Cutting tool expert Union Tool leads in the domain of hard milling. Here, the Japanese manufacturer demonstrates the optimal way of milling a solid block of 1.2344 steel, tempered to 54HRC to 100mm from each side.
The classic method of machining hard materials or hard milling is an alternative to grinding and electrical discharge machining (EDM). Hard materials are classified in two categories. The first, soft materials that have undergone hardening by a thermal or thermo-chemical treatment and the other is naturally hard materials like carbides, ceramics, minerals and many sintered materials. Materials with a hardness between 35 to 70 HRC are classified as hard. For steel, a minimum carbon content of 0.4% is essential for steel to harden sufficiently after tempering.
Union Tool, headquartered in Japan, has a Swiss-based subsidiary, Union Tool Europe, located close to Neuchâtel, a stone throw from Borotec that houses the European technical centre of the Japanese manufacturer. The demonstration workpiece described in this article was machined right here – a solid block of 1.2344 steel (X40CrMoV5-1) tempered to 54HRC to 100mm from each side. Prior to detailing the machining process and the tools used, it is worth mentioning trochoidal milling.
The advent of solid carbide end mills has opened many doors to HPC/HPSC (high performance cutting / high-speed performance cutting). In machining strategies associated with high-tech cutting tools, end mills are worn only on a particular part of the tool, normally, at the tip and on the first 30% of the tool. For such cases, interest to use solid carbide end mills in roughing operations is low, the preferred choice being inserts. However, under best possible conditions, a solid carbide end mill outperforms its insert competitor in milling depth, cutting forces, stability, surface finish and noise. Trochoidal milling enables the use of the full cutting length of the solid carbide end mill whilst reducing cutting forces and increasing tool life. Also known as vortex milling, trochoidal milling can be defined as a circular interpolation with a simultaneous forward movement called path. The end mill removes successive strips of material, following a continuous spiral trajectory along a straight line or profile.
The workpiece machined at Union Tool’s technical centre is a showpiece for all technologies associated with latest-generation cutting tools. An important point is, Union Tool manufacturers its own machines to grind and flute its own tools. This ensures very high quality and performance. The company also looks into the complex surface coating chemistry and the delicate processes for coating tools.
The milling was executed using the FAO Autodesk Power Mill 2018 software with the Vortex function. Safely mill this complex workpiece means reuniting a mix of conditions, beginning with the choice of the cutting tool. Union Tool chose a 5-flute corner radius roughing tool, the CXLRS 5120-20-48 with a 12mm-Ø, 2mm-corner radius and an effective length of 48mm. This tool allows for unequal pitch and helix angle, has a high-performance UT-coating and a positive cutting geometry, which ensures stable milling at high-speeds. The rigid design with integrated corner radius reduces cutting forces greatly. The CXLRS series is applied on a wide range of soft and tempered materials of up to 55 HRC. A hydraulic Schunk tool holder was chosen as the spindle-tool interface. The milling machine used was the 5-axis Hermle C22 U P machine.
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Today, to generate optimal milling strategy, one needs the FAO software, the company says. Vortex roughing from Autodesk’s Power Mill software has the “cutting path” option that enables a safer milling path with a deeper cutting depth, controlled tool engaging angle and optimum cutting conditions for all of the milling paths. With the controlled tool cutting angle and a constant feed rate, the milling process does not exceed the load on the tool, which warrants tool life. Raphael Gravois, technical consultant and application engineer at Union Tool explained that the milling of two workpieces with only one new tool is indeed very, considering the nature of the milled material.
It was essential that the most time was saved in roughing, which for this job, took about three hours. Cutting speed was 220 m/min for a feed per tooth of 0.2mm, corresponding to 5,000 rpm/min for 5,000 mm/min with a 0.3mm step-over.
The full cutting length of the CXLRS 5120-20-48 roughing end mill was utilised, corresponding to 24mm (2 x d) for the deepest milling paths. The software analysed ahead of every 0.4mm to detect rest material and recreate at each level an optimised milling path. This is the most efficient roughing method as it reduces the number of paths while increasing the cutting depth at each pass.
The superiority of this roughing strategy can be quantified by the subsequent roughing operations necessary to eliminate the last milled “steps” before the finishing operation. Without the “cutting path” option, the roughing operation would have been an hour less, but the subsequent roughing operations needed to remove the milled “steps” would have been two hours more – here it was only 15. For semi-finishing and finishing, a tool from the CFB series (CFB 3040-0600, 3-flute ball nose end mill with a 4mm-Ø, a 30° helix angle and a UT coating). The number of flutes ensures high in-feed and chip volume and the variable pitch guarantees an high stability as well as good surface finish. The cutting parameters were 184 m/min with a feed rate per tooth of 0.06mm, corresponding with 14,600 rpm/min and 2,450 mm/min, Ap = 1.2mm and Ae = 0.4mm.
Lastly, the workpiece was engraved “Power Mill Autodesk / Union Tool” with a 2-flute ball nose end mill from the CSEB series (CSEB 2006-0060 with a 0.6 mm Ø) in only four min 46 seconds. (24,000 rpm/min, 2,600 mm/min, Ap = 0.3mm, Ae = 0.09mm).
All the milling operations were conducted with water coolant to ensure the best possible thermal evacuation, thereby increasing the tool-life of the cutting tool.
Admittedly it was milling a 54 HRC hardened steel block but the challenge was no less complex and uses expertise in economical hard milling. Also, only coated solid carbide end mills, which are not much more expensive CBN tools were used. This holds out the complete milling process and controlling the temperature on the cutting edges was essential as well as enabling constant cutting forces, which was possible with the Power Mill software.
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