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Get a Grip on Toolholding

 

High-speed machining is driving improvements in toolholders

 

By Michael Tolinski
Contributing Editor
            
                                                          

As you do more machining at spindle speeds above 15,000 rpm, you may find there's much more to contend with: the effects of imbalance, vibration, and runout are magnified, potentially leading to shortened tool life, poor surface quality, or even spindle failure. Thus, choose the wrong toolholder and you're asking for trouble.

But higher speeds increasingly have to be dealt with, despite the demands on machinery and tooling. Weight-saving efforts by the automotive industry, for example, are leading to more applications for aluminum components that can be machined at higher speeds. Moreover, the higher throughput of high-speed machining (HSM) benefits manufacturers of engine blocks, drivetrain components, and dies and molds. HSM also makes lights-out manufacturing practical and dependable, because the lighter depth of cut reduces the chance of tool failure during unmanned operation.

For HSM, the choice of toolholder is important in terms of reliability, precision, and overall cost-effectiveness. The toolholder must be balanced or easily balanceable, must have high clamping strength, and must be flexible enough to accommodate a shop's practice and its various cutting tools. These demands eliminate many standard toolholder styles but, as shown below, strong candidates remain.                                   

For some time, shrink-fit toolholders have been heralded as the best choice for high-speed, high-volume applications. Their design concept is straightforward enough: the metal around the holder's bore is heated and expanded just enough to accommodate the cutting tool's shank. As it cools, the metal contracts around the tool for 360º clamping.

These systems seem to have convincing design advantages for HSM. They're inherently balanceable because of symmetry and lack of moving parts (like setscrews). Runout and concentricity is rated at 0.0001 - 0.0002" (2.5 - 5.0 µm) or less, and they offer extremely large gripping forces. As estimated by Briney Tooling Systems (Bad Axe, MI), shrink-fit holders provide an average clamping force of 10,000 lb (44.5 kN) on the cutting tool shank; this force would require 100 hp (75 kW) to turn the tool within the bore. The company says this gripping capability is over five times as great as that provided by mechanical or hydraulic chuck toolholders.

Yet shrink-fit holders are only slowly catching on. Why?

There may be several reasons. The first may be the fundamental concept of shrink-fitting itself. The idea of holding something without a setscrew or a chuck may appear counterintuitive for some people. Plus, shrink-fit holders must match specific shank diameters, so they're relatively inflexible for odd tool sizes (although this may not be a major problem in high-volume automotive production, where shank sizes are standardized and don't vary much).

Also a negative for some people is the need to heat the toolholder and wait for it to cool, even though tool-change cycle times can be short--sometimes under 30 sec. Briney's Shrinker System, for example, requires about 7 sec to heat and expand the bore, and cools quickly after the cutting tool is inserted.

And there are cost concerns. Shrink-fit holders can cost about 30% more than competitive types, and they require an induction heating unit that can cost from a few thousand dollars to over $10,000. Although much less expensive than other shop equipment, the heating unit typically must be budgeted as capital equipment--and then be cost-justified to survive budget-cutting.

But the real-world performance of shrink-fit toolholders shows they are indeed justifiable, as all the shrink-fit suppliers contacted for this article agree. The proof comes from direct comparisons with hydraulic-clamp or other mechanical-clamp holders. And the benefits are evident for machining at both high and low rpms, says John Stagge, president of Techniks Inc. (Indianapolis, IN). "Those who test and measure the results of shrink fit for high-speed machining purchase shrink-fit tooling."

Ultimately, the benefits show up as longer-lasting cutting tools and productivity improvements from HSM. Faster feed rates and lower carbide-tooling purchases help cover the cost of the heater/insertion machine. Stagge says that if a shop purchases more than a critical mass of carbide and diamond tools per month in its current operation, a shrink-fit system can be justified by future savings.

As one example, Stagge points to the case of Aisin Drivetrain (Crothersville, IN), a supplier of transmission cases and brake master cylinders. The company was using end mill holders and hydraulic holders with carbide and polycrystalline diamond (PCD) cutting tools respectively for the two applications. After testing with shrink-fit holders, speeds and feeds reportedly increased 20 - 30%, and productivity went up 400% for the transmission cases. For the PCD job, tool life quadrupled. The resulting savings in purchasing these expensive tools helped justify a shrink-fit system.

Both Techniks and Briney have also tried to make the systems more justifiable by offering more affordable and user-friendly shrink machines. Last year Briney announced its table-top Thermax shrinker, a lighter, less-expensive, user-friendly system for smaller shops. And recently Techniks introduced a $4000 induction shrinker, as well as a higher-end three-station machine with heating, tool-changing, and cooling stations combined on one machine footprint. The machine doesn't require the operator to handle a hot tool, further simplifying a process that Stagge implies isn't so complicated to begin with. For removing tools of most diameters from a holder, for instance, all you need is heat: "You can heat the holder up with a hand-held propane torch you purchase at Wal-Mart."

Meanwhile, collet-chuck toolholders haven't shrunk from taking on the challenge of higher rpms. As the most popular style of toolholder, collets already offer users a comfort level and flexibility. They rely purely on mechanical interference to hold tools, and can accommodate a wide range of tool-shank lengths and diameters. At least a few tooling providers are able to offer collet holders with HSM capabilities.

Besides their familiarity, collet toolholders have practical advantages over the heat-expandable, shrink-fit variety, argues David McHenry, product engineer for Rego-Fix Tool Corp. (Indianapolis, IN). "What we have heard the most is that customers, automotive included, like the fact there is no heat of any kind, making the system much safer for the general worker to use."

McHenry adds that a collet holder allows more accurate presetting of the tool in the Z direction, which would require a special presetter for shrink-fit systems. "The automotive industry loves the Z-axis preset ability; they can now change tools very easily, and know that they are making the parts the same from tool to tool."

Collet toolholders also offer flexibility. When changing to a tool with a different diameter shank, only the collet needs to be changed, not the entire holder. The penalty of flexibility is that some collets lack rigidity in holding certain tools, leading to more cutting-tool wear, and thus more purchased tools.  In terms of clamping force, Rego-Fix claims to have a collet system that rivals shrink-fit holders. Last year, the company introduced the powRgrip system, which is said to generate high clamping forces through an extreme interference-fit created between the collet and holder.                   

The system performs well after repeated tool changes, reportedly having been tested for more than 20,000 tool insertions and removals. And with collet chucks, the only element that wears down is the collet, usually a relatively inexpensive item to stock.

The powRgrip is similar to shrink-fit systems in that it requires a machine for tool insertion. The machine, a table-top mini-press, produces six tons of force, requiring about 15 sec for tool insertion or removal, for collets in sizes of 0.125 - 0.75" (3 - 19 mm).

A high-speed collet system is also offered by Lyndex-Nikken Inc. (Mundelein, IL). Said to be particularly suitable for die/mold finishing, the VC toolholder is balanced at 40,000 rpm, with a runout of 0.00012" (3 µm) at 4 X diam. Moreover, the VC doesn't require fixtures or machines to install or remove collets.

Various features make the toolholder highly accurate up to 40,000 rpm, according to the company. The collet has an extended straight section and a flat shoulder that maintains concentricity and proper force transfer. Moreover, the toolholder's smooth profile reduces air currents and cuts noise at high rpms. Harmonic vibrations are dampened and rigidity increased through machined-in features, a thicker wall section at the base, and an 8º collet design.

The system's ability to absorb and isolate vibration is particularly valuable at high speeds, because vibration cuts tool life, degrades surface finish, and wears the spindle. Its dense body design is said to dampen vibration better than shrink-fit holders. The VC toolholder also has a TiN-coated bearing plate that allows higher tightening torque by reducing friction during nut tightening, while a groove under the nut dampens microharmonic vibrations.

Another series of high-speed chuck toolholders, also balanced for 40,000 rpm, was announced last September by BIG Kaiser Precision Tooling Inc. (Elk Grove Village, IL). The Mega Chuck series uses the company's BIG-Plus dual-contact spindle system, which increases accuracy and rigidity at the spindle/holder interface (see "What's the Connection?" in Manufacturing Engineering's June 2004 issue).

The chucks have a notch-free nut that reportedly eliminates tool vibration at high speeds and reduces whistling noise and coolant splattering. Toolholders come with a special wrench that has a one-way clutch system with roller bearings, functioning as a ratchet for nut tightening.

Despite its name, the series is designed for the smallest of tools. Four different styles cover a tool diameter range of 0.010 - 1.500" (0.25 - 38 mm): the Mega Micro collet chuck for tight areas with small cutting tools; the standard New Baby collet for high-speed machining; the E collet for high-clamping-force end milling up to 0.500" (12.7 mm); and the Double Power Chuck, which has a dual-contact nut for higher rigidity between the toolholder body and nut.

Balance is key for high-speed machining, whether dealing with a collet or shrink-fit toolholder (see "Basics of Balance," in Manufacturing Engineering, March 2005). Unbalance leads to uneven cuts and chips and the vibration mentioned above. Adequate balance is critical for getting the fine finishes and longer tool life that high-speed machining offers, explains Jay Verellen of Seco-Carboloy (Warren, MI). "Balanced tooling will yield better results at any rpm. It becomes absolutely necessary when considering HSM. It's important to note what maximum unbalance your machine can withstand."

As a general rule of thumb, he says it's widely accepted to allow a maximum of 3 g-mm unbalance in toolholders for HSM, but this decreases to 1 g-mm at very high spindle speeds (>30,000 rpm). For comparison, a standard, generic end-mill holder can show as much as 250 g-mm unbalance. "Depending on the actual rpm used; this imbalance can be the difference between catastrophic failure and complete success."

Some toolholders are pre-balanced to a specified level for high-speed use. For example, ThermoLock shrink-fit toolholders from Command Tooling Systems (Ramsey, MN) are sold balanced to G2.5 at speeds from 15,000 to over 25,000 rpm. Below these speeds, balance is less of an issue. "At speeds below 10,000 rpm, the vibration caused by the toolholder will be less than the background vibration level of most machine tools," explains Dennis King, director of engineering.

The company counterbalances toolholders to correct for inherent body design and manufacturing process asymmetry. The pre-balance grade chosen by a user depends on the style of toolholder (BT, DIN, or CAT) and, for steep-taper toolholders, on whether pre-balancing should be done with the retention knob installed or not. Mounting retention knobs to the body creates additional imbalances that, depending on the application, need to be corrected, says the company.

Even with pre-balancing, other variables can sabotage balanced performance in a machining operation, such as poor spindle fit, bad or worn tooling, and loose assembly tolerances of components. Thus it's important to maintain toolholder balance in the shop. For example, Rego-Fix engineer David McHenry points to the method the company supplies with its toolholders. The holders are pre-balanced up to 15,000 - 25,000 rpm, but a ring system allows additional balancing, without drilling-out material.

"Each holder that we make comes with a set of ground grooves on them to which you can, when needed, add a set of balancing rings. The rings pass over the holder and snap into these grooves, making any Rego-Fix standard holder a balanceable holder." The rings are snapped on and, unlike other ring systems, use setscrews only for orientation and to keep the ring from rotating. The ring system is effective for speeds of more than 42,000 rpm and, McHenry adds, "As with any balanceable-type holder, a balancing machine is required to balance and check the holders after tool changes."

 Balancing machines are offered by some suppliers of pre-balanced toolholders. These allow users to balance toolholder assemblies for optimum use, and to reduce wear caused by centrifugal forces from unbalance at high rpms.It's important to keep toolholder assemblies under the unbalance threshold for a given rpm, says Command Tooling's Dennis King. This threshold goes down as rpms increase. "If the rpm is up at 30,000 or faster, then the balance threshold is so low that even small variations are not allowable. In these instances, customers would be well advised to only run toolholder assemblies that have been balanced after tool presetting."       

 

Forces increase with imbalance and spindle rpm, leading to more tool deflection and shortened tool life.

 

Even at lower rpms, it may be necessary to re-balance, particularly when changing retention knobs to run on different machines, changing between HSS steel cutters and carbide, or changing the design of the cutter.

Command's balancing machine can measure unbalance in one or two planes. The machine's software provides data that support various balancing methods, including balancing rings or drilling for metal removal. The company says it also uses laser marking to indicate the exact location of unbalance on the toolholder. The unit comes with a standard clamping spindle in CT 40 or 50, and HSK 40 or 63 forms; additional clamping spindles can be installed.



This article was first published in the September 2005 edition of Manufacturing Engineering magazine. 

 


Published Date : 9/1/2005

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