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 Tooling up with HSK

Autos and aircraft first to take off with German standard.

By Al Sanchez, Director of Product and Mfg Eng, Tooling Systems Division, Frankenmuth, MI

It has been a little over ten years since the German consortium of machining center manufacturers, end users, and tooling manufacturers, in conjunction with the Machine Tool Laboratory at the University of Aachen, developed the revolutionary HSK, Hollow, Short, Kegel (German word for taper) toolholder connection. Although relatively new to the US, HSK technology has had a major impact in Europe for the past seven years with estimates of more than 10,000 spindles now equipped with HSK connections.

Fig 1--HSK shallow taper toolholder (left) compared with typical steep taper V-flange.

The standards (DIN 69893) that were developed have survived the critical period of revisions and attacks from the skeptics and it now appears that the design is poised to be everything we thought it would be and more.

And why not? It's a fact that the steep angle taper toolholder (Fig 1) like CAT, BT, and NMBT have reached their performance limits for today's high speed machining processes used on machining centers, high production transfer machines with manual tool change, and other high speed metalcutting machines.

HSK benefits to the user include:

  • High static and dynamic rigidity. Bending load is 30% to 200% greater than steep taper toolholders.
  • High precision axial and radial reproducibility. The toolholder does not have the tendency to "such in" like a steep taper holder.
  • Low mass, low stroke length when tool changing.
  • Centered clamping with twice the force.

When the clamping process begins, the dimensional match between the taper and the end face initially leads to surface contact pressure being exerted between the surfaces on the spindle and the tool flange. Clamping from the inside outward utilizes centrifugal forces on the clamping fingers to increase clamping force, ensuring that the entire tool taper is in contact with the spindle taper which increases surface contact pressure in the taper.

HSK manufacturing challenge

The hollow taper HSK shank must be manufactured to exacting tolerances, and precise care must be exercised in the selection of materials, heat treatment, gaging, and precision grinding. For example, some of the early concerns with respect to some HSK failures involved improper heat treatment where carburizing case depth was too deep and fractures were occurring at the corners of the drive slots. When HSK holders are properly manufactured, core hardness should be about 370 Bhn to attain physical properties of 170 Ksi and 200 Ksi minimal tensile strength. The surface hardness should be in the 56-62 Rc range.

Producing toolholders to the DIN standard has been challenging. However, TSD is no stranger to manufacturing HSK toolholders, in that we have had a joint venture arrangement with Gildemeister DeVlieg GmbH in Bielefeld, Germany, who is one of the original consortium members and has been producing HSK toolholders since 1994.

Today at TSD, grinding and gaging of HSK toolholders are performed in an air-conditioned cell that maintains an ambient air temperature of 68 F. The toolholders are precision ground on a Kellenberger CNC UR 175/1000 grinder (Fig 2) with CNC B axis. The B axis has two OD wheels and one ID wheel. It can position within 5 arc sec and repeat within 2.7 arc sec. The workhead is guaranteed to run within 8µ´´ for the live grinding. The X and Z axes have glass scales with 4µ´´ resolution.

Fig 2--TSD's production cell for HSK toolholders includes a Kellenberger CNC UR 175/1000 grinder with CNC "B" axis.

The HSK shank has internal and external features that must be manufactured to close tolerances. These include the inner clamping angle L6, d2 and d3 diameters relative to the flange, and the position and radius of the drive slots at the end of the taper.

TSD provides the HSK connection for its complete toolholder product lines, including milling chucks, collet chucks, Mod-Flex™ modular toolholders, solid boring tools, and Shrinker® tools (Fig 3). There are nine sizes of HSK and six form variants. Currently, the most popular HSK design utilizes Form A (general purpose) and Form F (high speed). Form A is available in sizes 32A through 160A; Form F in sizes 50F and 63F. The numbers represent the flange diameter in millimeters. The letter is a form designation that defines the taper size, along with other features like drive keys and access holes for manual drawbar actuation.

Fig 3--TSD manufactures HSK connections for milling chucks; collet chucks; Mod-Flex™ modular toolholders; solid boring tools; and Shrinker® tools.
ual-plane balancing machine.

TSD also offers the full range of sizes 32 through 160 and all form variants A,B,C,D,E, and F toolholders and spindle tapers. All the toolholders are serialized and pre-balanced.

AT3 is not HSK

Many toolholder manufacturers are beginning to advertise their products as AT3 grade to meet the end user's demand for higher spindle speeds and increased precision as newer technology is applied in their plants. The Tooling Systems Division has long recognized the need for more accurate tapers with regard to precision and balance and has been traverse grinding NMTB and CAT-V tapers to the AT3 grade for more than 10 years.

Using the ANSI B5.10 (AT4-AT5) standard allows excessive clearance between the toolholder and spindle. For example, the ANSI B5.10 standard allows a tolerance of 83.3µ´´/in. length of taper, which may result in a taper angular deviation of about 17 arc sec. The tolerance on the toolholder is applied to increase the rate of taper while the spindle is toleranced to decrease the rate of taper. The resulting radial clearance at the small end of the taper can be 0.000 33´´ and 0.000 22´´ for #50 and #40 tapers, respectively. This clearance allows the shank taper to shift in the spindle causing runout and imbalance. While this may appear to be a small amount, it alone, exclusive of the spindle tolerance, could easily cause 0.000 66´´ runout of a tool point with a nose end-to-shank length ratio of 2:1.


Fig 6--HSK DIN 69893 dimension designations.

With HSK, the spindle taper angular tolerance is specified as AT3 while the toolholder taper is not defined with an angular tolerance but rather by two tightly toleranced diameters, d2 and d3, (Fig 6), separated by an axial distance L3. The resultant angular tolerance of the shank is substantially less stringent than the ANSI B-5.10 standard. For instance, the resultant angular tolerance for HSK 100 is 51 arc sec vs 17 arc sec for ANSI B-5.10. The excessive angular deviation of the two HSK components is removed as the drawbar pulls the toolholder into the spindle bore for a simultaneous fit between the toolholder taper/flange and spindle face, subsequently expanding and deforming the hollow taper of the toolholder to fit the spindle taper. The faces of the toolholder and spindle have total runout limits of 80µ´´ each and combined with the expanded toolholder taper, provide excellent axial and radial accuracy as well as repeatability.

Another significant factor governing the successful utilization of HSK on the production floor is absolute cleanliness.

Balancing essential

The balance of the toolholder is equally as important as taper fit to higher speed machining. Some toolholders are advertised as "production balanced toolholders" for speeds up to 20,000 rpm without being actually specified to the ISO 1940 tolerance grade. When tested, many of these toolholders are found to fail to meet quality G6.3 standards, much less the more stringent G2.5 grade often specified for toolholders. The user should always ask what "G" rating the toolholder is balanced to and at what speed (rpm). These two components define the maximum permissible vibration displacement of the center of mass. The higher the speed; the smaller the vibration displacement must be for a given "G" grade.998hsk4

Fig 4-All HSK toolholders are balanced on a computerized Hofmann precision dual-plane balancing machine.

At TSD, all HSK toolholders are balanced on our Hofmann computerized balancing system (Fig 4) to a quality Grade G2.5 for the customer's required service speed. Single plane is standard; dual plane is available if the length of the tool allows it.

Inspection verification

Verifying the critical d2 and d3 dimensions on an HSK toolholder requires sophisticated and, in most cases, costly computerized air/electronic gaging capable of measuring in the micron range with sub-micron resolution for both internal and external gaging. Currently, TSD utilizes a Stotz gaging system (Fig 5), capable of inspecting HSK sizes from HSK 32 to HSK 160.998hsk5

Fig 5--HSK toolholders are inspected by computerized air/electronic instruments.

Through a series of air and mechanical snap gages, dimensional data relative to the flange is measured for d2, d3 gage line diameters, "V" groove location, the depth of the ejection surface, and the location of the L6 clamping angle. Some of the dimensional data like the d2, d3, and L6 readings are recorded and sent with the toolholders to the customer.

We're convinced that HSK will become the system of choice throughout this decade and into the next, especially for new installations. We've seen it blossom in the automotive industry over the last two years, as well as with commercial airframe manufacturers, particularly in combination with Shrinker-type tooling.

Originally published in the September 1998 issue
of Tooling & Production.
Please Note
: some pictures or diagrams are only available through the printed media.