Structural and dynamic analyses of a new machining fixture help Porsche AG get production up and running faster. Porsche contracted with KTW Konstruktion Technik K. Weisshaupt GmbH of Friedrichshafen, Germany, to design and produce the fixtures required to hold automobile motor parts in place during drilling and high-precision milling. It has been noted that using a 3-D computer-aided design (CAD) solid modeler can point out weaknesses before a design leaves the computer. In order to avoid the need for costly generations of prototypes prior to the casting process, Speedy Engineering in Immenstaad, Germany, used finite element analysis software to perform linear static and dynamic (vibration) analyses of the motor component fixture geometry within only a few days of its design, to determine displacements and eigenfrequencies of the geometry. The results of the static and dynamic analyses enabled Speedy to specify maximum working loads for •production, which guaranteed success with the first use of this fixture design.
NOT LONG AGO, the company that manufactures Porsche automobiles- Dr.Ing.h.c. F. Porsche AG of Stuttgart, Germany-decided that it needed several fixtures to hold automobile motor parts in place during drilling and high-precision milling. The fixtures would be mounted on a palette and had to fit in a limited space. In addition, they had to allow access from all sides to the part being drilled or milled. The fixtures had to be constructed of cast iron, a strong material, in order to achieve a sufficient level of stiffness. Porsche contracted with KTW Konstruktion Technik K. Weisshaupt GmbH of Friedrichshafen, Germany, to design and produce the fixtures.
In this project, KTW used PRO/Engineer from Parametric Technology Corp. of Waltham, Mass., to create a 3-D CAD solid model of the fixture geometry. KTW then approached Speedy Engineering of Immenstaad, Germany, a company offering finite-element analysis software and services. Speedy was hired to interface the CAD model with FEA software to predict the behavior of the fixture design in advance. This analysis work was to take place before the first fixture prototype was produced, to assure smooth commencement of operations.
Manufacturers of fixtures that secure castings for drilling and milling can use a traditional approach to the design cycle. In this case, the design would be modeled in a 2-D CAD program based on experience and over-dimensioning. This method leaves a design's behavior to be discovered much later, during the first stage of prototype testing.
Using a 3-D CAD solid modeler can point out weaknesses before a design leaves the computer. Solid modeling also tends to reduce the error rate during design and construction because 3-D models are easier to understand than 2-D drawings. Design discussions are more efficient and less problematic when the object can be viewed in 3-D as well as in 2-D drawings. The ability to view and rotate shaded models on a computer adds even more realism to the visualization of designs.
To avoid the need for costly generations of prototypes prior to the casting process, Speedy used FEA software to perform linear static and dynamic (vibration) analyses of the motor component fixture geometry within only a few days of its design, in order to determine displacements and eigen-frequencies of the geometry. The eigen-frequency, or more precisely the eigenvalue natural frequency, is the frequency at which a given structure will naturally vibrate. If a power-driven device (such as a motor, for example) attached to the structure should produce such a frequency, the resulting resonance could, given sufficient power, destroy it. FEA can yield precise results, provided that the correct material properties: forces, and boundary conditions are applied. Speedy studied KTW's design using software from Algor Inc. of Pittsburgh.
KTW provided Speedy Engineering with a rough draft of the geometry in universal IGES file format, a common method of exchanging geometry when the CAD and FEA systems are on separate computers. On reviewing the first IGES file, it was clear that a change in export parameters was needed, but once the final CAD geometry file was received, final meshing and FEA computation were finished in two days. The results confirmed that the fixture was stiff enough to fulfill all of Porsche's requirements, and the casting process could begin.
To shorten the operation start-up time, Speedy suggested to KTW that it also analyze the dynamic behavior of the fixture component. Every machined part has to have a high-quality surface. If a vibrating tool causes resonance, the machined part's surface may be unacceptably rough. If the machined part, the fixture, the machine, or a combination of these items has resonance frequencies resembling those of the tool frequencies, the surface is destroyed and the part becomes unusable. It is important, therefore, to choose machining frequencies that will not cause resonance.
Typically, trial-and-error tests must be performed to determine resonant frequencies before a new machining component is put into use. The new part is used with varying support and rotational speeds until settings are found that avoid resonant frequencies. Despite considerable costs, this trial-and-error method avoids loss of product and production time later.
Dynamic analysis can reduce the need to use the trial-and-error method to test for resonant frequencies. A dynamic analysis can be carried out on the model used for the displacement analysis, provided that the FEA model's mesh is fine enough. A dynamic analysis determines the destructive eigenfrequencies and typical displacements for each mode. This is sufficient to define frequency windows, which can be used to set up the milling process.
Only the base frequencies and their first two harmonics are energetically relevant and have to be computed. In our case, the fixture needed to be fully constrained at the six mounting points on the palette (so no rigid body modes had to be computed) and the first 30 eigen-frequencies were computed. When looking at the displacement produced by each frequency, the analyst needs to consider two factors: whether the displacement is occurring in an area of the model that might cause damage to the surface of the component being milled, and how much energy the mode has (base and low frequencies have more energy than higher frequencies). By considering these two factors for each mode, Speedy determined which frequencies would cause undesirable vibration.
The accuracy of the computation depends on the precision of the material data. Based on the analysis results and accounting for the fact that the stiffness of cast iron often varies more than 10 percent, Speedy recommended a practical frequency window width between 25 Hz and 40 Hz-in other words, if such a distance is kept between the two adjacent eigen-frequencies, this should be sufficient to prevent the machining tools from causing resonance. Usually, in such cases, the weight and stiffness of the parts being machined have to be taken into consideration. A complete analysis would normally include the machined part as well as the fixture, since the total assembly could develop local eigen-frequencies that would result in undesirable effects on the surface quality.
However, since only relatively lightweight aluminum motor parts are machined and the fixture is cast iron, the additional mass is less than 5 percent of the fixture's mass and would not have a significant effect.
To verify the results, Speedy measured a physical prototype of the fixture acoustically. Engineers recorded the sound spectrum at different parts of the fixture with a simple microphone and a Pc. The resulting fast Fourier transform analysis gave them precise frequencies to compare with the computed eigen-frequencies. The measured frequencies differed from the analysis results by less than 3 Hz in the frequency range of 20 Hz to 300 Hz.
The results of the static and dynamic analyses enabled Speedy to specify maximum working loads for production, which guaranteed success with the first use of this fixture design. Knowledge of be ha vi or and displacement magnitudes of fixtures makes it possible to compute permissible forces for drilling and milling to ensure high efficiency for the creation of machining programs.
Performing static and dynamic analyses typically takes only a few days. This time investment pays off later in the production cycle, avoiding the need for reworking or even a second prototype, whose construction could delay the project by several weeks.