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Hollow Holding: HSK toolholders

HSK toolhol`ders were developed in
the early 1990s. HSK stands for
Hohl Shaft Kegel. Translated from Ger-
man, this means hollow shank taper.
The HSK design was developed as
a nonproprietary standard. The work-
ing group that produced the HSK stan-
dard consisted of representatives from
academia, the Association of German
Tool Manufacturing and a group of
international companies and end us-
ers. The results were the German DIN
standards 69063 for the spindle and
69893 for the shank.
The HSK working group did not
adopt a specific product design, but
rather a set of standards that defined
HSK toolholders for different applica-
tions. The group defined a total of six
HSK shanks. These shank styles are
designated by the letters A through F.
Each style is also identified by the di-
ameter of the shank’s flange in milli-
meters. Styles A, B, C and D are for
low-speed applications. Styles E and
F are for high speeds. The main dif-
ferences between the styles are the
positions of the drive slots, gripper-lo-
cating slots, coolant holes and the area
of the flange.

The shank itself is made as a hol-
low taper with a ratio of 1:10. The sur-
face inside the shank is cut with a 30°
chamfer, making it possible to clamp
the toolholder from the inside. The
wall of the shank is designed to be thin
enough to flex slightly. On the outer
surface of the shank flange is a tradi-
tional toolchanger V-groove and slots
for locating and orienting an automatic
toolchanger’s (ATC) gripper.
The principal difference between
styles A and B is the size of the taper.
The B-style shank has a taper one size
smaller than an A-style shank with a
flange of the same size. D and F shanks
have tapers one size smaller than C and
E shanks with the same flange diam-
eter as well. Styles C and D were de-
signed exclusively for manual use,
with the elimination of features to ac-
commodate ATCs.
To handle extremely high speeds
and machining of light materials,
styles E and F are totally symmetrical.
Their symmetry minimizes unbalance,
which can be a significant problem at
high speeds.

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 param-
eters. The shank and spindle both must
have precisely mating tapers and fac-
es that are square to the taper’s axis.
There are several HSK clamping meth-
ods. All use some mechanism to am-
plify the clamping action of equally
spaced collet segments.
When the toolholder is clamped
into the spindle, the drawbar force
produces a firm metal-to-metal con-
tact between the shank and the ID of
the clamping unit. An additional ap-
plication of drawbar force positively
locks the two elements together into
a joint with a high level of radial and
axial rigidity.
As the collet segments rotate, the
clamping mechanism gains centrifu-
gal force. The HSK design actually
harnesses centrifugal force to increase
joint strength.
Centrifugal force also causes the thin
walls of the shank to deflect radially at
a faster rate than the walls of the spin-
dle. This contributes to a secure con-
nection by guaranteeing strong contact
between the shank and the spindle.
The automotive and aerospace indus-
tries are the largest users of HSK tool-
holders. Another industry that is seeing
increasing use is the mold and die
“There are two ends to it,” said Dan
Springhorn, president of Diebold
Goldring Tooling U.S.A., Sharon,
Wis., which makes HSK toolhold-
ers. “High-speed machining is usually
what people think of when they think
of HSK, but a large number of people
are also using it for low-speed, high-
stock removal where a high stiffness it
Whatever the application, HSK tool-
holder use is definitely on the rise. “In
1996, when we opened, we estimated
that 3 percent of the market in the U.S.
was HSK,” said Springhorn. “I would
estimate today that it is somewhere
around 15 percent. I think eventually
people will look at the HSK standard
the way that people look at the CAT
standard now.”
Reference: “The Secrets of HSK,” by Dr. Eugene Kocherovsky and Bruce Travis, CUTTING TOOL ENGINEERING, September 1998.