Toolholders for High-Power Cutting
Various cutting forces make significant demands on the tool/toolholder interface
By Michael Tolinski
Ideally, a cutting tool and its toolholder should behave as one solid, perfectly balanced unit—but obviously this is never the case. They're two separate tools united only by clamping pressure at their interface, where the various forces of metalcutting tend to uncouple them.
These forces can sometimes be strong and multidirectional, especially in high-powered milling with carbide tools. However, different experts have different definitions of what might be called "high-force machining."
"Applications normally requiring higher gripping forces are first-operation [roughing] cuts, or applications involving higher-strength workpiece materials, such as hardened workpieces and the common aerospace materials," says Jeff Keith, product manager for tooling components, Schunk Inc. (Morrisville, NC). Higher cutting forces are also found during secondary finishing operations where the end-user is trying to reduce cycle time by increasing the depth of cut, width of cut, or table feed.
Especially in a cost-conscious industry like automotive, toolholders are under pressure from the higher feeds and speeds asked of them. "You also have the new specialty cutting tools with a high helix that are made to remove material faster—all of this plays into how you're going to hold onto [the tool]," says product engineer David McHenry of Rego-Fix (Indianapolis, IN).
Tougher workpiece materials increase the forces that tend to pull helical carbide cutting tools out of the holder, explains Jack Burley, vice president of Big Kaiser Precision Tooling Inc. (Elk Grove Village, IL). This applies especially to companies that do a lot of high-helix-angle end milling with "exotic" materials, such as Inconel and titanium, that are used in the aerospace arena, he says. When milling these materials with tool helix angles of 45 or 60°, "the end mill acts like a screw," pulling itself into the workpiece and out of the toolholder. Likewise, roughing out mold steel "requires exceptionally high gripping torque on the tool," asserts Preben Hansen, vice president, Lyndex-Nikken Inc. (Mundelein, IL).
But in talking about "high force" situations generally, it may be better simply to focus on the size of the cutting tool and the tool pressures it experiences, explains John Stagge, president of Techniks Inc. (Indianapolis, IN). End mills or drills at or above 3/4" (19 mm) in diameter, cutting at the highest pressures, qualify for Stagge's definition of applications that require high-gripping-force toolholders. The workpiece material is not critical, "because the softer the material is, the faster you can go." Thus, high forces can result even when cutting aluminum at a speed four times as fast as that employed when cutting steel.
Toolholders don't get much credit when they supply sufficient clamping forces under pressure—but they may get the blame when a tool slips or breaks. Often the source of a problem lies elsewhere, Jeff Keith says, for example with dull cutting tools, tools with poor shank tolerance, improper insertion of the tool into the toolholder, poor workpiece fixturing, or the use of an old machine with a "loose" spindle or inadequate power.
"Today's machine tools are lighter, less rigid, and have smaller main-spindle motors and axis-drive motors," adds Keith. "This has changed the method of machining from taking maximum depths and widths of cut at lower feed rates, to taking lighter depths and smaller widths of cut at much higher feed rates." On the bright side, this often means that "the clamping forces generated by the holder are much less important today than they were in the past."
Still, the choice of toolholder remains important, and there's a debate about which styles provide high clamping forces along with the best overall benefits for an operation. Mechanical, shrink-fit, and even hydraulic holders offer advantages for various kinds of tough cutting situations.
There's often a tradeoff between clamping force and other positive attributes such as low runout, vibration, and cost. Consider basic end mill holders with side setscrews for engaging a flat in the tools. For secure toolholding in many situations, "there's still no better holding method than this traditional, 'cheap,' if you will, solution," says Jack Burley of Big Kaiser. But the user may sacrifice rigidity and stability during cutting.
Higher-end toolholding that relies on friction for maintaining clamping force, such as the shrink-fit approach, is popular, but "there are pros and cons," Burley says. Shrink-fit toolholding depends on the tool and the holder having the proper tolerances, and the proper shrinking of the heat-expanded bore around the shank. And, like traditional collet holders, "there's no positive lock holding that cutter in place."
In Big Kaiser's evaluations comparing various toolholders' effectiveness in holding roughing end mills, mechanical milling chucks provide the strongest gripping power. They also provide pullout security, good balance, and coverage of a range of shank diameters.
Milling chucks provide high mechanical clamping torque for holding, such that "when someone talks about gripping torque, they're the first thing that should be considered," says Preben Hansen of Lyndex-Nikken. In practical use, the company's Ultra Lock milling chuck reportedly provides at least five times higher gripping torque than hydraulic and shrink-fit chucks, and its gripping torque increases exponentially as chuck size increases.
Although the company also supplies shrink-fit holders, Hansen cautions about the variations in shrink-fit bore tolerances that a user might see in holders from various suppliers. Good tolerances are critical for maximizing gripping force through a range of diameters.
Yet shrink-fit holders' clamping force typically makes them more than adequate for a job. John Stagge of Techniks says any toolholder might provide much more gripping power than is necessary for an application. "Tool slippage is not necessarily a big problem in this industry; what is, is trying to apply the right toolholder to the cutting tool. What it really comes down to is rigidity in the cut, and accuracy in the cut."
Stagge emphasizes that toolholder performance cannot be measured in an "unreal" laboratory situation. When a chuck is on an actual spindle, "there's an amount of deflection you get with that tool, based on the speeds and feeds that you're running." Low deflection indicates rigidity in the toolholding.
Shrink-fit toolholders provide the best overall performance, according to Stagge. The company's torque tests shows shrink-fit's superior clamping ability for 3/4" (19-mm) cutting tools. The "rigidity in the cut really helps you in the tool life and the quality of the cut," extending tool life 6–8 times more than milling chucks, at higher feeds and speeds.
Shrink-fit's effectiveness is maximized with carbide cutting tools, says Tom Wagner, manager of engineering for T.M. Smith Tool International Corp. (Mt. Clemens, MI). This is because the cutters have different thermal expansion from the holder's H13 steel. "When you use highspeed tool-steel shanks, the shrinker machine has to be able to put a lot of heat into the holder in a short period of time, so that the holder bore opens up before the shank of the tool starts to grow with it." With carbide shanks at H6 tolerance, he says the optimum grip will break the tool shank off before it can spin in the toolholder.
In automotive manufacturing, shrink-fit has grown in popularity. Wagner says some of the reasons relate to simplicity and consistency. "When you use a shrinker, your maintenance load is lightened because that shrinker tool is a one-piece item." By contrast, basic collet toolholders, for example, are multicomponent affairs requiring tightening nuts and wrenches. Thus collet setups can change person to person; by contrast, a shrinker machine, with no moving parts, can be fixed at one setting with automatic toolsetting gages to position each tool consistently.
Enhanced collet-style toolholding does have some advantages. Rego-Fix's powRgrip system uses a mechanical press to consistently generate high clamping forces. This pressed-in collet system is more controllable than a shrink-fit system, says engineer David McHenry.
He says the system also prevents a user from being able to damage a toolholder by setting the shrinker wrongly (such as by overheating, and thus annealing the tools). "Because you can control the amount of deformation of our toolholder when you press that collet in, we never destroy the steel or the way the toolholder works." The powRgrip also allows the user to create extra clamping force on the tooling when it's needed.
The system has been expanded to accommodate tool diameters up to 1" (25.4 mm) because of interest from the aerospace industry for roughing out the massive amounts of aluminum the industry cuts. McHenry adds that other industry sectors are pressuring toolholder specialists—specifically the cutting-tool and machine makers, who are pushing the envelope on speeds. "And we're kind of stuck in the middle, trying to figure out how long this aggressive cutting tool can stay in the machine and work properly."
Hydraulic toolholders are traditionally candidates only for low-power machining. Concerns about inadequate or lost hydraulic holding pressure during use have relegated them to reaming applications, such as for castings in the automotive industry.
"It's absolutely true that hydraulic holders do not generate the same levels of clamping force as mechanical or heat-shrink holding systems," says Jeff Keith of Schunk, which supplies multiple styles of toolholders, but is best known for hydraulic holders. However, hydraulic toolholders do completely engage the tool 360° around its shank with a very uniform clamping force, which "results in superior runout accuracy and overall cutting performance." And design improvements have made hydraulics more rigid, allowing greater depth of cut, width of cut, and/or table feed.
Another key advantage for hydraulic holders is vibration dampening. "Like shock absorbers on a car minimizing the roughness of the road, the fluid in hydraulic holders reduces the effects of cutting vibrations in the machining process," says Keith. This results in improved surface finish, dimensional accuracy, and tool life. "We feel the optimum choice in holder technology today combines adequate [versus maximum] clamping forces with workpiece accuracy and long life of the perishable tool."
Makers of cutting tools understand the importance of toolholders in rigorous cutting situations. "The toolholder does play a large part in creating success when dealing with quality tools," says Mark Menconi, solid carbide end mill product specialist, Mitsubishi Materials USA Corp. (Hoffman Estates, IL). By gripping a tool firmly, a good toolholder can "provide the best opportunity for a premium tool to optimize its performance."
Mitsubishi has worked with shrinkfit and other toolholding specialists to optimize carbide tools. "I've come to see that heat-shrink systems work best with Mitsubishi's product; they ensure the best hold by creating a higher percentage of contact with the tool," he adds.
One application, thread milling, demonstrates how certain cutting tools "create a very unique set of cutting forces that demand very rigid toolholding," says Mark Hatch, thread milling manager for Emuge Corp. (West Boylston, MA). In thread milling, the tooling encounters various cutting forces simultaneously: forces from the circular movement of the machine in the X and Y directions, and Z-axis forces for creating the threads. "So we not only have radial pressure, but we have axial forces in action during the threading cycle."
Radial pressure tends to create the most problems with the tooling, Hatch explains. The most common problem resulting from poor toolholding is thread taper from top to bottom. "A second issue would be premature failure of the cutter due to bending forces, which in most cases cause edge chipping."
Matters are complicated by the market's needs for thread lengths that extend up to 2xD, while end mills may normally run at only 1–1.5 L/D, he adds. Plus, threads are typically milled in tough, strong materials, such as alloys for the aerospace, oildrilling, and medical industries. "As a result of these higher-strength materials, in combination with unfavorable L/D ratios, we must have very rigid toolholding in place."
Many thread-milling operations use typical ER collet toolholding and cutters with straight shanks instead of flats for clamping. Straight shanks are fine with shrink-fit toolholding, Hatch says, but ER collets provide weak radial stiffness. Moreover, an ER collet holder's runout under load is poor, causing taper, breakage, and low tool life. He says that Emuge thread mills include a flat on the shank, making conventional end-mill style holders with side-locking screw acceptable options, and most shops already have these holders.
Emuge is also a licensed provider of Rego-Fix's powRgrip technology that's specific to thread milling. Here, "you get a comparable amount of clamping pressure as in shrink-fit holding, but the added benefit is that you have anti-dampening properties within that type of collet." This is important in thread milling, an oscillating cutting process, because the cutting tool can fall into harmonic vibrations, causing problems with thread finish and tool wear.
Tooling developers also consider tool/toolholder forces when perfecting new cutting strategies, such as the high-feed milling strategy of Seco Tools Inc. (recently relocated to Troy, MI). High-feed milling doesn't create as much side (radial) forces on the tooling as standard milling, according to Seco's Bob Goulding.
High-feed milling was originally developed as a process for pocketing in mold and die applications, but Goulding says it now fits the overall industry's trend towards low to medium-power machines. "Because of the way the tool is designed, it gives a very thin chip with a small depth of cut, which allows you to take very high feed rates. This pushes forces back up through the spindle, rather than in the radial direction that you experience with traditional milling." The lack of side forces reduces runout and surface-quality problems.
The strategy accommodates weaker spindles, and potentially allows lower-cost toolholding to be used, such as generic side-lock holders, says Goulding. Or hydraulic toolholders such as Seco's AccuFit can reduce runout of the flutes on the cutting tool—this equalizes tool wear and extends tool life.
This article was first published in the September 2008 edition of Manufacturing Engineering magazine.