HSK Force Gage

CAT/DIN/BT Force Gage

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By Dr. Eugene Kocherovsky

An HSK Primer
What can HSK do for you? Why replace steep tapers?

As more and more companies adopt the HSK system, fans of the CAT shank see a threat to that tried and true tooling connection. Enthusiasts for the HSK coupling, on the other hand, tout it as the solution for every possible tooling applications. To evaluate the HSK concept, let's look at how the system developed and what it can do.

Before HSK, as products were widely adopted, standards would evolve more or less organically. The CAT (flange) connection, a steep taper shank system with a ratio of 7:24 is a good example. Another steep taper with a 7:24 ratio, developed in Japan is called the BT shank; the SK taper is the European twin of the CAT shank. Standardization of the CAT shank only required some formal "legislation."

Not so with HSK. In the mid-1980s, end users, machine tool builders and tooling manufacturers found that the performance of traditional and proprietary interfaces had come to a standstill. End users wanted a new solution that would be widely available, free of patent infringement, and capable of addressing high-speed machining as well as conventional operations.

A working group in Germany, which included academics, machine tool builders, end users, cutting tool manufacturers, and the standardization community, convened to develop that new solution. After about five years, the new standard was published and the term "HSK" entered the manufacturing world’s lexicon. (HSK is an acronym for the German words Hollow Taper Shank.) Described in the Deutsches Institut fur Normung standard DIN 69893, the HSK has a ratio of 1:10. Its matching part, the spindle receiver, is described in a second standard, DIN 69093.

The new standard includes six types of shanks and 35 sizes. Exactly the same matrix was generated for HSK spindle receivers to match every type of shank (see table). Early in the 1990s, the working group presented these options to the manufacturing community for discussion. Eventually the final standards were established for HSK shank types A, B, C, and D. Types E and F still only match "preliminary standards," but they’re the most popular HSK solutions for high-speed machining.

Although HSK has become the primary tooling connection in Europe, on this side of the Atlantic the metal cutting community remains skeptical. Most critics compare HSK with CAT (SK, BT), and suggest that some improvement of the steep-taper connection can eliminate the need for HSK. Let’s test this hypothesis by comparing those characteristics of CAT and HSK which are critical to machining operations.     

Radial and axial stiffness are the most important technical properties of the tooling interface, and define the system’s chatter-free machining capacity.

The design of the HSK allows mounting the mechanical amplification mechanism that clamps the shank inside the shank. When a force is exerted on the end of the draw bar, twice as much force is delivered on the flange of the shank. This amplification positively increases the HSK system’s stiffness. Studies at the Aachen Technical School (Aachen, Germany) compared the SK taper and HSK connections. This work demonstrates that the radial stiffness of the HSK connection is five or more times greater than the CAT/SK/BT tooling interface for comparable sizes.

High radial stiffness allows an HSK connection to handle elevated bending loads, which means deeper depth of cut, and higher feed rates for milling and boring. In addition, high rigidity means that the cutting system has a higher natural frequency, allowing the system to run at higher rpm before resonance (chatter) begins. And, finally, higher stiffness or rigidity reduces deflection, thus producing more accurate machining and improved surface finish.

Because of the axial contact between flanges of the HSK shank and the spindle receiver, there is almost infinite axial rigidity in the spindle direction. Such axial rigidity guarantees a fixed position for the interface during boring and (especially) drilling operations, where axial thrust forces are stronger. In the direction opposite the spindle flange, HSK performs better than CAT/SK/BT because axial clamping force is twice as high for tools of comparable size, and because higher friction results in a self-locking effect caused by the 1:10 taper. So the pullout resistance of HSK is higher than that of the steep-taper connection.
Torsion stiffness of the HSK interface is comparable to that of a steep-taper connection. It’s achieved by careful design of drive keys in combination with two areas of friction. The first area is on the contact surface between the walls of the spindle and shank; the second is between clamping surfaces on the shank and the flexible fingers of the clamping mechanism.

Users can choose from three HSK options to achieve torsion stiffness:

Shank types E and F - low-torque, super-high-rpm connection,

Shank types A and C - moderate-torque, moderate-to-high rpm connection,

Shank types B and D - high-torque, moderate-to-high rpm connection.

Each type of interface permits the user to choose an optimum torque-transfer solution for a specific application. Torque-transmission tests done at the Aachen Technical School are summarized in Figure 4.

Tool runout and repeating accuracy of the HSK connection is defined by simultaneous contact between the shank and spindle receiver along the taper and flange. The elastic deformation of the taper shank walls makes this contact possible. As Figure 2 shows, the accuracy of the HSK interface falls within 0.0001" (0.003 mm) in radial and axial directions. While the CAT connection has comparable accuracy in the radial direction (defined by the natural characteristics of the taper-to-taper joint), in the axial direction accuracy can vary as much as 0.004" (0.10 mm).

Axial cutting force also influences the CAT connection’s axial repeatability, reducing the attainable level of machining accuracy. On the other hand, the HSK interface has flange-to-flange contact between shank and spindle, which eliminates the affect of the axial force on finished part accuracy.

Tool collisions, repair ability, and spindles are all influenced by HSK and steep-taper connections. A CAT shank will wear the front of a spindle and cause it to bell mouth. When machining at spindle speeds above 8000 rpm, spindle walls expand faster than the CAT shank, which is fairly rigid. As a result, the draw bar force pulls the shank axially into the spindle. This movement changes the Z-position of the tip and locks the toolholder inside the receiver when the spindle halts for a tool change.

These problems don’t occur with the HSK interface. Dual contact on taper and flange defines the constant position of the tool tip independent of rotation speed. Also, the HSK taper expands radially at a higher rate than the spindle receiver. This design feature ensures permanent contact between the spindle walls and the tool holder during low and high-speed machining. On the other hand, we don't yet have enough data to generalize about the wear rate of HSK spindles.

It’s not difficult to find a company that can regrind a CAT spindle and socket for reuse. But special material deposit (coating) techniques and grinding should be used to compensate for wear or damage on an HSK spindle. These techniques maintain the integrity between the socket and face. Not very many organizations provide this service, and regrinding an HSK spindle is a finicky, expensive operation requiring a skilled operator, a very precise machine, and an excellent gagging system.

On the other hand, during collisions a CAT spindle will be more severely damaged than an HSK spindle. During a collision the CAT shank, which is very strong, transfers high forces through the interface to the spindle. Because the HSK shank is hollow, thin, and lightweight, it acts as a fuse during a collision, breaks, and protects the more expensive spindle from severe damage.

Tool change stroke and time correlate. The low weight and moment of inertia of the HSK shank and the taper’s short length contribute to fast tool change times. It’s difficult to achieve comparable performance with a steep-taper or straight-shank interface. For comparable steep-taper and HSK shanks, the HSK gage line diameter is about half that of the steep taper. Also, the HSK interface does not require a retention knob for clamping, which further reduces tool change stroke.

Suitability for varying cutting conditions was a requirement faced by the HSK working group. As mentioned above, the CAT interface has serious problems at high rpm.

The HSK tooling and clamping system avoids these problems by preloading in the shank-receiver area and establishing a solid face-to-face contact between mating components. For low-speed, high-torque work, drive keys were optimized using FEA. A controlled amount of face draw helps generate friction between the HSK shank and spindle receiver, which increases torque transmission.

The six different types of HSK shanks and spindle receivers can handle a broad range of manufacturing needs. Types A and C are used for general machining (A for automatic tool change, C for manual tool change). The B and D units are intended for high torque transmission and for stationary (turning) applications. Type B is meant for automatic tool change, D for manual change. Finally, the E and F types are recommended for low-torque, super-high rotational speeds with automatic tool change. Completely symmetric, they’re designed as balanced tools.

These connections can be used for both milling and turning work, which helps to reduce the shop-floor tooling inventory.

Tool weight or mass of HSK tools is usually less than that of comparable steep-taper tools because of the HSK shank’s hollow design and shorter taper length. In cases where the front part of the component becomes heavier than the shank, however, the center mass is displaced toward the front of the tool. This displacement can create a toppling moment when the tool is automatically changed. In addition, because the HSK shank is hollow, it doesn’t allow use of the shank to locate its cutting-tool-holding features. So, in some instances, HSK end mill and collet chuck adapters can be longer than steep-taper adapters.

Automatic and manual tool changes are allowed for by the design of the HSK interface. Referring to Figure 3, shanks A, B, E, and F are recommended for automatic tool change. They include precision-ground V-grooves and auto operator location slots. Shanks A and B also have a radial cavity for locating an information microchip. This memory cells carries information about tooling nomenclature, preset dimensions, and wear level. During the tool change cycle, data are exchanged between the microchip and the CNC control.

Shank types C and D are designed for manual tool change. Depending upon the clamping mechanism employed, shanks can be locked by a remote draw bar or by activating a wedge-lock mechanism through radial access holes located on the periphery of the shanks. Type A and B shanks can be clamped with a manual clamping system, but the C and D shanks can’t be used in an automatic system.

Balancing HSK adapters is no more difficult than balancing CAT/SK/BT adapters. Type E and F shanks were designed with balancing in mind. Completely symmetric, they have no internal threads. Used in combination with heat-shrink tooling, these shanks provide the best solution currently available for high-speed machining.

Typically, manufacturers sell HSK adapters in an unbalanced state. Customers who want balanced units must specify balancing when placing an order. At speeds below 15,000 rpm, grade G2.5 is used for balancing. If an application requires high surface finish and accuracy, or if grade G2.5 does not guarantee chatter-free operation, users should employ grade G1.0. The DIN standard allows production of all types of HSK adapters in either unbalanced or balanced states, and suggests that each component of modular tools or multicomponent assemblies be balanced separately.

Comparing HSK and steep tapers, remember that the lower weight of the HSK shank isn’t always beneficial for balancing - especially when working with nonsymmetrical types A and C.

Internal coolant supply can be provided on both HSK and steep-taper shanks. With steep-taper shanks, use a retention knob with the coolant hole. With HSK shanks type A and C, use a coolant nozzle to deliver sufficient coolant. Types B and D use flange coolant channels bypass the clamping mechanism. Shank HSK-E, according to the DIN standard, allows a coolant supply. But there’s no explanation of how coolant can be provided on CNC centers that lack accommodation for the coolant nozzle. Type F HSK shanks have no provisions for coolant supply; however, this interface is recommended mostly for woodworking, so it’s not likely to be used with coolant.

During high-speed machining, HSK tooling operates at speeds exceeding 20,000 rpm. At these spindle speeds, because of the asymmetry of coolant channels in adapters or possible air and oil contamination, an internal coolant supply can destroy the static balancing of the spindle-toolholder system. In these situations, an external coolant supply is recommended.

Vendor and cost competition was important to those who developed the HSK standard. Once the DIN organization established the standard, it was made available to all interested parties with no risk of patent infringement. The idea was to encourage suppliers of the HSK system to enter the market.

HSK tools cost more than steep-taper tools largely because the manufacturing tolerances involved in making them are quite tight. Tolerances for HSK gage line diameters are higher than those for steep-taper designs, for example. Gagging equipment needed to make the HSK tools is also expensive, typically costing more than $100,000 a set.

There are some hidden expenses to consider when comparing the cost of HSK with steep taper. The HSK interface requires more attention to the cleanliness of the shank surfaces. Before installation of a shank in a spindle, the operator must check it to make sure there are neither chips nor metal residue on the mating surfaces. Contamination can reduce both the stiffness and the accuracy of the interface. This need for cleanliness is not a disadvantage. It’s a natural consequence of a move to high-technology manufacturing.

Because of the tighter specifications of the HSK system, wear influences the performance of HSK shanks and spindles more than it does steep taper. Many users will need to acquire gaging systems or use outside inspection to control their tooling periodically. Finally, the workforce must be educated to handle HSK tooling, or there will be trouble in River City.

Modularity is a strength of the HSK system. High interface accuracy and rigidity, and the range of sizes available, allow a user to design extensions and reducers that have a final rigidity close to that of solid adapters. Separate branches are typically connected with a wedge-lock clamping unit. The high force amplification and the efficiency of the clamping mechanism provide stable, chatter-free performance for modular tools.

Most cutting tool companies offer HSK modular tooling, including units capable of attaching proprietary products on top of the adapter. End users with new rigid CNC centers can use them with HSK spindles, along with their existing inventory of proprietary cutting tools. As HSK cutting tools become more standardized, and as more cutting tool companies begin making them, I expect the use of proprietary tools to decrease significantly.

One more point; HSK benefits tools that require presetting. In a steep-taper system, interface differences between spindles will show up in the axial position of the tool tip. In an HSK system, face-to-face contact guarantees the position of the tool tip on the machine, independent of possible variation between the machine spindle and the pre-setter spindle.

The standardization process for HSK tooling continues in Europe under the leadership of the DIN organization. ISO institutions are bringing the standards effort to the US. After introducing the fundamental DIN standards for HSK shanks and spindle receivers, the ISO technical committee is now finalizing its own draft standards for HSK shanks and receivers.

Further, the DIN organization has begun standardizing particular groups of cutting tools, rather than main components. The new standard DIN 6594 covers the geometric parameters of indexable drills with shank types A and C and internal coolant supply. Standard DIN 6597 describes the geometric parameters of indexable turning and boring toolholders with HSK shank types A, B, C, and D and an internal coolant supply. Finally, DIN 6598 sets the designation for each toolholder depending upon size and cutting geometry.

During the next decade, HSK and steep-taper systems will coexist. As the cost of HSK decreases, and its benefits become more widely understood, use of the steep taper will decline. In the end, it will join the Morse taper, as an interface that still exists, but is rarely used.

Want More Information?

Eugene Kocherovsky can be contacted by e-mail at: intelcon@hskworld.com

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