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Technology and Trends Spotted By The Editors of Modern Machine Shop

Rapid Traverse - An Alternative To HSK

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