Geometrical Inaccuracies Research Articles

Surface Quality of a Groove after Trochoidal Milling with a Monolithic Ceramic Milling Cutter

Hard-to-machine materials conclude a variety of materials. In this group of materials are high-strength, hardness-resistant steels, such as austenitic steels, but also non-ferrous alloys with high cor-rosion, heat resistance and strength based on nickel, titanium or cobalt, etc. For machining of these materials, it is necessary to choose suitable tools. The improper cutting tool can cause an increase in geometric inaccuracies, rapid wear, etc. Cutting material is an important factor during designing of cutting tools. A combination of a proper cutting tool with the trochoidal milling can reduce maching time, extend tool lifetime and reduce production costs.

Analysis of the causes of undesirable curvature of the workpiece during longitudinal rolling using numerical simulation

Undesirable additional deformation was observed during the experimental cold rolling of steel. A simulation was performed in Deform software to clarify the causes. The influence of the unequal coefficient of friction, geometric inaccuracies of the rolls and the conveyor, and the situation when the rolls did not have the same diameter or rotated at different speeds were investigated. Simulations with different roll speeds corresponded best to reality. The simulation of the undesired curvature of the workpiece at different speeds of rotation of the rolls and 30% reduction was subsequently performed as an elastoplastic problem in order to determine the amount of springback and residual stress. Furthermore, the press straightening of the workpiece was simulated. Inhomogeneous and accidental deformation and residual stresses must be taken into account, especially when researching the effect of cold rolling on the mechanical properties and structure of the material.

Calibration Dependencies and Accuracy Assessment of a Silicone Rubber 3D Printer

Silicone rubbers are relatively new in additive manufacturing, with only a few commercial printing services and reports on custom-built printers available. Publications and standards on calibration and accuracy assessment are especially lacking. In this study, the printhead calibration process of a custom-built silicone printer is explained, and a set of test objects is proposed and evaluated. The printer in use is based on an open-source filament printer, capable of multi-material printing with silicone rubbers and thermoplastic polymers. Three different high-viscosity single-component liquid silicone rubbers and one polylactic acid thermoplastic filament were used as printing materials. First, the calibration process of the silicone printhead was conducted, and the dependency of the dosing accuracy on silicone viscosity, nozzle diameter and extrusion speed was evaluated. Second, various test specimens were proposed and printed to characterize the accuracy and geometric limitations of this printer. These test parts contained features such as thin walls, slender towers, small holes and slots, unsupported overhangs and bridges. It was concluded that silicone viscosity strongly affects geometric inaccuracies. Design recommendations were deducted from the results, advising for wall thicknesses above 1 mm, slenderness ratios below 2, bridging lengths below 2 mm and unsupported overhang angles below 30°.

Using experimental non-contact methods to discuss on a damage process of laminated thin-walled L-columns under ends shortening

This paper analyses the behaviour of actual laminated angle columns under shortening at full load until their damage. The study was conducted on short columns with geometric inaccuracies, subject to local buckling. The geometric dimensions of the columns were selected to prevent any interactions between different eigenmodes and to keep eigenvalues from being too close. In order to eliminate the effects of stress concentration and the resulting damage at column ends, flexible pads were installed between the sample and stiff attachment plates. The geometric inaccuracies of actual samples were determined with a 3D scanner, which also allowed to determine actual sample dimensions. The behaviour of compressed samples was registered with the Aramis system and an acoustic emission system. The experimental study results were compared with FEM numerical solutions. A full non-linear analysis of the compressed columns was conducted using progressive damage analysis. In both cases an identical damage mechanism was observed and the results were consistent.

Optimisation of single contour strategy in selective laser melting of Ti-6Al-4V lattices

PurposeSelective laser melting (SLM) is increasingly used to manufacture bone implants from titanium alloys with particular interest in porous lattice structures. These complex constructs have been shown to be capable of matching native bone mechanical behaviour leading to improved osseointegration while providing numerous clinical advantages, encouraging their broad use in medical devices. However, producing lattices with a strut diameter similar in scale to a typical SLM melt pool or using the same process parameters and scan strategies intended for bulk solid components may lead to geometric inaccuracies. The purpose of this study is to evaluate and optimise the single contour strategy for the production of Ti-6Al-4V lattices.Design/methodology/approachHerein, the potential of an unfilled single contour (SC) scanning strategy to improve the reproducibility of porous lattices when compared with a single contour and fill approach (SC + F) is explored. For this purpose, two parametric analysis were carried out on Ti-6Al-4V diamond unit cell lattices with different strut sizes and scan strategies. Porosity and accuracy measurements were correlated with processing parameters and printing strategy to provide the optimal processing window for lattice manufacturing.FindingsSC is shown to be a viable strategy for production of Ti-6Al-4V lattices with a strut diameter below 350 µm. Parametric analysis highlights the limits of this method in producing fully dense struts with energy density presented as a useful practical tool to guide some aspects of parameter selection (design strut diameter achieved at approximately 0.1 J/mm in this study). Finally, a process map combining data from both parametric studies is provided to guide, predict and control lattice strut geometry and porosity obtained using the SC strategy.Originality/valueThese results explore the use of non-standard SC scanning strategy as a viable method for producing strut-based lattice structures and compare against the traditional contour and fill approach (SC + F).

A novel paradigm for feedback control in LPBF: layer-wise correction for overhang structures

In laser powder bed fusion (LPBF), it is common practice to select process parameters to achieve high density parts starting from simple geometries such as cubes or cylinders. However, additive manufacturing is usually adopted to produce very complex geometries, where parameters should be tuned locally, depending on the local features to be processed. In fact, geometrical features, such as overhangs, acute corners, and thin walls may lead to over- or under-heating conditions, which may result in geometrical inaccuracy, high roughness, volumetric errors (i.e., porosity) or even job failure due to surface collapse. This work proposes a layer-wise control strategy to improve the geometrical precision of overhanging regions using a coaxial melt pool monitoring system. The melt-pool images acquired at each layer are used in a control-loop to adapt the process parameters locally at the next layer in order to minimize surface defects. In particular, the laser duty cycle is used as a controllable parameter to correct the energy density. This work presents the main architecture of the proposed approach, the control strategy and the experimental procedure that need to be applied to design the control parameters. The layer-wise control strategy was tested on AISI 316L stainless steel using an open LPFB platform. The results showed that the proposed layer-wise control solution results in a constant melt pool observed via the laser heated area size starting from the second layer onward, leading to a significant improvement in the geometrical accuracy of 5 mm-long bridge geometries.

Carbonization Region Measurement in Vegetable Tanned Goat Leather using Machine Vision System for Evaluating Performance Measures of Leather Cut Contour Edges

Due to the widespread application and popularity of lasers in recent times, the usage of laser cutting for leather applications has increased as well. Laser technology is needed to provide more consistent and effective results while cutting leathers that include complicated geometries particularly in several sectors where leathers are often utilized such as footwear, apparel and fashion accessories. In this investigation, the diode laser was preferred for leather cutting due to its regulated power density, compact size and portability whereas the CO2 lasers are uncontrollable. The benefit of employing a diode laser is that it can overcome some of the disadvantages associated with CO2 lasers such as power consumption, carbonization layer and geometric inaccuracy. There is no technique available to measure the carbonization at the leather cut contour edges. Hence an attempt has been made to investigate the carbonization percentage with the help of a machine vision system to improve the machining process. The technique of measuring carbonization can be used effectively in the leather industry for the accurate measurement of carbonization. The lower duty cycle with moderate pulse width modulation (PWM) and amplitude could produce lower carbonization layer. PWM frequency has a high influential role on determining carbonization in leather cutting.

Multi-Objective Optimization Under Uncertainty of Part Quality in Fused Filament Fabrication

Abstract This work presents a data-driven methodology for multi-objective optimization under uncertainty of process parameters in the fused filament fabrication (FFF) process. The proposed approach optimizes the process parameters with the objectives of minimizing the geometric inaccuracy and maximizing the filament bond quality of the manufactured part. First, experiments are conducted to collect data pertaining to the part quality. Then, Bayesian neural network (BNN) models are constructed to predict the geometric inaccuracy and bond quality as functions of the process parameters. The BNN model captures the model uncertainty caused by the lack of knowledge about model parameters (neuron weights) and the input variability due to the intrinsic randomness in the input parameters. Using the stochastic predictions from these models, different robustness-based design optimization formulations are investigated, wherein process parameters such as nozzle temperature, nozzle speed, and layer thickness are optimized under uncertainty for different multi-objective scenarios. Epistemic uncertainty in the prediction model and the aleatory uncertainty in the input is considered in the optimization. Finally, Pareto surfaces are constructed to estimate the tradeoffs between the objectives. Both the BNN models and the effectiveness of the proposed optimization methodology are validated using the actual manufacturing of the parts.

Optimal and adaptive lattice design considering process-induced material anisotropy and geometric inaccuracy for additive manufacturing

Optimal and adaptive lattice design considering process-induced material anisotropy and geometric inaccuracy for additive manufacturing

On the Potential of Slicing Algorithms in Additive Manufacturing for the Optimization of Geometrical Part Accuracy

Despite its potentials, geometrical inaccuracies remain a major shortcoming for Additive Manufacturing in general and especially for Fused Layer Modeling, even though much research effort is put in enhancing mechanical properties and reducing process-specific deviations by optimizing process parameters. Novel slicing and path planning algorithms, such as non-planar printing, are suitable tools for addressing these problems and thus for improving geometrical part accuracy, but are subject of current research and only partly applicable. With the aim to show and evaluate the potential of these algorithms in the context of tolerancing, this paper proposes an approach for applying one pre-processing technique to 3D-printed parts. The exemplary application shows the applicability and the high potential of advanced slicing and path planning algorithms. The findings therefore indicate that aspects of pre-processing should be matched with the parts geometry and therefore have to be frontloaded into the product and the tolerance design for their consideration.

Investigation of dose profile across the junction of deep inspiration breath hold, breast with supra-clavicle fossa treatments.

Breast with supraclavicular fossa (Br+SCF) radiotherapy treatments can utilise a monoisocentric technique to concurrently treat the breast area (tangent fields) and supraclavicular area (opposing fields). The region where these treatment areas adjoin is known as the junction region, field junction, or match line. Dose variations that may occur about the junction region, due to geometrical inaccuracies, are typically feathered out in patient free-breathing treatments. However, there is limited information on how dose at the junction is influenced in deep inspiration breath hold (DIBH) treatments. This study aims to investigate dose variation at the field junction for a patient population undertaking a DIBH Br+SCF treatment course. GAFChromic EBT3 film was used to record the dose across the junction at skin surface for approximately one third of the 25 fraction treatment course for 11 patients undergoing DIBH Br+SCF treatment. Single fraction and summated fraction profiles for each patient were compared to profiles in the treatment planning system and assessed the: (1) local dose variations, (2) position of the 50% dose gradient, and (3) relative dose at the nominal junction. Local dose variations of 10% or greater, position displacement of the junction greater than 5 mm, and relative dose differences at the match line greater than 10% can be found within single fraction dose profiles. When these single fractions are summed over the treatment course, the position variations reduce to 2 mm and dose variations reduced to within 10% for 10 of the 11 patients. Only one of 11 patients recorded a summed dose difference greater than ±10% over their treatment, recording 76% ± 8% of the planned dose in this region. This was due to a small overall position displacement of 1.8 ± 1.6 mm from the nominal junction. A feathering of the dose at the junction is present for DIBH Br+SCF patient treatments. The feathering effect is sufficient, in the majority of cases studied, to reduce any differences in dose and displacement present in single fractions. This work also demonstrates that there may be exceptions from this observed behavior that should be considered. Further study in this area using a larger patient cohort is recommended.

Dislocation evolution during additive manufacturing of tungsten

Additive manufacturing (AM) frequently encounters part quality issues such as geometrical inaccuracy, cracking, warping, etc. This is associated with its unique thermal and mechanical cycling during AM, as well as the material properties. Although many efforts have been spent on this problem, the underlying dislocation evolution mechanism during AM is still largely unknown, despite its essential role in the deformation and cracking behavior during AM and the properties of as-fabricated parts. In this work, a coupling method of three-dimensional dislocation dynamics and finite element method is established to disclose the mechanisms and features of dislocations during AM. Tungsten (W) is chosen as the investigated material due to its wide application. The internal thermal activated nature of dislocation mobility in W is taken into account. The correlations between the combined thermal and mechanical cycles and dislocation evolutions are disclosed. The effect of adding alloying element Ta in W is discussed from the perspectives of tuning dislocation mobility and introducing nanoparticles, which helps to understand why higher dislocation density and fewer microcracks are observed when adding Ta. The current work sheds new light on the long-standing debating of dislocation origin and evolutions in the AM field.

A Comparative Investigation of Conventional and Hammering-Assisted Incremental Sheet Forming Processes for AA1050 H14 Sheets

Incremental Sheet Forming (ISF) is emerging as one of the popular dieless forming processes for the small-sized batch production of sheet metal components. However, the parts formed by the ISF process suffer from poor surface finish, geometric inaccuracy, and non-uniform thinning, which leads to poor part characteristics. Hammering, on the other hand, plays an important role in relieving residual stresses, and thus enhances the material properties through a change in grain structure. A few studies based on shot peening, one of the types of hammering operation, revealed that shot peening can produce nanostructure surfaces with different characteristics. This paper introduces a novel process, named the Incremental Sheet Hammering (ISH) process, i.e., integration of incremental sheet forming (ISF) process and hammering to improve the efficacy of the ISF process. Controlled hammering in the ISF process causes an alternating motion at the tool-sheet interface in the local deformation zone. This motion leads to enhanced material flow and subsequent improvement in the surface finish. Typical toolpath strategies are incorporated to impart the tool movement. The mechanics of the process is further explored through explicit-dynamic numerical models and experimental investigations on 1 mm thick AA1050 sheets. The varying wall angle truncated cone (VWATC) and constant wall angle truncated cone (CWATC) test geometries are identified to compare the ISF and ISH processes. The results indicate that the formability is improved in terms of wall angle, forming depth and forming limits. Further, ISF and ISH processes are compared based on the numerical and experimental results. The indicative statistical analysis is performed which shows that the ISH process would lead to an overall 10.99% improvement in the quality of the parts primarily in the surface finish and forming forces.

Accuracy and Sheet Thinning Improvement of Deep Titanium Alloy Part with Warm Incremental Sheet-Forming Process

Incremental forming is a recent forming process that allows a sheet to be locally deformed with a hemispherical tool in order to gradually shape it. Despite good lubrication between the sheet and the tip of the smooth hemisphere tool, ductility often occurs, limiting the formability of titanium alloys due to the geometrical inaccuracy of the parts and the inability to form parts with a large depth and wall angle. Several technical solutions are proposed in the literature to increase the working temperature, allowing improvement in the titanium alloys’ formability and reducing the sheet thinning, plastic instability, and failure localization. An experimental procedure and numerical simulation were performed in this study to improve the warm single-point incremental sheet forming of a deep truncated cone in Ti-6Al-4V titanium alloy based on the use of heating cartridges. The effect of the depth part (two experiments with a truncated cone having a depth of 40 and 60 mm) at hot temperature (440 °C) on the thickness distribution and sheet shape accuracy are performed. Results show that the formability is significantly improved with the heating to produce a deep part. Small errors are observed between experimental and theoretical profiles. Moreover, errors between experimental and numerical displacements are less than 6%, which shows that the Finite Element (FE) model gives accurate predictions for titanium alloy deep truncated cones.

Multistage Tool Path Optimisation of Single-Point Incremental Forming Process.

Single-point incremental forming (SPIF) is a flexible technology that can form a wide range of sheet metal products without the need for using punch and die sets. As a relatively cheap and die-less process, this technology is preferable for small and medium customised production. However, the SPIF technology has drawbacks, such as the geometrical inaccuracy and the thickness uniformity of the shaped part. This research aims to optimise the formed part geometric accuracy and reduce the processing time of a two-stage forming strategy of SPIF. Finite element analysis (FEA) was initially used and validated using experimental literature data. Furthermore, the design of experiments (DoE) statistical approach was used to optimise the proposed two-stage SPIF technique. The mass scaling technique was applied during the finite element analysis to minimise the computational time. The results showed that the step size during forming stage two significantly affected the geometrical accuracy of the part, whereas the forming depth during stage one was insignificant to the part quality. It was also revealed that the geometrical improvement had taken place along the base and the wall regions. However, the areas near the clamp system showed minor improvements. The optimised two-stage strategy successfully decreased both the geometrical inaccuracy and processing time. After optimisation, the average values of the geometrical deviation and forming time were reduced by 25% and 55.56%, respectively.

A new procedure for implementing the modified inherent strain method with improved accuracy in predicting both residual stress and deformation for laser powder bed fusion

As a metal additive manufacturing (AM) process, laser powder bed fusion ( L -PBF) has been widely used to produce parts with complex geometries. The large thermal gradient caused by the fast, intense, and repeated laser scanning induces significant residual deformation and stress to the as-built parts, which increase manufacturing difficulty and geometrical inaccuracy as a result. The modified inherent strain (MIS) method exploiting multiscale process simulations was developed to simulate residual deformation accurately and efficiently. However, the existing procedure of implementing the MIS method is found to give inaccurate residual stress prediction. In this work, a new implementation procedure for the MIS method is proposed to improve the simulation accuracy of residual stress without degrading the residual deformation prediction. The new procedure concerns the application of inherent strains to the part-scale layer-by-layer finite element model to obtain residual stress and deformation field. While the existing implementation of the part-scale MIS model involves only mechanical properties at ambient temperature, the new procedure adds one more solution step employing mechanical properties at an elevated temperature determined from the inherent strain extraction step. Both numerical and experimental studies are conducted to validate the proposed new implementation procedure. It shows that by using the new procedure, the MIS-based simulation can predict both residual stress and deformation of as-built L -PBF metal parts with good accuracy.

Design and construction of a customizable phantom for the characterization of the three-dimensional magnetic resonance imaging geometric distortion.

One of the main challenges to using magnetic resonance imaging (MRI) in radiotherapy is the existence of system‐related geometric inaccuracies caused mainly by the inhomogeneity in the main magnetic field and the nonlinearities of the gradient coils.Several physical phantoms, with fixed configuration, have been developed and commercialized for the assessment of the MRI geometric distortion. In this study, we propose a new design of a customizable phantom that can fit any type of radio frequency (RF) coil. It is composed of 3D printed plastic blocks containing holes that can hold glass tubes which can be filled with any liquid. The blocks can be assembled to construct phantoms with any dimension.The feasibility of this design has been demonstrated by assembling four phantoms with high robustness allowing the assessment of the geometric distortion for the GE split head coil, the head and neck array coil, the anterior array coil, and the body coil. Phantom reproducibility was evaluated by analyzing the geometric distortion on CT acquisition of five independent assemblages of the phantom.This solution meets all expectations in terms of having a robust, lightweight, modular, and practical tool for measuring distortion in three dimensions. Mean error in the position of the tubes was less than 0.2 mm. For the geometric distortion, our results showed that for all typical MRI sequences used for radiotherapy, the mean geometric distortion was less than 1 mm and less than 2.5 mm over radial distances of 150 mm and 250 mm, respectively.These tools will be part of a quality assurance program aimed at monitoring the image quality of MRI scanners used to guide radiation therapy.

Modified methods for estimation of local bifurcational loads including an effect of moderate geometrical inaccuracies

The problem of estimation of values of local bifurcational loads in plate structures burdened with moderate geometrical inaccuracies is dealt with. A way to include an effect of geometrical inaccuracies on the estimated value of bifurcational loads is suggested. If these inaccuracies are not considered, it can have a serious impact on underestimation of bifurcational loads of the real thin-walled structure and can yield a consequential scattering of the results obtained for a various series of samples. A theoretical value of local bifurcational loads is determined from a solution to the eigenproblem on the assumption that the structure is perfect and not burdened with geometrical inaccuracies. Under laboratory conditions, samples are always burdened with inaccuracies, and, what follows, imperfections always occur. A detailed procedure in which the classical plate theory (CPT) and the laminated plate theory (CLPT) within Byskov-Hutchinson’s approach to Koiter’s theory for a plate model of the thin-walled structure were assumed was developed. To verify the results, numerous finite element method (FEM) simulations were conducted. On one hand, solutions to the eigenproblem were compared to post-buckling behaviors of actual structures with imposed initial deflections, which were used to estimate values of bifurcational loads with the following modified methods: the P-w2 method, the intersection method and the Δ-w2 method. A very good agreement between the results attained with new methods and theoretical solutions was obtained.

Tolerance to geometrical inaccuracies in CBCT systems: A comprehensive study.

The last decades have seen the consolidation of the cone-beam CT (CBCT) technology, which is nowadays widely used for different applications such as micro-CT for small animals, mammography, dentistry, or surgical procedures. Some CBCT systems may suffer mechanical strains due to the heavy load of the x-ray tube. This fact, together with tolerances in the manufacturing process, lead to different types of undesirable effects in the reconstructed image unless they are properly accounted for during the reconstruction. To obtain good quality images, it is necessary to have a complete characterization of the system geometry including the angular position of the gantry, the source-object and detector-object distances, and the position and pose of the detector. These parameters can be obtained through a calibration process done periodically, depending on the stability of the system geometry. To the best of our knowledge, there are no comprehensive works studying the effect of inaccuracies in the geometrical calibration of CBCT systems in a systematic and quantitative way. In this work, we describe the effects of detector misalignments (linear shifts, rotation, and inclinations) on the image and define their tolerance as the maximum error that keeps the image free from artifacts. We used simulations of four phantoms including systematic and random misalignments. Reconstructions of these data with and without errors were compared to identify the artifacts introduced in the reconstructed image and the tolerance to miscalibration deemed to provide acceptable image quality. Visual assessment provided an easy guideline to identify the sources of error by visual inspection of the artifactual images. Systematic errors result in blurring, shape distortion and/or reduction of the axial field of view while random errors produce streaks and blurring in all cases, with a tolerance which is more than twice that of systematic errors. The tolerance corresponding to errors in position of the detector along the tangential direction, that is, skew (<0.2°) and horizontal shift (<0.4mm), is tighter than the tolerance to those errors affecting the position along the longitudinal direction or the magnification, that is, vertical shift (<2mm), roll (<1.5°), tilt (<2°), and SDD (<3mm). We present a comprehensive study, based on realistic simulations, of the effects on the reconstructed image quality of errors in the geometrical characterization of a CBCT system and define their tolerance. These results could be used to guide the design of new systems, establishing the mechanical precision that must be achieved, and to help in the definition of an optimal geometrical calibration process. Also, the thorough visual assessment may be valuable to identify the most predominant sources of error based on the effects shown in the reconstructed image.

High-resolution microscopy assisted mechanical modeling of ultrafine electrospun network

This paper demonstrates a novel hi-resolution microscopy-assisted computational approach to mechanical modeling of ultrafine electrospun networks (ESN). Morphological data extracted from X-ray microtomography (μCT) of ESN made of poly(lactic-co-glycolic acid) (PLGA), supplemented by data acquired from the nanoscale flexural test of a single fiber, was used to create a volumetric finite element (FE) model of the investigated ESN. The results of the carried-out FE simulation have been successfully validated against the results of the quasi-static tensile test. The accuracy and the reliability of the μCT-based ESN geometry have been proved by the comparison with the scanning electron microscopy (SEM) data and by the numerical analysis of fluid flow across the rendered ESN structure. The designed methodology enables the analysis of potential factors affecting the mechanical properties of electrospun structures made of aliphatic polyesters, like the depth of fiber fusion, the ESN geometric inaccuracies, and the fiber diameter distribution. Moreover, in the study, the novel experimental setup enabling for single fiber AFM flexural test was proposed, assuming deposition of fibers directly on polystyrene (PS) substrate containing crater-like wells. Additionally, the presented study contains the numerical analysis of the effect of the substrate stiffness on the results of the three-point bending test of a single fiber.