By David McHenry
As appeared in Manufacturing Engineering, June 2021
For manufacturers in aerospace, energy, automotive and defense, efficiently machining new materials means keeping up with the competition. These demanding sectors are often the first to use cutting-edge materials science in part design—and the properties that make these materials desirable for tough applications make them equally tough to cut. This is especially true for composite materials, which are being used more in machine shops and require fully optimized toolholding.
The practice of combining materials to increase their strength is as old as recorded history. This includes composites like concrete or newer ones like fiberglass that are used every day. Today, manufacturers in pursuit of lighter, tougher parts can use materials like carbon-fiber reinforced polymers (CFRP) and other combinations of advanced binding agents and reinforcing fibers made of various metals, ceramics and plastics. These materials can be stronger than most metals at a fraction of the weight, but their composition creates numerous difficulties for shops.
The high tensile strength of composites means manufacturers need tough cutting tools, much as they do with superalloys and other difficult-to-machine materials. However, with composites, much of that strength comes from reinforcing fibers that are incredibly abrasive and tend to sharply reduce cutting tool life unless super-tough polycrystalline-diamond (PCD) tools are used.
While these state-of-the-art and often expensive PCD tools can better handle the abrasiveness of composite materials, other challenges remain. CFRPs and similar materials often produce dust rather than chips when machined, for example, and applying coolant in this situation generally just creates an abrasive slurry that will quickly damage machines. Furthermore, in industries such as aerospace and medical, the risk of contaminating composite parts with coolant has led to strict regulations on its use. As a result, composites are usually cut dry, with dust-collection systems used to protect operators and remove swarf.
The unique composition of composites further complicates machining operations. In many cases, composites will threaten delamination when approached conservatively and uncut or pulled-out fibers will pollute holes or various material layers. To combat this, manufacturers apply much higher speeds and feeds. Using high-performance machine tools with spindle speeds exceeding 20,000 rpm is recommended for the best efficiency and surface quality.
However, performing high-speed machining operations in dry conditions means that temperature becomes a concern. The swarf created when cutting composites carries little heat away from the tool, which results in rapid heating and cooling cycles as the tool enters and exits the cut. This produces significant amounts of thermal stress on tools and toolholders.
Tools can generally be designed to handle thermal cycling with innovative coatings and DAVID MCHENRY Engineering & Technical Manager Rego-Fix Corp. The machining of composites, such as those used in the construction of turbine blades, is typically done dry, which generates heat that can cause certain types of toolholders to vibrate and lose clamping forces as they heat up. To combat these issues, manufacturers can incorporate Rego-Fix’s powRgrip holders. (All images provided by Rego-Fix) June 2021 | sme.org 17 geometries, but even the most cutting-edge tool will struggle in a toolholder that is unable to handle the heat. Certain types of toolholders will experience increases in vibration and lower clamping forces as they heat up, which quickly contributes to increased runout. Given the expense of PCD tooling, the reduced tool and toolholder life caused by subpar toolholding performance makes even the most efficient processes less profitable —and creates significant risk for parts that have often required substantial investments prior to machining.
Optimal Composite Machining
To achieve truly optimal composite machining, manufacturers require a toolholding solution that can handle all of the challenges created by these materials. One such solution, the powRgrip toolholding system from REGO-FIX, is designed for similar high-speed, high-performance cutting strategies, and its range of advanced features make it ideally suited for composite machining.
Unlike toolholders that require heat for clamping and unclamping, powRgrip uses a unique hydraulic press-fit assembly mounting system that mechanically clamps tools within collets with up to 9 tons of force. This produces a clamping torque rating of 1,100 Nm, which is sufficient for pullout protection at even the highest rotational speeds. This mechanical interface is unaffected by heat—and therefore allows it to retain all of its clamping force regardless of the thermal cycling produced during composite machining.
In addition to handling the thermal effects common with these materials, powRgrip provides other advantages to manufacturers. With a total system runout (TIR) of ≤0.0001″ (3 µm) and length adjustment repeatability of <0.0004″ (10 µm), these toolholders deliver high precision. The toolholders themselves are balanced by design to G2.5 at 20,000 rpm for steep taper holders and up to 90,000 rpm for HSK holders, which ensures that holders maintain their accuracy even at extremely high speeds during composite machining operations. This makes it much easier to achieve quality surface finishes when drilling, routing and profiling, a critical factor when meeting customers’ needs in demanding industries like energy and aerospace.
These industries also often require manufacturers to invest in high quality equipment, and protecting these investments is paramount. Thankfully, the strong toolholder-to-collet and collet-to-tool shank interfaces do more than promote greater accuracy and higher clamping forces—they also make powRgrip highly effective at vibration damping. As a result, the lifespan of high-performance PCD tools and high-speed spindles can be maximized.
The combination of high clamping forces, low total system runout and vibration damping makes powRgrip useful in terms of process security. As with any high-value workpiece, manufacturers can’t afford to scrap parts with pulled-out or dull tools, and downtime caused by unplanned maintenance can make it far more difficult to keep promises to customers. With powRgrip, shops can achieve fully secure processes and better predict their maintenance needs.
The system can also be expanded with options that improve efficiency and final part quality. Tabletop mounting units reduce clamping and unclamping times to seconds, while optional secuRgrip clamping collets, toolholders and accessories allow for greater precision and pullout protection for especially tough applications. And with a five-year, 20,000-cycle warranty, powRgrip users can be sure they’ll consistently achieve the guaranteed 0.0001″( 0.00254 mm)runout.
Whether producing high-performance skis, windmill housings, helicopter rotor blades or racecar bodies, shops working with composites have unique challenges. To overcome these obstacles, solutions like high-performance machines and PCD tooling can be leveraged for better results—and toolholding solutions like powRgrip protect those investments for high accuracy, process security and part-production efficiency.