Precision Milling Milling operations for mould making: Higher surface quality and hardly any reworking
Mould making specialist Braunform, in cooperation with Moldino Tool Engineering Europe, has reviewed its electrode milling processes, developed alternatives and tapped into new potential. The solution had such a striking result that the entire graphite machining process at the plastics specialist in Baden switched to tools from Moldino.
In addition to turn-milling, wire and die-sinking EDM, mould maker Braunform considers milling in particular — graphite machining likewise high-precision HSC milling of hard materials — to be a key technology in its manufacturing processes. “We have achieved a very high standard in milling over many years. Nevertheless, we always strive for continuous optimisations”, emphasises Volker Kreutner (Head of the CAM, HSC milling and electrode production department) and refers to graphite machining, which is done on three 5-axis Exeron HSC-MP7s that are capable of 24/7 unmanned operation. “Hence, process reliability is a major aspect for us. Our tools are mainly used to produce small and very precise parts, which are often within the visible range.” The requirements for accuracy, surface quality, near-contour cooling and demouldability are demanding. Thus, the tolerance requirements for the machining of electrodes in Bahlingen are within the range of 0.005 mm.
Braunform has been using Moldino products for hard machining for many years. The results with the tools from the Japanese manufacturer have been consistently positive, so using them for graphite milling seemed to be the obvious choice. Based on reference electrodes, the current situation was analysed and alternative production strategies were identified. The test objects included an electrode for the cover of a razor with a tolerance requirement of +/- 0.005 mm. Following a few days of evaluating the existing processes to calculate the optimisation potential, a time saving of approx. twenty percent was determined.
Looking at the electrode for the razor cover, the roughing process now starts with smaller tools. For this step a two-flute ball end mill with 2 mm from the D-EPDB series was implemented. Although roughing took a little longer, less residual material remained, which saved a lot of time during the residual material machining. Roughing is now performed with an increased stepover and stepdown (ae and ap) compared to the past. The residual material was removed from the corners with the smallest possible tool diameter (0.4 mm). This strategy also had the advantage of being able to get much closer to the final contour within the roughing allowances. For finishing, first a ball end mill with 0.4 mm diameter and 4 mm length was used, followed by another one with 0.3 mm diameter. In addition, larger feeds were applied, which were mainly achieved by higher stepover (ae). Since many segments were milled with the same tool, only four ball end mills from Moldino's D-EPDB series in total were needed for the contours of the complete electrode.
The result met all expectations. Thus, it was possible to significantly reduce machining time for this electrode by using new strategies and the D-EPDB ball end mill. The time savings were mainly achieved by having less residual material to be removed after roughing. There has also been a positive effect on accuracy and surface quality, due to the high level of geometric precision of the tools from Moldino. The characteristics of the ball have a crucial influence on the machining result. If the cutter tolerances exceed a certain threshold, this can be seen in the milling result of the electrode surface. The company from Baden made improvements in this area compared to the previous tools. The cutting edges of the Moldino ball end mills only showed little signs of wear after five hours. The previously used tool became almost unusable after only three hours.
As a result, the entire graphite machining process was transitioned to Moldino, which had a positive effect on the production costs. The profitability calculations based on the previous and new values for each electrode are convincing: “We achieve an annual saving of around 6500 hours with all three machines due to the reduced production times, which equals a free capacity of around eleven months,” concludes Volker Kreutner. “In addition, by switching to Moldino and thanks to the new strategies, we have better surfaces and almost no more rework.”