Despite its growing use and acceptance in the United
States, HSK technology remains widely misunderstood. Questions about its
proper use have created substantial resistance among those who are
accustomed to traditional, steep-taper shanks, including CAT, SK and BT.
Although a significant portion of the machine tools imported to the United
States from Europe incorporate HSK spindles, steep-taper shanks still
represent the most widely used tooling interface.
The acronym "HSK" is the German
abbreviation for "hollow taper shank." An HSK shank has a ratio of 1:10,
compared to CAT (BT, SK) shanks that have ratios of 7:24 (Figure 1,
below left). HSK shanks must be connected to machines via
compatible HSK spindle receivers. Whereas steep-taper shanks were developed
prior to standardization, HSK shanks were developed to address performance
problems associated with the traditional interfaces, particularly in high
speed machining applications.
After five years of testing by a working
group that comprised educators, machine tool builders, end users, cutting
tool manufacturers and standards organizations, the HSK standards were
created. The preliminary standards included six types of HSK shanks
designated as A through F (Figure 2, below right) and a
total of 35 sizes. Final standards have now been published for Types A, B, C
and D, but the most popular types for high speed machining--Types E and
F--are covered by the working group's "preliminary standards."
 |
 |
| Figure
1. Comparison of HSK and CAT (SK) Shanks. |
Figure
2. Overview of six types of HSK Shanks, per DIN 69893. |
HSK shanks address three different
application categories. Types E and F are designed for low torque and very
high spindle speeds on machines that incorporate ATCs. Types A and C serve
applications requiring moderate torque and moderate-to-high spindle speeds.
(Type A is for automatic tool changing, and Type C is for manual changing.)
Types B and D are designed for high torque applications with
moderate-to-high spindle speeds. (Type B is for automatic changing and Type
D is for manual changing.)
Comparing HSK With
Steep Taper
Although HSK has become the primary choice
for newly developed machine tools in Europe, substantial skepticism remains
in the United States. To alleviate some of this doubt, it's important to
explain some fundamental differences between HSK and conventional tooling
interfaces.
 |
| Figure
3. HSK shank clamping mechanism operates from the inside. |
The first category of comparison is radial
and axial stiffness-the most important aspects of any machining operation.
Unlike conventional shanks, an HSK shank is hollow and the clamping
mechanism operates from the inside (Figure 3, at right).
The end of a typical, HSK Type A shank incorporates two drive slots that
engage milled drive keys in the spindle receiver. The wall of the hollow
shank deflects slightly when it's clamped into the receiver. Radial access
holes in the shank's wall allow the clamping mechanism to contact an
actuation screw. The inner surface of the shank wall also incorporates a
chamfer to facilitate clamping.
Although different clamping methods are
available depending on the tooling manufacturer, all HSK receivers
incorporate segmented collets that expand radially under drawbar pressure to
bear against the inner wall of the shank. Because the collet's chamfer
matches the chamfer of the shank's inner wall, the shank is locked securely
into the receiver when the drawbar is actuated. When this occurs, elastic
deformation of the shank's walls creates firm metal-to-metal contact around
the shank, as well as mating the shank's flange with the receiver. (See
Figure 3, above right.)
 |
| Figure
4. Radial stiffness of SK40 and HSK-A63 Shanks. |
Assuming that equivalent force is applied
to the drawbar, twice as much clamping force is exerted on the flange of an
HSK shank compared to a steep-taper shank. This extra clamping force makes
the radial stiffness of HSK toolholders up to five times greater than the
value for CAT, SK or BT (Figure 4, at right). This makes
the tool more resistant to bending loads, thus allowing deeper cuts and
higher feedrates in milling and boring operations. Higher rigidity also
translates to a higher natural frequency for the cutting system. This allows
a tool to be operated at higher speeds before resonance or "chatter"
commences. Because tool deflection is reduced, machining accuracy and
surface finish also improve.
With firm contact between the HSK shank's
flange and the receiver, the axial position of the interface remains
constant during boring and drilling operations that exert the strongest
Z-axis forces. With its stronger clamping mechanism, HSK tooling is also
considerably more resistant to pull-out forces than conventional interfaces.
In terms of torsional stiffness, the HSK
interface is comparable to the CAT (SK, BT) connection. But HSK transfers
significantly greater torque than conventional shanks (Figure 5, at right).
Regarding accuracy and resistance to tool
runout, the HSK interface is equivalent to CAT (SK, BT) in radial accuracy,
while providing significantly better axial accuracy. In the axial direction,
the accuracy of a CAT (SK, BT) connection can vary up to 0.004 inch compared
to an HSK shank. This affects the repeatability of machining operations.
 |
| Figure
5. Torque transmission of SK50 and HSK-A100 and HSK-B125. |
Another factor that affects accuracy is
tool presetting. With a CAT (SK, BT) interface, variation between the
machine spindle and the pre-setter spindle changes the axial position of the
tool tip. This is particularly true in cases where bell-mouthing of the
machine spindle has occurred as a result of wear.
Conversely, the HSK interface (with
metal-to-metal contact both radially and axially) maintains a constant tool
tip position that does not depend on physical differences between the
machine and a pre-setter spindle. As the HSK connection wears during
operation, therefore, the tool's rigidity is affected--but not its static
accuracy.
The HSK interface also offers some key
advantages in relation to high speed machine spindles, tool collisions and
maintenance. Using a conventional interface (CAT, SK, BT) at spindle speeds
greater than 8,000 rpm, the spindle receiver expands at a much higher rate
than the toolholder shank. This causes the shank to be pulled back axially
into the spindle under the force of the drawbar. This changes the Z-axis
position of the tool tip and often locks up the toolholder inside the
receiver, thus making tool-changing difficult. Conversely, the design of the
HSK connection prevents the shank from pulling back into the receiver during
high speed operation.
When a tool collision occurs using a
conventional, steep-taper shank, the potential damage can be considerably
greater than is true when using an HSK shank. Because a CAT (SK, BT) shank
is solid steel, most of the collision load (and damage) transfers to the
spindle. With its hollow design, however, the HSK shank acts as a fuse
during collisions. When a cutting tool crashes, the toolholder breaks off
and protects the spindle, thus reducing repair costs and machine downtime.
CAT (SK, BT) spindles may be reground to
restore proper performance. Although regrinding must be done by a
professional, many companies offer this service. On the other hand,
regrinding of an HSK spindle is considerably more difficult, requiring a
highly skilled operator, an extremely precise grinding machine and the
proper gaging equipment. Because this work is beyond the capabilities of
many machine shops, the cost is higher than is true for regrinding
steep-taper spindles.
The tool-changing capability of HSK is
another improvement when compared to steep-taper shanks. Because of the
short length of the HSK taper (approximately one-half the length of a CAT
shank) and the lighter weight of its hollow shank, tool changes can be
completed more rapidly than is true with conventional toolholders. Part of
this time savings results from the fact that the HSK interface does not
require a retention knob to clamp the shank.
Variable cutting conditions can adversely
affect the CAT (SK,BT) interface. This applies particularly to modern CNC
machining centers that are used in flexible manufacturing systems. Under
these circumstances, machines may operate at low speed and high torque, as
well as high speed and low torque. Because conventional toolholders are
clamped from the outside, centrifugal force causes the spindle walls to
expand faster in relation to the shank at spindle speeds higher than 8,000
rpm. Consequently, the draw bar force pulls the shank deeper into its
receiver, changing the position of the tool tip and frequently locking up
the tool.
The HSK interface is not subject to this
problem because of firm contact between mating components. This contact is
enhanced at high speeds because, as the collet segments in the receiver
rotate inside the hollow shank, centrifugal force increases the clamping
force.
Additional Performance
Factors
In terms of tool balancing, HSK and SK
adapters are similar. HSK adapters are normally sold unbalanced but, if
balancing is required, the customer should specify this when placing an
order. Two methods are used to balance HSK tooling. The first method
balances by using a cutting tool to remove excess material from the adapter
housing. This method is recommended for heat-shrink tooling, and both the
tool and toolholder must be balanced (usually by the manufacturer).
The second method incorporates adjustable
components such as screws that allow fine-tuning of the tooling assembly
prior to use. Although this method is more accurate, it also requires
frequent user intervention to make balancing adjustments.
Both HSK and steep taper shanks allow for
use of an internal coolant supply. At spindle speeds exceeding 20,000 rpm,
however, internal coolant may destroy the static balancing of the
spindle/toolholder assembly. This can occur because of asymmetrical coolant
channels in the tooling or by contamination with air and oil. In these
cases, using external coolant may be necessary.
HSK tooling is manufactured according to
more rigorous specifications than steep-taper tools. One reason for tighter
tolerances is because the force of the clamping mechanism improves as the
clearance between toolholder and receiver is reduced. As a result of its
minimal clearances, the HSK interface requires even greater attention to
cleanliness of shank surfaces than is true when using conventional
toolholders. HSK tools are also more sensitive to wear than other types of
tooling. This means that users must have their own gaging systems or use
outside inspection services to control tooling quality. A shop's workforce
also must be properly educated regarding the care and maintenance of HSK
tools.
HSK In The Future
The HSK system is well suited for
modular tooling. Because of the excellent rigidity and accuracy of this
interface, tooling assemblies that incorporate shank extensions and reducers
may be used with results comparable to those attainable when using solid
adapters. The majority of cutting tool companies today offer HSK modular
tooling. This tooling is adaptable to the proprietary products of various
manufacturers, such as
Sandvik "Capto";
Komet "ABS"; and
Kennametal "KM". When
an HSK spindle is installed on a particular machine, therefore, the shop's
existing inventory of cutting tools may be used. If a spindle adapter is
used to attach HSK tooling, however, toolholders must be changed manually.
As HSK tools become more standardized and
additional cutting tool manufacturers introduce the system, the current
utilization of proprietary or unique tooling components will probably be
reduced. Furthermore, as HSK tooling becomes more widely used in America,
the current price gap between HSK and steep taper tooling may also be
eliminated. MMS
Dr. Eugene Kocherovsky
represents
Intelligent Concept (West Bloomfield,
Michigan), a firm that has developed an automated Internet design system
for HSK products. He has more than 20 years experience in the cutting tool
industry, primarily in the fields of product research and development. An
expanded version of this article, published with permission, may be
obtained from
www.hskworld.com.
|
