February 2008 Edition
SPOTLIGHT: TOOLHOLDING
Getting the Right Grip on Toolholding
Applying the advantages of either SK or HSK interfaces in higher-precision components can improve tool life and productivity by 25 percent or more according to some industry experts. This informational piece offers some insights for users.
Typical SK clamping sets
In today's machine tools, the cutting tools are changed automatically into the spindle in various ways and at high speeds. It is essential that the technology of the drawbar holding the tools ensures the highest level of precision. There must not be any significant loss of precision when changing the tools, even over a long period of time.
Here is an examination of trends in the toolholder interface, as machine builders and shops alike seek to gain greater control, speed, and accuracy in toolholding and toolchanging protocols, such as high-performance cutting – HPC – or high-speed cutting – HSC.
SK-Interface is an industry term for standard contours, taken from the German word steilkegel [literally, "steep cone"]. The more advanced interface, HSK-Interface, also originates from German: hohlschaftkegel [literally, "hollow shaft cone"].
Typically, drawbar systems are built to be modular. There are components for almost every operating range. After determining customer needs, design engineers work with manufacturing engineers to create an optimal system.
SK steep taper systems consist of the gripper with holder, the cylinder head with integrated force intensification, a hydraulic or pneumatic device for unclamping, as well as rotary unions for internal cooling via lubricant or air delivery through the spindle.
HSK systems have the same modular setup, but they consist of the HSK gripper assembly without integrated force intensification, because the pull-in force is amplified by the gripper assembly itself.
Trends in HSK Technology
SK systems benefit from a worldwide market base. Some small shops don't use HSK technology because it requires substantial capital investment and SK tool systems are less expensive and readily available.
However, there are more spindles on the market today for machines with an HSK interface. The reason for this trend is the axial accuracy provided by the tool flange when in contact with the spindle flange. Also, tool handling is simple because of the low weight and the dimensions of the HSK receiver.
Owing to the short tool changing times and the high rotation speed, HSK systems are state-of-the-art in high-speed machining.
SK Gripper With Holder
One company has developed SK grippers with holders for all nominal sizes, from SK 30, SK 40, SK 50, up to SK 60. All sizes are in compliance with DIN 69871/72 (ISO 7388/1/2 Type A), ANSI B 5.50-78 (ISO 7388/1/2 Type B) and MAS 403-1982 (BT 1 and BT 2). The unique characteristic of this gripper with holder is the standardized inside contour of the spindle. Machine and spindle producers can interchange the gripper assemblies without changing the inside of the spindle contour because of the universal contour design of this system.
SK Drawbars
Steep taper drawbars are built to have the highest pull-in forces. Such drawbars are built with an intensifying mechanism to increase the pull-in force of the spring by a factor of three. This results in smaller diameter spring O.D., which is important for HSC.
HSK Clamping Set for Each Application Area
HSK clamping sets are available in various sizes, forms, and designs. The standard clamping set can be used by the machine shop generalist for nearly every purpose. The special black-pearl coated clamping set has advantages for industries, such as automotive, where fast and frequent tool changes are needed. The optimized coating extends the lubrication intervals and allows it to be used virtually maintenance-free. Because of the geometry of the spacer, there is a compulsory guide on the gripper segments and this leads to better concentricity.
These clamping sets have been tested by one manufacturer up to nine million times. They are typically clamped and unclamped in about 0.13 seconds. The wear and the clamping force were shown to be minimal.
A new generation gripper assembly with special anti-friction coating also enables faster tool changing. After the machining is completed, the tool can be unclamped while the tool changer is in transit, before the changer takes the tool.
HSK Drawbars
An HSK power drawbar with coolant, dry-run capability, and hydraulics
For HSK drawbars, it is important to calculate the resonant frequency, position the necessary support, then layout and dimension the proper spring for entry force and durability. More advanced HSK drawbars provide long durability and high machine time availability over the life of the machine tool.
Rotary Unions
For applications under 10,000 rpm, hydraulic, two-channel rotary unions are used for unclamping the power drawbar and the combined two-channel, rotary unions are used for hydraulic unclamping on one channel and the delivery of the medium – air or cooling lubricant – on the second channel. At higher rpms, components with separate unclamp units and one-channel rotary unions are needed.
Recently, a company developed rotary unions for very high rpm applications. These unions include a permanently-closed seal kit. This allows standardized performance parameters of about 36,000 rpm at 80 bar pressure – the highest lifespan rating of the hybrid bearing.
These rotary unions are available for nearly every application. For rpms up to 60,000, the coolant pressure will need to increase up to 150 bar or a bigger diameter of the through-hole is needed for increasing the rate of flow. Every type of rotary union should be available with external mixing.
All rotary unions are usually balanced in manufacturing. The closed seal kit is relatively new on the market, but has been used in thousands of rotary unions. The advantage of this sealing kit is minimizing leakage. Also, the temperature generated is negligible and the guaranteed lifespan is about 6,000 operating hours. If it is used properly and the service is done to the specification, the practical lifespan could be even higher in use.
Unclamp Units
Unclamp units are used to unclamp the power drawbar. There are two kinds currently in use: hydraulic and pneumatic.
The force needed to unclamp depends on the rpms and the front bearing I.D. size. With SK systems, increased rpm leads to more force for unclamping the tool. This is because the combination of the centrifugal force and mass of the spindle causes the taper to expand. When the spindle is stopped, the taper shrink-fits onto the tool. Careful calculations are necessary in the unclamp unit design stage to ensure the tool can be released properly.
Analog Tool Sensing
An electronic analog sensor also can be included with the unclamp unit to check the position of the tool. This eliminates the need for proximity switches and externally mounted brackets. Almost any unclamp unit can be modified to accommodate the sensor. The sensor can determine the exact position of the tool at any time. The analog signal can be connected to the machine controller or can be converted into a digital signal by an optional position controller. The position controller shows the exact position of the tool as an output signal. It also counts cycles and hours of spindle operation, a useful feature to facilitate shop preventative maintenance programs.
Looking Ahead
Because of the importance of the toolholding technology in today's machine tools, designers and shops alike must be dedicated to ongoing research into faster tool change times. This ensures there will be advancement in the end product production with no loss in quality. It would not be too conservative to estimate that using such components as described in this article would improve tool life and a shop's productivity by 25 percent or more.
This article was taken from a white paper authored by Hubert Sykora, the sales and marketing manager at Ott-Jakob, Germany.
Ott-Jakob is an international company in the machine tool industry, know for components and tool management technology.
Visit www.rsleads.com/802mn-220 for more information
What do you think?
Will the information in this article increase efficiency or save time, money, or effort? Let us know by e-mail from our website at
www.ModernApplicationsNews.com or e-mail the editor at
pnofel@nelsonpub.com.
No Coolant at the Collet Face
Self-sealed ER Coolant Collets prevent
coolant from reaching the collet face without the need for
plugs, discs, or other hardware. The ER's patented staggered
slot design prevents coolant from reaching the collet face.
For a longer life, coolant collets are made of solid steel,
and contain no wearable rubber or silicone parts. Also, with
these coolant collets, drill insertion need not be longer
than the collet length. The collets are designed for use
with cylindrical shank tools – no flats – in diameters
ranging from 4mm to 26mm. Coolant pressure of 1,800 psi is
guaranteed. Lyndex-Nikken Inc., Mundelein, IL
For more information
(click here)
CoroGrip Offers Increased Precision
The CoroGrip pencil collets are capable
of reaching into deep cavities and pockets, and machining
challenging areas without compromising part accuracy, part
repeatability, or stability. They are useful in the die and
mold industry. The pencil collets are available for end
mills with a shank diameter of 6mm, 8mm, 10mm, and 12 mm.
Also introduced are CoroGrip precision power chucks, which
provide clamping forces and levels of stability for
high-speed machining. Together with CoroMill Plura endmills,
CoroGrip power chucks provide a combination for minimum
run-out and high levels of precision. Sandvik Coromant
Co., Fair Lawn, NJ
For more information
(click here)
No Torque Wrenches Necessary
SINO-R Universal Toolholders give customers a rigid
toolholder for rough milling applications with the same advantages as SINO-T:
easy operation, good vibration dampening characteristics, and a high
cost-performance ratio. The toolholders' clamping principle is based on
expansion technology with a solid body as a pressure medium. Different tool
shanks can be clamped by using the actuation wrench. Torque wrenches are not
necessary. The user tightens the actuation cap to the dead-stop position. This
reduces setup time. Cutting tools are clamped at a maximum torque capacity of
850 Nm for a clamping diameter of 32mm. Schunk Inc., Morrisville, NC
For more information
(click here)
Adjustment-free Rotary Broaching Tool Holders
The Adjustment Free Rotary Broaching Tool Holder comes in
shank sizes of 0.625", 0.750", 12mm, and 22mm. It is designed for Swiss-type
lathe and screw machines. The adjustment-free design lets operators use the
toolholder without the need for centering. The toolholder uses the standard
1.25" length rotary broaches and can be used for any type of CNC, Swiss, or
manual turning, milling, drilling, or screw machine. Slater Tools Inc.,
Clinton Township, MI
For more information
(click here)
Long Reach
The PG 10 toolholding system, part of the powRgrip system, is
suited for small part machining. The toolholder features long, slim contours for
long reach performance. Since the powRgrip system does not use heat, the PG 10
can be used for holding small diameters and combined with the PG 15 and PG 25
for longer extensions. All powRgrip PG 10 toolholders are balanceable up to
42,000 rpm and are coolant capable. The powRgrip system includes a holder,
collet, and mini-press that generates six tons of force. The system relies on
the interference between holder and collet to generate its clamping force. The
system uses the mechanical properties of the holder material to generate
tremendous gripping force with run-out below 0.0001". Interchangeable collets
are available from 1/8" up to 1", including metric sizes, and will hold both
carbide and HSS tools. Rego-Fix, Indianapolis, IN
For more information
(click here)