Machining Concepts Meeting the challenge of high-performance milling
The challenges of high-performance machining have become more complex and demanding. These include high-mix, low-volume production, dynamic machine concepts, materials and high-precision surfaces. Particularly high demands are placed on a stable tool system.
The advantages of high-performance milling include shorter machining times due to high metal removal rates, longer tool life, improved chip formation, better heat dissipation and reduced vibrations. Due to the corresponding cutting depths and feed rates, a powerful machine with a high torque and high available power at low speeds is required, as well as stable clamping of the often large and heavy workpieces. In order to provide its customers with new approaches for cost and time optimisation of production, cutting tool expert Paul Horn is in regular dialogue with its customers.
High-mix, low-volume manufacturers, who are faced with a wide product variety and shorter product life cycles, rely on low costs and short response times, with several different components having to be produced on one machine in shorter time intervals. Therefore, tool manufacturers need to define their tools in the most optimum way for the most diverse applications. One example is Boehlerit’s BETAtec 90P Feed multi-functional milling tool system, thanks to the fact that the tool holders may also be used with indexable cutting inserts for high-performance cutting. The end mills and screw-in milling cutters are available from diameters of 16 - 32 mm with different tooth pitches, the corner milling cutters from diameters of 40 - 80 mm with likewise wide and narrow tooth pitches. A narrow tooth pitch allows for high metal removal rates. The tool system enables face milling, full-track milling, shoulder and groove milling, trimming, copy milling, ramping and circular milling.
To machine 90-degree angles with high cutting speeds and precision, Horn recommends the BETAtec 90P version with 18 mm cutting edge length. The maximum cutting depth of up to 18 mm, in combination with a helical cutting edge, result in maximum productivity when finishing 90-° shoulders.
In addition to the tool holder, options to machine various materials have been designed to meet the new requirements. For the ISO groups, P, M, K, N, a wide range of carbide grades is available. In the case of the BCP25M grade (multi-range grade for milling unalloyed, low-alloy and high-alloy steel), for example, the Goldlox coating is available for milling steel with high wear resistance at high temperatures and increased tool life. Cutting parameters for tool steel (1.2379) are vc = 180 m/min, fz = 0.2mm, ap = 4mm.
Speed: High-feed milling
To reduce machining times, Horn compared conventional face milling with high-feed milling on a DMU80P duoBlock from DMG Mori (tool system DAHM37). On a workpiece made of heat-treated steel 1.7225, a material removal depth of 6 mm was to be realised with a width of 60 mm and a tool path of 500 mm.
Since the machining time results from milling path/feed rate (t = lf/vf), the high-feed cutter must cover three times the tool path compared to conventional machining to achieve the cutting depth of 6 mm at a feed speed of 8,950 mm/min (1,003 mm/min during conventional machining). As a result, machining time could be reduced from 0.49 min to 0.15 min at the same rpm.
A comparison of the material removal rate results in 1,074 cm3/min for high-feed milling, which corresponds to a factor of 3 compared to conventional face milling. The specific material removal rate is increased by a factor of two for high-feed milling. Both methods can be used under the given conditions, but high-feed milling is clearly more efficient. In order to utilise the tool system to its optimum capacity, conditions regarding the speed/power diagram must be taken into account.
In order to optimise the machining of a component for a hydraulic rotator on excavator attachments, CNC machine shop Bamann turned to Paul Horn. Second-generation Managing Director Jörg Bamann has been working with Horn for more than 20 years.
Case study: High-feed milling
Bamann produces around 300 rotators, which is made of the alloy steel grade 42CrMo4 (1.7225), quenched and tempered to 1000 N/mm2. Using a full-radius torus cutter fitted with indexable inserts, Bamann needed over 100 minutes to rough machine the axial groove which measures around 240 mm in diameter, 40 mm in width and just under 90 mm in depth without finishing allowance. The tool life was 30 grooves. While the roughing process was faster than axial grooving, it led to vibrations as well as a high noise level and the machining time was not ideal. Horn suggested using a five-insert version of the DAH high-feed milling system with a cutting diameter of 40 mm. Due to the load in the axial direction, the tool is mainly subjected to pressure and the transverse forces are relatively low. Due to the low tendency to vibrate, the tools can safely absorb the high loads at the usual feed per tooth of fz= 1 mm at cutting depths of up to 1.2 mm. The large radius on the main cutting edge of the three-edged insert produces a soft cut, ensures an even distribution of cutting forces and promotes long tool life. On the inside, a small cutting edge radius ensures smooth and fast plunging. A primary and secondary angle results in a stable wedge angle and very good cutting edge stability.
The workpieces are machined on a CTX 800 TC turning/milling centre from DMG Mori. The cutting speed is programmed with vc = 150 m/min. The tool plunges helically into the workpiece with a continuous infeed depth of ap = 1 mm. The radial feed rate (speed of the component) is vf = 4777 mm/min. The feed per tooth is fz = 0.8 mm. The new machining time of the groove is now only seven minutes per component. The tool life of the inserts increased to 90 components per machine set-up. “We are very happy with the result,” Bamann says. “By using the high-feed cutter, we were able to significantly reduce the cycle time. Furthermore, the load on the machine was reduced because, in addition to the machining time, the cutting pressure and the vibrations were also greatly reduced – and there is even more room for improvement in the machining process.”
Another challenge for tool manufacturers in the field of high-performance milling is the use of new and difficult-to-machine materials. In the automotive industry, “high performance” has long been an issue. One example here is “downsizing” (reducing the size of the engine displacement) with a simultaneous significant increase in engine performance. Often, the increase in power is achieved with the addition of turbochargers, which are made of high-temperature nickel-based alloys that are difficult to machine due to their alloying elements. Here, new geometries, substrates and coatings are the challenges for the toolmaker.
High temperature alloys such as 1.4849 are used in the automotive industry for turbochargers. The alloy consists of 39 % nickel with a chromium content of 20 %. The chromium content is an attempt to replace the expensive raw material nickel, but this results in difficult machinability because chromium has an abrasive effect. The challenge is to find a tool system with a suitable cutting material.
For machining the surface of a bolt-on turbocharger, Horn solved the problem for a customer using an arbour milling cutter with a negative axial rake angle and effectively positive insert geometry from Boehlerit. A tough substrate with a TiAlN layer was chosen to compensate for vibrations. The cutting parameters were selected according to the required cycle time. The tool system is very stable due to the negative axial angle of the insert. The insert is precision-sintered with a chip breaker, which allows for an effective positive cut. The tough base substrate was chosen to compensate for the vibrations that occur. The milling system met the requirements for surface, flatness, cycle time and tool life.
Due to changes in materials and geometries as well as the demands on the component (example: turbocharger), the specifications on the surfaces have changed. Where in the past an Ra or Rz value used to be sufficient, today combinations of surface roughness and waviness are often required; very often with part flatness or with fixed ranges from Rz 16 to Rz 25.
In order to find out where a manufacturing process such as grinding can be substituted under certain conditions, one first needs initial values. In this case, these were obtained using the example of a ground clamping plate for a machine vice made of tool steel. The measurements were carried out with a hand-held Mahr Perthometer. The measuring and probing distances were considered accordingly (DIN EN ISO 4288, ASME B46.1).
Two different finishing methods were compared. Single point finishing as a radial tool concept (wiper flat on the individual cutting edges smooths the surfaces) versus wide finishing as a tangential tool concept (large finishing radius with a single cutting edge smooths the surface). Horn chose a tangential milling cutter (diameter = 63 mm, Zeff = 8 with standard indexable inserts ER=0.4 mm and 2 wide finishing inserts) in carbide grade AS4B.
The results achieved with the milling process are almost identical to the ground surfaces. Both machining variants were in the range of fine finishing, namely in the roughness grades N5 and N6. The big difference was not only the achievement of better surface quality (finishing to fine finishing), but also the significantly higher feed rate due to the wide insert finishing process, as a significantly higher fz is possible because the wide finishing insert allows a greater feed per revolution. In the wide finishing process, the degree of roughness