Workpiece Tolerances Research Articles

Decreasing ramp-up durations of ultraprecision machine tools using reinforcement learning

Ramp-up phases in machining processes are often required to enable the desired workpiece quality. Within conventional CNC processes these phases mainly depend on the corresponding CNC program. But, especially in the field of ultra-precision manufacturing these ramp-up phases require adaptions deep within the control system of the machine tools. The control parameters of the axis servo-drives need to be optimized regarding the corresponding manufacturing process. This process inhibits an iterative nature, which causes multiple rejects because of not sufficient workpiece quality. Naturally this optimization process adds up to the overall manufacturing time. One of the main factors to reduce this time span is the machine operator's experience. But within the currently expanding industry of high-tech products, which require functional workpiece surfaces inhibiting complex geometries, requirements in workpiece tolerances are constantly increasing, resulting in individual and mostly unknown axis control settings for most newly developed workpieces. In a first approach of automating and accelerating the ramp-up phase, an artificial intelligence solution, based on reinforcement learning techniques is introduced. One of the main advantages of using reinforcement learning (RL) based models for this problem is their capability to adapt to feedback from their environment. The installed machine drives can serve as learning environments, but this approach results in extraordinary high training durations. The behavior of the machine axis can be efficiently simulated by applicating techniques from the field of control theory, which results in a drastic reduction of training times while the behavior of the real axis can still be emulated.

Machining Tool and Bearing Failure Analysis Using the Orthogonal Hilbert-Huang Transform on Vibration and Motor Current Datasets


 
 
 Early fault detection is the primary goal of condition-based maintenance (CBM); to identify an impending fault before failure occurs and provide the necessary maintenance in a timely manner. Machining tools, such as drill bits and milling inserts, require a significant amount of maintenance during industrial machining operations to ensure that workpiece tolerances are met. Faulty tools tend to machine outside expected tolerances and run the risk of causing permanent damage to the workpiece, resulting in added production costs and unwanted delays. Similarly, ball bearing maintenance is crucial for keeping a machine in acceptable working condition, and has been the primary focus of many CBM studies due to nonlinear bearing degradation.
 The orthogonal Hilbert-Huang transform (OHHT) is a recent improvement to the relatively new Hilbert-Huang transform (HHT) that is significantly more computationally efficient— compared to other improved HHT algorithms—and reduces the amount of energy leakage caused by non-orthogonal intrinsic mode functions. The HHT and its derivatives are being explored in CBM applications due to its adaptive nature in analyzing nonlinear and non-stationary phenomena, which is characteristic of machining induced vibrations. It has also shown promise in diagnosing machine tool wear from motor current signals compared to commonly used techniques such as wavelet analysis. This paper showcases the OHHT’s potential as a useful diagnostics tool for analyzing machine induced vibration and motor current signals.
 
 

Influence of workpiece tolerances on the fixtures design. Case study

The tolerances of a part influence both the operation of the assembly of which it is part and the technological manufacturing process. High tolerances can lead to an assembly that does not work at the designed parameters, and low tolerances require additional machining operations that increase the cost of the part. The manufacturing series is another factor that influences the technological process. Large series production requires special facilities in terms of processing and control equipment which are needed in the manufacturing process. The paper presents a case study on the influence of tolerances of a part being produced in large series on the construction of the orientation and clamping device used in machining. Starting from the dimensions indicated in the design drawing, a first version of the fixture is proposed in which the parts are fixed and the tolerance of the closing dimension of the assembly of parts is analysed using the Worst Case and Root-Sum-Square methods. Since the closure size tolerance is outside the recommended range, another variant of orientation and fixture is proposed. By re-analysing, the closing dimension tolerance by the two methods mentioned above it was confirmed the necessity of modifying the part width tolerance. The analysis of the tolerance of the closing quota is resumed by the two methods mentioned above and the result confirms that it is necessary to change the tolerance to the width of the piece. The tolerance verification of the closure dimension of the assembly formed by the parts fixed in the device is performed by Monte Carlo simulation.

Edge computing-based virtual measuring machine for process-parallel prediction of workpiece quality in metal cutting

Digital process twins enable in-line quality inspection and root cause analysis, thus sustainably optimizing the production ramp-up and quality control cycle. Current approaches reply on expensive computing units or platforms to perform material removal simulations followed by virtual metrology. This raises deployment costs and conflicts with industrial edge devices. In this paper, a novel analytical approach is proposed to build an edge computing-based digital twin of machining processes for the process-parallel prediction of workpiece tolerances by leveraging machine internal signals and contextualizing metadata sources along the digital process chain, i.e., the digital thread. This concept is validated with the results of a state-of-the-art digital process twin and conventional measurement data from a coordinate measuring machine.

Mapping and compensation of geometric errors of a machine tool at different constant ambient temperatures

Thermo-mechanical effects due to changes in the ambient temperature on the shop floor and internal heat sources caused by the manufacturing process significantly contribute to the geometric deviations of a machine tool and therefore, the geometric deviations of the manufactured workpiece. Minimizing these thermally induced geometric deviations is worthwhile since the requested tolerances of machined workpieces become continually smaller nowadays. To investigate the overall deformations of a machine tool structure due to variations in ambient temperature the geometric errors of a five-axis machine tool at different ambient temperatures by means of a portable climate simulation chamber are systematically mapped. While positioning and squareness errors of the linear axes are significantly influenced by the ambient temperature, straightness as well as rotational errors were less sensitive to temperature effects. For the investigated machine tool errors of the two rotational axes are negligible due to an active cooling of these axes. Through numerical error compensation of the linear axes, the geometric errors of the investigated machine tool can be reduced up to 80%. Finally, an outlook how a temperature-dependent compensation could be derived from previously measured compensation fields at discrete temperatures and afterwards applied on-the-fly during manufacturing is given.

Tool wear modelling using micro tool diameter reduction for micro-end-milling of tool steel H13

Micro components have been demanded increasingly due to the global trend of miniaturization of products and devices. Micro milling is one of the most promising processes for micro-scale production and differs from conventional milling due to the size effect introducing phenomena like the minimum chip thickness, making the prediction of micro milling process hard. Among challenges in micro milling, tool life and tool wear can be highlighted. Understanding tool wear and modelling in micro milling is challenging and essential to maintaining the quality and geometric tolerances of workpieces. This work investigates how to model the diameter reduction of a tool caused by tool wear for micro milling of H13 tool steel. Machining experiments were carried out in order to obtain cutting parameters affecting tool wear by considering the diameter reduction. Dry full slot milling with TiAlN (titanium aluminium nitride)-coated micro tools of diameter d = 400 μm was performed. Three levels of feed per tooth (fz = 2 μm, 4 μm and 5 μm) and two spindle speed levels (n = 30,000 rpm and 46,000 rpm) were used and evaluated over a cutting length of lc = 1182 mm. The results show that lower levels of feed per tooth and spindle speed lead to higher tool wear with a total diameter reduction over 22%. The magnitude of the cutting parameters affecting tool wear was determined by ANOVA (analysis of variance), and the model validation meets the statistical requirements with a coefficient of determination R2 = 83.5% showing the feasibility of the approach to predict tool wear using diameter reduction modelling in micro milling.

Measuring clamping forces during turning via a sensorised steel ring

The geometric tolerances of cylindrical workpieces are highly influenced by clamping forces. This relation is of special importance in slender workpieces such as thin rings. Better tolerances are achieved with lower clamping forces, but the disadvantage is that friction is reduced and the risk of slipping increases. Thus, in order to control the process, a key factor is achieving the lowest possible clamping force while still ensuring safety. Cylindrical parts are usually machined in lathes that have concentric plate chucks that are fixed either mechanically with wrenches or hydraulically by controlling pressure with valves. This paper proposes a method for measuring clamping forces in lathes during the turning process. The method allows the clamping force to be calculated from the torque applied by a dynamometric wrench or from valve controlled hydraulic pressure.

Measuring clamping forces during turning via a sensorised steel ring

The geometric tolerances of cylindrical workpieces are highly influenced by clamping forces. This relation is of special importance in slender workpieces such as thin rings. Better tolerances are achieved with lower clamping forces, but the disadvantage is that friction is reduced and the risk of slipping increases. Thus, in order to control the process, a key factor is achieving the lowest possible clamping force while still ensuring safety. Cylindrical parts are usually machined in lathes that have concentric plate chucks that are fixed either mechanically with wrenches or hydraulically by controlling pressure with valves. This paper proposes a method for measuring clamping forces in lathes during the turning process. The method allows the clamping force to be calculated from the torque applied by a dynamometric wrench or from valve controlled hydraulic pressure.

Quality-predictive CAM Simulation for NC Milling

Milling processes are primarily rated by productivity and resulting product quality. However, while modern CAM and NC simulation systems are capable of predicting productivity indicators (e.g. cycle times), no explicit forecast of the resulting product quality with respect to individual workpiece tolerances is possible, yet. On that account, a first-time-right approach is introduced, which enhances the physics-based modelling behind existing simulations by describing the impact of machine tool, fixture, cutting tool, technology parameters and environmental factors on part quality. As a result, CAM planners are presented with a simulated quality report for the workpiece to be machined.

Novel Research on a Fibration Knowledge-Based Solution for Engineered Surface Tolerances within the Modern GPS Context

Geometrical product specification and verification (GPS) is the only worldwide tolerancing specification language available for unambiguously communicating functional requirements and precisely expressing geometrical tolerances of workpieces on technical drawings. Although the understanding and application of GPS are of vital importance for modern manufacturing industry, the latest GPS standards contain huge amounts of data, information and knowledge that are obscurely linked together, so they are very difficult for designers, engineers and metrologists to understand, remember and comprehensively use them. This is especially true for surface texture that is one of the most complicated geometrical characteristics in GPS domain. To overcome the implementation barrier of GPS surface texture, a novel knowledge-based solution supported formally by a unified knowledge modeling of the complete GPS surface texture standard chain has been researched. This knowledge model can serve as a formal basis for developing more intelligent and reliable tolerance design and measurement systems. This paper starts with an introduction on how to build up a formal knowledge model based on category theory, called categorical fibration model, to unified acquisition and representation of all surface texture knowledge. It then moves on to define a recursive knowledge refinement process for the devised model to enable the recursive multi-level GPS knowledge transformations and refinements. This knowledge model is validated naturally by checking rules identified from category theoretic notions. Finally, this paper presents a general framework for applying the modeled GPS knowledge in practice, from the aspects of the integrated and complete modeling of all surface texture knowledge, detailed knowledge acquisition and modeling examples for four sub-knowledge bases of the proposed software solution, core functional features of this software, and a comprehensive case study for testing its validity and usability. This knowledge-based solution facilitates life cycle collaboration and integration of the complete GPS surface texture chain to reduce product development cost and lead time, and to ensure functional requirements of geometrical workpieces.

Laser beam oscillation for fillet welding

In today’s automotive frame-and-body construction, laser welds are typically carried out as square butt welds in lap joints. These welds are increasingly produced by remote laser welding optics with working distances of 500 mm and more. This enables simple path programming because clamping devices are traversed over and therefore low cycle times with improved productivity is achieved. However, workpiece tolerances lead to part displacements and varying joint positions over time. These displacements have to be acted against by appropriate flange length that contain for positioning deviations. Using this concept, aspired light-weight optimisations, e.g. reducing flange length, are difficult to achieve. By using seam tracking sensors, part tolerances are automatically detected and counteracted for. In addition, joint edge detection allows constructive changes on the type of weld. Fillet welds reduce flange lengths’, improve force flow and open up possibilities for visual quality monitoring. Apart from that, vertical displacements in the form of height tolerances still occur. This needs consideration by adapting the set of welding parameter to the current welding situation. In this respect, one main welding parameter is the lateral beam offset to the upper sheet. Since body-in-white welding applications mainly comprise of zinc-coated steel sheets, special requirements for the welding process are given. Especially zero-gap-welding and welding of joints with gaps larger than 0.2 mm are critical. Using a laser beam oscillation process can stabilise these situations. Approaches to finding parameter sets are presented in this paper.

Continuous variable feed rate: a novel method for improving infeed grinding processes

This paper presents a novel method for improving infeed grinding processes based on the application of continuous variable feed rate (CVFR). Nowadays, infeed cycle configuration is defined by selecting constant feed rates and stock removals for each consecutive stage, normally roughing, finishing, and a final spark-out, for the purpose of achieving required workpiece tolerances in a fixed time while avoiding process limitations and instabilities. CVFR leads to more efficient cycles without the difficulty of defining feed rate and stock removal values. To that end, CVFR has been implemented in a time-domain simulation of infeed grinding processes to analyze theoretically the influence of variation parameters in process forces, workpiece roughness, roundness and size tolerance, or dynamic behavior. Then, tests have been performed to validate the simulation approach, comparing CVFR cycles with equivalent conventional ones. Different types of downward variations for the feed rate have been applied, concluding that grinding processes can be improved with this method regarding productivity and workpiece geometrical and surface tolerances.

Stability of the friction stir welding process in presence of workpiece mating variations

This paper explores common process variations encountered in friction stir welding (FSW) and the limits to which acceptable joint strength is maintained while welding with a robotic FSW system. Part fit-up and mating variations are common in manufacturing, yet the limits to which a friction stir welding process can weld without major process adjustment are unclear. The effects on joint strength and mechanical properties of several of the most common mating variations (i.e., faying surface gap, misalignment, mismatch, etc.) are experimentally determined as individual effects as well as among common welding parameters. Experimental results on 5-mm-thick aluminum alloy 5083-H111 show that ultimate tensile strength, yield strength, and elongation begin to decrease from nominal weld conditions when either the tool offset distance from weld centerline or gap in abutted plates exceeds 25% of the average pin diameter (6 mm). In addition, vertical plate mismatch and lack of penetration can be tolerated up to 2.5% and 10%, respectively, before adverse effects on mechanical properties are observed. The work also indicates that of all the mating variations tested in this study, tool misalignment, followed by travel angle, has the most significant effect on the measured joint strength. Process stability testing has shown that the FSW process is able to endure part fit-up and mating variations within a defined tolerance, giving the practitioner an awareness of how well stock workpiece tolerances must be controlled before joint strength is adversely effected.

Intelligent Design and Metrology for Higher Quality, Accuracy and Improving Production Efficiency

Adequate knowledge in the areas of intelligent metrology and design are important presuppositions to achieve waste free production and low costs of manufacturing and accuracy at the same time within the sophisticated production systems. This is of extreme importance in present time of worldwide international competition in industry and production engineering and at the same time increasingly higher costs of energy and raw material. The prescription and consumption of material and energy to achieve the necessary and required workpiece accuracy in series manufacturing depends to a great extent from the (geometrical) workpiece tolerances of any kind (roughness, form, positional, dimensional) which are prescribed for the production and the fulfillment of these tolerances and therefore for the function of the produced workpieces and their fitness for practical application and none the less of the economy of production altogether. This requirement is of great importance at the time being which is characterized as described above.

Image-based creation and positioning of welds for accelerated prototyping

In most laser welding applications the welding result is highly sensitive to the weld position. The initial determination of process parameters therefore requires time-consuming adjustments to be made. When manufacturing individual parts with low quantities, this is a significant cost driver. State-of-the-art computer-aided design/computer-aided manufacturing systems ease the creation of weld seams significantly, but position deviations resulting, e.g., from workpiece tolerances or displacement must still be compensated. Seam tracking systems are rather complicated to handle during process qualification as they require adjustment to a start position and are subject to be misguided by disturbances in the material. Besides, it is difficult to apply seam tracking to scanner-based microwelding operations, which are considered in this paper. Therefore, the authors developed a system that enables the user to position welds easily and accurately by marking the intended weld position in a calibrated live image of the workpiece, which serves as reference. Thereby, positioning errors are compensated. When executing the welding process it is also possible to observe it in the live image and to assess the geometrical properties of the weld surface immediately. Moreover, the paper describes how this idea is extended to a system for creating more complex weld paths and concludes with an approach for automated image-based path generation by extracting the joint geometry from the workpiece image.

A new perspective on the stability study of centerless grinding process

Regenerative chatter is one of the most complex dynamic processes in machine tools. It is characterized by the presence of self-excited vibrations during machining, limiting the achievable tolerances in the workpieces. In order to predict the set-up conditions that produce these vibrations, it is necessary to model the regenerative mechanism responsible of their appearance accurately, so that the system stability can be studied solving the characteristic equation of the chatter loop. Although the dynamic behavior of machining processes like milling, turning or drilling is governed by a time delayed differential equation with one time delay term, a very particular problem is presented in centerless grinding. In this process, in addition to the dynamic instabilities, geometric instabilities must be analyzed, which are another important factors limiting the workpiece tolerances and lead to three time delay terms in the modeling procedure. This fact complicates its study remarkably, and the resolution of the characteristic roots of the dynamic process of these kinds of machines has not been tackled in the specialized literature as extensively as in other machining processes, being this field a challenging research line. According to this, in this paper an original and efficient method is presented to solve the roots of the characteristic equation of the centerless grinding process, based on the application of the root locus method. The main features of the proposed procedure are its ability to obtain the solutions accurately and that it is capable of determining the origin of the instabilities, so it constitutes a powerful tool to predict machine response for different set-up conditions. These interesting properties are demonstrated through the simulation results presented in this paper.

Increasing the Accuracy of Off‐Line Generated Tool Paths by Compensation of Workpiece Tolerances

Due to its high flexibility and its high accuracy laser machining has become the common process to perform three‐dimensional cutting on deep‐drawn workpieces especially when small lot sizes are to be machined. In the last few years the programming of laser machines has changed from teach‐in to off‐line programming, where the tool path definition is done in a CAM system based on a CAD model of the workpiece. Owing to tolerances in workpiece geometry and positioning, the off‐line programmed tool paths often do not fit exactly the surface of the real workpiece. In this publication, a software system is proposed which adapts the CAD models to their corresponding real workpiece by compensating deviations in shape and location. As a result cutting quality and accuracy are improved.

Effect of tool bit insert-holder assembly on the quality of machined workpieces

Different techniques are normally used for assembling insert-holders of metal cutting tools. For instance, mechanical clamping, brazing, and adhesive bonding are common techniques for assembling metal cutting tools insert-holders. The present work addresses the impact of the assembly technique on the dimensional accuracy and geometrical tolerances of machined workpieces. Extensive cutting tests conducted on mild steel specimens using different insert holder assembly techniques revealed that bonded assembly ranked ahead in preserving dimensional accuracy and geometrical tolerances, when compared with mechanical clamped and brazed assemblies.

The Abbott curve — well known in metrology but not on technical drawings

The tolerances of workpieces decrease continuously since the beginning of the twentieth century. This lies in one line with the perspective of nanotechnology and also recent reports give evidence on the effect of constantly tightening workpiece tolerances. As tolerances of workpieces decrease the interaction between dimensional tolerances and surface finish becomes more important. In this study ideas of the authors to use the Abbott curve for specification of surface finish are presented and a proposal is given how to specify it in engineenng drawings.

Nanometrology for industrial applications

The development of continuously decreasing workpiece tolerances lies in one line with the perspective of nanotechnology. To fulfil metrological demands since about 1982 new high resolution and high precision measuring devices have been developed. These are computerized instruments and the measuring results are very illustrative figures of tested specimens. But the problem of calibration in the nanometric range is still an open question which normally is done only with high uncertainty. New ideas to find a solution for the problem of sufficiently accurate calibrating will make possible that this measurement technique will be used on a broad basis also in industry.