TedA (Stranger
)
10/09/06 01:42 PM


You mentioned in your post that CETOL is based on vectorloop technology. This is no longer a valid comment, even though the original research with Sigmetrix and Dr. Chase involved vector loop models, and Dr. Chase continues to use vector loop models for some of his publications and explanations on the ADCATS website. Early releases of TI/TOL (the precursor to CETOL) were based on a vectorloop model. That was from 1994 through 1998. There are many inherent limitations to the vectorloop approach. The current CETOL software uses a much more sophisticated modeling approach that has two components: a kinematic assembly modeler with advanced joints based on precise contacting surfaces and a part modeler based on feature constraint technology. This modeling approach supports the precise modeling of complex 3D parts and assemblies for tolerance analysis.

TedA (Stranger
)
10/09/06 01:53 PM


You have presented many interesting points with respect to vectorloop modeling and Monte Carlo simulation, although it is not really valid to make a comparison of these two technologies. Vectorloop modeling is a constraint system definition, not a solver technology. The public information available on this subject is a little confusing and limited. Most of this information doesn't precisely discuss defining the assembly functions and part feature relationships through different modeling methods vs. how the functions are initially solved, and then how the solved solution is finally analyzed/reported. There are generally two tolerance solver technologies used in current commercial applications: Monte Carlo Simulation and derivativebased analysis, both of which are supported in CETOL. A vectorloop model can be solved with either of these solution approaches.
In your post, you mentioned the fact that Monte Carlo simulation supports the ability to account for variations that occur due to manufacturing methods and machines. Actually, that capability is supported by both of the solution approaches (Monte Carlo and derivativebased analysis). In fact, that is much more conveniently accomplished in the derivativebased analysis approach because the manufacturing variation can be incorporated into the analysis as a postprocessing function. Once the derivatives (sensitivities) have been calculated, the results can be recalculated quickly (almost instantaneously for most models) when manufacturing variation data (or any other input data, for that matter) is entered. With a Monte Carlo approach the entire simulation of the original model with the new manufacturing variation information must be rerun, as it would with any change in input data. This is one of the inherent problems with Monte Carlo simulation.
There are conditions where one solver may be more beneficial than another. Both are capable of generating results for large, complex problems However, with Monte Carlo solvers it is difficult, if not impossible, to ascertain the validity of the model that is being solved or, more importantly, the accuracy of the results. A derivativebased analysis provides the derivatives (true sensitivities) and contributions that show the precise mathematical relationships between the measurements and dimensions. Thus, the sensitivities and contributions can be used to verify the validity of the model and accuracy of the results. This is the most significant differentiation in the overall benefits of one solver method vs. the other: the ability to use the output to clearly understand the model, eliminate the error associated with modeling assumptions, and provide higher confidence in the results' ability to predict and correlate with the real manufactured product.
Many of commercial applications that use the Monte Carlo solution approach provide tools for estimating the sensitivities and contributions. However, these calculations are incompatible with many of the models created in those applications because of the discontinuities and nonlinearities inherent in the modeling approach used. Thus the sensitivities and contributions that are reported in those applications can be highly misleading.

Tom R. (Unregistered) 10/30/06 07:18 AM


We use both VisVSA and CETOL were I work (QMC).
We have expert users for both tolerance analysis packages.
I worked for VSA for 8 years and used the VSA software for over ten years.
I have used the CETOL software tool for the last 6 years to conduct tolerance analysis studies of large complex systems. If you have any questions about the application of either package please contact me at www.qmc.com.

Dave S. (Unregistered) 11/07/06 08:09 PM


The way VSA worked was easy for anyone using it to understand. This was because VSA generated an intermediate plain language file that was then compiled and processed.
Want to see how it worked? Read the source code that was fed to its compiler.
This situation gave a huge advantage in that any program that could be gotten to talk could be included into the VSA analysis  Have a cylinder combustion chamber temperature program that depends on the compression ratio? Take the outputs of the varied diameter, stroke, wristtocrown variations, et al, and use those to generate temp outputs and sensitivity contributors to same.
The feature VSA did not have was the ability to handle variation due to default angular plus/minus tolerances. It did best with features controlled by feature control frames and their geometric characteristic symbols. OTOH nobody I know handles default angular plus minus tolerances in their analysis.
Vector loop or Monte Carlo, shop floor info is only necessary if yields are to be predicted, otherwise it doesn't matter. At least the models I built never depended on shop floor data. We just checked to see if, when within tolerance, the parts would function.

Norm (Unregistered) 11/28/06 10:31 AM


I reccomend NOT using +/ angular tolerances in favor of GD&T angular conrol that provides a total width zone versus the pie shaped zone.
The absolute need for a +/ angular tolerance has never happened for me. So, if there is a case in your applications, I would like to learn about it.

Srini (Unregistered) 11/29/06 11:45 AM

Looking for opportunity in Tolerance Analysis
[re: DavidO] 
 
Hi
I am looking for a VSA GD&T Tolerance stack up position.
Can any one help me? I have 10year exp in product development and 3years in using VisVSA software for different assemblies like Chassis, Suspension,Airbags, Electronic packagings like PCB packaging.
srini_chetlapalli@yahoo.com
