Alternatives to steep tapers: Drilling Performance

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.

“Sticking” Together

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 above, offer advantages steep-taper styles can’t. Image courtesy of IBAG North America.

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.

For people who think they can’t take advantage of this technology because they don’t plan to buy a new machine, they might want to check with their distributor, as their machine may already be equipped for BIG-PLUS. “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.”

Revving Up

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. And 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.

Sandvik Coromant’s Capto has a polygonal geometry that reportedly gives it greater torque-transmission capabilities and accuracy than competing systems. Image courtesy of Sandvik Coromant.

HSK was developed during the late ’80s at RWTH Aachen University in Germany as a collaboration among various machine tool builders, cutting tool manufacturers and industry experts. (HSK is the German acronym for hollow taper shank.)

Unlike CAT, BT and BIG-PLUS, which have solid bodies, HSK and other hollow toolholders expand along with the spindle cartridge as the rpm increases. This alone improves accuracy and rigidity at elevated spindle speeds, but these systems also incorporate dual face and taper contact, similar to BIG-PLUS. They’re also much lighter than solid-body designs, making them more suitable for extreme spindle speeds.

HSK is available in six styles, HSK-A through HSK-F, as well as the newest style, HSK-T, which is specifically for multitask, or mill-turn, machines. Common sizes range from 32mm (1.259″) to 125mm (4.921″), as measured at the face diameter, with some capable of running at 50,000 rpm or higher. Dual-contact designs such as this are more difficult to produce, requiring extreme accuracy in toolholder and spindle taper alike. There is also a new U.S. standard for HSK, recently published by ASME.

Perusing Polygons

Sandvik Coromant Co., Fair Lawn, N.J., produces HSK toolholders at several of its facilities in Germany. Jim Grimes, senior product specialist for machine tool integration, said HSK beats steep-taper toolholders hands down across a range of machining applications, but, for multitask machines at least, he recommends Coromant Capto.

Modern spindles are capable of high spindle speeds and extreme accuracy. Image courtesy of IBAG North America.