Erik L. J. Bohez

Five-axis milling machine tool kinematic chain design and analysis

Five-axis CNC machining centers have become quite common today. The kinematics of most of the machines are based on a rectangular Cartesian coordinate system. This paper classifies the possible conceptual designs and actual existing implementations based on the theoretically possible combinations of the degrees of freedom. Some useful quantitative parameters, such as the workspace utilization factor, machine tool space efficiency, orientation space index and orientation angle index are defined. The advantages and disadvantages of each concept are analyzed. Criteria for selection and design of a machine configuration are given. New concepts based on the Stewart platform have been introduced recently in industry and are also briefly discussed.

Morphological study of the proximal femur: a new method of geometrical assessment using 3-dimensional reverse engineering

This study presents a new method of using computerized tomography images combined with the reverse engineering technique to obtain and analyse the three-dimensional inner and outer geometry of the proximal cadaveric femur. Three-dimensional models were reconstructed from the computerized tomography images and approximated with 2D and 3D fitting algorithms based on reverse engineering methods. The following parameters were calculated for each femur: femoral head diameter, femoral neck axis, femoral shaft axis, anteversion angle and neck-shaft angle. These data represent the geometry of the studied proximal femur, and can be used for the design of proper size and shape of femoral prostheses and trochanteric nail systems.

Medical rapid prototyping applications and methods

Purpose – Aims to investigate medical rapid prototyping (medical RP) technology applications and methods based on reverse engineering (RE) and medical imaging data.Design/methodology/approach – Medical image processing and RE are applied to construct three‐dimensional models of anatomical structures, from which custom‐made (personalized) medical applications are developed.Findings – The investigated methods were successfully used for design and manufacturing of biomodels, surgical aid tools, implants, medical devices and surgical training models. More than 40 medical RP applications were implemented in Europe and Asia since 1999.Research limitations/implications – Medical RP is a multi‐discipline area. It involves in many human resources and requires high skills and know‐how in both engineering and medicine. In addition, medical RP applications are expensive, especially for low‐income countries. These practically limit its benefits and applications in hospitals.Practical implications – In order to transfe...

Compensating for systematic errors in 5-axis NC machining

The errors introduced during 5-axis machining are higher than the intrinsic repeatability of the machine tool. It is possible to identify such systematic errors and compensate for them, thus achieving higher performance. A group of systematic errors can be compensated for directly in the inverse kinematics equations. Other systematic errors can be combined and compensated for through the total differentials of the inverse kinematics relations. A new general approach on how to compensate for the systematic errors based on the closed loop volumetric error relations is presented. The errors due to the 5-axis toolpath generation in current CAD/CAM and CNC are analyzed in detail. A new strategy for measurement and compensation is presented.

A geometric modeling and five-axis machining algorithm for centrifugal impellers

Abstract Unlike in three-axis machining, in five-axis machining the flexibility in tool orientation in space is extremely high. To obtain the full advantage of five-axis machining, the computer-aided manufacturing (CAM) system must have similar flexibility in tool orientation. Unfortunately, most CAM systems that support five-axis machining do not provide the required flexibility. A centrifugal impeller is a perfect example of a part that can be efficiently designed and manufactured with the help of a computer. Machining these types of complex shapes requires a CAM system with a high degree of flexibility in tool orientation. The use of programming languages is one of the best methods for eliminating the drawbacks of existing CAD/CAM systems and to obtain the flexibility required. This paper explains how flexibility is obtained using interactive geometric modeling and a programming language provided by a CAD/CAM system to access the geometry.

Design for medical rapid prototyping of cranioplasty implants

Design methods for medical rapid prototyping (RP) of personalized cranioplasty implants are presented in this paper. These methods are applicable to model cranioplasty implants for all types of the skull defects including beyond‐midline and multiple defects. The methods are based on two types of anatomical data, solid bone models (STereoLithography files – STL) and bone slice contours (Initial Graphics Exchange Specification – IGES and StrataSys Layer files – SSL). The bone solids and contours are constructed based on computed tomography scanning data, and these data are generated in medical image processing and STL slicing packages.

Systematic geometric rigid body error identification of 5-axis milling machines

A 5-axis milling machine has 39 independent geometric error components when the machine tool is considered as a set of five rigid bodies. The identification of the deterministic component of the systematic error is very important. It permits one to improve the accuracy close to the repeatability of the machine tool. This paper gives a new way to identify and compensate all the systematic angular errors separately and then use them further to identify the systematic translational error. Identification based on a new mathematical method and a stable numerical solution method is proposed. The model explains from first principles why some error components have no effect in a first order model. The identification of the total angular systematic errors can be done independently from the translation errors. However, the total translation error depends on the angular errors and the translation errors of each machine tool slide. The main problems solved are to find enough linear independent equations and avoid numerical instability in the computation. It is important to separate numerical problems and linear dependence. The very complex equations are first analyzed in symbolic form to eliminate the linear dependencies. The total of linear independent components in the model is reduced from 30 to 26 for the position dependent errors and from 9 to 3 for the position independent components. Secondly, the large system of linear equations is broken down in many smaller systems. The model is tested first with simulated errors modeled as cubic polynomials. An artifact-based identification is proposed and implemented based on drilling holes in various locations and orientations. New ways to measure the volumetric error directly are proposed. Direct measurement of the total volumetric error requires considerably less measurement than measuring all 6 components of each machine slide especially in the case of a 5-axis machine.

Optimization of rotations of a five-axis milling machine near stationary points

Abstract We consider a new algorithm designed for five-axis milling to minimize the kinematics error near the stationary points of the machined surface. Given the tool orientations, the algorithm optimizes the required rotations on the set of the solutions of the corresponding inverse kinematics equations. We solve the problem by means of the shortest path scheme based on minimization of the kinematics error. We present an application of the proposed algorithm to tool-path planning and demonstrate the efficiency of the proposed scheme verified by practical machining.

Applying linguistic criteria in FMS selection: fuzzy‐set‐AHP approach

This paper presents a computational framework that combines both fuzzy sets and analytical hierarchy process (AHP) for selecting the best‐ranked flexible manufacturing system from a number of feasible alternatives. Fuzzy sets are employed to recognize the selection criteria as linguistic variables rather than numerical ones, which, in turn, makes the framework quite user‐friendly. AHP is used to determine the due weight of the selection criteria, in accordance with their relative importance. In total, 14 criteria are considered, grouping them into flexibility, cost, productivity, and risk. The criteria under the first three groups are independent (i.e. their own fuzzy sets evaluate them) and the criteria under risk are indirectly evaluated by using the fuzzy sets of the criteria under flexibility. The proposed framework is implemented by developing an expert system called FmsExpert, using Borland C++. The performance of this system is also demonstrated by using an example.

The sweep plane algorithm for global collision detection with workpiece geometry update for five-axis NC machining

A new algorithm based on the sweep plane approach for global collision detection for five-axis NC machining is presented. This algorithm takes into account not only collisions between the tool and workpiece, but also collisions between the other parts of the CNC machine, especially the change of the workpiece geometry is included in the detection process. The workpiece and machine bodies are firstly approximated by an octree of bounding spheres. Collision detection is conducted between these spheres. If there is any interference between these bounding spheres, their subspheres are further tested. The subdivision process is recursively performed until the resolution reaches the desired precision level. If there is no interference between the spheres, there is no need to subdivide any more. When the interference is detected between the spheres in the last octree level, the slices within these colliding spheres are further checked by using the sweep plane algorithm to determine whether the enclosed objects really collide with each other. In the sweep plane algorithm, most of the slices of the moving bodies stay parallel and their collisions are detected by checking the interference between these parallel slices using 2D polygon clipping. Whereas, if the slices are not parallel to the reference slicing direction (due to the rotary axes), the interference detection is conducted by examining overlaps of the projections of these slices on the three perpendicular planes XY,YZ, and ZX. The accuracy of the algorithm can be adjusted by changing the distance between the sweep planes. The algorithm can be applied to any five-axis CNC machines.