Improving the high-speed finishing of forming tools for advanced high-strength steels (AHSS)

Forming tools manufacturers have extensively incorporated high-speed milling technology for the finishing of large punch or die tools. The main objective is to achieve a good surface quality directly form machining, without any additional, tedious, manual work. Currently, new advanced high-strength steels (AHSS) are being used for car body parts. In this case, there are two special changes related to the forming tools: a higher proportion of harder surfaces on the working area, and long try-out iterations, due to their great springback. Tempered surfaces and insert blocks harder than 60 HRC are needed to withstand the forming charges with a good working life expectancy. From this, two problems arise with regards to highspeed finishing: first, the deflection of the tool due to the cutting forces, which can produce unacceptable dimensional errors; and second, in the same finishing operation, zones with sharp changes in hardness must be machined with the same CNC program and cutting tool. The key to solving both problems will be the use of newly developed utilities in the preparation stage, and the elaboration of CNC programs using CAM software. In the first case, the deflection of tool is dealt with by a milling model which obtains the values of cutting forces. This model characterises the couple tool/material with six coefficients, which are previously obtained for ball-end milling tools and base die materials. Inputs provided by the CAM user include feed per tooth, and radial and axial depths of cut. The problem of surfaces with areas of different hardness and poorly defined boundaries is solved with a special postprocessor coded in C language. Once the CAM user has defined (on the CAD model) the theoretical boundaries of the tempered areas, the insert blocks or the deposition material areas, this utility includes changes of the programmed feed function in the CNC program. In this paper, these approaches are applied to medium size workpieces with the same features of the actual punch and die for AHSS forming. Results are provided to die manufacturers for application in real forming tools. This technological model of the milling process estimates values of cutting forces and offers manufacturers a reduction of production and lead times.