Because an HSK
toolholder is hollow, it's good for high speed machining. Because an HSK
toolholder is hollow, it's NOT so good for high speed machining. The hollow
shank of the HSK is both a strength and a weakness. A new toolholder design
that looks like HSK but isn't hollow seems to solve this paradox. This
alternative has the short, shallow-tapered shank like HSK, but the shank is
solid. This gives it better vibration damping characteristics than HSK, its
developers say, and makes it less likely to wear or break under load.
This new style of toolholder is the NC5 from Nikken Kosakusho Works, Ltd. of
Osaka, Japan (represented in the United States by
Heartech Precision Inc., Elk Grove Village, Illinois).
Like the HSK toolholder, the shallow taper of the NC5 means it can equal the
static stiffness of a steep-tapered toolholder such as the Cat 40 or 50 but
be much shorter. The shorter shank allows spindle designers to move the
bearings at the front end of the spindle closer to its face for greater
rigidity.
And like the HSK toolholder, the NC5 features two-point contact with the
spindle. The back of the V-flange contacts the face of the spindle at the
same time the tapered shank makes full contact with the tapered spindle
socket. This two-point contact is critical in high speed machining
applications. Conventional steep-tapered toolholders contact the spindle
only on the tapered shank.
 |
| The NC5 toolholder is
short like HSK but not hollow. The diagonal slit on the taper is
critical to two-point contact with the spindle. |
When a spindle is rotating at high speed, centrifugal
force causes the spindle socket to open slightly. When it does, the force of
the drawbar pulls the conventional toolholder deeper into the spindle. This
causes the Z-axis offset to change in mid-cycle. This effect can't happen
when the V-flange is up against the face of the spindle, as it is on HSK or
NC5 holders.
Where HSK and NC5 are not at all alike is in how holder-to-spindle contact
is achieved. The HSK shank is hollow so that an internal clamping mechanism
can be used. This clamp pushes on the toolholder from the inside. As the
spindle opens up at high speed, centrifugal force causes the outward force
of the clamp to increase as well, expanding the shank through elastic
deformation. This expansion is very slight yet adequate to compensate for
the opening up of the spindle socket.
The NC5 takes a very different approach. The shank of the
toolholder consists of two parts: a solid cylindrical core inside a
cone-shaped shell. This shell is split. In the illustration, you can see a
diagonal slit running up and down the taper. The shell, though split, is
preloaded against the V-flange with a Belleville disc spring. When the
spindle drawbar pulls the toolholder into the spindle socket, the tapered
cone makes contact with the tapered surface of the socket. Because the shell
is slit, it can give a little, contracting slightly to conform with the
socket. This action makes complete contact between mating surfaces yet
allows the tool to be centered very precisely. As the toolholder is pulled
farther into the spindle, the springs between the shell and flange are
compressed until the rear of the flange butts up against the face of the
spindle. Because the action absorbs slight gage line error, near-perfect
two-point contact is achieved even in cases where the spindle has
experienced some wear or has been reground.
The split shell has another critical function. As the spindle opens at high
speed, the Belleville disc spring pushes the split shell upward, thus
maintaining the full contact with both taper and flange, its developers say.
Apparently, this approach has several advantages over the hollow taper. Even
though an HSK toolholder is hollow, the clamping mechanism inside leaves no
room for the shank of the cutting tool, whereas the NC5 allows the cutting
tool's shank to be inserted deeper, reducing overall length. The developers
also say that the mass of the solid shank provides more vibration damping.
The slit in the tapered shell and the spring action on the flange also keep
vibration down. Solid construction resists breakage in case of an accidental
collision, too.
Stiffness combined with vibration damping means reduced potential for
chatter. Because chatter is the limiting factor in high speed machining, the
NC5 will outperform HSK, allowing further increases in speeds and feeds,
developers say. They also note that the centering action of the taper design
reduces runout at the tip of the cutting tool, extending its life
considerably.
Nikken's NC5 tooling represents a complete system, with compatible end mill
holders, tap chucks, drill holders, modular boring tools and so on, with
extenders and accessories. The NC5 is available in sizes corresponding to
30-, 40-, 45- or 50-taper Cat style toolholders.
HSK was a major development in response to the
extraordinary forces encountered in high speed applications. But new
insights and further innovation are to be expected. The appearance of
alternatives to HSK, such as the NC5 and several others, is not surprising.
They deserve close examination.—MDA |
