As appeared in Cutting Tool Engineering, December 2021
From Rene 41, 718 stainless steel and 625 Inconel to titanium and carbon fiber, the materials used in the aerospace and defense industry sector as well as space exploration continue to present significant toolholding challenges.
Magnifying these challenges are complex workpiece geometries with deep cavities, features that require long tool overhangs, and jobs requiring small-diameter cutters. Toolholding is just as important as the machine that runs it, and sub-par tooling can keep a machine from running at its full potential. For these reasons, shops turn to high-quality, precise toolholding.
Because many of these materials are hard on cutting tools, shops constantly strive for the longest possible tool life. For one shop, the simple switch to a high-precision mechanical-type toolholding system skyrocketed its tool life from machining 100 steel parts per cutter up to 700.
Toolholder reduces TIR
When it comes to toolholding, one key to increased cutter life is a holder that provides ultra-low Total Indicated Runout (TIR). The goal is to get as close to zero TIR as possible. With every 0.0001″ of TIR reduction, the company says that tool life can increase by upwards of 50%.
Another example when reduced cutter TIR is required are long tool overhangs to reach complex part features in parts that do not provide much clearance. Shops within the Aerospace, Defense and Space Exploration sectors often run cutters as small as 0.128″ diameter in a CAT 40 machine tool spindles with the tools hanging out up to 8″ from the spindle nose. To achieve TIR of 0.0002″ and under in such overhang applications depends almost entirely on the precision of the toolholder.
Part characteristics and features that involve 5-axis machining, such as those typically associated with 3D metal printed parts, compound the risk of cutter runout and breakage. In the Aerospace, Defense and Space Exploration sectors, 3D metal printed parts made from materials such as 718 Inconel and titanium are commonplace, and some of these parts can take as long as 40 hours to produce. Shops have one chance to machine them correctly, and must use toolholders that not only provide low TIR, but also offer the necessary strength and stability.
Cutters create dillemma
In addition to challenging parts, solid ceramic cutters for machining nickel-based and other heat-resistant alloys present a dilemma for shops in the aerospace, defense and space sectors.. Solid ceramic cutters generate significant amounts of heat that can affect the holding integrity of certain types of toolholders. For instance, toolholders that use heat or hydraulics for clamping would be at risk from the higher-than-normal levels of heat generated during machining with ceramic tools. Mechanical-based systems, however, are unaffected by extreme heat.
Beyond the challenges of metal workpieces, those produced from carbon fiber composites also cause issues with tool life and for toolholding. Shops must use tools designed specifically for the materials, which are extremely abrasive. While cutting, these tools force the part material downward and upward simultaneously to prevent delamination of layers.
When holding these tools, maintaining low TIR is once again critical to achieving part quality and the longest-possible tool life, but tool pullout is also a major concern, especially when shops want to run cutters at maximum rpm. To combat this, toolholders need tremendous gripping strength paired with some type of locking tool-retention mechanism.
Because shops cannot use conventional coolant when machining carbon fiber composites, many have incorporated the use of CO2-based cooling systems. These systems use supercritical CO2 not only to cool, but also to lubricate and to evacuate chips. Not all toolholders are suited for CO2 coolant delivery, and with some, tool changeovers can be time consuming as well. Combined with the latest cutting tool technology and the right toolholders, CO2 cooling systems can provide productivity increases in the 70% range.
Compared to shrink-fit toolholding systems, mechanical-based systems offer optimum CO2 delivery and the fastest, easiest tool changeovers. For heat shrink, the delicate ruby nozzles used to deliver the CO2 to cutters would need to be removed prior to heating the holder to change out a tool – a task that can require hours.
Conversely, some mechanical toolholding systems incorporate modified collets specifically for either through-tool or external CO2 cooling that have no effect on tool changeout time. CO2 coolant also can be supplied through modified toolholding collets with 150µm slots that direct coolant around the shank and along the exterior sides of a composite tool.
CO2 cooling offers especially beneficial performance when profiling and drilling carbon fiber composites. Those benefits include dramatic cooling of the cutting tool tip, which reduces wear and extends tool life. Most importantly, however, the tool will cut through the carbon fiber layers much better with fewer, if any, ragged edges.
For drilling, instead of the large through holes used with flood coolant, the holes for CO2 are much smaller, in the 150 to 300 µm range. These tiny holes not only cool the drill but also the material, so the drill makes clean entries and exits. Both internal and external type CO2 toolholders ensure drills cut through the last layer, as opposed to punching through it and leaving ragged edges.
High tool usage rates provide the first indication that a shop may not be using the right toolholding. Regardless of workpiece material or geometry, a shop that runs through many instances of the same tool for one part needs a better toolholder, especially one that is high precision, makes for fast and easy tool changeovers, provides extreme gripping force and, most importantly, delivers near-zero TIR.
powRgrip achieves near-zero TIR toolholding
Out-of-balance, poorly manufactured toolholders generate vibration that increases exponentially from the toolholder to the cutting tool tip. So, the longer the cutting tool, the higher the levels of runout. While the complete elimination of TIR and vibration is impossible, the REGO-FIX powRgrip toolholding system reduces both to near-zero levels.
The powRgrip system consists of three basic components – holders, collets and press-fit assembly mounting units. In manual or automatic configurations, the hydraulic press-fit assembly units quickly press collets into holders with up to nine tons of force. The collets have high-precision tapers that match with equally high-precision internal tapers in the holders, all of which creates the system’s extreme levels of transferable torque.
Unlike other clamping systems where heat or hydraulics are used, the powRgrip system uses the mechanical properties of the holder material to generate its tremendous gripping force. Such taper-to-taper collet-holding systems deliver extremely high levels of precision, and as a whole their TIR is typically less than 3µm, which ensures accurate finished parts. Such precision optimizes the machining of complex, critical Aerospace and Defense industry components through the use of high speed cutting (HSC) or high performance cutting (HPC) strategies. The lower the TIR, the less tool vibration, and the less vibration, the longer cutting tools last, which in turn reduces overall tooling costs.