In this paper, an automatic determination method of part build orientation for laser powder bed fusion is presented. This method includes two steps. First, an existing facet clustering-based approach ...is applied to automatically generate the alternative orientations of a laser powder bed fusion part. Second, support volume, volumetric error, surface roughness, build time and build cost are considered. Their values in each alternative orientation are estimated by certain estimation models. The weights of these factors are determined via pairwise comparison. The weighted sum model is used to calculate a summary value of the factors in each alternative orientation. According to the calculated summary values, an optimal orientation to build the part is generated. To demonstrate the method, a set of part orientation cases are tested, and effectiveness analysis and efficiency and characteristic comparisons are reported. The demonstration results suggest that the proposed method can work for both regular and freeform surface models, produce stable and reasonable results and provide desired efficiency.
The orbital angular momentum (OAM) holography has been developed and experimentally demonstrated the capability of holographic multiplexation and high-security encryption. However, the helical phase ...of an OAM mode can only be encoded into one hologram associated with a target image, and this severely limits the information capacity in practical applications. The modulated orbital angular momentum (MOAM) holography is proposed by imposing multiple modulation phase modes onto one OAM mode. Three essential properties, including the MOAM-preservation, MOAM-selectivity and MOAM-multiplexation are investigated. This method can significantly enhance the holographic information capacity and it has broad prospects for optical encryption and beam manipulations.
A novel modulated diamond cutting (MDC) technique is proposed for the generation of complicated micro/nanofluidic channels. The MDC adopts a turning configuration through a four-axis ultra-precision ...diamond lathe. A motion modulation based milling operation is introduced by extending the virtual spindle technique. This unique principle makes the MDC more suitable to generate micro/nanofluidic channels through compromising certain inherent advantages of both diamond turning and milling. Moreover, taking advantage of axial servo motion modulation as well as tool mark modulation using the re-cutting effect, complicated channels can be effectively generated having spatially-varying shapes as well as hierarchical micro/nanostructures. Through both numerical simulation and experimental cutting, capability and outperformance of the MDC are demonstrated well. The result suggest that the MDC is capable to generate ultra-smooth channel surfaces with complicated shapes and superimposed surface nanostructures, exhibiting significant superiority for the generation of micro/nanofluidic channels with high flexibility, high efficiency, and high universality.
•A novel MDC technique is proposed for the generation of micro/nanouidic channels.•Diamond turning and micro-milling are fundamentally synthesized in the MDC.•Motion modulation and tool mark modulation strategies are combined.•Micro/nanouidic channels with complicated shapes and hierarchical structures can be generated.
In this paper, a closed-loop virtual simulation system has been developed to simulate the milling process considering the interactions between manufacturing processes and machine tool dynamics. The ...system consists of cutting force module, machining stability module, and surface generation module. The synchronous effects of the machining parameters, tool geometry parameters, and the dynamic performance of the machine tool system are considered in the model, and the instant machine dynamic motion error is compensated in the model as a feedback to correct the cutter trajectories. Instantaneous tool-workpiece contact status is used to calculate cutting force, and the peak-to-peak cutting force plot is used to predict the machining stability in time domain under different depths of cut and cutting speeds. The envelope curve of the cutting tool is used to reconstruct the machined surface texture. Moreover, to verify the feasibility of the proposed system, micro-milling experiments are conducted with results showing that the simulation system enables the effective prediction of micro-milling process such as the cutting forces and machined surface quality. It can be potentially applied in production on processing parameter optimization and surface topography prediction.
In this paper, a novel facet clustering based method is proposed to generate alternative build orientations for laser powder bed fusion. This method consists of two steps. First, a hierarchical ...clustering algorithm is applied to divide facets of the design model in standard tessellation language format into different clusters, each of which shares a similar normal vector. Second, alternatives of each cluster are computed and the final set of alternative build orientations is generated by combining and refining the alternatives from all clusters. To illustrate and validate the method, a set of examples including both regular and freeform surface models are tested, and qualitative and quantitative comparisons between the method and the existing methods are reported. The results suggest that the proposed method is feasible and effective for both regular and freeform surface parts. It is evident that the proposed method can output stable results, provide satisfying efficiency, and work well with facet clusters of varying probability density.
The coded aperture correlation holography (COACH) is a powerful three-dimensional imaging technique. However, the conventional COACH method can only restore the three-dimensional images of the ...samples under test, but cannot specify the physical characteristics. We propose a full-dimensional measurement method based on the polarized coded aperture correlation holography. Specifically, the dynamic phase and geometric phase distributions can be separated from two holograms associated with the horizontal and vertical polarization components, then the Stokes parameters, surface topography and reflectivity can be calculated simultaneously. This method achieves the same spatial resolution for the two terms, which greatly improves the reliability and practicability with respect to conventional measurement methods.
In order to enhance the measurement availability for manufacturing applications, on-machine surface measurement (OMSM) is integrated onto the machine tools, which avoids the errors caused by ...re-positioning workpieces and utilizes the machine axes to extend the measuring range as well. However, due to the fact that measurement probe actuation is performed using the machine tool axes, the inherent kinematics error will inevitably induce additional deviations onto the OMSM results. This paper presents a systematic methodology of kinematics error modelling, measurement, and compensation for OMSM on an ultra-precision turning lathe. According to the measurement task, a selective kinematics error model is established with four primary error components in the sensitive measurement direction, based on multi-body theory and a homogeneous transformation matrix (HTM). In order to separate the artefact error from the measurement results, the selected error components are measured using the reversal method. The measured error value agrees well with the machine tool's specification and a kinematics error map is generated for further compensation. To verify the effectiveness of the proposed kinematics error modelling, measurement, and compensation, an OMSM experiment of an optically flat mirror is carried out. The result indicates the OMSM is the superposition of the sample surface form error and the machine tool kinematics error. With the implementation of compensation, the accuracy of the characterized flatness error from the OMSM improves by 67%.
Freeform surfaces are featured with superior optical and physical properties and are widely adopted in advanced optical systems. Slow tool servo (STS) ultra-precision machining is an enabling ...manufacturing technology for fabrication of non-rotationally symmetric surfaces. This work presents a theoretical and experimental study of surface topography generation in STS machining of freeform surfaces. To achieve the nanometric surface topography, a systematic approach for tool path generation was investigated, including tool path planning, tool geometry selection, and tool radius compensation. The tool radius compensation is performed only in one direction to ensure no high frequency motion is imposed on the non-dynamic axis. The development of the surface generation simulation allows the prediction of the surface topography under various tool and machining variables. Furthermore, it provides an important means for better understanding the surface generation mechanism without the need for costly trial and error tests. Machining and measurement experiments of a sinusoidal grid and microlens array sample validated the proposed tool path generation and demonstrated the effectiveness of the STS machining process to fabricate freeform surfaces with nanometric topography. The measurement results also show a uniform topography distribution over the entire surface and agree well with the simulated results.
A concurrent multiscale model coupling discrete dislocation dynamics to the finite element method is developed to investigate the plastic mechanism of materials at micron/submicron length scales. In ...this model, the plastic strain is computed in discrete dislocation dynamics (DDD) and transferred to the finite element method (FEM) to participate in the constitutive law calculation, while the FEM solves the complex boundary problem for DDD simulation. The implementation of the whole coupling scheme takes advantage of user subroutines in the software ABAQUS. The data structures used for information transferring are introduced in detail. Moreover, a FE mesh-based regularization method is proposed to localize the discrete plastic strain to continuum material points. Uniaxial compression tests of single crystal micropillars are performed to validate the developed model. The results indicate the apparent dependence of yield stress on sample size, and its underlying mechanisms are also analyzed.
Fused silica is widely used as a material for optical lenses owing to its excellent optical properties and low thermal expansion coefficient. However, as a hard and brittle material, there is very ...limited option of processing technologies to machining fused silica with surface structures. In this paper, a picosecond laser based single crystal diamond tool fabrication technology is proposed to generate micro milling tools with different geometrical designs, and the tool cutting performance is experimentally tested through micro-milling of fused silica under different cutting conditions. An optimal picosecond laser processing path is proposed to inhibit the graphitization of diamond tool and improve the concentricity of tool blades, and a multi-edge milling tool with a minimum rotary diameter of 0.4 mm can be obtained. The effects of rake angle on cutting force and the degree of brittle damage on the subsurface of fused silica are studied by micro milling tests of fused silica using the laser-shaped tools. The results show that the fused silica machined by diamond milling tool with a rake angle of −30° has the best surface finish (Ra = 41.2 nm). Using this laser-machined milling tool, a plurality of micro Fresnel lenses with aperture of 1.6 mm were successfully machined on a fused silica sheet.
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