Toolholding technology keeps pace with the requirements of high-speed machining
It is getting more difficult to remember the days when machining was performed at slow speeds with shallow cutting depths. Today, machine shops of all kinds, not just the ones working with high-tech alloys on multi-axis autonomous machining centers, are laser-focused on speed.
This is not just speed for speed’s sake. The goal is to produce more good parts, more profitably, and more safely. High-speed machining (HSM) is a means to achieve this goal by reducing cutting forces, increasing metal removal rates, improving heat dissipation, and imparting better surface finishes.
Manufacturing Engineering interviewed a number of industry experts to get their views on HSM with a particular focus on toolholding. One thing became clear in our discussion—the high-speed evolution will continue with the toolholder playing a crucial role.
‘Your Mentality Quickly Changes’
Many technologies must meld together to get maximum benefit from HSM. “When you move to a HSM operation, your mentality quickly changes,” said Chris Herdman, technical field support engineer for Rego-Fix Tool Corp., Whitestown, Ind. “You have to be aware of the interlinking of machining center, programming, high-speed spindles, advanced cutting tools, balancing, and high-performance toolholders in order to take full advantage of HSM.”
Different Things to Different People
The metalworking industry first adopted HSM in the 1990s. The term “high-speed” means different things to different shops.
“High-speed machining is a high-rpm, high feed-rate strategy, but with a low depth of cut,” said Brendt Holden, president of Haimer USA LLC, Villa Park, Illinois. “Even with a lower depth of cut, at these higher speeds and feeds the material removal rate actually increases, which is the ultimate goal in that it allows shops to make parts faster.”
Though there is no specific rpm demarcation line, our experts agreed that spindle speeds in the 15,000-30,000 rpm range are definitely high-speed. In fact, Herdman of Rego-Fix made note of toolholders operating in the 120,000 rpm range in specialized medical and electronics applications. “What we see now as high speed are less rigid, four- and five-axis machines operating at 12,000 rpm or above,” Herdman said. “The applications are low radial cut, high tooth count, high feed rate, and most of them are utilizing some type of dynamic milling software.”
Matt Goss, applications engineer/project development for Greenleaf Corp., Saegertown, Pa., said his company considers high speed to be a combination of two factors: high spindle speed and a high feed rate. “This translates to a high feed speed (ipm), which helps to not only maximize the amount of material removed, but also leads to reduced cycle times.”
Preben Hansen, president of Platinum Tooling Technologies Inc., Prospect Heights, Illinois, said that as spindle speeds increase above 20,000 rpm, shops begin to approach the work a bit differently. “You may not take as much material during each revolution of the tool, but feed rates are typically higher,” he said.
Overcoming HSM Challenges
As speeds increase, so does the potential for serious problems with balance and accuracy. A perfectly balanced toolholder in the spindle is essential, according to Dan Doiron, milling product manager for Emuge Corp., West Boylston, Mass. “Even minor fluctuations will lower repeatability, reduce tool life, and possibly damage workpieces. With time and money invested in each component, the slippage or pullout of a cutting tool as a result of using the wrong toolholder is not an option.”
Conventional chucks (such as ER collet holders and Weldon flats) can easily be pushed beyond their limits and usually cannot be counted on to work reliably in the HSM environment. “ER collets are weak in clamping force and runout and side-lock chucks are weak in runout and dampening in high-speed applications,” Doiron said.
At any speed, the spindle is subjected to centrifugal force. At high speed, the centrifugal force is enough to make the spindle bore grow slightly. “This expansion can draw steep-taper tools (CAT, BT, SK, etc.) up into the spindle, causing a change in the Z-axis dimension,” said Holden of Haimer. “Hollow shanked taper (HSK) tools, in contrast, grow with the spindle at high rpms, maintaining face contact and eliminating the Z-axis movement.”
Herdman of Rego-Fix noted that any vibration quickly becomes magnified as tools rotate faster. “The majority of HSM applications necessitate staying away from steep tapers,” he said. “For this reason, HSK and Capto style toolholders are going to be the best choices.”
When you get into the newer and more exotic ceramic cutters that operate at extremely high speeds, heat becomes an issue. Heat generated at the cut can be transmitted into the toolholder, which may compromise holding integrity. “This can lead to vibrations during the machining process and eventually pullout of the tool,” Goss of Greenleaf said.
Another challenge related to HSM is maintaining spindle life, according to Holden. “The spindles are not very massive and the concern becomes the effects of unbalance on the spindle bearings,” he said. “This has led to the development of prebalanced toolholders with no moving parts or added accessories.”
In the tapping world, there are specific challenges for toolholding when it comes to high-speed operations, according to Kyle Matsumoto, product engineer II for OSG USA Inc., St. Charles, Illinois. “Synchronization is key in the tapping operation and errors can be very problematic in terms of quality and tool life,” Matsumoto said. The errors occur at two points: the beginning of the cycle when the tap begins to rotate, and at the point when the tap transitions from forward motion to retraction. “If the tap is not allowed to reach its targeted rpm before it touches the material, and/or if it begins retracting before reaching the targeted rpm, the additional thrust loads produced can reduce thread quality and lower the tool life, or even break the tool,” Matsumoto said.
Trusting in Trochoidal Milling
In situations where a part is being produced from solid material and has complex profiles and slots, HSM can deliver productive, low-load roughing via trochoidal milling. This technique creates a slot wider than the tool’s cutting diameter on a programmed toolpath that maintains a low radial depth of cut and a high axial depth of cut, utilizing the entire length of the cutting edge. This shapes the part very close to its final form.
“For trochoidal milling, due to the long tools (up to 5×D) and number of flutes (up to seven) on the end mill, the clamping force is extremely important,” Doiron of Emuge said. He cautioned against total reliance on friction-fit toolholders in trochoidal applications. “High gripping torque by itself does not guarantee pullout protection,” he said. “The incredible cutting forces generated by faster machine spindles and aggressive toolpath strategies have exposed the limitations of even the best friction-fit clamping systems.”
Holden expanded on this thinking. “The challenge has been to secure the cutting tool in the holder with friction-fit systems (shrink-fit milling chucks, high-precision collet chucks, hydraulic chucks, press-fit collet systems, etc.),” he said. “Without a form-fit, the cutting tool can spin in the toolholder and ultimately pull out of the chuck. There have been cases where the operator didn’t realize the tool was pulling out until it prematurely broke. This drove some users to go backwards and return to the 100-year-old method of securing a tool via a side-lock end mill holder, which of course has terrible balance repeatability and poor runout.”
The key characteristics of a high-performance holder for HSM are balance, rigidity, clamping strength, and runout accuracy. It is important to match the toolholder to the application. Roughing applications typically benefit from a toolholder with the highest clamping strength. In finishing operations, where precise tolerances and fine surface finishes are the goal, toolholders with minimum runout and maximum repeatability would be the first choice.
All of our experts recommend a high-performance toolholder for HSM applications and caution against trying to save a few dollars on the toolholder. It does not make economic sense to gain productivity advantages from HSM if it is squandered on lower tool life, slow changeovers, or quality losses, they said.
“The biggest return on investment when purchasing a high-performance toolholder is the increased tool life, hands down,” said Doiron of Emuge. “We have customers who saw a tool life increase of five times simply by changing over to our high-performance toolholder, without changing the speeds or feeds that they were running.”
The high-performance toolholders cited were either mechanical, heat-shrink, or hydraulic chuck. Each has its advantages and limitations in specific applications. Shrink-fit uses a heated bore that expands to accept the cutter and then clamps as it cools. Hydraulic toolholders use hydraulic pressure to actuate a membrane or other device to grip the tool.
“Shrink-fit is quite popular and has very good runout and balancing behavior, but does not excel at vibration dampening, which is very often limited by the maximum feed rate,” Doiron said. “Improved hydraulic chucks have been developed during the last few years and their advantage is good vibration damping. But their limitation is the clamping force and the absence of pullout safety features.”
“The preferred approach to toolholding for HSM is going to be a precision holder that can hold runout of the tool to within 0.0001" [0.00254 mm] or less measured at up to 3×D,” said Goss of Greenleaf. “Holding these extremely tight runout tolerances leads to better overall tool performance, repeatability, and more predictable tool life.”
“Make sure that you purchase prebalanced toolholders and that once you assemble the toolholder, it is still balanced as an assembly,” Haimer’s Holden said. “Try to find a system with as few moving parts as possible. If you don’t, you’ll get reduced tool life and poor surface finish. Or, the spindle will blow out fast and you will have machine downtime and the high cost of replacement. We recommend obtaining an in-house balancing machine to confirm that your assemblies are balanced before you put them into the machine. For steep taper applications (CAT, BT, SK), we recommend the purchase of high-quality pull-studs that have a pilot to guide the pull-stud to the center of the toolholder for the best balance.”
For tapping applications, Matsumoto of OSG recommended the SynchroMaster holder to absorb the thrust loads of higher speed operations. OSG claims that the holder “turbocharges” tapping performance on CNC machines with synchronous spindles by compensating for synchronization errors during the threading process. The SynchroMaster has an integral micro-tension compression float that absorbs loads in the thrust direction and significantly reduces thrust forces during reverse rotation. The result is longer tool life, consistent tapping depth and improved thread quality. Short chamfer spiral tap tool life is extended from 231 holes to 1,000 holes, a factor of 5×.
Doiron cited Emuge’s rigid FPC mil/drill chuck with three tons of traction force to hold a tool securely. According to Emuge, the patented FPC has a 1:16 worm gear pulling a special collet with extremely high ratio into a flat angled cone, absorbing virtually all vibration. The mechanical drive is actuated with a hex wrench, permitting quick tool changes. Clamping forces are extremely high and independent from the tolerances of the tool shaft. With a 3×D tool length, variation in concentricity is less than 3 µm, which extends tool life and improves surface finishes. All models are factory-balanced to G2.5, 20,000 rpm. In a speed comparison with four competing chuck designs, the FPC chuck enabled the feed rate to be increased by 30 percent with no loss in performance, according to Emuge.
Herdman recommends the Rego-Fix powRgrip series of holders, collets, and automatic or manual pressing systems. The collet is pressed into the holder with up to nine tons of pressure. Rego-Fix claims that tool changes can be accomplished in 10 seconds with powRgrip and that total indicated runout is less than 3 µm at depths of cut up to three times the tool’s diameter, warranting its performance for five years and 20,000 tool changes. For applications that require additional cutter retention measures, Rego-Fix offers the secuRgrip anti-pullout system, which locks the tool to the collet.
Holden cited Haimer’s “friction- and form- fit” solution, called Safe-Lock. The system combines the accuracy of a shrink-fit, high-precision collet, or hydraulic chuck, with a form at the back of the bore to engage with the grooves on the shank of a cutting tool. The company claims there is no possibility of tool pullout with the Safe-Lock system. All Haimer holders come prebalanced and the shrink-fit chucks come drilled and tapped to accommodate balancing screws that can be added to correct the potential unbalance of the full toolholder assembly.
Goss said that Greenleaf recently announced XSYTIN-360, a new line of high-performance solid-ceramic end mills that deliver significant increases in productivity by allowing users to apply chip loads similar to solid-carbide end mills with the higher speeds common to ceramic machining. The new ceramic end mills offer 10 times higher productivity and major cost savings, according to the company. Greenleaf also launched its line of quick-change toolholders, which are designed to maximize tool life in carbide and ceramic turning applications. The holders have high-pressure coolant nozzles to aid in chip removal and improve tool life. Positive V-bottom inserts use replaceable nests. Negative inserts utilize the side rake angles to offer better tool life with Greenleaf ceramic inserts.