Residual stresses in milled β-annealed Ti6Al4V

Residual stresses can cause part distortion especially in the case of large components such as structural parts in aerospace industry. Therefore, this paper investigates the machining induced residual stresses for milling of a workpiece material with increasing usage in industry, the β-annealed titanium alloy Ti6Al4V. This thermal treatment results in a large grained material structure. For this reason X-ray diffraction, the standard residual stress measurement method, cannot be used for stress determination. In this paper an adopted indirect measurement method, the layer removal method is discussed. With respect to the material removal, two different methods are investigated, electrochemical material removal and laser ablation. Finally, the influence of the tool wear on the residual stress state after face milling is analyzed.Lower Saxony Ministry for Economics, Labour and TransportPremium Aerotec GmbH, Vare

Tools and Strategies for Grinding of Riblets on Freeformed Compressor Blades

A major goal in the design of turbomachinery is the increase of efficiency. To attain this increase, the flow losses must be reduced. A substantial proportion of the losses is generated by skin friction between compressor blades and working fluid. With respect to smooth surfaces, micropatterns (riblet-structures) reduce skin friction in turbulent flow by up to 10%. Grinding with multiprofiled wheels is an effective method for the manufacturing of riblet-structures on large plane surfaces. However, the grinding wheel wear affects the accuracy of the riblet geometry and the efficiency of the manufacturing process. Therefore, this paper shows the potential of different grinding wheel types for the manufacturing of riblet structures on an industrial scale with regard to tool wear. The results show that vitrified bonded tools are not suitable for the structuring of compressor blades. Here, axial forces lead to high profile wear. In contrast, grinding wheels with a metal bond are more wear resistant. However, the dressing process of metal bonded tools is time-consuming and causes 80% of the total machining time. As a consequence, just one blade can be structured per day. To increase the efficiency, a new grinding wheel was developed, which is bionically inspired by beaver teeth. The tool is constructed of alternating layers consisting of metal bonded diamonds and pure resin respectively. With this layer-by-layer setup, the tool does not have to be dressed and enables structuring of up to 50 compressor blades per day.BMBF/03V047

A New Tool Concept for Milling Automotive Components

Due to the rising number of car variants, the production systems in automobile industry are driven by a strong demand for flexible production processes. In the production of thin-walled workpieces, forming and cutting processes stand in concurrence to each other. In many applications, cutting processes facilitate higher flexibility regarding possible workpiece geometries. However, the required productivity is demanding. In this paper, a multi-sectional milling tool is developed to reach the required cutting performance by minimizing secondary processing times. Tool geometry is optimized with statistical methods to enable a target oriented tool development and reduce iterative development steps in milling tool design processes.BMBF/02PN218

A novel tool concept for roughing and finishing operations

In this paper, a new tool concept suitable for simultaneous roughing and finishing operations is presented. The developed end mill has two radial recessed roughing teeth with a flank face chamfer and two sharp finishing teeth. While the chamfered cutting edges ensure a high process stability due to process damping, the sharp cutting edges generate the final surface. In order to investigate the roughing and finishing capabilities, the tool was compared with a roughing tool with chamfered teeth only and a finishing tool with sharp teeth only. With all three tool concepts milling experiments were carried out, in which forces, process stability and surface quality were analyzed. The generated surface quality for the new tool concept could be significantly improved compared to the roughing tool. However, the aimed surface quality of the finishing tool with sharp edges could not be achieved. Moreover, experimental results show that the process stability of the new tool concept is significantly higher than the process stability of the finishing tool

Material removal and chip formation mechanisms of UHC-steel during grinding

The grinding process is still an important manufacturing process for the machining of automotive components. For power train components, ultra-high carbon steel (UHC-steel) is a promising new innovative alloy because of its low specific density. Results from turning of UHC-steel showed that the texture of UHC-steel significantly differs from conventional steels. Furthermore, extremely hard carbides, which are embedded into a soft ferrite matrix, result in a UHC-steel specific machining behavior and a high tool wear rate. Therefore, UHC-steel is marked as a difficult-to-cut material. So far, there are no research results available for the grinding of UHC-steel. Therefore, fundamental investigations were conducted in order to analyze the material removal and chip formation mechanisms. Scratching tests with a geometrically defined cubic boron nitride cutting edge showed ductile material removal mechanisms for a single grain chip thickness variation from hcu = 1.5 up to 14 μm. Analysis of the contact zone by means of an innovative quick stop device confirms these results. The final publication is available at Springer via http://dx.doi.org/10.1007/s00170-017-0270-9BMBF/02PN205

Chip formation in machining metal bonded grinding layers

Gears demand increasingly high quality regarding acoustic emissions, surface roughness and lifetime. Therefore, grinding is often the last step in the process chain of gear manufacturing. Grinding wheel grain sizes of 30 micrometers lead to high surface quality and metal bonded CBN-grains allow a high wear resistance and profile stability of the grinding tool. Consequently, an increase of the material removal rates and thus productivity is possible without increasing the thermal load on the workpiece due to the grinding wheels' high thermal conductivity. However, the time and cost intensive dressing process in combination with the high profile requirements for gear grinding prevent the wide application of metal bonded tools for this application. This challenge can be solved using a new dressing approach with geometrically defined cutting edges. Metal bonded CBN-grinding layers have a structure similar to metal-matrix-composites, which can be machined by using the turning operation. The aim of this work is to verify the machinability of metal bonded CBN-grinding layers. In the present work, the chip formation for metal bonded grinding layers is presented

Machining of Micro Dimples for Friction Reduction in Cylinder Liners

In combustion engines a large percentage of mechanical losses result from friction, for example within the cylinder liner. In order to reduce the friction between cylinder liner and piston, micro dimples in the surface decrease the internal friction of combustion engines. These dimples hold back lubricant from being pressed out of the tribological contact zone which enhances the friction behaviour. Competing processes such as laser material removal allow micro dimples to be generated at high productivity. Therefore the process time of machining micro dimples needs to be reduced in order to maintain low production costs. For this purpose a rotating single-toothed tool is used in an axially parallel turn-milling process. The aim of this paper is to show the investigation of productive machining and the effect of the micro dimples in heavy duty cylinder liners. The cutting behaviour is investigated by analyzing micro dimples cut at high speed concerning their burr formation and geometrical deviations

Residual stresses in grinding of forming tools with toric grinding pins

The subsurface residual stress state of forming tools is an important factor for the lifetime of these tools. This is especially important for tools used in processes like sheet-bulk metal forming, where very high process loads occur in the tools during the forming operation. Grinding as one of the last process steps for manufacturing of these tools significantly affects the subsurface residual stress state. For five-axis grinding, toric tools are advantageous, because constant contact conditions are realized even on complex free form surfaces. Previous work identified the major process and tool parameters for influencing the residual stress state due to grinding with toric grinding pins. This paper investigates the quantitative correlations between the main parametersfeed rate and cutting grain size and the resulting residual stresses in a full factorial experimental design for the lateral grinding strategy. An empirical model is determined from the results of the experiments, which allows to predict these residual stresses for toric pin grinding. Additional grinding force measurements and cutting simulations are conducted to gain additional insight in the generation of residual stresses through grinding with toric pins

Prediction of the Principal Stress Direction for 5-axis Ball End Milling

While regenerating damaged components, e.g. compressor blades, the removal of excess weld material called re-contouring often determines the surface integrity including the residual stress state. A load-specific residual stress state is beneficial for lifetime. This leads to the necessity to predict the resulting residual stress state after machining. The paper describes two models, which predict the principal stress direction as a residual stress characteristic for 5-axis ball nose end milling of Ti-6Al-4 V. One model uses process force components, the other is based on the microtopography of the workpiece, which is influenced by the kinematics of the process.DFG/Collaborative Research Centre/87

Material Removal Mechanisms in Grinding of Mixed Oxide Ceramics

The technological basis for a cost-effective and reliable grinding process of mixed oxide ceramics requires a fundamental understanding of the prevailing grinding mechanisms to maintain surface quality and strength requirements. However, these material removal mechanisms are not yet fully understood. This paper presents an innovative quick stop device for the interruption of cut during grinding. This appropriate method allows a detailed analysis of the interactions of grains along the contact zone. The results reveal correlations between the prevailing grinding mechanisms, the tetragonal to monoclinic phase transformation of the zirconia based ceramics as well as the resulting bending strength. © 2016 The Authors. Published by Elsevier B.V