SAN-OM-29
Fixture design for the machining of thin compliant workpieces
LITERATURE REVIEW
Fixture design for the machining of thin compliant workpieces:
In aerospace engineering, one of the most important requirements regards low weight. Since thin plates are not considered as the primary structure of an aircraft, pockets are proposed in their design to reduce the weight of the structure without affecting the stiffness. Thin plates are usually machined using back-plate setup for full support. However, because of the high cost of the tooling system produced specifically for each part model having different and complex surfaces, flexible systems for such parts have been the subject of different researches in the last years. The fixture setups are required to meet the conditions such as positive location, repeatability, rigidity, interference, and positioning fundamentals.
Machining errors and their prediction :
In machining of thin components, geometrical errors generally appear because of the cutting deviations (machining deflections) of the process. Machining deflections often occur in the milling process due to the low-rigidity of these components that are thin-walled (floored). There are three major errors issue from the cutting processes; geometric, thermal and cuttingforce induced errors (Ramesh et al., 2000, Ramesh et al., 2000). The geometric error is also due to the inaccuracy of the machine tool and the components in the system-assembly.
In support to other researches focusing on prevention and compensation of the cutting deflections occurring during the machining of thin components, many scientists have attempted in their research works to predict the cutting errors due to the deflections of cutting tool system and thin walls during machining (Figure 1.4). Kline et al. (1982) developed a mathematical model for the prediction of the surface error profile with combining cutter deflection and workpiece deflection under effect of cutting forces. The authors developed three models of the cutting force system, the deflection of the cutter, and the deflection of the workpiece. The chip load of the cutting tool was applied to compute the cutting forces in different elements. The cutter was considered as a cantilever beam deflected under the action of the cutting forces in the X and Y direction at each cutting forces’ center. A thin walled workpiece was examined with three edges clamped while the rest of edge was free to deflect. The finite element method was applied to predict the workpiece deflection under effect of Y cutting force located at the center of the force distribution in the Y direction. In this research, the authors considered the combination of the cutting forces as well as the cutter and workpiece deflections into their modeling of cutting error prediction. However, their works suffer from the fact that the effect of the cutter and workpiece deflections were not taken into account in their cutting forces calculation.
Error compensation in machining of thin compliant workpieces:
In avoiding the geometrical errors of machined parts following the machining process, many research works have been carried out to reduce the cutting deviations in machining of thin parts. They can be classified as two groups: deflection avoidance and deflection compensation. Researches in the group of deflection avoidance have attempted to improve the rigidity of the machining system. However, this kind of method requires a high cost to develop and manufacture the physical structures of the system. Researches related to deflection compensation regard the development of cutting models to optimize the tool paths in order to compensate the cutting deviations. By computing the machining deflections, a new tool path is optimized until the cutting deviation converges to a reasonable value.
Wang et al. (2002) developed a static/quasi-static error compensation system composed of an interpolation algorithm based on shape functions for error prediction, and a recursive software compensation procedure. Based on the proposed schemes, a practical error compensation system incorporated with an automatic NC code identifying/rewriting system was developed for multi-axis machines.
Summary of literature review :
Recent research works related to fixture design for the milling of thin components proposed different fixture methods using adjustable support pins at the backplate of the workpieces. The pins in these cases provide support in order to avoid the material deformation of plates during the machining process; as well, the special mechanisms for the adjustable pins are designed to match the fixture structure. However, for the machining of large sized thin workpieces, geometrical errors still appear in the un-supporting areas.
In this research work, we propose a test bed system offering partial support to the backface of the workpieces during machining in order to reproduce existing commercial systems. A model using the finite element method is also proposed in order to predict the cutting deviations during the milling process of thin plates using the introducing flexible setup system. Recent research works for the analysis of the machining process of thin walled components are referenced to our proposed model related to the milling process of thinfloored components. The proposed test bed is utilized to mill slots with different machining conditions in order to validate the results of the proposed analysis model.
Finally, the compensation process using a mirror technique is applied to compensate the cutting deviations during machining of thin plates using the proposed system.
EXPERIMENTAL METHODOLOGY:
As discussed, the removal of support to the bottom face of a workpiece leads to geometrical errors after machining of thin/compliant plates using the proposed flexible setup configuration. In this section, experimental tests under different cutting conditions are carried out to verify the results of the prediction model regarding the cutting deviations during the slot milling of thin plates using the proposed flexible setup.
Raw material preparations:
As already mentioned, the raw material consists of aluminum 6061 – T6 plates of 120 x 120 x 6.35 (mm). Four holes of 8.4 mm diameter on the corners are used to fix the plate while two holes of 6mm diameter are machined to be utilized as references during the inspection process.
A lead-in pocket (blind hole in the middle of the part) was also prepared to avoid the penetration of the tool during the beginning of the machining, as shown in Figure 3.1. The depth of the lead-in pocket equals the depth of cut utilized in the slot milling tests and its diameter of 19 mm, is a little larger than the cutting tool’s diameter to avoid interference. The depth of cut of the slot is 4.0 mm.
The preparation process is conducted using a backplate support to prevent any deformation at this stage.
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Table des matières
INTRODUCTION
CHAPTER 1 LITERATURE REVIEW
1.1 Fixture design for the machining of thin compliant workpieces
1.2 Machining errors and their prediction
1.3 Error compensation in machining of thin compliant workpieces
1.4 Summary of literature review
CHAPTER 2 MODELING OF CUTTING ERRORS DURING MACHINING OF THIN COMPONENTS USING A FLEXIBLE SETUP CONFIGURATION
2.1 Problem definition
2.2 Methodology
2.2.1 Workpiece’s geometry and geometrical profile inputted to analysis
2.2.2 Material property
2.2.3 Cutting forces simulation
2.2.4 Meshing
2.2.5 Application of cutting forces
2.2.6 Boundary conditions
CHAPTER 3 EXPERIMENTAL METHODOLOGY
3.1 Raw material preparations
3.2 Identification of geometrical profiles of workpieces before and after machining
3.3 Machining tests using the flexible configuration setup
CHAPTER 4 RESULTS AND DISCUSSIONS
4.1 Cutting forces
4.2 Central point displacements
4.3 Geometrical errors
CHAPTER 5 COMPEANSTIONS OF CUTTING DEVIATIONS IN MILLING OF THIN COMPONENTS USING THE FLEXIBLE SETUP
5.1 Methodology
5.2 Results and discussions
CONCLUSION
LIST OF REFERENCES
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