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Intelligent Concept,
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HSK: Characteristics And Capabilities.
HSK Differs From Standard Toolholders And What Advantages It Offers
Despite its growing use and acceptance in the United States, HSK technology remains widely misunderstood. This primer examines how HSK differs from standard toolholders and what advantages it offers.

By Eugene Kocherovsky, Ph.D.
Intelligent Concept,
West Bloomfield, Michigan

    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."

HSK vs. CAT Overview of HSK Shanks
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.

HSK shank clamping mechanism operates from the inside.
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.)

Radial stiffness of SK40 and HSK-A63 Shanks.
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.

Torque transmission of SK50 and HSK-A100 and HSK-B125.
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