CNC Controller Research Articles

Review on the development of a hybrid incremental sheet forming system for small batch sizes and individualized production

Asymmetric Incremental Sheet Forming (AISF) has been developed as a flexible process for low-volume production of sheet metal parts. In AISF, a part is obtained as the sum of localized plastic deformations produced by a simple forming tool that moves under CNC control. In spite of about 20 years of research and development, AISF has not had much industrial take-up yet. The main reason for this is that attempts to improve, among other limitations, the accuracy, speed and range of feasible geometries of the process by adapted process strategies has not brought about general solutions. This paper presents an overview of the current state of development of hybrid asymmetric incremental sheet forming processes at RWTH Aachen University. The goal of the development of hybrid ISF processes is to allow for a quantum leap of the capabilities of AISF in order to enable a broader industrial use of AISF. Two hybrid process variations of AISF are presented: stretch forming combined with ISF and laser-assisted AISF. It is shown that the combination of stretch forming and AISF can improve the time per part, sheet thickness distribution and accuracy of the final part. Laser-assisted AISF is shown to enable the flexible forming of non cold-workable materials such as magnesium and titanium alloys when the forming conditions are adapted to the temperature and strain rate dependent formability of the sheet metal. In addition, first results of the forming of hybrid aluminum-steel sheet metal are shown.

Interest of Multiphysics and Multilevel Simulation Approaches to Enhance the Machining Process

The rise of high speed manufacturing gave birth to high capabilities machine tools; meanwhile the constraints have increased. To face these challenges, several simulation solutions have been proposed. They cover many aspects of the manufacturing process such as CNC controller behavior, cutting forces estimation or geometric errors. However, most of them center on a specific view of the machining process. In contrast, combined multiphysics and multiscale approaches are beneficial, in particular for surface integrity improvements purposes. This paper discusses the interest of using simulation approaches at different levels. The first target is to analyze existing parameterizations. Then, a comparison of several parameters tuning extends the space of acceptable solution and provides new enhancements. Finally, a simulation method for form defect of machined surfaces identification is introduced. Several simulation solutions developed in the laboratory are taken as illustrative examples.

Adaptive Control in CNC Sheet Metal Processing

Three different and complementary ways of adaptive control are presented, each working on a different time scale. All three of them are made possible due to evolutions and developments in CNC controls. The first and most popular way of adaptive control is adaptive process control. Since the process variables in sheet metal equipment change significantly in milliseconds, the control loop must also perform on a sub second time scale. The second adaptive loop is adaptive production control. Modern CNC controls know what they are manufacturing and talk with higher production software. Instead of just executing a planning that came top down, the CNC has enough knowledge of its environment that it can propose an intelligent job sequence and hence react more effectively on unforeseen situations. These adaptive loops perform on a time scale of hours and days. The third adaptive loop is adaptive resource control. To produce sheet metal parts, we need several kinds of resources: human resources, material, consumables, energy and equipment. CNC sheet metal equipment is a considerable investment. However most production managers do not know how effective their resources are being used. By applying the principles of overall equipment effectiveness, we can close the resource control loop. This loop performs on a time scale of weeks and months.

Investigation on Incremental Sheet Forming Combined with Laser Heating and Stretch Forming for the Production of Lightweight Structures

Asymmetric Incremental Sheet Forming (AISF) is a process for the flexible production of sheet metal parts. In AISF, a part is obtained as the sum of localized plastic deformations produced by a simple forming tool that, in most configurations, moves under CNC control. Flexible processes with low tooling effort like AISF are suitable for sectors with small lot sizes but premium products, e.g. for the aviation and the automotive sector. Four main process limits restrict the range of application of AISF and its take-up in industry. These are: (i) material thinning, (ii) limited geometrical accuracy, (iii) the process duration and (iv) the calculation time and accuracy of process modelling. Moreover, the material spectrum of AISF for structural parts is mostly restricted to cold workable materials like steel and aluminum. This paper presents some new investigations of incremental sheet forming combined with laser heating or stretch forming as possible hybrid approaches to overcome the above mentioned limitations of AISF. These hybrid incremental sheet forming processes can increase the technological and economical potentials of AISF. A possible application is the fabrication of lightweight sheet metal parts as individual parts or small batches, e.g. for the aerospace industry. The present study provides a short overview of the state of the art of AISF, introduces the new hybrid process variations of AISF and compares the capabilities of the hybrid processes and the standard AISF process. Finally, two examples for applications are presented: (i) the production of a part used in an airplane for which the manufacturing steps consist of die manufacture, sheet metal forming by means of stretch forming combined with AISF and a final trimming operation using a single hybrid machine set-up; (ii) laser-assisted AISF for magnesium alloys.

STEP-NC Compliant Intelligent Process Planning Module: Architecture and Knowledge Base

STEP-NC (ISO 14649) is a new feature-based machine tool control language between CAM and CNC. In contrast with conventional G&M code (ISO 6983), it could provide richer and more machining information for CNC controller at shop floor. Therefore, it's feasible to develop an embedded micro process planning module in CNC controller. Meanwhile, some other information models such as manufacturing resources are essential for completing the intelligent process planning. The main thrust of this paper is focused on two fields: i) Proposition of a framework for STEP-NC compliant controller, ii) Development of knowledge base with OWL for the process planning in CNC controller. A novel STEP-NC compliant controller named OntoStep CNC is presented, which could accept ISO 14649 machining models and then generate optimized tool path based on knowledge base and rules at the shop floor level.

A Unified Communication Framework for Intelligent Integrated CNC on the Shop Floor

The CNC controller is the termination executor located at the end of the whole manufacturing chain. The current CNC is relatively isolated from the upper planning and manufacturing modules owing to the older data interface ISO 6983. Furthermore, lack of effective integration of heterogeneous information at the distributed automation environment is becoming increasingly evident. As a result, the development of CNC's intelligence and integration is heavily restrained. In this paper, a unified communication framework is proposed, which takes STEP-NC as the information input from CAD/CAPP/CAM and adopts the real-time industrial Ethernet fieldbus for controlling and monitoring at the device level. Based on this framework, a knowledge base modeling approach using ontology theory is addressed. The validation of combining this communication framework with the open and intelligent CNC is also discussed in the end.

Expanding the Use of Robotics in Airframe Assembly Via Accurate Robot Technology

Serial link articulated robots applied in aerospace assembly have largely been limited in scope by deficiencies in positional accuracy. The majority of aerospace applications require tolerances of +/-0.25mm or less which have historically been far beyond reach of the conventional off-the-shelf robot. The recent development of the accurate robot technology represents a paradigm shift for the use of articulated robotics in airframe assembly. With the addition of secondary feedback, high-order kinematic model, and a fully integrated conventional CNC control, robotic technology can now compete on a performance level with customized high precision motion platforms. As a result, the articulated arm can be applied to a much broader range of assembly applications that were once limited to custom machines, including one-up assembly, two-sided drilling and fastening, material removal, and automated fiber placement.

Design of a STEP compliant system for turning operations

The changing economic climate has made global manufacturing a growing necessity over the last decade, forcing companies from East and West to collaborate beyond geographic boundaries in the design, manufacture and assembly of products. The ISO 10303 and ISO 14649 Standards (STEP and STEP-NC) have been developed to introduce interoperability into manufacturing enterprises so as to meet the challenges of responding to production on demand. This paper describes and illustrates a STEP compliant CAD/CAPP/CAM System for the manufacture of rotational parts on CNC turning centers. The information models to support the proposed system together with the data models defined in the ISO 14649 standard used to create the NC programs are also described. A structured view of a STEP compliant CAD/CAPP/CAM system framework supporting the next generation of intelligent CNC controllers for turn/mill component manufacture is provided. Finally a proposed computational environment for a STEP-NC compliant system for turning operations (SCSTO) is described. SCSTO is the experimental aspect of the research and is supported by information models that and have been constructed using a structured methodology and object-oriented methods. SCSTO was developed to generate a Part 21 file based on machining features to support the interactive generation of process plans utilizing feature extraction. A case study component has been developed to prove the concept of using the milling and turning parts of ISO 14649 to provide a turn-mill CAD/CAPP/CAM environment.

A new digital chain for additive manufacturing processes

Additive manufacturing is, nowadays, a higher process than most traditional processes (machining, etc.). But there still remains a domain where this process is not very competitive: that being its digital chain. Indeed, it is too poor in information to allow for the development of advanced additive manufacturing operations and therefore to compete with the traditional processes. This paper proposes to use the STEP-NC concept, which contains high-level information, in order to integrate the additive manufacturing processes in a complete STEP-NC digital chain in accordance with the norm work group committee ISO TC 184/SC 1. This paper proposes to develop this digital chain with a classical CNC controller, which is also present in machining equipment, and to hence benefit from their development possibilities.

Tool-path generation of planar NURBS curves

It has been widely used in CAD field for many years and gradually applied in CAM area with the prevalence of NURBS interpolator equipped in CNC controllers. But few of them provide the tool radius compensation function. In order to achieve the goal of generating tool-path, an algorithm was presented to offset NURBS curves by an optimum process for CAD/CAM systems in this paper. NURBS format is ideal for HSM applications, but not all NURBS outputs are equal and standard. Basically, there are two different ways to generate NURBS tool-paths; one is to fit a NURBS curve to the conventional tool-path output, the other one is to generate a NURBS tool-path from the start. The main targets for the tool-path of this paper are: (1) To keep a constant distance d between progenitor curve C( t) and offset curve C d ( t) on the normal direction of C( t); (2) to alternate the order k of the basis function in offset curve C d ( t); (3) to oscillate the number of control points of offset curve C d ( t) and compare it with progenitor curve C( t). In order to meet the tolerance requirements as specified by the design, this study offsets the NURBS curves by a pre-described distance d. The principle procedure consists of the following steps: (1) construct an evaluating bound error function; (2) sample offset point-sequenced curves based on first derivatives; (3) give the order of NURBS curve and number of control points to compute all initial conditions and (4) optimize the control points by a path searching algorithm.

An object-oriented architecture for sensorless cutting force feedback for CNC milling process monitoring and control

Intelligent Manufacturing Systems require fault-tolerant components capable of preventing and detecting failures, and optimizing their own performance. Although there are a number of process monitoring and control systems (PMCS) available for these tasks, most of them require special equipment that increases cost and setup time. This work proposes a standard software architecture and methodology for implementing a sensorless PMCS which separates data representation from its algorithmic processing. Two different case studies are introduced showing the feasibility and applicability of the proposed design, one for cycle time optimization and another one for online tool condition detection. A good performance was obtained in both case studies based on the proposed PMCS architecture, which can be implemented using different distributed software technologies.

Method for graphically evaluating the workpiece’s contour error in non-circular grinding process

To find and compensate the dynamic contour error of workpieces on CNC non-circular grinder, a graphic display and evaluation method is introduced. The programmed set and actual contours of workpiece are displayed through the outlines of grinding wheel at a series of set and actual positions. The dynamic contour error can be recognized directly on CNC control screen without extra measuring equipments as usual. The set and actual positions of grinding wheel related to workpiece are collected from the CNC controller and servo drives and further evaluated. This process is carried out after a dry-run cycle on the machine. Based on this method, a display and evaluation software module has been developed and integrated in the controller of a CNC non-circular grinder. It helps to find contour error and generate the compensation data. Grinding an eccentric test piece shows good result and advantage of the proposed method.

A nurbs curve interpolator with jerk‐limited trajectory planning

The control of the maximum acceleration and jerk in the trajectory planning of the CNC controller is very important because it affects the smoothness of the machined surface substantially. We proposed a parametric interpolator composed of a look‐ahead stage and a real‐time sampling stage for jerk‐limited acceleration planning. A pre‐sampling process was implemented in the look‐ahead stage to identify the segment points, where the acceleration changed across zero. A check of the acceleration and jerk was implemented and the feedrate was modified accordingly. In addition, a backtracking process was carried out during the deceleration stage to ensure that the modified feedrate was satisfied. In the real‐time sampling stage, an acceleration profile based on constant jerk was implemented for each region between two segment points. With this algorithm, we ensured that both the chord error and the maximum acceleration and jerk were within the allowable limits. Also, it kept the similarity of the speed profiles for adjacent cutting paths in zigzag machining.

Strategies to improve the geometric accuracy in asymmetric single point incremental forming

Asymmetric incremental sheet forming (AISF) is a manufacturing process for the small batch production of sheet metal parts. In AISF, a sheet metal part is formed by a forming tool that moves under CNC control. AISF currently has two dominant process limits: sheet thinning and a limited geometric accuracy. This paper focuses on the latter limit. It is shown with a pyramidal part that multi-stage forming can yield an increased accuracy compared to single-stage forming. However, due to residual stresses induced during forming, the accuracy of the as-formed part can be lost if the part is trimmed after forming. A case study with a car fender section shows that the geometric accuracy of the final part can be improved compared to single-stage forming by a combination of multi-stage forming and stress-relief annealing before trimming.

Robust real-time NURBS path interpolators

In a review of the real-time non-uniform rational B-splines (NURBS) path interpolation method in CNC controllers, it was found that none of the NURBS interpolators described in the literature has the necessary robustness against an extreme knot distribution. The problems begin with the calculation of the total length of the NURBS path: most interpolators handle knots as a global curve parameter and may deliver incorrect results if the knots are of an extreme distribution. Further, Taylor's-expansion-based NURBS interpolators may overlook the path portion of extremely small knot spans. To solve these problems, methods for improving the robustness of Taylor's-expansion-based NURBS interpolators are proposed in this study, and an improved robust fast NURBS path interpolator is described. These new methods process NURBS data on the basis of knot spans: the adaptive quadrature method is applied to each knot span, and the calculated lengths of all knot spans are summed to build the total path length. Further, the inverse length functions are also generated based on the knot spans, and a control mechanism is introduced to prove the validity of the resulting inverse length functions. Experimental results have proved the effectiveness of the proposed robust fast NURBS path interpolator.

Interoperable CNC System for Turning Operations

The changing economic climate has made global manufacturing a growing reality over the last decade, forcing companies from east and west and all over the world to collaborate beyond geographic boundaries in the design, manufacture and assemble of products. The ISO10303 and ISO14649 Standards (STEP and STEP-NC) have been developed to introduce interoperability into manufacturing enterprises so as to meet the challenge of responding to production on demand. This paper describes and illustrates a STEP compliant CAD/CAPP/CAM System for the manufacture of rotational parts on CNC turning centers. The information models to support the proposed system together with the data models defined in the ISO14649 standard used to create the NC programs are also described. A structured view of a STEP compliant CAD/CAPP/CAM system framework supporting the next generation of intelligent CNC controllers for turn/mill component manufacture is provided. Finally a proposed computational environment for a STEP-NC compliant system for turning operations (SCSTO) is described. SCSTO is the experimental part of the research supported by the specification of information models and constructed using a structured methodology and object-oriented methods. SCSTO was developed to generate a Part 21 file based on machining features to support the interactive generation of process plans utilizing feature extraction. A case study component has been developed to prove the concept for using the milling and turning parts of ISO14649 to provide a turn-mill CAD/CAPP/CAM environment.

Investigation into a new hybrid forming process: Incremental sheet forming combined with stretch forming

Asymmetric incremental sheet forming (AISF) is a process for the flexible production of sheet metal parts. In AISF, a part is obtained as the sum of localised plastic deformations induced by a simple forming tool that moves under CNC control. Three main problems exist in AISF: material thinning, geometric accuracy and the process duration. These limits restrict the range of applications of AISF. This paper focuses on a new hybrid process, the combination of AISF and stretch forming. First results are presented that show the positive impact of this hybrid process on the process limits.

A Novel Information Modelling Approach for Representing Parallel Kinematic Machine Tools

Today Parallel Kinematic Machines (PKM) are re-attracting attention in the manufacturing practice due to their advantages over traditional serial kinematic structures. Still very few PKMs are on the market due to limited CNC controller support. In addition, most commercial postprocessor development tools lack the capability to represent all aspects of parallel kinematic structures. A novel universal approach is proposed in this paper for representing PKM structures as well as serial kinematic machine tools. This universal approach can be adopted as a basis for developing a generic postprocessor to support serial and parallel kinematic machine tools.

STEPNCMillUoA: a CNC system based on STEP-NC and Function Block architecture

STEPNCMillUoA is the prototype of a new CNC system that utilises the STEP-NC data model and IEC 61499. STEP-NC provides a high-level data model and enables feature-based machining whereas the enabled layered Function Block architecture simplifies the design of the CNC controller. The architecture layers are responsible for data processing, storage and execution. The object-oriented Model-View-Control design pattern supports the system architecture and the design framework, in which simulation of the machining becomes natural and inherent part of the design process, with seamless transition from simulation to actual machining. This system possesses interoperability, portability, re-configurability and distribution characteristics. The system was tested through simulation and actual machining.

Dealing with feature interactions for prismatic parts in STEP-NC

Determining the precedence of machining features is a critical issue in feature-based process planning. It becomes more complex when geometric interaction occurs between machining features. STEP-NC, the extension of STEP (ISO 10303) standard developed for CNC controllers, is a feature-based data model. It represents all the geometric and topological product data minus feature interactions. In this paper, machining precedence of interactive and non-interactive STEP-NC features is discussed. Local and global precedence of machining features are defined on the basis of geometric constraints, such as geometric interaction of features and feature approach face and technological constraint such as access direction of the cutting tool. A software tool has been developed to visualize the STEP-NC part model and to generate the graphs of feature interaction and feature precedence. The output can be then used to augment the STEP-NC data in order to generate the optimal sequence of operations.