Abstract
Presented are an experimental setup and affiliated methodology to measure the specific cutting forces in the milling process with proceeding tool wear at simplified orthogonal milling kinematics. The cutting forces, cutter rotation angle and chip temperature are acquired by a time sensitive measuring system consisting of a synchronized dynamometer, ratio pyrometer and spindle encoder. The approach allows the accurate acquisition of cutting forces under defined engagement conditions and thus constitutes a valuable basis for cutting force modelling and tool wear monitoring approaches. The results show uniformly and linearly increasing forces over the entire range of undeformed chip thickness due to wear. Besides a mechanical view on the cutting process, also the thermal domain was included into the analysis. Therefore, a ratio pyrometer was used as part of the synchronized measurement system tracking the chips backside temperature in order to estimate a virtually continuous heat flow into the chip. This heat flow increased with wear and process power, which indicates that the chip’s temperature can be used as process monitoring variable for tool wear.
Highlights
Introduction and state of the artIn cutting technology the force is a key indicator of the process capability in order to reach quality related as well as economic manufacturing targets
The energy required and transformed in the cutting process is in particular relevant for the manufacturing of safety critical workpieces made of super alloys such as Inconel 718 or other difficult-to-cut materials
It should be noted that the process power increases from 48 W at lc = 114 mm to 94 W at lc = 144 mm, whereas the estimated heat flow into the chips only increases from 32 to 37 W. This indicates an increasing heat partition into the tool and probably in particular into the workpiece, since the principal heat partitioning takes place between chips and workpiece. These results indicate the usefulness of the ratio pyrometer as an indirect measurement system for tool wear in machining under dry conditions, and with regard to wear related increase in heat flows and temperatures with regard to surface integrity considerations
Summary
In cutting technology the force is a key indicator of the process capability in order to reach quality related as well as economic manufacturing targets. Schwenzer et al [11] conducted a comparative study on optimization algorithms for online identification of an instantaneous Kienzle force model in milling They aimed for decreasing the experimental effort to calibrate mechanistic force models and at the same time provided an online calibration to consider uncertainties inherently (such as the tool wear state or material characteristics) for an accurate force control in milling. With regard to the mentioned potentials on force based tool wear monitoring and the huge advances in online parameter identification a methodology is desirable in order to effectively determine wear-dependent force coefficients under known engagement conditions. The state-of-the-art method is the orthogonal cutting test providing definite engagement conditions [17] It lacks from effectiveness since multiple cuts are required in order to obtain a representative tool wear state [8]. This setup enables the analysis of forces and the monitoring of transient temperature fields and heat partitions in the workpiece, the tool and the chip
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