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Milling Faster than Eroding

Modern Milling Technology Improves the Production of Die-Cast Parts

| Author / Editor: Klaus Vollrath / Alexander Stark

In the production of die casting molds modern machining centers for HSC milling offer significant economic advantages, as the eroding process can be omitted. A medium-sized mold manufacturer reports on his experience in this field.

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A view into the interior shows the Quadroguide Z-axis, which is particularly rigid thanks to a total of four guide rails.
( Source: Klaus Vollrath )

"Our customers are primarily die- casting foundries that produce aluminum, magnesium or zinc die casting parts for the automotive industry and its suppliers as well as other high-tech industries," says Jürgen Jung, owner and managing director of Formenbau Jürgen Jung GmbH located in Gransdorf. In addition, they produce injection molds. After many years of working in the die casting foundry, where he had already completed his training, Mr. Jung started his own business in 1991 as a sideline and in 1997 as a full-time business. Meanwhile he supplies die casting foundries throughout Germany and Switzerland, where the quality "Made in Germany" produced by him and his experienced team is well-known and appreciated. The customer base includes about 80 % contract foundries and 20 % foundries that produce for their own consumption. A cooperation with in-house foundries has clear advantages, as there is a possibility of influencing the component design, which ultimately saves costs and reduces reworking efforts of the casting tool.

Development Know-How Used for the Customer's Benefit

"Since I made my first experiences at a time when the casting tools were based on drawings and not on 3D CAD data and the machine axes were moved manually, I followed the development of modern IT technologies particularly closely," adds J. Jung. Thanks to his many years of experience in die-casting tool making, he has gained many different experiences regarding the possibilities and limits of the die-casting process. The experience gained is incorporated into the design process or as a consulting service for the construction of the casting tools. This cooperation has a positive effect on the productivity of the process, the durability of the tool and the properties of the die-cast part. His staff is also familiar with the special requirements for die casting tools and pay attention to the important points right from the start of production.

J. Jung uses Visi CAD software in its own CAD/CAM department when working with customers. This software offers optimal continuity — from the interfaces used for the data transfer of various customer data formats to the postprocessors of the machine tools in his production. An engineer with many years of foundry experience was recruited to further expand the internal expertise, who also supports customers in designing the technical casting elements of the die casting tools. An experienced external service provider is involved in the casting simulation calculations. His cooperation with research institutions such as the universities in Aalen and Aachen also has a positive impact.

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Precision and Flexibility During Fire-Fighting Operations

"We attach great importance to high precision in the production of our molds, as this has a positive effect on the service properties of the casting tools and the quality of the cast parts," adds Jan Petri, tool mechanic and right-hand man of J. Jung. During machining, the focus is mainly on the hard, contour-giving active mold parts and outsource less critical operations, such as the manufacturing of mold frames, to suppliers. The company's key advantages also included high flexibility and fast reaction times in the event of breakdowns or urgently required repairs to customer casting tools. This applies to tool breakage (partial break-outs of the contour surface) as well as, for example, welding and reworking worn contour areas. In cases like these, the strengths of the medium-sized business managed directly by the owner would take effect. The boss knows his resources and production planning inside out and can make timely decisions about new or rescheduled machine capacities and personnel. These types of urgent orders are also accepted for molds whose original manufacturers would have problems in flexibly rescheduling their capacities accordingly. In addition, according to Mr. Petri, the processing machines are working independently, which means that process chains can be broken up and reassembled without problems.

Recently, a customer placed a follow-up order for an identical successor tool to an older mold, built years ago, which at that time had been produced mainly by die-sinking EDM. In this case the experts decided to switch the production of the new tool largely from eroding to HSC milling. Instead of using eight electrodes on the ejector side, only one was needed this time, and of the two electrodes on the nozzle side, only one remained. "Although the cost of this redesign of the production technology was considerable, it has paid off in terms of the overall cost calculation," reports Jürgen Jung.

HSC Milling Faster Than Eroding

"Over the last few years, we have gradually switched from eroding to HSC milling," explains J. Jung. Eroding is a process that creates harmful tensile stresses on the die surface and a white layer which can lead to premature contour wear on the die casting mold if the polishing depth is insufficient. This white layer must be removed after eroding and involves considerable time and cost. He was also not always satisfied with the accuracy that can be achieved during eroding. Four years ago, the company therefore invested in a five-axis milling machining center with swivel head and rotary table from Matek. This marked the company’s entry into 5-sided machining of molds. The cost advantages quickly became apparent, which then also paid off for the customers. In the meantime, much less erosion is taking place than a few years ago because HSC milling is faster and more accurate. As a result, the share of milled parts increased significantly, while eroding decreased significantly. After three years, a second system had already been considered, but it had to meet higher demands with regard to accuracy and driving dynamics than the existing system.

Röders Recommended by Colleagues

"An important contribution to the decision for the new production system came from the experience of colleagues," says J. Petri. Of course, they did some research, looked through brochures and compared data, but the main focus was on the practical long-term experiences of friendly mold makers. It became clear that with the Röders machine excellent long-term accuracy could be achieved and that these machines also offered a high degree of reliability. Röders also offered a fast and competent service. In view of the importance of the investment, a company that already used two Röders machines, was also visited. The company’s recommendation in favor of Röders was decisive. Another important point was the possibility of using the system both for HSC machining of the hardened contour parts of the molds and for milling graphite electrodes, as eroding remains indispensable in certain areas, e.g. with deep, tight ribs, despite the decreasing proportion. To date, around 90 % of the demand for electrodes had to be purchased. After the introduction of the new Röders machine, almost all electrodes are now manufactured in-house. Another factor was the competence of Röders' sales staff, who advised on the design of the machine to meet J. Jung's requirements. In September/October 2016 the plant — a five-axis HSC milling machining center RXU 1001 — was put into operation.

Milled Parts Production Running Since Commissioning

"In principle, we were able to produce right from the first day of commissioning," recalls J. Jung. Practically from the beginning of the training, machining was carried out on real milled parts. It proved to be an advantage that the CAM system used by Jung is also used at Röders. Thus, the people there know the switches and icons exactly, allowing for information to be exchanged one to one via Teamviewer during consultation of optimal HSC strategies. The Windows-based Röders control system is pleasantly simple, intuitive to operate and very easily memorized; even feedback received during processing, it is sorted according to relevance, which is very helpful. Positive aspects also include the very clear manual, which can also be accessed via the control, as well as the plain text support of commands. For example, as an alternative to the usual M command, you can simply enter "Oil on". CNC programs could be added manually without great effort, for example to drill a hole or repeat operations from another origin. As far as Röders' service is concerned, he could only say that he has a good reputation, because he has not really been challenged in the one and a half years since commissioning.

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HSC-Milling Machine Works Very Precisely

"The precision of the new HSC milling machine is impressive and makes our work much easier," says J. Petri. Both hot work tool steels with a hardness of 44-47 HRC and graphite electrodes are processed. For the cleaning of the graphite dust the machine is equipped with a powerful dust suction system. The machining tools also achieved long service lives during hard machining, probably also because of the low vibrations. This is why the system consisting of processing tools and a Röders machine is also convincing in terms of surface quality. Milling machines can be used down to diameters of 0.3 mm and, if necessary, the smallest corner radii of only 0.2 mm can be achieved in the molds.

In the past, sliders had to be milled in advance and then reworked manually. Today, they can be removed from the machine and usually pushed immediately into the mold. Since the new Röders milling machine was used, the time required for checking the molds has also been considerably reduced in most cases. The quality of the molds is now so good that the supplied die casting molds are accepted on average with only one sampling run. In addition, possible post-processing often concerns the casting technique rather than the part contour. Working with such results is a lot of fun.

The zero-point clamping system used, which is installed throughout all production lines and whose clamping spigots are mounted directly on the underside of the workpiece, offers decisive advantages in terms of set-up time. In combination with the coordinate measuring machine, the workpieces and the clamping devices used can be precisely measured even before they are inserted into the machining center, so that the time previously required for set-up in the machine can be reduced to a minimum.

This article was first published by Schweizer MaschinenMarkt

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