Tool Encyclopedia

3. Hollow-taper-shank Tooling: HSK

HSK Axial Turning Holder

Comparison with CAT/DIN/BT

HSK features a hollow cone that is held on the inside by grippers and this is one of the keys to its success at higher speeds. As the centrifugal force increases, the grippers expand and push out so that the toolholder remains in contact with the cone of the spindle. There is no gap, no radial movement and no scalloping effect. Also, unlike BT, HSK features face contact: a precision milled faceplate on the toolholder that interfaces with a milled face on the spindle. So the toolholder cannot be pulled upwards; it can only get longer - a known axial movement that can be compensated.

Fig.1. HSK Axial Turning Holder

BT relies on taper contact alone, as the centrifugal force increases, it expands at the bottom (where there is less mass), allowing radial movement of the tool. The result is a scalloping effect that affects surface finish and tool life.

Torsional rigidity is another factor to bear in mind. Research has shown that a BT40 spindle offers only 670 Nm/mm and the BT50, 2,400 Nm/mm, whereas the HSK63 provides 3,600 Nm/mm and the HSK100, 17,000 Nm/mm. This superiority minimises spindle deflection, which in turn minimises run-out and provides a repeatable concentricity within microns.

Clamping force is one of most important parameters of modern tooling system. Machine tool manufacturers specify nominal clamping force, generated by spindle draw bar.

HSK spindles clamp tools with greater and more consistent force than most standard-taper spindles do. CAT and BT spindles generally depend on a pack of belleville spring washers for axial retention forces. (NMTB spindles use a mechanical drawbar and an internal shank thread.) At high spindle speeds, a spring-activated retention assembly doesn't offer much security.

An HSK spindle's hydraulic retention assembly provides higher, longer lasting clamping force than a spring-activated assembly does. The retention claws of an HSK spindle apply consistent radial and axial clamping forces to the holder. Because the positive locking action is enhanced by centrifugal force, the retention claws hold the toolholder more securely at higher speeds.

The coefficients of thermal and centrifugal growth can produce undesirable results in standard-taper toolholders and spindles. The walls of a standard-taper holder are much thicker than the spindle walls. This difference can cause the spindle walls to expand more than the toolholder walls as heat and centrifugal force act on them at high speeds. Therefore, at high spindle speeds, a significant loss of rigidity occurs due to the lack of surface contact between the toolholder and the spindle. The coefficients of thermal and centrifugal growth of an HSK holder's hollow, thin-wall shank more closely approximate those of the spindle. As a result, the two surfaces remain in contact even at high speeds. While a standard-taper holder may become loose and chatter inside the spindle, HSK (and NMTB) holders are mechanically locked into position.

An HSK assembly also owes its rigidity to its simultaneous-fit design. This means the toolholder's flange face contacts the spindle's face at the same time the taper is completely drawn into position. Because the toolholder is 0.00025" shorter than the spindle socket, an interference fit occurs at the toolholder's flange face. The resulting axial force, or mechanical preload, between the flange face and the spindle ensures full contact of the mating surfaces. This increased bearing surface area gives HSK holders better resistance to radial deflection than standard-taper holders have.

Because there has been no standard regarding a qualified gage length, variations up to 0.020" are not uncommon in standard-taper toolholders. HSK toolholders have much better repeatability than standard-taper holders, because the flange-face contact ensures that the tolerances always remain the same. HSK form A holders offer radial and axial repeatability of 0.000098" in all three axes.

Hollow-taper-shank Tooling: What is HSK? Why HSK tooling system? HSK types. HSK advantages. HSK disadvantages

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