1D Vs 2D and 3D Problems
EZ Tolerance Analysis enables you to solve 1-dimensional (1D) stackup problems, but not the 2 or 3 dimensional (2D or 3D) stackup problems. However, in most scenarios when the defined stackup is not 1D, a warning appears.
One-Dimensional Problems
• In a 1D tolerance stackup problem, the critical distance being analyzed and all dimensions that contribute to the variation of that distance act in the same direction.
• Linear variation of the surfaces back and forth in the direction of the stackup is considered.
• Angular variation of the surfaces relative to each other is not considered and the effects of such variation are mostly ignored; keeping the analysis 1D as a result. However, when significant differences exist in the size of the surfaces involved in the stackup, angular variation in some of the smaller surfaces can have a greater effect on the edges of the larger surfaces.
• If larger surfaces follow the orientation of the smaller surfaces, they move back and forth in the direction of the analysis by more than the simple translation of the surfaces would permit. A warning message appears in these scenarios.
• In a 1D problem, the sensitivity of the overall stackup distance to each contributing standard dimension is typically 1.0 or -1.0. Sensitivity to size dimensions such as diameter or width can be 0.5 or -0.5.
In a 2D tolerance stackup problem, the distance being analyzed and all dimensions that contribute to the variation of that distance are represented in a single plane. In a 3D tolerance stackup problem, dimensions can contribute to the variation in any direction. Both usually involve complex trigonometric calculations to determine the sensitivity of the measurement to each dimension in the assembly.
Where 1D Analyses Fall Short
EZ Tolerance Analysis provides a warning when the defined 1D stackup problems may benefit by being evaluated with a 2D or 3D tolerance stackup method. EZ Tolerance Analysis highlights scenarios where rotational variation which is not included in the 1D method, may have significantly more impact than corresponding translational variation of the same surface through the permissible tolerance limits. Although such effects are not included in the calculations, you are notified of such cases. You can then assess the severity of the requirement and determine whether further investigation with a 3D tolerance tool like CETOL 6σ is required.
Although all such cases are not detected, the following are the scenarios in which notifications appear:
• When the surfaces that define the critical stackup distance do not overlap.
• When some of the assembly constraints between parts are outside the coverage for the surfaces selected for the critical stackup distance.
• When the size of the surfaces constraining the parts to one another are significantly smaller that the surface involved in the stackup distance definition.
• When a part is attached with a pattern of two or more fasteners, and the measurement direction is in the plane of the mating faces to a surface that is outside of the pattern.
EZ Tolerance Analysis may flag stackups that truly can be modeled with a 1D method. The most common situation when this occurs is when several equivalent assembly constraints are present between parts in the assembly. For example, a shaft mounted on each end into a bearing or bushing or a circuit board mounted on several standoffs with multiple fasteners. These stackups are flagged because the 1D stackup definition involves creation of a single dimension loop through the parts, and the other constraints that help to stabilize the part in the assembly are not included in the stackup definition. The flag is a warning message about considering the relationship between the parts and assessing whether modeling a more advanced tool like CETOL 6σ is required.