Research projects of Metal Cutting
In the case of deep boreholes with high demands on machining quality, the requirements for high accuracy and surface quality are increasingly being joined by those relating to the edge zone of the borehole wall, i.e. maintaining a defined residual stress condition or guaranteeing specified structural properties. In order to achieve the requested results reliably, machining processes such as autofrettage or roller burnishing and deep rolling are used after the borehole generation. It would be advantageous to set defined bore properties already during the course of the machining of the borehole in such a way that, ideally, subsequent machining operations can be dispensed with.
The main goal of the research project carried out within the priority program SPP 2086 "Surface Conditioning in Machining Processes" funded by the DFG is therefore to measure influencing variables in the course of the drilling process in order to be able to change the process conditions by means of a controller and thus to influence the properties of the bore edge zone (e.g. residual stress, hardness and roughness) in a defined manner
A partial goal of the research project concerns the development and supply of a sensor-integrated single-lip deep-hole drill, with which an in-process acquisition of the thermo-mechanical condition in the chip formation zone and the occurring tool vibrations is possible. Within the scope of these developments, a combined numerical-analytical model will be created that shows the correlation between the measured variables and the actual machining temperature in the chip formation zone.
A further sub-goal concerns the development of a machining model and a material model with which the correlation between the thermo-mechanical load condition in the chip formation zone and the properties of the bore edge zone can be mapped and predicted without time constraints. A further analytical, in-process-capable model, also to be created within the project, is the basis of the actual process controller. This model compares the output variables of the cutting and material model with the measured data of the sensor-integrated tool and derives suitable control variables from the evaluation of the deviations. The controller can be based on this model, which communicates via a suitable interface with the control of the deep-hole drilling machine and provides it with specifications for cutting parameters such as cutting speed or feed rate in order to be able to adjust the surface properties of the borehole in a targeted manner.
In the Ad-Proc-Add project, the additive-subtractive manufacturing chains (ASM) are investigated. The aim is to find out the detailed dependences and interactions of material and component properties of additively manufactured and reworked workpieces with regard to process parameters, manufacturing strategies and boundary conditions. A visionary component here is to adapt the workpiece properties with regard to geometry, surface and substrate along the ASM process chains in such a way that predefined requirements can be met. This enables the specific design and implementation of additive-subtractive production chains in various industrial applications. To this end, an international network of research associations and institutes has been set up, in which the necessary expertise in the areas of additive manufacturing, machining processes, process monitoring and control, machines and systems, simulation and optimization as well as energy efficiency analysis is bundled.
Extensive experimental studies into the correlations between parameter settings and influences on workpiece properties in additive and subtractive process steps, correlation analyses, the detailed investigation of physical effects as well as methods for process monitoring and quality control are implemented in a networked research approach that makes optimum use of synergies and capacities of the partners involved.
From the findings of this project, new tool concepts, strategies for 3D printing of metal parts, methods for a high-quality design of reworking processes, system prototypes for monitoring devices and fixtures, and new services are derived. In addition, the knowledge generated will enable parts manufacturers as well as system and service providers to develop new products with enhanced functionality. At the same time, it enables end users to implement ASM process chains with higher productivity and economic as well as ecological efficiency.
The project results will lead to a digital compendium based on a new performance measurement system that can be used by design and production engineers as well as service staff. The project is accompanied by a user committee from all relevant market segments: toolmaking, 3D printing systems, machine tool manufacturers, software and service providers and end users. Intensive communication with the industrial partners ensures the development of industrially relevant and usable results.
This project dealt with the potential for substituting ceramic cutting tool materials for cemented carbide ones in the production of sawing tools.
The goal of the project was to optimize the production of ceramic cutting tool materials using the example of sawing tools, in order to be able to provide economical alternatives to cemented carbide cutting tool materials for the mass market. The scientific objective was to understand the efficiency of the cutting edge in interrupted cutting and to design joining and grinding processes of ceramic saw teeth. The findings gained were applied to concrete cases (milling tools, circular and band sawing tools).
The essential results of the project were that the achievable tool life in the most important machining processes with interrupted cutting (e.g. sawing) depends on the changing mechanical and thermal loads as well as on the geometry and the wear behaviour of the tool. For that reason, a model process based on the classic recessing operation was elaborated, with which it was possible to model the conditions of circular sawing and compare cutting tool materials. The workpiece bodies were designed so that the loading of a tooth in the sawing process could be modelled. Figure 1 shows the set-up of the model test in the lathe (Gildemeister CTX 420 linear) used as well as an exemplary workpiece specimen made of S355JR structural steel.
In the project, experiments were not only carried out with S355 structural steel but also with GG25 and GGG60 grey cast iron and EN-AW-7075 aluminium. Ceramic recessing inserts made of oxide (SN 60), non-oxide (Beta-Si3N4) and mixed ceramics (Alpha/Beta-SiAlON) were used for the qualification in interrupted cutting. Recessing inserts of MG 12 cemented carbide with the same recessing width were examined as reference tools.
It could be proved that the developed model test is suitable for a quick and cost-effective qualification of ceramic cutting tool materials as well as an optimization (design) of the cutting edge geometry in interrupted cutting. The results gained showed that the non-oxide and mixed ceramics have a great potential for the machining of GG25 cast iron in interrupted cutting. Hence, these two ceramic cutting tool materials could be used for substituting cemented carbide in this concrete application.
The project was carried out in cooperation with Fraunhofer IPA and the KFS of Furtwangen University as well as several industrial enterprises.
Funded by: Baden-Württemberg Ministry of Finance
Development of an adaptive control (Adapt-i-Cut) – Subproject: „Development of the theoretical basis for predicting the resultant forces to optimize a CAM path planning“
For the process planner, it is extremely informative to know the resultant forces and the load curves, for example, in order to detect critical areas along the tool path. New analytical cutting models make it possible to quickly calculate the resultant forces along a complete NC tool path.
Modern CAM systems are capable of creating not only a classic, predefined path layout but also a tool path on the basis of geometrical engagement conditions that are as constant as possible. Despite of this approach, it is not always possible to avoid substantial alterations in the kinetic cutting characteristics of individual path segments as well as idle travel. There may be peak resultant forces of up to 70% in relation to the nominal values. This leads to an increased tool wear and risk of tool breakage, a reduced accuracy and quality of the machined component as well as to unstable processes in unfavourable circumstances.
A possible improvement may be to change from geometrically to kinetically constant engagement conditions between tool and workpiece. However, a guarantee of kinetically constant engagement conditions requires to determine the resultant force for each path point in advance.
For that purpose, a novel, completely analytical cutting model for milling processes was developed and implemented in this project. The crucial advantage here is an essentially more general modelling, allowing a valid prediction of resultant forces at any time even in the case of greatly varying engagement conditions and dynamically variable technology parameters – without time-consuming series of tests.
In a further integration step, this makes it possible to additionally compensate the deflection and compliance in the form of a location-adapted derivative path action already during the NC programming in the CAM system. In this way, it is possible to considerably improve the net shape and accuracy, especially when using long, slim tools.
Funded by: Central Innovation Programme for small and medium-sized enterprises - ZIM/AiF
In this research project, the dynamic and static characteristics of circular sawing tools were examined and simulation models were made to reproduce these characteristics. The investigations were focused here on the influence of the laser ornaments made into circular saw blades and their fillings as well as on the flange joint It was analysed here how the decisive parameters, such as the surface quality in the joints and the contact pressure, influence the characteristics of the system. In model tests, the individual effects were examined in an isolated way. Then the complexity of the tests and simulations were being gradually increased in the course of the research project, in order to be able to model the interactions of the influencing factors. The collected results were applied to the entire rotating model including clamping and then validated by experiment, in order to finally carry out a sensitivity analysis of the influencing parameters. The results gained in the analysis showed that neither the microslip in the flange interface nor the characteristics of the filling had a considerable influence on the system behaviour in the unloaded system. By means of the shape and the arrangement of the slots as well as the prestress applied by the flange, it was, however, possible to clearly influence the system’s static and dynamic characteristics.
The project was carried out in cooperation with the Institute for Nonlinear Mechanics (inm) at the University of Stuttgart.
Funded by: DFG
The goal of this research project was to better understand the abrasive process in the ultrasonically assisted machining of stone. Influencing factors and working mechanisms that are relevant to the process were investigated here as they have not been sufficiently described yet. It was possible to gain fundamental insights into the process flow by means of linear scratch tests with a single diamond grain fastened to an ultrasonic transducer. The contact process between the diamond grain vibrating at high frequency and the stone specimen moved perpendicular to it in a linear motion was visualized by means of a high-speed camera. The evaluation of the video recordings and the force progressions showed that microhammering and a three-phase contact process (impact contact, penetration and separation) arose in ultrasonic processes. In addition, it could be proved that there was a correlation between the dominating chip forming mechanism and the cutting speed in vibration superposition. At low cutting speeds, chips were formed due to the fact that the diamond flank face hit the specimen surface (microhammering and secondary chip formation). In contrast to that, chips were mainly formed in front of the cutting edge (primary chip formation) at high cutting speeds. Moreover, two mechanisms that were acting in exactly the opposite way and influencing the process force were detected in ultrasonically assisted scratch tests. On the one hand, an increase in the vibration amplitude in the contact zone led to a decrease in the resultant force due to the growing velocity of vibration and the resulting reduction in friction. On the other hand, a higher vibration amplitude affected the effective depth of cut so that the chip cross-sectional area and the resultant force increased.
The above-mentioned process was described with models verified by experiment. For that purpose, the process and the system were divided into the following subsystems: tool, material removal, process forces, kinematics and ultrasound system. Afterwards, these subsystems were modelled separately and then put together to an iterative total model. The calculated process load was imposed on the ultrasound system, and the resulting vibration behaviour of the system was established. Each iteration step was carried out with the recalculated kinematic boundary conditions. To couple the submodels, a generic framework was developed with which all sorts of and any number of submodels could be connected. The created total model presented a useable tool for designing the system components.
By means of model-based parameter studies, the bearing of the ultrasonic transducer was found to be essential for the vibration behaviour under load. A correspondingly optimized transducer-tool combination was established with the total model as well as an optimization algorithm and then built. Drilling and milling tests were carried out with both vibration structures. The optimized system showed a greater reduction in process forces (68-90 %) than the initial system (41-80 %). In addition, the optimized vibration structure proved to be more robust towards changes of the process parameters. The optimized system also showed an improved behaviour with regard to electric power.
Funded by: DFG