By Jim Barnes
Dampening vibration in extended reach toolholders boosts productivity
Extended-reach tooling can provide significant productivity benefits through reductions in setup times.
However, there is a price to pay; the laws of physics dictate that vibration and rigidity may become potential problems as toolholders get longer.
Applications for extended-reach toolholders are numerous and include work in aerospace, automotive and tool and die. “They may be used for large workpieces that are mounted on tombstone-style fixtures,” says Mike Marshall, milling applications engineer, Hardinge Inc., Elmira, NY.
“In automotive, we have seen it in engine blocks and in special brake components where you have to reach through fixturing. In aircraft, it’s all over the place, because the aluminum and titanium pieces are getting so big now,” notes David McHenry, senior product engineer, Rego-Fix Tool Corp., Indianapolis, IN.
“Many people are doing high precision work where they are not willing to refixture the tools,” he says. If an extended reach toolholder can do the job, they will use it.
The physical demands of the process must be kept in mind. “When you are that long out, you can’t put a lot of deformation force on the tool,” says McHenry, “not to mention what it does to the spindle.”
“Most extended reach t oolholders are fairly slow, say, roughly 5,000 rpm and slower,” according to McHenry.
Everything depends on the application. If you’re making holes with an axial-load application, it isn’t doing much to the spindle, says McHenry. “It’s when you start trying to mill at the bottom of a pocket that the operator really has to think about the torque. If he’s reaching out 14 in. and he wants to apply a load, he has to consider what that’s doing to his toolholder, to his interface and to his spindle. “It definitely has to be looked at, application-wise,” he says.
Several companies have been making efforts to increase rigidity and accuracy in their extended-reach toolholders, says Marshall. The use of a one-piece design provides the greatest amount of rigidity and accuracy. Dual Contact toolholders increase rigidity by utilizing a simultaneous face and taper contact between the spindle and toolholder, according to Marshall.
Extensions vs. integral holders
The basic choice to be made in equipping yourself for extended-reach cuts is whether to rely on extensions or longer integral toolholders. The decision is cost and flexibility versus rigidity.
“Most people are leaning toward the extensions. An integral holder is usually much more expensive. Extensions give you the versatility, where you have one common holder and a variety of different extensions,” says Mike Roy, general manager, Schunk, Mississauga, ON.
In making the decision on extensions, “You have to take into account things like rigidity and accessibility,” says Roy. “There are many different technologies, like there are with a toolholder, such as heat-shrink extensions, hydraulic extensions and side lock extensions, so you have to look at the technologies available as well,” says Roy.
Integrated toolholders will provide the best rigidity versus tool extensions,” says Marshall. “Extensions provide the ability to reduce toolholder inventory by allowing the user to adapt each tool to the specific needs of the application.”
Manufacturers have devoted a lot of development time on the vibration issue, and the approaches vary. Many are proprietary.
Making toolholders from heavier metals, such as tungsten, is one tactic. “These materials are denser, thus reducing vibration and bending, but this alternative can be a little expensive,” notes Marshall. Design is a big part of the answer: he adds that the general design of the Dual Contact system limits vibration throughout the machining system.
“Vibration and surface finish are critical. We see people leaning towards hydraulic extensions for that reason,” says Roy. Hydraulic chambers are built into the toolholders, with the fluid providing effective dampening. In finishing applications, “we focus on hydraulic vibration dampening, but we also have a proprietary system called Tribos.” That system supports extremely low tool clearance and includes extensions that permit the use of many standard cutting tools. “The absorption technology is not incorporated in the holders, it’s in the extensions themselves,” says Roy.
In cases of applications with radial loads and side-cutting forces, more rigidity may be required and something like a heat-shrink extension might be appropriate, says Roy.
McHenry says his firm uses a proprietary technology called micro-friction dampening. “It is part of our assembly process, it’s the way we assemble the two-piece holder,” explains McHenry.
“Everybody has their own way of dampening the vibration. Just make sure you are using something accurate at the end of it,” he says.
“The tool you are using on the extended toolholder – and how it is being used – is as important as the holder itself. Finding a tool or using machining techniques that transfer most of the forces into axial force rather than radial force is a must for any extended-length toolholder application,” says Dale McClelland, milling manufacturing engineering, Hardinge Inc.
“Extended-length milling requires a strong toolholder-to-spindle connection. Dual Contact toolholders such as BIG-Plus, HSK, and Coromant Capto provide better support and rigidity over standard steep-taper toolholders. This strong base allows for higher material removal rates at higher spindle speeds increasing productivity. ATC repeatability of dual contact toolholders will also reduce runout for improved surface finishes when fine milling with extended reach,” says Alan T. Miller, engineering manager, product manager-Tooling Systems (BIG), BIG Kaiser Precision Tooling Inc., Hoffman Estates, IL.
“Presetting is sometimes an issue with long tools like that. People want to preset them offline, so the nice thing about a hydraulic-type holder is you have axial control so you can adjust the height through the bottom of the toolholder. You can typically adjust those up to ten mil.,” says Roy.
Misconceptions about the technology are often based on experience with older technology.
“In the past, people dealt with some lower quality products. To reach out really long distances, they had a collet chuck and an extension, and an extension inside an extension, and they were trying to drill a hole at the very bottom of a pocket. They were not used to having accurate options,” says McHenry.
Current technology may offer unexpected versatility. “The design of the BIG-Plus spindle provides the ability to use standard taper tooling and BIG-Plus tooling as the user requires,” says Marshall. Users need only purchase BIG-Plus tooling for specific applications, potentially reducing tool inventories.
Many suppliers caution that extended-reach tools have definite limits. “We get people who want to use an end mill holder, way out at the end of that long reach. It’s hard to explain to them that when you are out that far, the last thing you want to use is an end mill holder, because it is inherently offset–which causes an imbalance issue which causes vibration,” says McHenry.
Keep an eye on your load and understand that the rigidity is probably a fifth of what you are used to,” says Roy.
“The biggest thing I would stress is looking at each application individually. Nine times out of ten, it’s a lightweight finishing application, so vibration, rpm, balancing–things like that–are all critical,” says Roy. SMT
Jim Barnes is a Toronto-based writer with 30 years of experience in writing about manufacturing technology.