Giving Aerospace Shops a True Grip

The REGO-FIX powRgrip is a precise, safe, and cost-effective press-fit system for high-speed, high-performance cutting applications.

Today’s aerospace machining operations present several toolholding challenges – tough to machine materials, aggressive metal removal rates, and cumbersome assemblies requiring long awkward tool overhangs. In these situations, toolholders must perform flawlessly and provide strong gripping force, high precision, and vibration control, because a toolholder failure at any stage of part machining in an aerospace shop can spell huge losses in time and money.

To avoid such outcomes, many aerospace shops are incorporating mechanical toolholding systems in place of other popular holder systems that function using heat or hydraulics. Advanced mechanical toolholding systems not only provide some of highest levels of gripping, but also deliver the best possible vibration dampening for extremely low TIR rates to increase tool life and generate excellent part surface finishes.

One of the main reasons aerospace shops switch to mechanical systems is plain and simple: they have experienced failures, often due to cutter pullout, vibration, or runout. Consider, for instance, a typical aerospace shop operating within a JIT delivery environment that has put a week’s worth of machining hours into a massive aircraft wing spar. Then, toward the end of the process, the milling cutter pulls out of the toolholder a barely noticeable amount of 0.060".

The effects of cutter pullout are further magnified when machining parts from expensive materials such as Inconel or titanium.

For instance, it is not uncommon for a large aerospace component to be machined from a raw piece of titanium costing $100,000 and three-quarters of the way finished be worth $500,000, so in addition to the wasted machining time, cutter pullout at this point is a huge mishap.

Pullout Prevention
In efforts to prevent cutting tool pullout, aerospace shops will often take a do-it-yourself approach. Some of these homemade solutions include EDMing holes in cutters and driving pins into them to hold the cutter in place.

While mechanical toolholding systems provide incredible holding power, tooling manufacturers continue active development of mechanical systems that offer even more security against pullout by locking tools in place.

“With systems that involve altering a cutting tool’s shank, holder manufacturers will typically develop a patented system and establish agreements with cutting tool companies to modify their cutters for that particular holder system,” explains David McHenry, senior product engineer, REGO-FIX Tool Corp. “However, the down side is that shops are then forced to use only those cutting tools from manufacturers that have an agreement with the toolholder company. This limits a shop’s cutter choices, and there is often additional cost involved with the cutters due to the added manufacturing process of modifying cutter shanks.”

There are also other locking systems that use special ER collets with added lengths. This extra solid portion will have a nub, or pin, that rides inside a cutter’s flute as the tool is turned into position from the backside of the collet. The nub butts against the end of the flute to lock the cutter in place once the system is tight.

The drawback, according to McHenry, is that the end of cutter flutes are rarely straight, so there would not be a solid stop for the nub to butt up against since there is typically a taper. For maximum holding force, the whole ER collet length has to clamp around the cutter, but with a solid portion at the top of the collet, only the back part of the collet is actively clamping.

As opposed to pins or grinding special groove patterns on cutter shanks, a new tool-locking system, the REGO-FIX secuRgrip, uses a special threaded insert or key that eliminates the need to alter cutters. The simple and effective design is part of the company’s well-established powRgrip mechanical toolholding system and one that lets shops use any off-the-shelf tool as long as it has a common standard Weldon flat on its shank. The secuRgrip further enhances the extreme holding capability of powRgrip holders.

REGO-FIX's secuRgrip anti-pull system locks the collet and cutter to the toolholder body to eliminate cutter slippage during demanding metal removal applications.

To lock a cutter in place, the small insert of the secuRgrip system is placed in the Weldon flat of a cutter. The bottom profile of the insert matches that of the Weldon flat, and its exposed side has a thread pattern that matches with those of internally threaded PG collets. Users hold the insert in place while sliding the tool into the collet. The collet is turned so that its threads engage with those of the insert, and the tool is then screwed all the way into the collet. This cutter-collet assembly is pressed into a powRgrip system, and a special external cap nut is tightened onto the holder for added pullout security.

Shops with existing powRgrip holders can transform any of them into a secuRgrip holder simply by threading the outside of any powRgrip system PG 25 or PG 32 holder for accepting the cap nut.

The secuRgrip holders accommodate cutter diameters from 0.472" up to 1.000". With a holder body tensile strength higher than that of the cutting tools it will hold, the holders will withstand cutting forces strong enough to break the cutter before damaging the secuRgrip holder.

Holding, Vibration Dampening
According to McHenry, aerospace shops are always pushing for longer cutting tool life. When machining exotic materials for turbine blades, for instance, getting a few extra minutes of cutting time or even one more additional part from a cutting tool may seem like small gains, but they amount to significant reductions in part cost and lead times. In addition, many of the cutters used in aerospace applications are expensive and often the type that cannot be re-sharpened, so long tool life helps reduce tooling costs in those instances.

Achieving the maximum life from today’s advanced cutting tools depends heavily on the toolholder that connects them to machine tool spindles. As machine tool spindle speeds and feedrates continue to increase in aerospace shops, the more critical a toolholder’s vibration dampening capabilities become. The better a toolholder controls or even eliminates vibration, the tighter its TIR. The tighter a toolholder’s TIR, the more it helps increase tool life as well as improve part accuracies and surface finishes.

Mechanical toolholding systems can provide TIR ratings down around a couple microns. The powRgrip system, for instance, ensures concentricity (T.I.R.) with deviations of less than 3µ (0.0001") for tool lengths up to 3xD (diameter) and length pre-adjustment with a repeat accuracy of less than 10µ (0.0004").

The secret to these results lies in the interior of powRgrip. The powRgrip achieves high vibration dampening due to the functional contact surfaces between the toolholder and collet and the collet and tool shank. The concept absorbs vibrations better than do non-mechanical systems such as heat shrink holders.

Vibrations start at the cutting tool – or even from the fixturing securing the part being machined – and travel up through the cutting tool, into the toolholder system, to the machine tool spindle and back into the workpiece surface finish. This occurs because vibrations are not being absorbed anywhere along the spindle-toolholder interface.

The powRgrip system absorbs vibrations by creating what McHenry refers to as material breaks. The process starts with a cutting tool, typically made from anything from high-speed steel to carbide to cobalt, with each material having its own specific vibration frequency or harmonics. The cutting tool is held in a collet that is also made from a particular type of steel and then inserted into a powRgrip toolholder made from a different type of steel.

According to McHenry, the use of different materials, all with their own unique harmonics/frequencies, creates breaks or gaps. In testing conducted by leading European universities, the vibration dampening capabilities of the standard powRgrip holder are proven to exceed those of heat shrink-type systems.

Additionally, the level of stress and strain that is put into a powRgrip toolholder body is a known factor, and one that remains unchanged every time a tool is pressed into the holder. There is a given amount of growth that never reaches the yield strength or the plastic deformation of the toolholder material. Staying under the material limits, so to speak, gives the system its high accuracy repeatability because the holder material remains undamaged, as opposed to other systems that can exceed a toolholder’s material yield strength.

REGO-FIX worlld headquarters is located at a newly expanded 75,000ft2 facilty  in Tennikin, Switzerland.

As a mechanical system, powRgrip is also faster than other systems when it comes to exchanging tools. Removing a tool from a holder and installing another takes about 10 seconds, while heat shrink holders, for instance, must be heated, the cutter installed, and then put in a chiller for two or three minutes before the tool can be used.

In addition to toolholders that provide high gripping force and vibration dampening, aerospace shops are incorporating more extended length mechanical toolholding systems. The reason for this increase is that shops must often mill deep pockets, some of which can be more than 14" deep and in titanium workpieces. In addition, they may need long holders to reach areas that need machining on huge parts, such as landing gear components.

Another reason for the increased use of extended length toolholders is the fact that more aerospace shops are producing full assemblies, as opposed to just pieces parts. What happens is that there is often required machining needed as components are added to the assembly, but as more components are joined and the assembly grows in size, reaching the areas that need machining becomes a challenge. Extended length holders allow shops to reach in, out, and around multiple assembly components.

However, extended reach toolholders – because they put the cutting tool so far away from the machine tool spindle/toolholder interface – create a situation that is ripe for severe vibration problems. For dampening vibrations, tooling manufacturers will typically incorporate types of internal systems or use high-density materials. REGO-FIX, on the other hand, has developed a completely new vibration dampening technology for its powRgrip Xtended Length (XL) toolholders.

The new technology is Micro-Friction Dampening (MFD) that allows the toolholders to dissipate vibrations faster than other standard long-reach holders. XL toolholders are actually assemblies of multiple components that are joined using a special connection design that uses REGO-FIX’s same frequency interruption concept to dampen vibrations.

With the right toolholding system, aerospace shops can increase cutting tool life, experience significant cost advantages, and confidently run their cutting tools at the highest speeds and feeds to increase productivity while also improving part surface finish quality.

REGO-FIX Tool Corp.
Indianapolis, IN
www.rego-fix.com