By Dr. Eugene Kocherovsky and Bruce Travis
Published by CUTTING TOOL ENGINEERING Magazine , September
1998/Volume 50 Number 6
To put these high-speed toolholders to work
in North American machine shops, tooling manufacturers and end users must understand the
intricacies of the design.
The HSK toolholding system developed in Europe has been touted in the United
States as the system for advanced machining. It is the one toolholding system designed
specifically for high-speed, close-tolerance machining. To some extent, HSK has lived up
to its promise. U.S. manufacturers, like their European counterparts, have found that
using HSK toolholders will improve quality and productivity. But some end users in the
United States and Canada have reported lackluster performance using HSK toolholders. There
are subtle, yet fundamental, reasons for these disappointing results related to
misunderstandings of the proper utilization of HSK. Having invested thousands of hours of
R&D time in this new toolholding technology, engineers at Valenite Inc., Madison
Heights, MI, are in a good position to clear up some of these misunderstandings and offer
guidelines on the proper implementation of this new technology. Through their work with
HSK, these engineers have learned what designs, materials, and manufacturing techniques
will produce high-performance toolholders that will satisfy the North American market's
needs and conform to the habits of U.S. manufacturers. Their research has also revealed
ways to tweak the HSK system for better performance in extraordinary situations.
HSK toolholders are a German
development. The abbreviation HSK stands for "Hohl Shaft Kegel," which,
literally translated into English, means "hollow-shank taper." Among U.S. end
users it is more commonly referred to as "hollow-taper-shank tooling."
The HSK design was developed as a nonproprietary standard suitable for both
rotary and stationary applications. Its developers believed a single standard design was
preferable to the growing number of tooling interfaces that were proliferating at the
time. The working group that produced the HSK standard consisted of representatives from
the academic world, the Association of German Tool Manufacturing, and a group of
international companies and end users, including Valenite. Their effort produced the
German DIN standards 69063 for the spindle receiver and 69893 for the shank.
It is important to note that the HSK working group did not adopt a specific
product design, but rather a set of standards that defined HSK toolholders for different
applications. As these industry representatives considered design alternatives, they had a
very specific set of performance criteria in mind. They were looking for a toolholding
system that would be rigid, lightweight, and capable of high radial and axial accuracy. In
addition, the system had to be suitable for very-high-speed rotary applications.
In the end, the group chose to define not one, but a total of six HSK shanks.
These shank styles are designated by the letters A through F. Each model is also
identified by the diameter of the shank's flange in millimeters. Styles A, B, C, and D are
for low-speed applications. E and F are for high speeds. The main differences between the
styles are the positions of the drive slots, gripper-location slots, coolant holes, and
the area of the flange. Each style was defined for certain applications. It is critical
that the proper type of HSK tooling be specified on high-speed machine tools, not just the
|Figure 1: These cutaway drawings of A-style (left) and B-style HSK
shanks show the basic features included in the DIN standard
Figure 1 illustrates a typical shank design
and its basic features. The shank itself is made as a hollow taper with a ratio of 1-to-10
(taper angle 2° 51' 78"). Two drive slots are located in the end of
shank style A. Their unequal depths ensure that the toolholder can be oriented in only one
way. Figure 1 shows the drive slots, which feature a cylindrical profile to increase the
surface of the contact and reduce stress as torque is transmitted from the spindle to the
toolholder. The surface inside the shank is cut with a 30¡ chamfer, making it possible to
clamp the toolholder from the inside. As we shall see later, internal clamping offers some
unique advantages. The wall of the HSK shank is designed to be thin enough to flex
slightly. Radial access holes are located in the wall some distance from the contact face.
These access holes allow a manual clamping mechanism to reach the actuation screw (not
Inside the shank, there is a groove for a rubber O-ring and a central
coolant-nozzle hole. They are designed to protect the internal clamping components from
corrosion. The coolant nozzle is an optional feature that is not included in some
clamping-mechanism designs. When the nozzle is positioned in the holder, it has about
±1¡ of angular motion. This degree of freedom is provided to compensate for any
misalignment of the clamping-mechanism components during the clamping process. On the
outer surface of the shank flange, there is a traditional toolchanger V-groove and slots
for locating and orienting an automatic toolchanger's gripper. In addition, there is a
radial bore for a tool-identification microchip.
|Figure 2: On the left is a typical HSK50A shank with a 50mm-dia.
flange. The HSK50B shank on the right, which also has a 50mm-dia. flange, has a shank one
The principal difference between shank styles
A and B is the size of the taper. The B-style shank will have a taper one size smaller
than an A-style shank with a flange of the same size (Figure 2). For example, an HSK50B
shank will have the same 50mm-dia. flange that an HSK50A shank has, but its taper will be
equal to an HSK40A taper. This is true of the D and F shanks as well; all three will have
tapers one size smaller than A, C, and E shanks with the same flange diameter. The smaller
taper leaves more room on the flange's face to mate with the face of the receiver. The
bigger flange area allows manufacturers to locate drive slots on the flange and combine
drive slots with gripper-locating slots to transfer high torque. The bigger flange also
makes it possible to supply coolant through the face if it is necessary to reroute coolant
to protect the internal clamping mechanism from corrosion. Shank style C was designed
exclusively for manual use. It is a copy of shank style A, with the elimination of
features that style A has to accommodate automatic toolchangers. Style D differs from
style B in the same way.
To handle extremely high rpm and the machining of light materials, shank styles E
and F were designed to be totally symmetrical. Their symmetry minimizes unbalance, which
can be a significant problem at high speeds. Torque is transferred from the spindle to the
toolholder only through friction between the tapers and faces of the mating shanks and
|Figure 3: The basic features of an HSK spindle receiver are shown
in this series of cutaway views.
Figure 3 presents cutaway views of typical A-
and C-style receivers. The receiver design includes an internal taper with a 1-to-10 ratio
and milled drive keys with heights that match the unequal depths of the shanks' drive
slots. The partial profile of the drive keys also is cylindrical. The internal space of
the receiver is designed to house the clamping mechanism, which is not yet standardized.
If a manual clamping unit is incorporated, the receiver's radial access hole can be used
for clamping or releasing tooling shanks. When the access hole is to be used, a protective
ring covers it to keep out coolant and chips. An internal coolant supply can be included
as part of the clamping unit.
Receivers for B and D shanks are slightly
different from the A- and C-style receivers. B - and D -style receivers feature external
drive keys similar to the design of traditional steep-taper spindles. In these receivers
the coolant supply can be routed through the flange, bypassing the clamping unit. Some
people in the industry believe that A - and C - style shanks are interchangeable with B -
and D -style shanks. This is not true. The HSK developers were going to make these shanks
interchangeable, but they abandoned this idea in the final version of the standard.
Receivers for E - and F -style shanks, like
the shanks themselves, are totally symmetrical. Because the receivers do not have any
drive keys, torque must be transferred entirely by friction.
How HSK Works
|Figure 4: In the first phase of the HSK clamping action (top), the
mechanism is in the released position and the shank is out of the receiver. In the second
phase (middle), the shank has been inserted, but the mechanism has not yet been actuated.
In the final phase (bottom), the drawbar has pulled the sleeve in, pushing the collet
segments out to engage with the chamfer on the shank's inner diameter.
An HSK connection depends on a combination of
axial clamping forces and taper-shank interference. All these forces are generated and
controlled by the mating components' design parameters. The shank and receiver both must
have precisely mating tapers and faces that are square to the taper's axis. There are
several HSK clamping methods on the market. All use some mechanism to amplify the clamping
action of equally spaced collet segments. The steps illustrated in Figure 4 show how a
typical clamping mechanism is actuated.
In the first phase of the clamping process, the mechanism is in the unclamped
position. The drawbar (2), which is located in the spindle (1) and is rigidly connected to
the sleeve (4), is released. This allows the collet segments (3) to contract inside the
sleeve's cavities. With the segments in this position, the operator can insert the shank
into the receiver, engaging the drive keys (7) and (8) of the spindle with the mating
slots on the shank.
In the second phase of the clamping process, the shank is in the receiver, but
the clamping mechanism is not yet actuated. Because the gage diameter of the shank is
slightly larger than the gage diameter of the receiver, the toolholder does not slide
fully into position. As a result, there is some clearance between the flange and the face
of the receiver.
In the third phase, the mechanism is actuated. At this point, the drawbar pulls
the sleeve in, away from the toolholder. As a result, the collet segments expand radially,
and their 30¡ chamfer engages the mating chamfer on the shank. This amplifies the force
of the drawbar. This force is transformed into pullback forces that are equally spaced and
applied to the circumference of the 30¡ chamfer inside the shank.
As the collet segments engage, the pullback force causes the shank to deform
slightly, pulling the shank further into the receiver until contact is made between the
flange and the receiver's face. The fact that the two tapers are in intimate contact due
to the elastic deformation of the shank ensures the positioning accuracy and repeatability
of the joint, both radially and axially, to 0.0001". At the end of the clamping
cycle, the sleeve engages the coolant nozzle (6). The drive keys (7) and (8) transfer
When the toolholder is clamped into the receiver, the drawbar force at first
produces a firm metal-to-metal contact between the tapered shank and the inner diameter of
the clamping unit. An additional application of drawbar force positively locks the two
elements together into a joint with great radial and axial rigidity. During the clamping
process, some of the clamping unit's energy will be spent compressing the shank taper to
pull the shank deeper into the receiver. Depending on the amount of clearance, up to 20%
of the axial clamping force may be needed to pull the toolholder in. The larger the
clearance between the mating faces, the more energy will be needed to bring them together.
Take the connection of an HSK40A toolholder as an example. If the clearance
distance is 0.005", the clamping unit will have to exert about 1200 lb. of force to
achieve initial face-to-face contact. If the clamping unit is capable of generating a
total of 5200 lb. of axial force, it will be able to exert 4000 lb. on the face of the
connection after bringing the holder and the receiver together. But if the unit can
generate only 2500 lb. of axial force, it will be able to exert only 1300 lb. of force on
the connection after using 1200 lb. to close the gap between the flange and the face.
To avoid wide gaps that waste clamping-mechanism energy, the HSK standard
dictates tight tolerances for taper gage-line diameters. Such accuracy is needed because
each 0.001" deviation in the gage-line diameter will result in a 0.001" error on
the face clearance when the taper ratio is 1-to-10.
So far, this discussion of
clamping forces has not taken into consideration the beneficial effect the toolholder's
rotation has on the holding power of the mechanism. It is because of this effect that HSK
is considered the ideal design not just for the 15,000-rpm milling that passes for
high-speed machining today but for the 40,000- to 50,000-rpm operations that will be the
standard on the next generation of high-performance machine tools.
The HSK design actually harnesses centrifugal forces to increase joint strength.
As the collet segments rotate, the clamping mechanism gains centrifugal force in
accordance with the formula:
Where F = centrifugal force in pounds, n = the number of revolutions per minute,
W = the revolving body's weight in pounds, and R = the perpendicular distance from the
axis of rotation to the center of mass in feet (or for practical use, to the revolving
body's center of gravity). As a rough example (assuming amplification and friction cancel
out each other), let's say the weight of a single collet segment in an HSK50A clamping
mechanism is 0.18 lb., and the segment stands 0.51" from the body's center of
gravity. At 40,000 rpm, centrifugal force on this segment will be 4200 lb. This force,
which rises with the speed of rotation, positively contributes to a dependable connection.
Centrifugal force also causes the relatively thin walls of the tapered shank to
deflect radially at a faster rate than the wall of the spindle. This contributes to a
secure connection by guaranteeing strong contact between the shank and the spindle. The
changes that centrifugal force causes on the inside of the clamping mechanism won't affect
the axial position of the cutting edge, because this is determined by the face-to-face
contact between the flange and the receiver.
The North American Difference
The Valenite researchers'
studies of the HSK connection and the problems that North American users were having with
the toolholder led them to conclude that many holders are constructed of inappropriate
materials. The DIN specification does not cover materials, so most tooling suppliers make
their HSK components out of the same materials they use for steep-taper designs.
A typical rotary adapter is made from alloy steel that is either case hardened or
surface treated to create a wear-resistant skin over a strong, relatively ductile core.
This traditional technology works well for conventional solid-shank tooling. However, it
does not work well for flexible HSK shanks that have smaller structural sections and work
under higher stress. In fact, the use of these treated alloys is a major contributor to
short tool life and erratic performance under these conditions. As a result of the
researchers' findings, Valenite's HSK shanks are made of different steel grades and
receive a different heat-treatment than the company's conventional steep-taper shank
Having been developed in Europe, the HSK standard reflects machining practices on
that continent to some degree. When Valenite researchers compared practices in Europe to
those in North America, they found typical metal-removal rates in North America to be
higher. This difference has a profound impact on the real-world performance of HSK shanks
on this side of the Atlantic.
One of the most fundamental requirements for high removal rates is strong
connections between the toolholder and the receiver. The most obvious way to achieve
sufficient strength is to increase the clamping force. The DIN standard recommends minimum
clamping forces that range from 1530 lb. for a No. 40 HSK connection to 10,136 lb. for a
No. 100 HSK joint. Valenite recommends increasing these forces wherever possible. The
force should be doubled for HSK sizes up to No. 63, and it should be increased 30% for all
Increased clamping force is especially needed at low machining speeds, when
centrifugal force does not contribute significantly to the mechanism's holding power.
There is some risk in increasing the clamping force, however. The higher force levels can
overstress the clamping mechanism's components. It is up to the machine tool builder to
ensure that the mechanism's design can withstand this stress. An increase in the clamping
force also will compensate for variations in the clearance between the mating faces of the
HSK shank and the receiver. The DIN standard allows for a certain degree of deviation in
this clearance. For example, the clearance between an HSK63A toolholder and the receiver
can range from 0.0015" to 0.0047". The clamping mechanism must exert enough
force to pull the toolholder in tightly even at maximum clearance.
Tweaking the Design
Standard HSK tooling may not
satisfy every need. In some cases, a shop's existing equipment may force it to machine
parts in conditions beyond recommended guidelines, and the HSK design may need to be
tweaked to avoid overloading the system. In other cases, a shop may need a different
flavor of HSK to reap the benefits of the concept when the dimensional constraints of its
equipment or some historical or company preference make it difficult to use standard
Many shops would like to take advantage of the HSK design, but they do not have
machines equipped with an HSK spindle. Valenite provides a special HSK adapter for these
end users. The adapter is mounted on the existing spindle and aligned so that runout is
close to 0". In most cases where a spindle adapter is installed, HSK shanks are
changed manually. Although the connection is less rigid than an HSK toolholder mounted
directly in the spindle, it still offers high precision and fast manual changing.
Valenite offers custom-modified HSK tooling for a broad range of special
requirements. Its special tooling can be modified to provide performance characteristics
ranging from high-torque resistance for large-diameter milling cutters to high radial
stiffness for long, thin boring tools. Even though these special shanks are optimized for
specific uses, they all conform to the basic HSK receiver standards and can be used in a
common HSK spindle. Valenite produces these specials on a case-by-case basis, optimizing
its designs to satisfy each customer's requirements.
Even standard HSK tooling can be improved with small design tweaks. Valenite
found that balancing the shank/clamping unit connection could contribute significantly to
HSK tooling's high-speed performance. The company developed special static and dynamic
balancing techniques to ensure the high-speed stability of the HSK connection. It also is
possible to optimize HSK tooling for different applications by making small dimensional
changes to the shank. This provides some variation in total clamping force, even though
the amount of drawbar pull available in any given machine is essentially a fixed quantity.
At this point, there is no need to use this option, but it is available for future
All HSK tooling manufacturers
have been challenged by the extremely tight tolerances the HSK standard demands. To
achieve such a high degree of precision, manufacturers must use special inspection
technology with extremely fine resolution. Gages and instruments have to be recalibrated
to measure in the 0.000010" range. Additionally, the HSK manufacturer must use
special high-resolution sensors and processing devices and locate its final grinding
operation in a temperature-stable environment. To build HSK products to these demanding
standards and still deliver them to end users quickly, Valenite built an integrated
design-to-manufacturing system. The system can produce all necessary documentation and NC
codes for any tailored-standard custom tool in 30 to 40 minutes. This two-year-old system
is currently being used for 100% of the HSK toolholders Valenite produces.
The North American auto industry has adopted Valenite HSK toolholders as the de
facto performance standard for this category of products. One of the Big Three used a
transfer line completely equipped with Valenite HSK tooling as the benchmark in developing
its specification for all future system purchases.
Properly specified and applied, HSK tooling can optimize time in the cut for all
machine tools, while it ensures consistent radial and axial accuracy. It is lighter,
shorter, stiffer, and more precise than any competitive quick-change tooling technology,
and it is the only technology currently available that is designed to perform at
tomorrow's spindle speeds. With these advantages going for it, the HSK revolution is ready
About the Author
Dr. Eugene Kocherovsky is a product research engineer and Bruce Travis is business
manager for Valenite Inc., Madison Heights, MI.