Residual stress distribution in PVD-coated carbide cutting tools - origin of cohesive damage

PVD-coatings for cutting tools mean a substantial progress for tool lifetime and cutting conditions. Such tools, however, hold the risk of cost intensive sudden process breaks as a result of cohesive damage. This damage mechanism does not consist of a coating adhesion problem, but it can be traced back to the residual stress distribution in coating and substrate. This paper shows how residual stresses develop during the process chain for the manufacturing of PVD-coated carbide cutting tools. By means of different methods for residual stress determination it is shown that the distribution of residual stresses within the tool finally is responsible for the risk of cohesive tool damage.DFG/DE 447-50-

Marker-free identification of turned, ground and deep rolled workpieces using wavelet transformation

This paper presents a marker-free component identification of cylindrical workpieces produced by the manufacturing processes turning, grinding and deep rolling. The position of unique features from a 2-D profile in the 3-D frequency is detected for identification. Therefore, this work presents an approach using an industrial camera for surface measuring to clearly identify individual cylindrical components. In addition, wear tests are carried out to investigate the method's robustness. The results after the wear tests indicate a false positive rate of 10-2

Cutting inserts made of glass and glass ceramics

Against the background of the increasing cost and scarcity of raw materials that are required for the manufacture of cutting tools, the question of alternative cutting materials arises. Glasses and glass ceramics represent a possibility for this, the use of which has hardly been considered so far. This thesis is devoted to the question of whether cutting tools can be made from glass and glass ceramic materials at all. In addition, the question of how such tools can be used for which purposes is dealt with. First results on both questions are presented. The grinding of indexable inserts from the materials examined was possible without breaking corners and edges. Plastics can be easily machined with the tools produced. When machining aluminum, however, the tools made of glass fail completely, while those made of glass–ceramic show good results here too. These first results are intended to pave the way for further research in this area

FEM-based simulation of continuous wear of CrAlN-coated tools

The understanding of the correlation between the coating-specific properties of PVD-coated cutting tools, the thermomechanical loads on the cutting wedge and the resulting tool wear, is necessary to avoid costly iterative test series. To obtain this knowledge a hybrid approach based on experimental tests and FEM-based chip formation is used in this study. In this respect, in a first step, a suitable wear rate model is derived and parameterized on the basis of wear analogy tests and experimental machining investigations. This wear rate model is then coupled with the FEM-based chip formation simulation to predict continuous tool wear

Investigation of the material separation behaviour of rocks using scratch tests for the design of tool grinding processes

The use of natural rocks as cutting tool material poses an environmentally friendly alternative to conventional cutting tool materials. So far, however, the basics of tool grinding processes for rock tools have not been systematically investigated. This study, therefore, presents an investigation of the material removal mechanisms of four different types of rocks and a mono mineral via scratch tests analogous to a face plunge grinding process used in tool grinding. The aim is to contribute to a knowledge-based design of tool grinding processes for rock tools. This also includes a characterization of their mechanical properties. The occurring material removal mechanisms identified by SEM-images as well as width and depth of the scratches are used to evaluate the influence of single grain chip thickness and cutting speed on material removal mechanisms. The results show that ductile material removal is possible for all rocks in certain areas of single grain chip thicknesses ranging from 0.28 µm to 3.75 µm depending on the rock used and the applied cutting speed. Besides this, the results show optima for ductile material removal at single grain chip thicknesses that are up to 87-times higher than predicted by an analytical model. Additionally, recommendations for the design of the tool grinding process of the investigated rocks based on the obtained results are presented

Elementary studies on the inducement and relaxation of residual stress

In order to qualify residual stress relaxation as an indicator of mechanical overloading of machined parts, an individually designed residual stress profile has to be allocated. Even though numerous investigations have been carried out in the past, residual stress profiles cannot be predicted to a satisfactory degree. For this reason, essential studies on the reproducibility of residual stress profiles for several external cylindrical turning parameters are conducted and it is demonstrated that identical residual stress profiles can be induced successfully. Subsequently, specimens with defined residual stress profiles are loaded in bending tests with various numbers of test cycles. The amount of residual stress relaxation in the specimen's surface layer is measured to determine the influence of theapplied load on the stress relaxation. By applying single tensile and compressive loads below and above thematerial's yield and ultimate strength, the stress relaxation can be evaluated in detail.DFG/CRC/SFB/65

Estimation of load history by residual stress relaxation

Focusing on the impact of machining on structural integrity and fatigue life of components the surface and subsurface properties are of major importance. It is well known that machining induced residual stresses have a significant influence on the fatigue life of a component. Due to thermal and mechanical loads during a product's life cycle these stresses relax, which is undesired in most cases. The presented approach utilizes relaxations due to mechanical load to estimate the load history of a component. It is intended to qualify residual stress relaxation as a load sensor and to determine the limits of this approach. Therefore, it is demonstrated, how the residual stress state induced by turning of AISI 1060 determines the critical load causing relaxation. Subsequently, the influence of load stress and the number of load cycles is used to build up a model. The presented approach accesses load information from mass production components. Until now, this information is typically limited to prototypical developments or high price parts equipped with external sensors. One application of life cycle data is condition-based maintenance. This technology allows to extend service intervals and prevent a premature replacement of undamaged components. Thus, cost and resource efficiency are augmented. It is demonstrated that based on the changes of residual stress, possible mechanical loads and number of load cycle combinations can be identified. The changes are used to estimate the experienced loads.DFG/CRC/65

Design of tool grinding processes for indexable inserts made of rocks

Using natural rocks as alternative cutting tool material poses a possibility to meet actual environmental, economic and geopolitical challenges. The present state of knowledge, however, is not sufficient to allow a knowledge-based design of the tool grinding process of cutting tools made of rock. For this reason, this study presents an investigation of the significance of the grinding process parameters and grinding tool specifications for the flank face and cutting edge roughness as well as for the cutting edge microgeometry besides an analysis of the scatter of the grinding results in tool grinding of rock inserts. Thus, the study contributes to a knowledge-based design of tool grinding processes of rock tools. In this context, confocal and focus variation microscopes are used besides SEM images to investigate the above mentioned factors in the plunge face grinding of rock inserts from five different rocks. The results identify the axial feed velocity of the plunge face grinding process as a highly significant influence factor for cutting edge roughness and microgeometry, while cutting speed only shows a significant influence on cutting edge microgeometry. Besides that, highly significant influences of the used rock type and the abrasive grain size are identified for all three mentioned factors. Grinding result analyses show a scatter between 0.04 and 25.00 µm depending on the parameter and rock investigated. Additionally, recommendations for the design of the tool grinding process of rock tools are presented deduced from the obtained results

Process-related characteristic–based topography evaluation of wear conditions on grinding wheels

Non-productive auxiliary processes affect the single part and small badge production of milling tools. The key production process grinding is inevitably linked to the auxiliary conditioning process. The time demand of those process steps decreases the overall productivity of the manufacturing process. However, today the machine operator decides on conditioning cycles individually by the use of experience. Until today, there is no objective data based approach available that supports the initiation of these conditioning processes or the adaption of the grinding process itself in order to improve its process efficiency. For this purpose, a process-related topography evaluation method of the grinding wheel surface is developed within this study. For the measurement, an optical method based on laser triangulation is used. The measurement system is implemented into a common tool grinding machine tool. In addition, characteristic topography values are defined that show the wear conditions of the grinding tool. Moreover, the data is summarized in a database of wear conditions. The developed measurement method can save grinding and dressing tool resources, process times and minimizes scrap parts. In addition, an adaptation of the process and a targeted launch of auxiliary processes can be enabled. The novel characteristic-based topography measurement creates the opportunity to enhance the tool life of the grinding wheels up to 30% without losing productivity

Impact of Hard Machining on Zirconia Based Ceramics for Dental Applications

Since the late 90s the use of zirconia based all-ceramic restorations increases. Many manufacturing steps are necessary, like pre-sintering, soft machining (pre-sintered condition), sintering and hard machining (fully sintered) in combination with a final staining or veneering step. All these techniques, especially hard machining, are associated with the production of flaws in different scales, in conjunction with thermal and residual stresses and phase transformations. These are inter alia capable to induce failure. This work investigates the impact of hard machining on the material properties and attempts to establish a correlation to failure. © 2016 Published by Elsevier B.V