Machine Coordinate System Research Articles

Automatic part localization in a CNC machine coordinate system by means of 3D scans

Due to the rough nature of surfaces produced by current additive metal processes, a deviation between the nominal and actual pose of the part arises when fixturing in an NC machine. Modern 3D scanning systems are capable of generating high density measurements that may be used to localize the part and compensate for these errors; but require that scan data be transformed from the scanner to the CNC coordinate systems. In this work, precisely located fiducial features are automatically detected in the scan data and used to compute the pose of the scanner. This information is used to register each scan to the CNC machine space, and thereby create a model of the workpiece as-built and as-mounted in the machine, for use in process planning and localization. An implementation of the system was built and tested against a machined part that was offset to simulate uncertain position and orientation. The initial performance evaluation of the system indicates that it is capable of successfully registering surfaces to the machine coordinate system with accuracy in line with the scanner performance specification.

Sub-scale Machining of Large Components

A post-processing algorithm for CNC machine NC code was developed for altering NC code based on measurements from a stand-alone coordinate metrology system that measures the orientation and offset of the part and machine coordinate systems (CS). With this post-processing algorithm, the ability to machine a monolithic part larger than the working area of a machine in multiple sections becomes possible. A series of test parts were machined with a Haas Vertical Milling Machine (VMM) and a Leica laser tracker as proof of concept and to determine the errors introduced into the machining process by adding the additional metrology system. Monte Carlo simulations confirm that adding the stand-alone metrology system increases the magnitude of errors to approximately the accuracy of the metrology system itself, about 50 micrometers. The initial set of test parts machined showed errors at a factor of 25 larger than expected. It was determined that a component used to determine the machine's spindle location was introducing significant error into the process. The component was replaced and new parts machined that showed a significant improvement but still with errors larger than expected. Machine squareness errors showed the importance of measuring CS features within the same plane. Finally, it was determined that the definition of the part CS is sensitive to how the part is fixtured to the machine. Sub-scale machining is a viable process for machining large components with a slight increase in feature location error.

Realization of Online Measurement Technology and its Application in the Intelligent Machining Cell

In view of the continuous unmanned machining of revolving workpiece, this paper has integrated the industrial robot with machine vision, mill-turning center and trigger online measurement systems into an intelligent machining cell. It has achieved the automatic loading and unloading material, automatic adjustment of the Machine Coordinate System (MCS) of workblank, automatic compensation of machining errors and workpiece diameter and roundness of the on-machine inspection. In order to ensure the accuracy of continuous unmanned machining of the intelligent machining cell, the main factors that affect the accuracy of online measurement system and the corresponding solutions are analyzed. On this basis, a machining instance is made to verify the validity of the trigger online measurement technique’s application in continuous unmanned machining of the revolving workpiece. The results of this study show that the intelligent machining cell can ensure stable unmanned machining with high precision for several hours.

Assembly tolerance analysis method based on the real machine model

The position errors of an objective feature of a machine is affected comprehensively by the geometric errors and assembly errors of the parts on the assembly chain. In order to investigate the effect of assembly datum errors on the transformation and propagation of the geometric errors in an assembly chain, a modeling and analysis method of assembly error accumulation based on the real machine model is proposed. In this approach, the calculation process of the real position of geometric feature is consist of four steps. Firstly, the deviation geometric feature are represented by the control point variation model , and the deviations of geometric feature from its normal position are simulated by the Monte Carlo method. Secondly, the normal position of the deviation geometric feature in the datum reference frame (DRF) is determined by theoretical dimension. The third step, the DRF establishment method is proposed, and the measure DRF of the objective feature is established with the datum geometry example. The fourth step, the assembly contact model is proposed, the relationship between the measure datum reference frame and assembly datums is calculated according to the assembly sequence and the deviation geometry example of assembly datum surface. The real position of all related parts on the assembly chain in machine coordinate system are calculated by above procedures, and the transformation and propagation of the geometric errors of assembly datum in an assembly chain are obtained. The procedure to simulate the real machine assembly is decomposed into four modular processes, which is useful in realizing the automation of assembly error analysis. A assembly error accumulation example of pin-hole assembly with the three orthogonal datum planes is investigated, and the proposed approach is verified.

Thermal Error Measurement, Modeling and Compensation for Motorized Spindle and the Research on Compensation Effect Validation

The motorized spindle is a primate part of the machine tool and its thermal characteristics have great influence on the accuracy of the machine tools. The thermal errors of the spindle of a certain type of precision horizontal machining center were measured with PLC acquisition system. By multivariate linear regression method, the axial thermal error model was built. Online real-time error compensation was implemented by applying the FANUC 18i CNC system external machine coordinate system shift function. A verification method was proposed which include three steps: model validation, compensation validation and experimental machining verification. The accuracy of the model was 84.1%, 64.9% and 49.4% respectively. The quantitative analysis results showed the precision was effectively improved and the compensation method was reliable.

Constant Depth Scoring of Fibre Reinforced Plastic Structures to Prevent Delamination

Scoring of the outer layers of a workpiece with small engagement (ae) using circular saws is a well known method to avoid unwanted delamination of these layers, such as coatings of wooden particle boards. In edge milling fibre reinforced plastics (FRP), delamination also occurs in the top or bottom layer of the component and requires time intensive rework or scrapping of material. A combined process of scoring and milling is presented in this paper as a means to avoid delamination during machining of FRP structures such as aircraft carbon FRP parts. For the scoring process a diamond electroplated circular wheel is used. To guarantee a constant width of the scored slot, a constant depth of engagement is needed due to the shape of the grinding wheel. Therefore the profile of the scoring path on the workpiece surface is measured with respect to the machine coordinate system to correct a variation in the workpiece thickness and to compensate for uneven clamping. Next, the workpiece is scored with constant depth of engagement (ae,s < 0.5mm) and a high feed velocity of up to 16 m/min. Subsequently, the scored component is machined with a conventional polycrystalline diamond (PCD) milling tool. From experiments it is proven that a delamination-free edge is achievable even with a worn tool (cutting edge radius rn > 50μm) at an increased feed velocity (vf,m = 4.5 m/min). Without scoring, flawed workpiece edges have been obtained with cutting edge radii beyond rn > 23μm (progressive uniform wear stage) and a feed velocity of vf,m = 1.5 m/min. Hence, high performance cutting (HPC) edge milling of composites may be realized by an initial scoring step to fulfil quality requirements of the machined edges and simultaneously increase the tool life of the milling tool.

Total differential methods based universal post processing algorithm considering geometric error for multi-axis NC machine tool

Multi-axis numerical control machining for free-form surfaces needs CAD/CAM system for the cutter location and orientation data. Since these data are defined with respect to the coordinate of workpiece, they need converting for machine control commands in machine coordinate system, through a processing procedure called post processing. In this work, a new universal post processing algorithm considering geometric error for multi-axis machine tool with arbitrary configuration. Firstly, ideal kinematic model and real kinematic model of the multi-axis NC machine tool are built respectively. Difference between the two kinematic models is only whether to consider the machine tool's geometric error or not. Secondly, a universal generalized post processing algorithm containing forward and inverse kinematics solution is designed to solve kinematic models of multi-axis machine tool. Specially, the inverse kinematics solution is used for the ideal kinematic model, while the forward kinematics solution is used for the real kinematic model. Then, a total differential algorithm is applied to improve the calculation speed and reduce the difficulty of inverse kinematics solution. Realization principle of the total differential algorithm is to transform the inverse kinematics solution problem into that one of solving linear equations based on spatial relationship of adjacent cutter locations. Thirdly, to reduce the complexity of geometric error calibration experiment, effect weight of geometric error components is determined by the sensitivity analysis based on orthogonal method, and then the real kinematic model considering geometric error is established. Finally, the universal post processing algorithm based on total differential methods is implemented and demonstrated experimentally in a five-axis machine tool. The results show that the maximum error value can be decreased to one-fifth using the proposed method in this paper.

Development of Post Processor for Five-Axis Machine of Non-Orthogonal Head Tilting Type

A five-axis machine can be developed with various mechanisms as it simply adds two axes to three coordinate axes. Therefore, five-axis machining utilizes post-processor for conversion to NC code, which the five-axis machine can read, with tool path CL data created by CAM depending on the mechanism of each device. This study introduces a post-processor program that tilts from axis B, of which the head is inclined at about 45 degrees, moves straight toward the X and Z axes, rotates the table from C, and then moves along axis Y. The post-processor mechanism was applied to the program after calculating the rotation angles and tilting angles using inverse kinematics and mapping the part coordinate system by machine coordinate system. Then, to evaluate the established post processor formula, it was verified twice. First, a tool path that was identical to that created by a commercial post-processor program, Post Works, was post-processed and comparably reviewed to, verify the performance of the proposed method. Secondly, a test specimen was manufactured by creating NC data from the developed post processor program. After manufacturing the test specimen, the compatibility was verified, suggesting the possibility to commercialize the five-axis machine and the post -processor program.

Geometric error model and measuring method based on worktable for five-axis machine tools

This article presents a new integrated geometric model that takes worktable as reference for five-axis machine tools. It could simplify the description of different machine kinetic structures and uniform the measurement coordinate system and machining coordinate system. Based on this model, a new method of measuring geometric errors with conventional instruments is proposed. It could break through some existing limitations, such as special instrument, specific machine kinetic structures, or errors incompletion. In addition, some deviation errors in measuring process could be eliminated to improve accuracy further. Finally, a series of experiments are conducted on a five-axis machine tool with rotary worktable and tilting head. The results show that the error model and measuring method are effective and applicable.

Research on Machining Simulation of Large Scale Spiral Bevel Gear Machine Tool

The machining coordinate system of spiral bevel gear machine tool is established through analyzing the machining mechanism of spiral bevel gear, and the three dimensional structure model of large scale spiral bevel gear machine tool is also established using the new cutting feed method. The numerical control machining model of large scale spiral bevel gear machine tool is proposed. Finally, the machining simulation of one pair of given large scale spiral bevel gears has been done, the result indicates that the machining simulation model meets the prospective design requirement.

Study on Post-Processing of Five-Axis Cnc Machining

According to the feature of five-axis CNC machine tools’structure, do researches on post-process technique. By means of analysis of the kinetic model of five-axis CNC machine tools, Position and orientation of the tool in the workpiece coordinate system transforms to that of the machine coordinate system. Besides, Because of the rotation，Five-axises maching will produce the non-linear error．This article provides an easy algorithm to find the maximum non-linear error. Finally, an impeller, for example, simulates in the computer and machine by a 5-axis machine tool.

Cutter Radius Compensation for Five- Axis Peripheral Milling in CNC System

This paper presents a 3D tool radius compensation for five- axis peripheral milling in CNC systems. Using a generalized kinematics model of five- axis machines and uniform algorithm, it is possible to calculate the complete analytical equations for the tool orientation relative to part. The tool orientation relative to part selecting as Z-axis is used to establish compensation coordinate system. The points in the machine coordinate system are converted to the compensation coordinate system. The compensation points in the compensation coordinate system using these points can be calculated. This computation procedure is similar to the calculation of cutter compensation C. The compensation points in machine coordinate system can be obtained by converting the compensation points in the compensation coordinate system to machine coordinate system. The presented methodology was used in the CNC system, which makes the CNC system more efficiently and systematically

Research on Milling Spiral Bevel Gear Using CТС Machine Tool

From the NC machining principle of spiral bevel gear, the motion relationship of machine tool, cutter and workpiece during machining spiral bevel gear was analyzed. Based on the machine coordinate system, the tooth surface equation and the cone equation of cutting edges of spiral bevel gear were put forward. 3D solid model of spiral bevel gear was constructed using Pro/E Wildfire2.0 software. The NC milling simulation was carried out by the VERICUT software, and at the same time, the experiment was researched. Simulation and experimental results show that the spiral bevel gear can adopt the method of CNC machining, and can ensure the high quality, especially for single or small batch production.

Virtual Machining on Helical Bevel Gear with New Type Heavy Duty Helical Bevel Gear Milling Machine Tool

The machining coordinate system of helical bevel gear machine tool is established through analyzing the machining mechanism of helical bevel gear, and the three dimensional structure model of the machine is also built adopting the new cutting feed method. The numerical control machining model of the machine is proposed. Finally, the virtual machining on one pair of given heavy duty helical bevel gears has been done, the virtual machining result indicates the virtual machining model meets the prospective design requirement.

Modeling and Analysis of Multi-station Dimensional Deviation Stream Based on Linear State Space Model

In order to solve the problem of implicit expression of system matrix in the state space model which describes the transfer of product dimensional deviation,and existence of nonlinear factors in traditional multi-station manufacturing systems,part model consists of surface orientation vector,position orientation vector,size vector and its variation model are given on the basis of definitions of part,fixture and machine coordinate systems;by using homogeneous transformation method and matrix theory,mutual transformation between three coordinate systems and linear state space model with explicit system matrix are derived.The relationship built between dimensional deviation and its corresponding deviation source enhances the usability of traditional state space model,and the effectiveness of the model is verified by a case study.

Characterization of residual stresses in heat treated Ti-6Al-4V forgings by machining induced distortion

To provide a solid base for improved material exploitation in dimensioning calculations it is necessary to determine the stress state in the part prior to service loading. In order to achieve higher material strength at elevated temperatures, the surface temperature gradient with respect to time has to be sufficiently high during heat treatment. This results in non-negligable residual stresses that can reduce the allowable load level upon which yielding occurs. For titanium alloys there are two common heat treatments, namely solution treatment and mill annealing. The latter one is the method of choice within the presented project. Mill annealing is utilized in order to significantly reduce the residual stresses in the parts without loosing much of the improved strength at elevated temperatures. Quantification of residual stresses is done by solving an inverse problem. From the measurement of distortion, induced by dividing the investigated part, the residual stress state can be calculated via analytical modeling or correlation with finite element models. To assure a minimum perturbation of the residual stress state during specimen production, dividing of the part is accomplished by electric discharge machining.The parts of interest are v-shaped prisms with a length of approximatly 450 mm and a thickness in the cross sectional area from about 20 mm to 45 mm. Figure 1(a) shows the forged part and 1(b) the dimensions of the cross section in millimeters as well as the material properties considered in the finite element model. The heat exchange between the part and the environment is modelled as heat transfer by convection superimposed with heat radiation. Since the parts are exposed to air during forging and heat treatment, the surface develops a strongly adhesive oxide layer, the so called alpha-case. After forging the parts are cooled in air and heat treated at a temperature of 720° C for a duration of 120 min. Subsequent air cooling and removing the alpha-case by shot peening and chemical treatment in a mixture of nitric and hydroflouric acid finishes the processing. The residual stress state in the parts is quantified through correlation of results obtained by finite element simulations and high precision deflection experiments. Experimental measuring errors are minimized by using the capability of the wire cutting machine to measure locations on the specimens with respect to the fixed machine coordinate system. A sophisticated design of cutting operations allows an analysis of the part without removing the fixture and hence makes it possible to achieve a very high accuracy of the displacement measurements of about 4 µm. The quality of the obtained results also depends on the finite element models describing the mechanics of the experimental investigations as precise as possible. Since numerical modeling of shot peening and chemical milling is rather complicated, the whole processing route of the shapes is simplified assuming that the individual contribution of all process steps to the residual stress state can be described by virtual heat transfer coefficients. The overall virtual heat transfer coefficient for the whole processing route is calculated from the linear superposition of the individual heat transfer coeffcients for each processing step. Detailed analysis of the obtained overall virtual heat transfer coefficients in comparison with experimentally obtained ones shows, that other processing steps besides the heat treatment considerably influence the residual stress state. In order to make the conducted simulation scheme applicable in practise, advanced finite element modeling techniques are developed. The experimentally derived deflection curves are correlated to the finite element results via a least square fit.

Towards a Noninvasive Intracranial Tumor Irradiation Using 3D Optical Imaging and Multimodal Data Registration

Conformal radiotherapy (CRT) results in high-precision tumor volume irradiation. In fractioned radiotherapy (FRT), lesions are irradiated in several sessions so that healthy neighbouring tissues are better preserved than when treatment is carried out in one fraction. In the case of intracranial tumors, classical methods of patient positioning in the irradiation machine coordinate system are invasive and only allow for CRT in one irradiation session. This contribution presents a noninvasive positioning method representing a first step towards the combination of CRT and FRT. The 3D data used for the positioning is point clouds spread over the patient's head (CT-data usually acquired during treatment) and points distributed over the patient's face which are acquired with a structured light sensor fixed in the therapy room. The geometrical transformation linking the coordinate systems of the diagnosis device (CT-modality) and the 3D sensor of the therapy room (visible light modality) is obtained by registering the surfaces represented by the two 3D point sets. The geometrical relationship between the coordinate systems of the 3D sensor and the irradiation machine is given by a calibration of the sensor position in the therapy room. The global transformation, computed with the two previous transformations, is sufficient to predict the tumor position in the irradiation machine coordinate system with only the corresponding position in the CT-coordinate system. Results obtained for a phantom show that the mean positioning error of tumors on the treatment machine isocentre is 0.4 mm. Tests performed with human data proved that the registration algorithm is accurate (0.1 mm mean distance between homologous points) and robust even for facial expression changes.

Accurate calibration of a stereo‐vision system in image‐guided radiotherapy

Image-guided radiotherapy using a three-dimensional (3D) camera as the on-board surface imaging system requires precise and accurate registration of the 3D surface images in the treatment machine coordinate system. Two simple calibration methods, an analytical solution as three-point matching and a least-squares estimation method as multipoint registration, were introduced to correlate the stereo-vision surface imaging frame with the machine coordinate system. Both types of calibrations utilized 3D surface images of a calibration template placed on the top of the treatment couch. Image transformational parameters were derived from corresponding 3D marked points on the surface images to their given coordinates in the treatment room coordinate system. Our experimental results demonstrated that both methods had provided the desired calibration accuracy of 0.5 mm. The multipoint registration method is more robust particularly for noisy 3D surface images. Both calibration methods have been used as our weekly QA tools for a 3D image-guided radiotherapy system.

SU‐FF‐J‐13: A Simple Method to Calibrate Stereo‐Vision System In Image‐Guided Radiotherapy

Purpose: To calibrate a 3D stereovision system (or 3D camera in short) with complex planning and treatment systems to implement a 3D image‐guided radiotherapy system. Method and Materials: A calibration template with a printed grid is used to correlate the treatment machine coordinate system to the 3D camera image coordinate system. The calibration template is carefully placed on the top of the treatment machine table and precisely aligned with the corresponding machine axes. A 3D surface image of the template is captured by a 3D camera mounted on the ceiling of treatment room. Nine points of the 3D surface image are manually digitized and used to calibrate two coordinate systems. Two algorithm, analytical method and multi‐points registration method are conducted to find the transformation parameters. The analytical method only needs 3 points to find a rigid transformation between two coordinate systems. Trigged by least‐squares approximation, the multi‐points method uses 9 points to find an optimum rigid transformation between two coordinate systems. Results: Results from a set of experiments demonstrated that both methods can provide acceptable accuracy (&lt;1‐mm) for the image‐guided radiotherapy. Ideally, all points of the template surface transformed from the calibration should be in the O‐XY plane but due to image nose and calibration uncertainty, large error (up to 2 mm) can be introduced by the analytical method. In contrast, for the same template image, the multiple‐point registration approach provided image transformation with the maximum error in the template plane less than 1.0 mm under the noisy circumstance. Conclusions: Both methods can be used for accurate image system calibration. However, the multiple‐point registration method yields more reliable results than that of the analytical method. Both methods are simple and the calibration can be conducted weekly with the QA of radiotherapy systems.

SU‐FF‐J‐84: Integration Of 3D Stereovision System In Image‐Guided Radiotherapy: A DICOM‐Based Method

Purpose: To seamlessly integrate a 3D stereovision system (or 3D camera in short) with complex planning and treatment systems and implement a 3D image‐guided patient positioning for radiotherapy of breast cancer. Method and Materials: A high speed commercially available 3D camera is installed on the ceiling of treatment room to capture real‐time 3D surface images of patient. After transformed from camera coordinate system to treatment machine coordinate system, the real‐time 3D surface image is matched with a 3D reference image which is defined in planning systems. A stack of external contours with position information from DICOM RP and RS files are re‐sampled to generate a 3D reference image. Regions of interest (ROIs) of both images are defined by the PTV parameters from DICOM RP files. An Iterative Closest Points (ICP) algorithm is adopted to conduct the registration of ROIs. The parameters of patient positioning derived from the registration can be used to adjust the patient position. Results: Outliers and noise of captured 3D surface images are automatically and effectively removed when applying PTV parameters to define ROIs. Phantom test and An institutional IRB approved clinical trial indicates a typical case of image‐guided RT, including loading reference images, capturing real‐time images, and image matching can be done in less than 1 min. By double checking with online EPID, a millimeter positioning accurate can be achieved without considering deformation of images. Conclusions: This work shows the clinical potential for utilizing a 3D camera in image‐guided RT. The integration of 3D camera in image‐guided RT based on DICOM standard provides faster and more precise patient positioning than other image‐guided RT. How to deal with the deformation of ROIs will be our research direction in future.