A task-dependent approach to minimum-deflection fixtures

Presents an approach to planning minimum-deflection fixtures for tasks whose characteristics are well understood. Based on an accurately defined notion of deflection, we define a quality measure that characterizes the workpiece's deflection with respect to a set of external wrenches determined by the tasks. A scheme is proposed to model task wrenches, which can be used for practical manufacturing operations. This task modelling scheme is then used to obtain a convenient formulation of the task-dependent quality measure, which allows the quality measure to be efficiently computed. An example is presented to show that our approach can be effectively employed for planning compliant fixtures that are best suited to specified tasks

Constructing minimum deflection fixture arrangements using frame invariant norms

This paper describes a fixture planning method that minimizes object deflection under external loads. The method takes into account the natural compliance of the contacting bodies and applies to two-dimensional and three-dimensional quasirigid bodies. The fixturing method is based on a quality measure that characterizes the deflection of a fixtured object in response to unit magnitude wrenches. The object deflection measure is defined in terms of frame-invariant rigid body velocity and wrench norms and is therefore frame invariant. The object deflection measure is applied to the planning of optimal fixture arrangements of polygonal objects. We describe minimum-deflection fixturing algorithms for these objects, and make qualitative observations on the optimal arrangements generated by the algorithms. Concrete examples illustrate the minimum deflection fixturing method. Note to Practitioners-During fixturing, a workpiece needs to not only be stable against external perturbations, but must also stay within a specified tolerance in response to machining or assembly forces. This paper describes a fixture planning approach that minimizes object deflection under applied work loads. The paper describes how to take local material deformation effects into account, using a generic quasirigid contact model. Practical algorithms that compute the optimal fixturing arrangements of polygonal workpieces are described and examples are then presented

A stiffness-based quality measure for compliant grasps and fixtures

This paper presents a systematic approach to quantifying the effectiveness of compliant grasps and fixtures of an object. The approach is physically motivated and applies to the grasping of two- and three-dimensional objects by any number of fingers. The approach is based on a characterization of the frame-invariant features of a grasp or fixture stiffness matrix. In particular, we define a set of frame-invariant characteristic stiffness parameters, and provide physical and geometric interpretation for these parameters. Using a physically meaningful scheme to make the rotational and translational stiffness parameters comparable, we define a frame-invariant quality measure, which we call the stiffness quality measure. An example of a frictional grasp illustrates the effectiveness of the quality measure. We then consider the optimal grasping of frictionless polygonal objects by three and four fingers. Such frictionless grasps are useful in high-load fixturing applications, and their relative simplicity allows an efficient computation of the globally optimal finger arrangement. We compute the optimal finger arrangement in several examples, and use these examples to discuss properties that characterize the stiffness quality measure

Evaluation of ceramics for stator application: Gas turbine engine report

Current ceramic materials, component fabrication processes, and reliability prediction capability for ceramic stators in an automotive gas turbine engine environment are assessed. Simulated engine duty cycle testing of stators conducted at temperatures up to 1093 C is discussed. Materials evaluated are SiC and Si3N4 fabricated from two near-net-shape processes: slip casting and injection molding. Stators for durability cycle evaluation and test specimens for material property characterization, and reliability prediction model prepared to predict stator performance in the simulated engine environment are considered. The status and description of the work performed for the reliability prediction modeling, stator fabrication, material property characterization, and ceramic stator evaluation efforts are reported

Design study for a magnetically supported reaction wheel

Results are described of a study program in which the characteristics of a magnetically supported reaction wheel are defined. Tradeoff analyses are presented for the principal components, which are then combined in several reaction wheel design concepts. A preliminary layout of the preferred configuration is presented along with calculated design and performance parameters. Recommendations are made for a prototype development program

The effect of tool fixturing quality on the design of condition monitoring systems for detecting tool conditions

Condition monitoring systems of machining processes are essential technology for improving productivity and automation. Tool wear monitoring of cutting tools is one of the important applications in this area. In this paper, the effect of collet fixturing quality on the design of condition monitoring systems to detect tool wear is discussed. The paper investigates the difference in the system's behaviour and the changes in the condition monitoring system when the cutting tool is not rigidly fastened to the collet. A group of sensors, namely acoustic emission, force, strain, vibration and sound, are used to design the condition monitoring system. Automated Sensor and Signal Processing Selection (ASPS) approach1 is implemented to address the effect of the tool holding device (collet) on the monitoring system and the most sensitive sensors and signal processing method to detect tool wear. The results prove that the change in the fixturing quality could have significant effect on the design of the condition monitoring system and the behaviour of the system

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Automated fixture design for a rapid machining process

Rapid prototyping techniques for CNC machining have been developed in an effort to produce functional prototypes in appropriate materials. One of the major challenges for rapid machining is to develop an automatic fixturing system for securing the part during the machining process. The method proposed in this paper is the use of sacrificial fixturing, similar to the support structures in existing rapid processes like Stereolithography. During the machining process, sacrificial supports emerge incrementally and, at the end of the process, are the only entities connecting the part to the stock material. This paper presents methodologies for the design of sacrificial support structures for a rapid machining process and illustrates them using a complex sample part machined in the laboratory

Analytical and finite element modeling of a machining system to minimize inaccuracy in milling and using rapid prototyping for die manufacturing

The end milling process is used extensively in a gamut of manufacturing areas. It accounts for up to 40% of the cost of fabrication of non-electrical parts for a high performance aircraft. This economically justifies the effort to find ways to reduce inaccuracy caused in milling by workpiece deformation, fixture deflection and cutter deflection to improve the quality of parts. The process is also used extensively for roughing and finishing of dies. However, the conventional die manufacturing process, which uses the milling process, is too time consuming because of the extensive CNC programming involved. Furthermore skilled labor required for CNC programming accounts for the high cost of die manufacturing. Therefore new processes need to be developed that will eliminate CNC programming and possibly reduce the usage of the milling process thereby reducing the cost and time required to produce parts;An analytical non-linear optimization model has been developed which can determine the maximum inaccuracy due to workpiece deformation and the optimal clamping forces that are required to minimize work piece deformation while ensuring that the workpiece will not slip during machining. However, this model assumes rigid fixturing elements and is only suitable for simple workpiece shapes;A finite element model and a simple novel algorithm has been developed which has the same objective as the analytical non-linear optimization model. This model can be used for any complex shaped workpiece or fixture. The model also takes into account the flexibility of fixtures;Inaccuracy in machining is also caused by deflection of the tool. A study of the deflection of a milling cutter due to the action of the cutting forces was performed. An analytical equation was developed to determine the deflection of an end mill under a cutting force. The equation was verified by modeling the complete geometry of a four flute milling cutter using the finite element analysis module of I-DEAS software. The deflections obtained by the finite element model were exactly the same as those obtained by using the analytical equation. Previous researchers modeled the milling cutter as a simple cylinder which resulted in some error in the result;Three die manufacturing processes are proposed, namely, the casting prototype process, the EDM milling process and the copy milling process. All three processes use rapid prototyping to eliminate costly and time consuming CNC programming. All the three processes are economical compared to conventional processes provided there are large number of surfaces on the part. If the part has very few surfaces the conventional process will require less time for CNC programming making it more efficient. The Casting prototype process does not use milling whereas the EDM milling uses milling for rough machining. These two process would minimize inaccuracy in parts by eliminating milling or using milling to remove the rough stock only. The copy milling process uses milling but the models developed here can be used to minimize error in this process. All three processes have the additional advantage that they are more time efficient and economical than the conventional process of making dies