moldmakers learn the benefits of running small diameter cutting tools at
high rotational speeds, the toolholding system for the machines that run
those jobs becomes a key success factor. The HSK tooling system offers
tremendous advantages to mold shops in terms of accuracy, repeatability,
weight and fast tool changing cycles.
Originating in Germany in the later 1980s, the HSK family of standard
toolholders was developed to meet the needs of ever increasing spindle
speeds. Steep-taper tooling, using CAT V-Flange, SK and BT toolholders,
had begun to show its limitations at spindle speeds in excess of 8,000
rpm. Already widely accepted in Europe, HSK is still relatively new in
the United States. American automotive and aerospace manufacturers have
been the early adopters of the technology.
Holding the hefty mass of a size 40 or 50 CAT/SK/BT steep-taper
toolholder in your hand gives you a certain sense of security. By
comparison, the HSK adapter of comparable size seems quite light. At
high rotational speeds, though, the HSK system - toolholder and receiver
- has the dynamics of motion working with it, much as the seemingly
light structure of an airplane is strong in flight.
element of the design is dual-surface location of the toolholder when it
is engaged in the receiving member of the spindle. The holder is located
radially by the fit of the toolholder's 1:10 ratio tapered shank O.D.
with the receiver's matching taper I.D. Simultaneously, the holder's
flange comes face to face with the spindle nose's front flange for
positive axial location. This positive seating of the flanges sets the
HSK system apart from the CAT/SK/BT products, which locate on the 7:24
ratio taper alone.
block spindle with an HSK 50 Mapal tool interface.
spindle speeds increase, there is a tendency for the spindle shaft and
its receiving taper to heat up and grow, both radially and axially.
Radial growth in a spindle using CAT/SK/BT steep-taper tooling allows
the tool retention mechanism to pull the toolholder deeper into the
taper, resulting in a loss of Z-axis accuracy. At the same time, the
mass of the toolholder may prevent it from warming as quickly as the
spindle taper. The slower growth of the holder causes the solid fit of
the taper to be lost and radial stiffness is lost, as well. The loss of
the solid fit also opens opportunities for resonance - or chatter - to
using the HSK system also are subject to thermal effects. What is
different is how the HSK system handles them. Since the HSK adapter is
located axially by flange-to-flange contact with the spindle nose, the
tool retention system cannot pull it from its place. And since the HSK
toolholder is hollow, its tapered shank will heat more evenly and grow
with the spindle taper.
rotational speeds also have a positive effect on the HSK design's tool
retention system. Since the retention fingers are positioned to be
inside the hollow area of an installed toolholder and grip outward, the
centrifugal force of the rotation increases their grip, keeping the
toolholder's flange tight against the spindle's nose flange.
Payoff: Greater Radial and Axial Stiffness
The precision fit of the HSK adapters combined with the spindle and the
compatibility of the HSK design and the forces at work in high-speed
rotation all contribute to measurably better radial and axial stiffness
than that of spindle systems using steep-taper CAT/SK/BT toolholders.
The stiffness is what contributes to repeatable accuracy and high
quality surface finish.
stiffness advantage begins with the tool retention system, mentioned
above. Its clamping force is typically double that of CAT/SK/BT designs.
The simultaneous, two-surface seating and the basic precision of the HSK
adapters and receivers finish the job. Testing has revealed the HSK
interface's radial stiffness advantage to be up to five times that of
steep-taper tool systems. The greater the radial stiffness of the
assembly, the less chatter and tool deflection that can occur.
Outstanding axial stiffness - a result of the flange-to-flange mating -
of the HSK holder and adapter also is measurably superior to the
Applying Torque to the Cutter: A Range Of Choices With HSK
The HSK system is a family of designs, all featuring the 1:10 ratio
mounting taper. Six types of toolholders give users an opportunity to
choose the toolholder best suited for their application. A range of
sizes in each type of holder rounds out the offering with a total of 35
|A 60-100 mm
cartridge spindle with an HSK tool interface.
of a particular type or style of adapter can be based on the amount of
torque to be applied to the work, the speed range necessary for the job
or whether automatic or manual tool changing cycles are desired. From
the standpoint of torque transmission and spindle speed, user's choices
are as follows:
- HSK adapter types A
and C are designed for moderate torque and moderate to high rotational
- HSK adapter types B
and D are designed for high torque and moderate to high rotational
- HSK adapter types E
and F are designed for low torque and super-high speeds, with
Toolholders intended for
automatic tool changing applications include orientation guides.
Tool Changes - Automatic or Manual
Automatic tool changes in machining centers are faster with HSK tooling
than comparable sized CAT/SK/BT steep-taper toolholders. The tapered
mounting shank of the HSK design is approximately one-half the length of
the traditional tapers. The low-mass design of the HSK units eases
braking and handling issues, as well.
changing of adapters also is fast and easy. HSK toolholders feature
simple release/lock systems. They also are shorter than their automatic
change counterparts and are potentially stiffer and more accurate in
A and B holders have the additional advantage of being suitable for both
manual and automatic tool changing cycles. These two styles also have
provision for installing a microchip that can share cutter information
with the machine's CNC controller.
Balancing HSK adapters is no more difficult than balancing traditional
CAT/SK/BT toolholders. Types E and F, the holders for super-high-speed
applications, were designed with ease of balancing in mind.
Minimizing tool crash damage is another benefit of the hollow HSK
adapter design. When the energy of a serious crash is transmitted to the
toolholder, it will break, protecting the spindle, similar to the way
"crumple zones" on automobiles absorb energy to protect passengers.
of HSK tooling is coming down. Tool manufacturers are becoming familiar
with the fine points of producing the standardized designs and overall
demand for the product line is increasing. As acceptance increases, this
trend will continue.
HSK is a system of toolholders that can handle a very wide range of
machining needs, both in the traditional low-speed/high-torque
applications and the increasingly productive high-speed/low-torque world
of hard milling. Its advantages in stiffness and accuracy have been
clearly shown. Steep-taper CAT/SK/BT tooling that has served so well
over many years in low-speed cutting is not going to get the job done in
the future. The time to start getting on-board for the future with HSK