Within reach: Drilling Performance

However you define them, extended-reach toolholders are indispensible in certain applications.

Choosing the right toolholder can mean the difference between making good parts and making scrap. And the rule of thumb is keep the tool assembly—the toolholder and the cutting tool—as short as possible for the intended application.

But what if parts and part features require a relatively long reach? Extended-reach toolholders are used regularly in aerospace and die and mold machining, two industries with many applications that require long, slender toolholder configurations. The holders are used to reach into large workpieces fixtured on tombstones, and in deep-pocket milling and deep-hole boring.

“Extended-length toolholders typically are used in situations where, due to the part geometry or the fixturing, you need a slim profile to reach the work area,” said Jeff Keith, product manager, tooling components, Schunk Inc., Morrisville, N.C. “Dies and molds where you have to get deep down into contours would be good examples.”

The definition of an extended-reach toolholder varies among the tooling manufacturers interviewed for this article; there is no set standard or limit. Some consider extended reach to be 6 ” and over, some 8 ” and over. The maximum is usually 12 ” or 14 “, but holders can be much longer.

“It is usually a length-to-diameter ratio,” said Mike Gadzinski, a member of the Special Tooling Team at Iscar Metals Inc., Arlington, Texas. “When you start getting past an 8 times diameter ratio, it starts to become pretty unique.”

Machining Operations

Extended-reach toolholders can be used in most types of machine tools and for most types of machining operations. “Extended-length toolholders can easily be used in any operation with linear cutting, such as drilling, reaming and tapping,” said Justin London, sales engineer/draftsman, Briney Tooling Systems, Bad Axe, Mich. “Those operations have single-axis travel that follows the linear centerline of the toolholder.” Therefore, most of the cutting forces are going to be axial and directed into the spindle.

Things become more complicated when milling and boring. “With milling and boring, the forces are transferring from axial to more radial, which forces the toolholder to start to bend,” said Jack Burley, vice president of sales and engineering, BIG Kaiser Precision Tooling Inc., Hoffman Estates, Ill. “Bending creates vibration problems. So the only thing you can do is reduce the bending moment by reducing the speed and feed and taking a lighter DOC.”

In general, milling with a long overhang is for lighter cutting with lower metal-removal rates. “Operations where you have heavy side loading, such as aggressive milling where you are removing a lot of material, are the most difficult,” Keith said. “You just need to be careful that you are not overloading your tools.”

Trochoidal milling, however, can be done efficiently with extended-reach toolholders. Trochoidal milling paths change the cutting forces to be less radial and more axial, which means users can increase machining parameters. “Trochoidal works very well,” Gadzinski said. “It has to do with ‘radial thinning.’ Trochoidal milling removes a very small amount of material in a circular path at high speeds. Removing less material reduces the amount of twisting and bending you have with an extended-length toolholder.”

Courtesy of BIG Kaiser

With BIG Kaiser’s CKN Connection modular system, the desired length of a large boring tool can be achieved fairly easily without adding excessive weight. The black pieces are made from aluminum.

Rigidity Challenge

The main challenge with extended-reach toolholders is rigidity. “Anytime you are hanging out from the nose of the machine tool spindle, you are giving up rigidity,” said David McHenry, product engineer, Rego-Fix Tool Corp.,Indianapolis. “You’re giving up a lot of your machining capability because you are out so far. You can’t push the tool as hard, you can’t go as fast or as deep and, generally, you have to decrease the feed rate.”

Strong contact between the toolholder and the machine tool spindle provides rigidity. Unlike single-contact taper toolholders, dual-contact toolholders have simultaneous taper and face contact. “This added rigidity helps support long, heavy tools and reduce the problems associated with toolholder ‘droop’ and runout,” said BIG Kaiser’s Burley.

One-piece, solid construction provides the highest possible rigidity and accuracy for extended-length toolholders. Typically, holders are made of a conventional tool steel, such as 4340. As an alternative, extended-reach toolholders are made from heavy metal, such as tungsten, according to Gadzinski of Iscar. “This material is a lot more dense, which reduces bending and vibration,” he said. “But because heavy metal can be expensive, people only use it if they have to.”

Dampening Vibration

The greater the density of the toolholder material, the more harmonics dampening properties it has. To help combat vibration in extended-reach applications without using expensive heavy metals, Rego-Fix fuses two types of steel pieces for its new Xtended Length toolholders. The fusing process creates a microfriction dampening effect, which breaks up harmonics. The company developed the MFD technology specifically for extended-length toolholders.

Another vibration-dampening technology is Sandvik Coromant’s Silent Tools technology for its extended-reach boring bars and milling tools. “With this technology, a dampening device—a tungsten rod suspended in oil—inside the milling tool or boring bar, which is made from tool steel, counteracts any vibration that occurs with a long overhang,” said Jim Grimes, machine integration manager for Fair Lawn, N.J.-based Sandvik Coromant Co.

Courtesy of Briney Tooling

A CAT 50 shrink-fit tool assembly machining flats.

Silent Tools technology lets users keep the same DOCs, feeds and speeds as shorter tools. “You might have to make some adjustment, but the dampening device helps the tools react the same way a shorter tool would,” Grimes said.

The Modular Connection

Extended-length toolholders made of solid tool steel can be somewhat unwieldy, according to Burley. “For toolholders more than 12 ” long, when you make them out of a piece of steel, they also become heavy,” he said. “For example, an integral CAT 50 toolholder made from high-alloy tool steel can exceed 30 lbs. Although this may not seem that heavy, it is the tool’s weight moment (calculated by multiplying the tool weight by the distance to its center of gravity from the gauge line) that is the issue. Many machine tools can handle the weight, but the automatic toolchanger can struggle with the moment.”

As a possible solution, BIG Kaiser offers a modular toolholder, called CKN Connection, which users can build up to the length they need. Mating components in the middle are made of aluminum, which helps reduce weight. A three-screw connection provides rigidity and high clamping forces, according to the company (see photo on page 39).

Other companies offer modular extensions as well, which they say provide sufficient rigidity. “We can build extensions with interchangeable pieces that can shorten or lengthen the overall length of the toolholder assembly and still not lose a lot of the rigidity,” Gadzinski said.

In addition to solid one-piece toolholders with extended-reach capability, Schunk offers three types of extensions—hydraulic, shrink-fit and polygonal clamping. “If someone has a base holder (Schunk or another brand) that is too short to reach, and they don’t want to invest in a longer-length holder, they can simply buy an extension,” Keith said.

Shrink-fit toolholders, in particular, have proven effective for extended-reach applications. “Shrink-fit systems provide much less loss of concentricity and a high clamping strength for extended-length toolholders,” Gadzinski said. “Other methods, such as hydraulic clamping, provide good clamping pressure, but shrink-fit is the best system for users who need a slim design without a lot of extra bulk.”

Balancing Act