Today’s aerospace machining operations present several toolholding challenges – tough to machine materials, aggressive metal removal rates and big 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 devastating 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 T.I.R rates to increase tool life and generate superior part surface finishes.
One of the main reasons aerospace shops switch to mechanical systems is that, plain and simple, they’ve experienced failures, which are often due to cutter pull out, 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 affects of cutter pull out are further magnified when parts are machined 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 already be worth half a million dollars, so in addition to the wasted machining time, cutter pullout at this point is a huge costly mishap.
In efforts to prevent cutting tool pullout, aerospace shops will often take a do-it-yourself, in-house approach. Some of these homemade solutions include EDMing holes in cutters and driving pins into them to hold the cutter in place. Another common solution is to grind certain profiles on the bottoms of cutting tool shanks that will interlock with a pin that has been driven into the bottom of a toolholder, or shops will revert to using old style endmill holders.
While mechanical toolholding systems provide incredible holding power, tooling manufacturers continue to actively develop mechanical systems that offer even more security against pullout by locking tools in place. Many of these systems involve special grooved patterns, such as reverse helixes, or other modifications, that must be ground into a cutter’s shank.
“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,” explained David McHenry, senior product engineer at REGO-FIX. “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 is butted against the end of the flute to lock the cutter in place once the system is tightened.
The drawback, according to McHenry, is that the end of cutter flutes are rarely straight and are often tapered, so there wouldn’t be a solid stop for the nub to butt up against. And 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 with such a system.
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.
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 easily transform any of them into a secuRgrip holder by simply threading the outside of any powRgrip system PG 25 or PG 32 holder for accepting the cap nut. REGO-FIX will supply shops with the necessary thread specifications or provide them factory-threaded holders.
The secuRgrip holders accommodate cutter diameters from 0.472” up to 1.000”. And 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 actually break the cutter first before ever damaging the secuRgrip holder itself.
High-precision holding and 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 leadtimes. Plus, many of the cutters being 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 T.I.R. The tighter a holder’s T.IR., the more it helps increase tool life as well as improve part accuracies and surface finishes.
Mechanical toolholding systems can provide T.I.R. ratings down around a couple microns. The powRgrip system, for instance, ensures concentricity (T.I.R.) with deviations of less than 3 microns (0.0001”) for tool lengths up to 3 x D (diameter) and length pre-adjustment with a repeat accuracy of less than 10 microns (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. These gaps – between the cutter and collet, collet and toolholder body – provide a natural vibration-dampening capability. In testing conducted by leading European universities, the vibration dampening capabilities of the standard powRgrip holder were proven to far exceed those of heat shrink-type systems.
Additionally, by being mechanical, 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.
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, then put in a chiller for 2 or 3 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 over 14” deep and in titanium workpieces. Or 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 and fixturing to access those surfaces.
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.
Tooling innovations such as REGO-FIX’s secuRgrip and MFD continue to provide aerospace shops with solutions for their machining challenges. 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.