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Horn Blog

COMBINING expertise in tool, model and MOULD MAKING

January 2022
28
Author: paulhorn
Company: Hartmetall-Werkzeugfabrik Paul Horn GmbH
COMBINING expertise in tool, model and MOULD MAKING

When three specialists come together and push the boundaries of their respective fields, they can achieve great results. A solution for tool and mould making was the culmination of one such project developed for an online seminar – and the result is testament to the group’s expertise and successful partnership. The team comprised workholding equipment specialist SCHUNK, programming professionals from OPEN MIND and tool experts from Horn. New approaches were combined with tried-and-tested technologies: from zero-deformation magnetic workpiece clamping with mobile pole extensions through new HPC milling tools for fast processing and high machining volumes to efficient and cost-effective programming. The seminar was of interest not only for users in tool, model and mould making, but also to those in other areas of machining.

“When Uwe Weil from SCHUNK called me and told me about the idea behind this project, I was immediately interested. I agreed to take part shortly afterwards”, recalls Horn product manager Andreas Jenter, whose speciality at Horn is milling with solid carbide tools. “Clemens Bangert from CAD/CAM manufacturer OPEN MIND also immediately agreed to take part in the project. The hyperMILL® expert developed and programmed the 3D model of the complex mould”, explains Uwe Weil, who is responsible for product and technology training at SCHUNK. Weil continues: “After four days of close collaboration, we had a machining process in place. Some aspects might look different in practice, but we wanted to use different approaches to show how such a complex component could be machined in a cost-effective way”.

Magnetic workpiece clamp

Although magnets are perhaps most commonly associated with surface grinding machines, magnetic clamping technology is also used in milling. “People still have misconceptions about magnetic workholding in milling applications, but we wanted to use this example to show that magnetic technology is actually very well suited to production”, says Weil. The magnetic plate is attached to the SCHUNK zero-point clamping system on the machine table using an aluminium base plate and appropriately arranged zero-point clamping bolts. The magnetic clamp holding the workpiece is completely free from deformation. This is achieved by using a fixed pole extension to set the height of the workpiece in combination with pole extensions. They compensate for any unevenness in the surface of the component, which ensures that the part is not distorted by the clamps. Once the external dimensions have been face-milled, the workpiece is clamped in the fixed pole extensions. “The depth of penetration of the magnetic field in the component is around 10 mm (0.394") at the highest level of magnetism. One of the misconceptions about magnetic clamping technology is that the workpieces themselves become magnetised once clamped. But this isn’t the case. The low penetration depth of the magnetic field means that chips don’t stick to the surface even when milling a deep mould”, explains Weil.

Jenter uses the Horn DAH84 high-feed milling system to face mill the periphery. “The insert has eight usable cutting edges, resulting in a low cost per edge and a high level of cost effectiveness. Despite the negative mounting position, the positive cutting geometry ensures a smooth and soft cut combined with good chip removal”, says Jenter. The large radius on the main cutting edge of the indexable insert results in a soft cut, ensures even distribution of the cutting forces and, in turn, extends the tool life. The tangential type 409 milling system was used to finish the surfaces.

Programming expertise

For roughing, Clemens Bangert made use of a function from the hyperMILL® MAXX Machining performance package. “To ensure the machining process was dynamic and efficient, I used 3D-optimised roughing. Machining takes place in trochoidal tool paths and the milling cutter ramps into the part helically. It’s particularly important that we can program separate speeds for entry, the dwell after entry and cutting. This ensures that the process remains stable throughout”, explains Bangert. The programmed dwell provides the spindle with sufficient time to reach the correct speed so that the pre-milling of the mould in trochoidal movements can commence. As far as possible, helical movements are used for roughing the mould. “This ensures that the tool always cuts gently at a constant speed with no retraction, and that it never makes full cut contact”, says Bangert.

Jenter uses a Horn VHM end mill from the DS system for roughing the cavity. “We developed HPC milling cutters specifically for the purpose of milling high-strength steels with high material removal rates”, explains Jenter. The system particularly excels in dynamic roughing applications as well as in standard roughing cycles. The first roughing process uses an HPC milling cutter with a diameter of 12 mm (0.472") and four cutting edges. “Given the contour, we deliberately refrained from using a larger diameter to reduce rest machining”, says Jenter. The cutter enters the workpiece in a helical movement at an angle of 5 degrees. The cutting depth is ap = 20 mm (0.787"). The other cutting data are vc = 140 m/min (5,511.81"/min) and fz = 0.08 (0.003"), while radial depth of cut ae = 3 mm (0.118"). The different helix angles create an irregular tooth pitch, making operation exceptionally smooth. The tools’ optimised face geometry reduces the cutting pressure when circular or linear ramping. Improved chip spaces ensure optimal process reliability during chip formation and removal.

A high-feed milling cutter with a diameter of 12 mm (0.472") is used to rough mill the free-form surfaces. The milling cutters have a double radius geometry, which favours the flow of forces in the axial direction of the spindle and reduces the radial force. “Thanks to this geometry, we can maintain high feed rates even with long tool overhangs, without causing any vibration in the tool”, explains Jenter.

TENDO E compact hydraulic expansion chuck

The tools are clamped in SCHUNK hydraulic expansion chucks for roughing. Weil uses the TENDO E compact series for roughing; the short design is ideal for this process. “I am often asked how much torque I need to apply to tighten the chuck. With SCHUNK, this is an easy question to answer: All you have to do is turn the clamping screw as far as it will go and this gives you the optimum concentricity and best possible torque transfer to the tool”, says Weil.

To accommodate the guide bolts of the tool, four holes need to be reamed in the corners of the workpiece. Schunk uses the TENDO Zero hydraulic expansion chuck to clamp the reaming tools. The four opposing Torx screws on the collar of the clamping chuck can be used to adjust the concentricity to a high level of precision. The user can check the reaming tool using a presetter and then make the final adjustments directly on the machine using a dial gauge. This approach allows the concentricity to be set with micron precision. “With a reaming tool length of over 100 mm (3.937"), we can achieve run-out below 2 µm (0.00008"). This is a great result”, says Weil.

High-performance DR reaming system

The DR reaming system from HORN is used to ream the four holes. With an internal coolant supply, the cutting speed was vc = 110 m/min (4,330.71"/min) with a feed rate of 0.84 mm/rev (0.033"/rev). The retraction feed rate was programmed at 4 m/min (157,48"/min). “With a long projection and a through bore, it is important that the tool doesn’t protrude from the hole by more than 2 mm (0.079"). Otherwise, there is a risk of the tool oscillating”, explains Jenter. The HORN reaming system features a modular design and can be combined with countless interfaces. The repeatability of insert position after changeover is just 4 µm (0.0002"). With standard inserts, the system is capable of reaming materials up to a hardness of 58 HRC.

“The four external threads were machined with three lateral infeeds to ensure an exact fit. I used the hyperMILL® ‘Thread milling’ function for machining. This function automatically calculates the value for the lateral feed based on the tool and thread, which means that it can support both single-edged or multi-edged tools”, says the hyperMILL® expert. The Horn DC thread milling cutter machined the threads with a cutting speed of vc = 80 m/min (3,149.61"/min) and a feed per tooth of fz = 0.02 mm/min (0.0008"/min). The tool was clamped in a SINO-R expansion chuck. The clamping system is based on PU elements rather than being hydraulic. This provides excellent vibration damping, which ensures that the entire system remains stable during thread milling.

High surface quality during finishing

“When finishing a mould with different ball nose end mills, there are three key factors that enable you to achieve the required surface quality: the precision of the tool, powerful CAM software for precise machining, and the accuracy of the workholding equipment. We produce the milling cutter radii with a maximum form deviation of +/- 0.005 mm (0.0002")”, explains Jenter. The importance of this precision becomes clear when different milling cutters are used for finish machining a mould. Bangert has programmed the mould to be machined with a 6 mm (0.236") and a 4 mm (0.157") ball nose end mill: “Before programming the free-form surfaces, we always check the requirements for the component first. These include the required surface quality, the form tolerances and the transitions during finishing”. The machine kinematics – the interplay between the workholding equipment, tools and machine control system – also play a key role.

The standard version of the hyperMILL® CAM software includes countless strategies for high-precision machining. The “High-precision surface mode” option, for example, boosts the quality of the surface finish. This function was used when machining the mould.  Bangert explains: “We calculate the tool paths based on the actual CAD component surfaces rather than a mathematical model. This means that we can achieve tolerances to within microns. I also used the “Soft overlap” function to blend the transitions between different surfaces, even when these transitions had been machined with another tool or infeed. This is an efficient way to achieve a seamless surface finish”. Clemens Bangert also deployed the “5-axis radial machining” function: “This strategy allows us to achieve the best possible surface quality. With a radial projection method, tool paths – for bottle moulds, for example – can be calculated much faster. It also allows the user to respond flexibly to the actual component in front of them”.

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