a well-known company that makes CBN diamond wheels and cutters. Others include the manufacturers of the toolholders themselves, who use our equipment to audit product quality after machining. There are countless balancing needs out there, and we address pretty much all of them.”SMT