Heller Machine Tools Inc. (Nuertingen, Germany; Troy, MI) doubled metal removal rates for roughing operations in Ti-6Al-4V with Haimer’s Power Shrink and Heavy-Duty Shrink Chucks with Safe-Lock system. Prior to Safe-Lock, the tool pulled out of a standard shrink-fit chuck and broke when trying to machine a slot deeper than 1×D. The test study was conducted when a group of representatives from the aerospace industry, the Technical University of Dortmund, and Technical University of Hamburg-Harburg, visited Heller. The Safe-Lock system features special drive keys in the chuck and slots in the tool shank that prevent milling cutters from turning or being pulled out of the chuck during machining.
Heller Machine Tools is known as a source of machining innovations when working with materials that are difficult to process like titanium alloys. Heller is known for its high-quality four- and five-axis CNC machining centers, CNC mill/turn centers, CNC machines for crankshaft and camshaft machining as well as flexible manufacturing systems.
According to Werner Kirsten, a Heller technologist responsible for the area of difficult-to-machine materials, “Our service includes optimizing the machining processes together with our customers and suppliers. We often achieve this aim with an optimized roughing operation which ultimately results in a reduced finishing process. By maintaining the same technology values, but shortening the finishing depth of cut by 50%, the overall machining time is reduced to half. However, this requires process reliability and controllable systems during roughing,” said Kirsten.
In this regard, Kirsten said all machining components in the process chain have to be considered in order to improve productivity. “The machine tool is the most obvious component of the machining process, however, the tool, the toolholding system, the coolant supply and other elements are also essential for a successful operation. In the end, the weakest link of the process chain limits its success.”
In Kirsten’s opinion, most machine shops don’t pay attention to the toolholder even though it is especially significant for high-performance cutting. “Many of the trials which we carried out during the last few years have proven this point. We realized that the toolholder has an incredible influence on the machining process. In the case of reinforced shrink-fit chucks, for example, the vibration node is closer to the bearing point due to the larger mass. The result is a smoother machining process with less vibration and a better surface finish quality while using the same tool, machine, process parameters, and fixturing technology,” Kirsten said.
By selecting the right toolholder, good productivity and surface quality results using basic standard cutting tools can be achieved. Kirsten refers to comparative tests with basic four-edge cutting tools without an inner coolant supply. As an alternative to a standard shrink-fit chuck, he selected a Haimer Power Shrink Chuck with Cool Flash which ensures that the coolant is transported directly to the cutting edges. “Compared to using a normal shrink-fit chuck and external coolant, we were able to achieve significantly better results,” said Kirsten.
The Heller machining center selected for the test was the Heller H 5000 with a gear unit and HSK-A100 spindle, which yields torques up to 2290 N•m. To demonstrate the machining potential, different 25-mm diameter end mills were used to mill full slots into a titanium plate. To simulate holder conditions used in the aerospace industry, the tools were clamped into reinforced shrink-fit chucks. At an axial cutting depth of 0.5 to 1×D, the machining process was found to be very reliable. Since the spindle wasn’t running at full capacity, the trial participants agreed to increase full slotting depth to 2×D. “Under these conditions, the tool pulled out of the shrink-fit chuck during the machining operations, creating a slot closer to 2.5×D as it progressed through the part. Towards the end of the slot, the tool finally broke as a result of increased cutting forces.”
Heller’s milling specialists all agreed that the clamping force of the chuck was the limiting factor in this process, even in a high gripping torque system such as shrink-fit chucks. As soon as the axial pressure and process-related vibrations are too high the tool moves out of the chuck and further into the workpiece. At the same time, the cutting process increases such that in the end the tool breaks and the workpiece is irreversibly damaged.
After discussing reasons and possible countermeasures, Kirsten developed the idea that the tools are forced to navigate towards the spindle when the holding forces are exceeded. At this point, Kirsten became aware of the patented Haimer Safe-Lock system, which in addition to frictional clamping forces, has the same helix pattern as the tool. He realized that even if the tool got loose while using Safe Lock, it would be pulled into the holder through the helical drive keys and not into the part—a movement that can be easily prevented through the use of length presetting screws.
Kirsten contacted Haimer in order to test the Safe-Lock system. “We wanted to know if our ideas could be put into practice using the Haimer system in an even less forgiving environment. The Heller technology developer repeated the described trial with a few major changes. Haimer Safe-Lock grooves were subsequently added on similar solid-carbide tools, but the application would be run again with a reduced spindle taper interface (HSK 63 instead of HSK 100) with a less rigid five-axis gear-driven spindle and a less stable diagonally fixtured workpiece.
Despite these additional limiting factors, the tool was able to process a full 50-mm slot of 2×D with complete process reliability. “This way we were able to indirectly prove that Safe-Lock works and that the shrinking technology has even more potential with HSK-A100, especially on five-axis machining centers with gear spindles,” said Kirsten. “Roughing with the Haimer Safe-Lock system is a reliable process. It is a practical, easy to handle and process reliable addition to the shrinking technology which we already utilize, especially in connection with standardized tools of other manufacturers,” Kirsten said.