Distortion Engineering - Identification of Causes for Dimensional and Form Deviations of Bearing Rings

Distortion of power transmission parts after heat treatment is influenced by each step of the process chain, ranging from material production to hard-machining. However, an experimental analysis for each production step in the manufacture of bearing rings has shown that distortion is significantly influenced by turning before heat treatment. Elastic ring deformation caused by fixing the workpiece in the chuck leads to a variation of the depth of cut and polygonal form deviations occur after machining. In addition the cutting process induces locally varying residual stresses which also contribute to dimensional and form deviations of machined rings. In this paper results from experimental investigations of the principle physical causes for the distortion of bearing rings are presented and discussed. Finally, strategies for the minimization of distortion are derived.     

Advances in Modeling and Simulation of Grinding Processes

In the last decade the relevance of modeling and simulation of grinding processes has significantly risen which is caused by industrial needs and is indicated by the number of publications and research activities in this area. This keynote paper results from a collaborative work within the STC G and gives an overview of the current state of the art in modeling and simulation of grinding processes: Physical process models (analytical and numerical models) and empirical process models (regression analysis, artificial neural net models) as well as heuristic process models (rule based models) are taken into account, and outlined with respect to their achievements in this paper. The models are characterized by the process parameters such as grinding force, grinding temperature, etc. as well as work results including surface topography and surface integrity. Furthermore, the capabilities and the limitations of the presented model types and simulation approaches will be exemplified.     

Assessment of Grinding Fluid Effectiveness in Continuous-Dress Creep Feed Grinding

Creep feed grinding is a high-productivity abrasive removal process that is often limited by thermal damage and high wheel wear. A review of current industrial practices in the area of fluid supply optimisation in grinding shows that very little knowledge of the pressure, flowrate and method of application exists in industry. This paper presents an experimental procedure to evaluate fluid supply conditions in grinding on a continuous-dress creep feed grinder. Using tapered workpieces, the authors have evaluated the influence of wheel speed and material removal rate on grinding fluid effectiveness, based on the material removal rate at the position of the wheel along the ramp when burn starts to occur and the corresponding spindle power surge. Correlations are investigated between visible discoloration, metallurgical examinations and change in spindle power, in order to establish the onset of grinding burn. This procedure serves to determine the upper limit of material removal rate or - respectively - the lower limit of fluid flow rate for given grinding systems consisting of specified wheel type, material type, fluid type and fluid supply nozzle. The advantage of the presented method is its easy and time saving application in industry, but it is also of help to researchers who need to optimise fluid supply conditions prior to their grinding tests.     

纵向旋转切削加工对环形零件畸变的影响

通过旋转试验和有限元分析介绍了工件在切削加工过程中产生的畸变情况，分析了工件的装夹方式、切削速度、切削深度和进刀量对100Cr6钢环圆度的影响。通过去应力退火释放冷加工诱发的残余应力后工件的圆度与切削参数有关。另外测试了被试验环的表面残余应力，其表面残余应力与装夹方式有关。将测量的装夹力作为计算参数输入，通过有限元分析方法测试了装夹方式对工件变形的影响。协同测量结果示出了装夹方式影响工件变形的一个主要因素，表面残余应力与工件的径向变形有关，最大的拉伸应力位于夹口位置。旋转切削试验结果表明，提高切削速度圆度会稍有增加；随着切削深度的加大，圆度呈下降趋势，尽管切削力增加了；进给量的增加会导致更高的切削力，因此圆度值也增加；常规的去应力退火可使被加工环的圆度值增加。     

Consideration of Core Segregations on the Formability of Bearing Steel

In this paper the influence of core segregations on the formability of workpieces made of bearing steel is analysed and discussed on the basis of compression tests. First the necessary material properties and flow curves, taken at different regions of the bar stock material, were investigated. The core segregation material shows significantly higher flow stress at higher strain rates than the surrounding segregation‐free material. Further a strong influence of the forming temperature on the flow stress was found. On basis of these findings an FEM‐model was developed, that considers the core segregation properties, its shape and position. The results of the compression simulation with this model show a clear impact of the inhomogeneous material properties on the stress distribution.     

Application potential of coarse-grained diamond grinding wheels for precision grinding of optical materials

Fine-grained resin bonded diamond tools are often used for ultra-precision machining of brittle materials to achieve optical surfaces. A well-known drawback is the high tool wear. Therefore, grinding processes need to be developed exhibiting less wear and higher profitability. Consequently, the presented work focuses on conditioning a mono-layered, coarse-grained diamond grinding wheel with a spherical profile and an average grain size of 301 µm by combining a thermo-chemical and a mechanical-abrasive dressing technique. This processing leads to a run-out error of the grinding wheel in a low-micrometer range. Additionally, the thermo-chemical dressing leads to flattened grains, which supports the generation of hydrostatic pressure in the cutting zone and enables ductile-mode grinding of hard and brittle materials. After dressing, the application characteristics of coarse-grained diamond grinding wheels were examined by grinding optical glasses, fused silica and glass–ceramics in two different kinematics, plunge-cut surface grinding and cross grinding. For plunge-cut surface grinding, a critical depth of cut and surface roughness were determined and for cross-grinding experiments the subsurface damage was analyzed additionally. Finally, the identified parameters for ductile-machining with coarse-grained diamond grinding wheels were used for grinding a surface of 2000 mm² in glass–ceramics.     

Manufacturing of advanced smart tooling for metal forming

Forming is widely used due to its high efficiency in material utilization and its high production rate in general. Most forming processes control the geometry of final products through a set of tooling. The increasing demands on lightweight products have challenged the performances and functionalities of tooling. This paper provides a systematic review of recent advancements related to tooling performance and functionalities, including tool materials, tooling fabrication processes (e.g., machining, heat treatment, coating, surface texturing, and additive manufacturing), sensing, and data analytics. Finally, recommendations on future research directions for metal forming tooling are provided.     

Superfinishing and grind-strengthening with elastic bonding system

This paper presents with a new grinding strategy for a surface superfinishing and a grind-strengthening of the surface layer of steels in a single grinding step. To grind-strengthen a surface layer both abrasive material removal and a plastic deformation are needed. This combination together with an appropriate grinding wheel design offers the possibility to ensure high precision surfaces with a consistent quality finish that meets the highest standards. This kind of grinding process could be applied to bearings, for example. During the run-in period, the initial contact surfaces change until they stabilize to a running-in condition and have a new surface topography. Grinding with the new process strategy leads to benefits because a grind-strengthened surface layer eliminates initial wear and abrasion within the system, reduces surface finish roughness, extends fatigue strength, and reduces risk of part failure during operation.     

Potentials of precision machining processes for the manufacture of micro forming molds

Molds for micro forming processes require specific functional surfaces to provide optimized tribological condition for forming processes. Such topographies with microscopic dimensions can for instance be generated by lithographical methods. However, a main disadvantage of these processes is their low flexibility, e.g. changing the structures to be generated causes extensive adaptations in the manufacturing process. Therefore, mechanical precision machining processes like micro milling, micro grinding and micro polishing are appropriate processes for the manufacture of micro molds of manifold shapes and with well suited micro topographies.     

Modelling and optimization of grinding processes

The paper describes different methods for modelling and optimization of grinding processes. First the process and product quality characterizing quantities have to be measured. Afterwards different model types, e.g. physical–empirical basic grinding models as well as empirical process models based on neural networks, fuzzy set theory and standard multiple regression methods, are discussed for an off-line process conceptualization and optimization using a genetic algorithm. The assessment of grinding process results, which build the individuals in the genetic algorithm's population, is carried out using a target tree method. The methods presented are integrated into an existing grinding information system, which is part of a three control loop system for quality assurance.