O. Remus Tutunea-Fatan

Determination of Geometry-Based Errors for Interpolated Tool Paths in Five-Axis Surface Machining

Five-axis computer numerical control (CNC) machining is characterized with a multitude of errors. Among them an important component comes from the computer-aided manufacturing software known as the geometry-based errors. A new and accurate method to determine these errors is presented in this paper as opposed to the conventional chordal deviation method. The present method allows establishing the exact linearly interpolated tool positions between two cutter contact points on a given tool path, based on the inverse kinematics analysis of the machine tool. A generic procedure has been developed to ensure wide applicability of the proposed method. Analytical derivation of the geometry-based errors provides insights regarding the origin of these errors and their affecting parameters. Due to the highly non-linear characteristics of the problem, analytical solutions can only be obtained for simple surface geometry. Numerical computation is able to determine the errors for general surface shapes but it would be difficult to uncover further insightful information from the calculated error values. Besides the local surface geometry, the configuration of the kinematic chain of the CNC machine has been found to be the primary factor controlling the resulting value and type of the geometry-based errors. Implementations with a typical complex free-form surface demonstrated that the conventional chordal deviation method was not reliable and could significantly underestimate the geometry-based errors.

Comparing the kinematic efficiency of five-axis machine tool configurations through nonlinearity errors

Five-axis CNC machines represent a particular class of machine tools characterized by superior versatility. Little attempts were made in the past to compare directly their performances through a common indicator. In this sense, the present study proposes nonlinearity error as a valuable method to quantify the kinematic efficiency of a particular five-axis configuration. Nonlinearity error is defined as the maximum deviation of the cutter-location point from the reference plane generated by the initial and final orientations of the tool during linearly interpolated motions of the cutter along the intended tool path. The proposed concept has demonstrated that nonlinearity error occurs approximately around the middle of the linearly interpolated interval and therefore has validated the current post-processing practice of halfway cutter-location point insertion. The employment of nonlinearity error in the evaluation of the kinematic efficiency of vertical spindle-rotating five-axis machine tools revealed that for an identical machining task, configurations involving the vertical rotational axis tend to move more than those involving only horizontal rotational axes.

On the B-spline interpolated tool trajectories for five-axis sculptured surface machining

B-spline interpolation scheme is now available on modern five-axis Computer Numerical Control (CNC) machine tools. With this newly implemented interpolation scheme, a cutting tool can be directly commanded to trace B-spline trajectories, which approximate ideal 3D curved trajectories, in sculptured surface machining. The approximation of ideal tool trajectories by B-spline interpolated tool trajectories inevitably leads to machining errors, referred to as the geometry-based errors in the present work. It is essential to ensure synchronisation of the movements of the three translational and two rotational joints of a five-axis machine tool to reduce the geometry-based errors. This paper presents an effective method to achieve synchronisation of the machine joint movements. It first fits a 3D B-spline for the three translational joints and then uses a knot inheriting procedure to fit a 2D B-spline for the two rotational joints. Evaluation of the presented method was made through the machining of a typical bi-cubic Bezier surface on a five-axis machine tool capable of performing non-uniform B-spline interpolation. It was found that the resulting geometry-based errors, which were varying along the given isoparametric tool paths, were able to be maintained below 25m.

Framework for evaluation of the relative contribution of the process on porosity–cutting force dependence in micromilling of titanium foams

Porous titanium, characterized by interconnected and large open-cell structures, constitutes one of the most promising bone substitutes that are currently available for surgical orthopedic and dental implantation procedures. Since little is known about the behavior of this highly porous material during material removal operations, the main objective of this study was to develop a framework capable of evaluating the effect of cutting speed, cutting depth, and feed rate on the interplay between porosity and cutting force signatures, as experienced during microslot cutting experiments. The comparisons performed between optically determined porosity and cutting force profiles by means of standard random data analysis metrics (correlation coefficient, power spectral density, and coherence) revealed that the presence of a material discontinuity has a prevalent effect on cutting force variation in the case of micromilling processes characterized by (1) less intensive machining regimes and (2) larger cutter/workpiece engagement zones. The proposed methodology is useful in selection of the investigative approach to be taken in assessment of the micromachining-related behavior of highly porous foams subjected to micromilling operations.

Application of collision detection to assess implant insertion in elbow replacement surgery

An important aspect of implant replacement of the human joint is the fit achieved between the implant and bone canal. As the implant is inserted within the medullary canal, its position and orientation is subjected to a variety of constraints introduced either by the external forces and moments applied by the surgeon or by the interaction of the implant with the cortical wall of the medullary canal. This study evaluated the implant-bone interaction of a humeral stem in elbow replacement surgery as an example, but the principles can also be applied to other joints. After converting CT scan data of the humerus to the parametric NURBS-based representation, a collision detection procedure based on existing Computer-Aided Engineering techniques was employed to control the instantaneous kinematics and dynamics of the insertion of a humeral implant in an attempt to determine its final posture within the canal. By measuring the misalignment between the native flexion-extension (FE) axis of the distal humerus and the prosthesis, a prediction was made regarding the fit between the canal and the implant. This technique was shown to be effective in predicting the final misalignment of the implant axis with respect to the native FE axis of the distal humerus using a cadaver specimen for in-vitro validation.

Experimental analysis of the process parameters affecting bone burring operations

The experimental quantification of the process parameters associated with bone burring represents a desirable outcome both from the perspective of an optimized surgical procedure as well as that of a future implementation into the design of closed-loop controllers used in robot-assisted bone removal operations. Along these lines, the present study presents an experimental investigation of the effects that tool type, rotational speed of the tool, depth of cut, feed rate, cutting track overlap, and tool angle (to a total of 864 total unique combinations) have on bone temperature, tool vibration, and cutting forces associated with superficial bone removal operations. The experimental apparatus developed for this purpose allowed a concurrent measurement of bone temperature, tool vibration, and cutting forces as a function of various process parameter combinations. A fully balanced experimental design involving burring trials performed on a sawbone analog was carried out to establish process trends and subsets leading to local maximums and minimums in temperature and vibration were further investigated. Among the parameters tested, a spherical burr of 6 mm turning at 15,000 r/min and advancing at 2 mm/s with a 50% overlap between adjacent tool paths was found to yield both low temperatures at the bone/tool interface and minimal vibrations. This optimal set of parameters enables a versatile engagement between tool and bone without sacrificing the optimal process outcomes.

Fabrication of right triangular prism retroreflectors through 3½½-axis ultraprecise single point inverted cutting

ABSTRACTRetroreflectors (RR) are passive optical structures that are capable of returning incident light back to the source. The focus of the current study is represented by the right triangular prism (RTP) geometry which could be a more efficient alternative to the traditional inverted corner cube geometry. While current manufacturing practices rely solely on the use of conventional pin-bundling techniques, the work reported in this study presents further enhancements of the previously introduced ultraprecise single point inverted cutting technique which can be used in a manner approximately similar to 3½½-axis kinematics. The experimental results obtained have demonstrated both the feasibility of the proposed fabrication approach as well as the optical viability of the fabricated RTP elements.

Minimization of Bone Removal through Optimal Humeral Implant Alignment in Total Elbow Arthroplasty

Total elbow arthroplasty (TEA) represents one of the surgical procedures performed on the upper limb in order to replace the diseased joint with a prosthetic device. According to current surgical standards, TEA is carried out with little information on the amount of bone to be removed in order to allow the installation of the implant within the medullary canal of the humerus. To address this, the present study proposes a numerical technique capable to estimate both the amount and location of the bone to be removed from the canal. As a first step, the developed method entails the extraction of the outer and inner contours of the bone based on the raw CT data. Then, global optimization search built on a gradient-based solver was used to identify the implant posture which minimizes the total interference amount as quantified across the entire length of the analyzed humeral specimen. After the proposed approach was tested on three different specimens and compared with a computationally-intensive baseline, cli...

Assessing the performances of collision driven numerically-simulated implantation in elbow replacement surgery

Total elbow arthroplasty is a common surgical procedure used to replace diseased joints with an implant attempting to restore at least partially the lost functionality of the articulation. Given the relative paucity of studies attempting to simulate implant kinematics during insertion motions, the primary objective of the present study was to assess the feasibility and performances of conventional numerical computer-aided engineering (CAE) techniques in this biomechanical context. The results obtained revealed that while both CAE-driven and experimental navigated implantation techniques will yield comparable FE axis misalignment errors, the numerically-simulated approaches seem to be capable of providing more insight on the motion dynamics/kinematics due to their inherent level of maturity. Based on this, it was concluded that numerically-simulated techniques offer less invasive and more comprehensive means for implant motion control and visualisation, and therefore they should be further perfected for implant design as well as preoperative virtual surgery applications.

The Effect of Backing Profile on Cutting Blade Wear During High Volume Production of Carbon Fiber-Reinforced Composites

As vehicle lightweighting continues to become a widespread trend in the automotive industry, production methods for composite materials must continue to improve. Carbon fiber SMC (sheet molding compound) utilizes rotary chopping to produce sheets of composite material which can be molded to form lightweight vehicle parts, but high blade wear rates are seen when cutting carbon fibers. Experiments were performed to examine the wear progression of cutting blades during rotary carbon fiber chopping and to investigate the effect that profiled backing has on blade wear. Blade wear measurements were obtained by measuring worn regions and blade tips, and extensive micrographs of blade surfaces at different wear levels were collected. Most intentionally-profiled backings did not improve blade wear rates; however, backings prone to forming their own deep grooves reduced wear rate by up to 65%. Several explanations of how wear occurs and progresses under different conditions were developed.