Below are excerpts from a Cutting Tool Engineering article by the same title. To read the entire article please click HERE.
Author Kip Hanson, Contributing Editor, Cutting Tool Engineering
Kip Hanson is a contributing editor for Cutting Tool Engineering magazine. Originally Published: September 12, 2017 - 3:00pm
Shopping for a machining center was simpler when buyers had only two basic spindle choices: CAT or BT. Both of these “steep tapers” have an angle of 3.5 in./ft., or 7" in 24" (7/24), and are based on the 1927 patent by Kearney & Trecker Corp., Brown & Sharpe Manufacturing Co. and Cincinnati Milling Machine Co.
With the development of automatic toolchangers in the late 1960s, machine tool builders in Japan modified the patented design and invented the BT standard. In the 1970s, tractor manufacturer Caterpillar Inc., Peoria, Ill., changed things again with a flange design now known as CAT, or V-flange.
During the late ’80s, machine tool builders began offering vertical and horizontal CNC mills with spindle speeds higher than the 6,000 to 8,000 rpm common at the time. As rpm increased, so did problems with steep-taper toolholders.
Chief among them is the tendency for the mating spindle and toolholder tapers to stick together. This is caused by the expansion of the spindle housing at high speeds, which allows the toolholder to be pulled upward into the spindle taper, jamming it in place.
HSK spindles, like the one shown in the illustration below, offer advantages steep-taper styles can't.
One way to eliminate this problem is by extending the toolholder flange upward, thus creating a hard stop against the spindle face and preventing further Z-axis movement.
This is the approach taken by BIG KAISER Precision Tooling Inc., Hoffman Estates, Ill. Jack Burley, vice president of sales and engineering, said the BIG-PLUS system—developed in 1992 by BIG Daishowa Seiki Co. Ltd., Osaka, Japan—relies on a bit of elastic deformation in the spindle to provide dual points of toolholder contact at its face and taper, eliminating upward holder movement as the spindle expands.
He said it’s also more rigid, with tests showing that the deflection on a CV40 BIG-PLUS toolholder measured at 70mm (2.755") from the spindle face is only 60µm (0.002") when subjected to 500kg (1,102 lbs.) of radial force, roughly half that of a traditional V-flange toolholder.
“There are now roughly 150 machine builders that either offer BIG-PLUS or have it as a standard,” Burley said. “The beauty of the system is that it can use either standard toolholders or BIG-PLUS interchangeably. So for drilling and reaming work, you can use a conventional collet chuck, but for heavy milling cuts or profiling operations at higher spindle speeds, BIG-PLUS improves accuracy and tool life.”
Burley does not recommend BIG-PLUS for older machines that have never seen these toolholders, because CAT and BT taper-only contact holders tend to bellmouth the spindle over time, leading to undesirable results.
BIG-PLUS, like any dual-contact toolholder, requires particular attention to cleanliness, as chips caught between the spindle face and the toolholder can cause serious problems.
He also recommends staying below 30,000 rpm when using 40-taper holders, noting that higher speeds are better handled by HSK spindles and holders.
Keep It Clean
Bill Popoli, president of IBAG North America, North Haven, Conn., said the company started building steep-taper spindles in the late ’80s, but 95 percent of its work has since transitioned to HSK spindles. As mentioned earlier, the extreme accuracy needed to guarantee near-simultaneous contact between the spindle face and taper is challenging, requiring micron-level tolerances in toolholder and spindle alike.
These requirements were impossible to meet when steep taper was first developed, Popoli said, resulting in looser standards overall for CAT and BT spindles than the ones applied to HSK spindles and toolholders. Because of this, purchasing an HSK or equivalent toolholder automatically makes one “part of the club” when it comes to balance, accuracy, repeatability and tool life.
That’s not to say, however, that shops firmly married to steep tapers should settle for less. Popoli recommends purchasing the highest-quality tooling possible and paying close attention to the stated tolerance.
Always stay below 20,000 rpm with 40-taper holders, and reach no more than 30,000 rpm with 30-taper ones. Use balanced holders and high-quality retention knobs that have been properly torqued—otherwise distortion at the small end of the taper may occur. And whatever the taper type, keep the spindle and toolholder clean at all times.
Bob Freitag agreed. The manager of application engineering at Minneapolis-based metalworking products and services provider Productivity Inc. said the lines are evenly split between traditional 40- and 50-taper CAT or BT tooling (much of which is BIG-PLUS) and HSK.
“It really depends on the application,” Freitag said. “Most of our die and mold machines in the 20,000- to 30,000-rpm range will have an HSK63A or HSK63F. When you get up around 45,000 rpm, you’re probably looking at an HSK32. But in horizontal machining centers and lower-rpm, high-torque verticals, you’ll see mostly steep tapers, as this is generally preferred for deep depths of cut and lower feed rates, where you’re removing a lot of material at once.”
For shops that want to make the leap to an HSK machine but are leery of investing in new toolholders, Freitag advised:
“Anytime you buy a new machine, you should buy new toolholders to go with it. If not, the imperfections of the old toolholders will soon transfer themselves to the spindle on the new machine.”
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HSK is short for the German words "Hohl Shaft Kegel" or, in English, Hollow Shank Taper.
There are several major differences between "steep taper" toolholders, like NMTB, CAT & BT and HSK.
As machining spindle speeds have increased, BT and CAT tooling systems tend to lose accuracy due to centrifugal forces from running at high RPM. The mouth of the machining center spindle can grow, "bell mouthing". As it grows the BT and CAT tool is under constant drawbar pull, moves up the expanded taper. This causes the Z-axis offset to change. This can also cause the toolholder to stick in the spindle.
The HSK contacts the spindle taper and flange on the spindle face to make a solid union in both the axial and radial planes. In operation, HSK tool holders are resistant to axial movement because the face contact prevents the toolholder from being pulled up into the spindle at high speed.
Cutting tools generally takes higher radial forces because the flange contact and taper contact combine to resist deflection.
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