GRIPPING Rotating Round TOOLS
Here's how it's done with precision systems
By Jim Lorincz
When it comes to toolholding for rotating round tools, you might say there are old-fashioned conventional systems and there are newfangled devices, and a lot of gripping options in between. The challenge is to transmit force from a rotating machine spindle to a metalcutting tool—an end mill, a drill, or a tap—for accurate, efficient, and safe metal removal for precision machining results.
Toolholding systems represent a substantial investment. In the past, before the emergence in importance of high-performance machining (i.e. high speed and high torque), toolholders were regarded more as a commodity item. They weren't very high in the precisionmachining food chain, yet they were expected to perform consistently in producing quality machined parts.
Times changed with the emergence in the 1990s of high-speed machining, typically spindle speeds over 10,000 rpm, and, more recently, high-torque machining of especially difficult-to-machine metals like titanium and Inconel for aerospace and other demanding applications. High spindle speeds challenge the security of toolholders, and emphasize any unbalance in the toolholding system.
"Toolholding has emerged as a critical component in the search for better machining accuracy, better tooling performance, and improved total tool life cycle," says Justin London, application engineer, Briney Tooling Systems (Bad Axe, MI).
"Briney Toolholders are made of high-quality steel, and have specialized heat treat processes and are manufactured on high-precision machines. Toolholders that provide maximum concentricity and rigidity and minimum runout extend tool life and reduce scrap. Good concentricity and balance distribute the chip load evenly on the cutting edge so that users can take full advantage of cutting tool technology at today's high speeds and feeds," London says.
Toolholders depend upon highly engineered and precisely machined components, from the back-end interface with the machine spindle to the front-end seating and clamping of the cutting tool, to produce the least amount of vibration, runout, and unbalance to the whole toolholding system.
Technology has come a long way from the day when a collet, a drawbar, and a threaded cylinder hanging out of the toolholder's back end secured a tool in a Bridgeport mill. Before Caterpillar introduced the steep-taper CAT-V flange toolholders, there were as many toolholders, as there were OEMs, tooling suppliers, and manufacturers designing their own solutions. Once one of the largest manufacturers in the world established its steep-taper design, OEMs, tooling suppliers, and others quickly fell into line. Or, like the Japanese and the Europeans, they designed their own steep-taper system or HSK standard spindle interface.
Today, the toolholder connection to the spindle (the back-end) is pretty much a question of standards: CAT (ISO), BT (JIS), HSK (DIN), KM (ISO), Coromant Capto (ISO). And one proprietary system, the BIG PLUS spindle system from BIG Kaiser Precision Tooling (Elk Grove Village, IL). Each has its particular strengths for milling, turning, and drilling, and each requires precision machining (usually grinding), gaging, and advanced inspection to be manufactured to required specifications.
The BIG-Plus spindle system uses the taper and face of the machine spindle and toolholder contact simultaneously, when clamped with the retention system. A BIGPlus steep taper toolholder is supported by the gage-line diameter and flange-face diameter. It uses elastic deformation of the machine spindle to achieve simultaneous fit of taper and flange face. Before clamping, the toolholder taper and spindle are in contact with each other to establish good taper fit, but face contact hasn't yet occurred. When the spindle's retention knob clamping system is engaged, the toolholder is pulled into the machine's spindle, which expands by elastic deformation until the faces of the spindle and toolholder contact each other. This contact completes the simultaneous fit of the toolholder to the spindle and results in higher rigidity and precision.
Toolholder repeatability for BIG-Plus is 1µm for positional location. The spindle system is interchangeable with existing toolholders and machines. Conventional toolholders can be used in BIG-Plus machines, and BIG-Plus toolholders can be utilized in machines equipped with conventional spindles. The full benefit of the spindle system, however, occurs when machining centers are equipped with the BIG-PLUS spindle, that is when spindle face is precision ground to match the precision ground surfaces of the toolholders.
"When it comes to the front end of the holder, it's pretty much a free-for-all," says Andreas S. Weber, president, Rego-Fix Tool Corp. (Indianapolis). The most common front-end holder is the end-mill holder, the side-lock Weldon milling chuck. When an end mill is locked into the holder, it will push the tool out of center, a condition that must be corrected to achieve balanced cutting.
"You might consider the Weldon holder as a low-performance system, and the ER system where higher quality than Weldon holders is required," says Weber. For the highest levels of performance, the three main proprietary systems are Rego-Fix's powRgrip system, Sandvik Coromant's CoroGrip, and Schunk's Tribos system. Shrink-fit systems offer another option.
The ER collet system patented by Rego-Fix in 1972 became a DIN standard in 1992. One of its advantages is that it has a 1-mm clamping range, so that a collet can be used to clamp inch or metric, reducing the number of collet sizes that must be stocked. A 13-mm ER collet can be used for 13-mm round tools, half inch (12.7 mm), or 12.5 or even 12.0-mm diam tooling. Collets come in one of three basic versions: standard, sealed for coolant through, or with a square in the back for tapping.
"The industries that have benefited most from powRgrip are the ones that measure their tool life, scrap rates, and tolerances very closely for high-end processes, because they have a benchmark for performance improvement. Companies that benchmark are often found in the medical device, aerospace, and die/mold industries," says Weber.
The success of a toolholding system is determined in large part by the force with which the tool is pulled back into the spindle taper. Advanced Machine & Engineering (AME; Rockford, IL) is a supplier to machine tool OEMs like the MAG group and all the major spindle manufacturers. AME is the exclusive distributor in North America of the Ott-Jakob product line. "The Ott-Jakob drawbar can be married with other proprietary systems, including the KM and BIG Plus spindle systems and others, as well as the HSK interface," says AME's Harold Goellner, product manager for the spindle interface group.
"The Ott-Jakob's universal contour solved a problem of both the old and new-style CAT steep-taper toolholder, which required tightening the drawbolt by hand to pull the tool tight against the spindle [old style]," Goellner explains. "The universal contour enabled builders to make them to different standards, ISO, JIS, DIN etc., by changing out the gripper assemblies without changing out the spindle shaft. The inner contour is the same but some of the fingers were set further in and further out on the tool standard."
The key to this approach is an intensifying mechanism that utilizes three wedges and balls behind the springs that amplify the pull force of the springs by three to four times. "Because of that design, we can design our drawbar in much smaller packages. If you are limited by the spindle shaft diameter, because you don't want big bulky spindles, and your bearing sizes are getting too big, we can work in smaller packages with our springs that are significantly smaller in diameter," says Goellner.
Ott-Jakob is making toolholders smarter, too. "Using analog sensing built into the drawbar we are able to provide a means for electronically sensing tool position," says Goellner. "In other words, between tool changes the spindle wants to know whether there is a tool in the spindle, whether or not you are clamped with a tool, or clamped without a tool. The old way of doing this was using mechanical proximity switches supplied by the customer. Our approach also gives us a way of counting drawbar cycles and hours of spindle rotation. Knowing this is very important for warranty considerations," Goellner points out.
Aerospace machining, especially of aluminum, titanium, and composites, continues to be an important breeding ground for new toolholding innovations. According to Brendt Holden, president, Haimer USA (Villa Park, IL; Igenhausen, Germany), high productivity in the aerospace industry depends on the high-performance cutting (HPC) that new cutting tools can deliver. These advanced tools resist the high torque, high feed, and tensile forces encountered during the aggressive machining required when doing aerospace work. There is, however, the danger of this increased force pulling these very expensive cutters out of the chuck.
"This is, at least, the case for tool holders that offer precise clamping with high run-out accuracy, like shrink-fit chucks, milling chucks, hydraulic chucks, or press-fit chucks. They use friction to clamp the tool and, therefore, their clamping force is limited and sometimes insufficient for HPC," Holden explains.
"As an alternative, many users use conventional set-screw end-mill holders whose clamping screw grips the tool via a screw locking down on a flat. With that arrangement, you can generally transmit any torque you want until the cutter finally breaks. With the set-screw end-mill holders, however, the users have the disadvantages of imprecise run-out, which causes poor clamping as the shank in the chuck needs a little clearance, and a short cutting tool life, which is very expensive in this case."
Haimer developed and patented a solution called SafeLock, which combines a shrink-fit chuck or another high-precision chuck with locking elements. The locking principle results from helical grooves that are ground on the cutter's shank for form- closed drivers. The helical form of the groove protects the tool against overturning and pulling out. The drivers, which can be balls or rods, are integrated in the chuck, and produce high clamping accuracy and positive locking. With the helical form of the grooves, the tool's length can be adjusted. The clamping process is easy. The chuck is heated as usual and the tool is inserted with a turning move. The balls or rods locate themselves into the grooves. A spring supports the accurate fitting of the tool. After a few seconds the holder cools so that there is a friction- and form-closed connection.
"The modification to the cutting tool shank is straightforward and symmetrical. In fact, cutting tool suppliers have found that they are able to add the SafeLock grooves to standard stocked end mills, even end mills that have an existing Weldon flat. The grooves do not greatly affect the balance [which is an issue with the Weldon flat], and this type of modification keeps the cutting tool stronger [since the grooves are very shallow, as opposed to a deep Weldon flat cutting through a great portion of half the shank]," Holden says.
The principle of the SafeLock-System is not limited to Haimer shrink-fit chucks. It can be integrated as well in a collet chuck, milling chuck, or in a hydraulic chuck. Safe-Lock is catching on with cutting tool manufacturers. Dauphin Precision Tool LLC and Berkshire Precision Tool LLC have become licensees, enabling them to add the modification to the shanks of their Brubaker, Data Flute, Fastcut, and Weldon cutting tools.
The new Steadyline shell mill holder from Seco Tools Inc. (Troy, MI) is designed for vibration-damping machining for difficult-to-access machining areas, such as large, complex workpieces and deep cavities that use long- overhang tools. Without damping, cutting conditions must be matched to the frequency of the toolholder, with the user dialing in speeds and feeds to match the toolholder's natural frequency. Steadyline holders eliminate this problem to allow higher cutting speeds and up to 4x depth of cut, and up to three times greater dynamic rigidity.
The new KSN6/HD Softsynchro tap holder from Emuge Corp. (West Boyleston, MA) has a capacity up to M76/2.5"(63.5 mm) and is designed for synchronous spindle with length compensation and internal coolant-lubricant supply in wind turbine manufacturing.
This article was first published in the September 2009 edition of Manufacturing Engineering magazine.