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The machine tool is the most evident component of the machining process, however the tool, the tool holding system, the coolant supply and other elements are also essential for a successful operation

Image: Haimer

Workholding & Fixtures Ensuring reliability in heavy machining process

Sep 21, 2016

The article highlights the importance of a tool holder and its role in increasing productivity for heavy duty machining, which requires high process reliability when working with difficult to machine materials

The importance of the tool holder is still understated, especially when it comes to roughing and heavy duty machining. Metal removal rates in these types of processes are absolutely crucial for the productivity of the machining operation. By using special shrink fit chucks with drive pins and spiral grooves in the tool shank, it is now possible to perform high-feed full slotting of up to 2 x D (50 mm) or more in difficult machine materials.

Application studies conducted by the machine tool manufacturer, Heller have proven the effect of the HAIMER Safe-LockTM system. Especially within the aerospace, energy and mechanical engineering sector, production managers have to bridge the gap between economic efficiency and high process reliability when working with materials that are difficult to machine. Fortunately, the machine technology innovations continue to develop to meet the ongoing demands of manufacturing.

Roughing is a promising process

Werner Kirsten, who is working in the Technology Development Department at Heller and is responsible for the area of ‘difficult to machine materials’, explains, “Our service includes optimising the machining processes together with our customers and suppliers. To support such services with practical trials, our technology centre is equipped with a variety of machines.”

In most cases, the aim is to increase the productivity without compromising the process reliability. Technologist Kirsten adds, “We often achieve this aim with an optimised roughing operation which ultimately results in a reduced finishing process. By maintaining the same technological values, but shortening the finishing depth of cut by 50%, the overall machining time is reduced to half. However, this requires process reliable and controllable systems during roughing.”

In this regard, all machining components in the process chain have to be considered in order to improve productivity. The machine tool is the most evident component of the machining process, however the tool, the tool holding 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 the success,” Werner Kirsten emphasises.

In his opinion, most machine shops don’t pay attention to the tool holder 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,” Kirsten explains and adds, “We realised that the tool holder 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.”

Tool holding: important for productivity

By selecting the right tool holder, one can achieve good productivity and surface quality results using basic standard cutting tools. 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.”

When a group of representatives from the aerospace industry, the Technical University of Dortmund and Technical University of Hamburg-Harburg visited Heller, an especially extensive milling application study was conducted in Titanium Ti-6AI-4V.

Gaining new insights

The available machining centre was a four axis Heller H 5000 with a gear unit and HSK-A100 spindle, which yields torques up to 2.290 Nm. In order 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 1.0 x D, the machining process was found to be very reliable. Since the spindle was not running at full capacity, the trial participants agreed to increase full slotting depth to 2 x D. Kirsten explains, “Under these conditions, the tool pulled out of the shrink fit chuck during the machining operation, creating a slot closer to 2.5 x D as it progressed through the part. Towards the end of the slot, the tool finally broke as a result of increased cutting forces.”

The milling specialists all agreed that the clamping force of the chuck was the limiting factor in this process. 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 pressure increases such that in the end, the tool breaks and the workpiece is irreversibly damaged. Reasons and possible counter-measures were discussed intensively.

In the end, Kirsten developed the idea that the tools are forced to navigate towards the spindle when the holding forces are exceeded. This is how he ultimately noticed the patented HAIMER Safe-LockTM system which in addition to frictional clamping forces, has the same helix pattern as the tool. Theoretically, he realised that even if the tool got loose while using Safe LockTM, 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.

Prevent tool pull-out

Kirsten got in contact with HAIMER, the European market leader for tool holding technology, in order to test the Safe-LockTM system and shared, “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-LockTM 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 5-axis gear driven spindle and a less stable diagonally fixtured workpiece.

The result was that despite these additional limiting factors, the tool was able to process a full slot of 2 x D = 50 mm with complete process reliability. Kirsten says, “This way, we were able to indirectly prove that Safe-LockTM works and that the shrinking technology has even more potential with HSK-A100, especially on 5-axis machining centres with gear spindles.” Afterwards, this sample machining on Ti-6AI-4V was carried out various times, including during the Airshow in Farnborough, whereby it was proven that the previous limit of 1 x D = 25 mm full slotting in titanium could be doubled with the use of Safe-LockTM. Kirsten summarises the results and shares, “For roughing operations, this is a reliable process. Therefore I consider the HAIMER Safe-LockTM system to be a practical, easy to handle and process reliable addition to the shrinking technology which we already utilise, especially in connection with standardised tools of other manufactures.”

Licensing programme

From the onset of its creation, HAIMER recognised the potential of the Safe-LockTM system, which is why the company created a licensing programme to give tool manufacturers the ability to integrate the patented pull-out protection system into their tools. The result of their vision to create a more open system has been a success, with leading cutting tool manufacturers having now signed licensing agreements, including Walter, Widia, Sandvik Coromant, Seco Tools, Sumitomo, Kennametal, Helical, Emuge Franken, Data Flute, Niagara, OSG, SGS Tools and recently, Mapal.

Through the steady increase of Safe-LockTM tooling in the market via Haimer and its licensed partners, the system has become the new standard for heavy machining and roughing. It is also replacing the old Weldon clamping system, particularly when new milling strategies such as trochoidal (dynamic) milling are involved. Andreas Haimer, President of HAIMER Group concludes, “With our Safe-LockTM system, we found a solution that combines a high run-out accuracy with 100% pullout protection. The system has established itself as a standard in the aerospace industry and for roughing and heavy duty machining.” ☐

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