Line-to-line dimensions

Release Notes

This section reflects additions or changes that apply to Enventive Concept v4.3.1 and later, which changed how sensitivity is reported in tolerance analysis results. We strongly recommend upgrading to the most current version of Concept to take advantage of new features and bug fixes. If you are running an earlier version of the software, please open the Online Help for your version using the Enventive Concept Help menu.

Line-to-line linear dimensions are used to constrain parallel lines at a specified measurement, as shown in the example below.

A line-to-line dimension uses a dimension and a parallel constraint to constrain two degrees of freedom. If there is no parallel constraint on the two lines, Enventive Concept automatically adds one. If a parallel constraint does not exist and cannot be created due to other existing constraints, Enventive Concept displays a solver warning.

Note: If you delete the parallel constraint that is associated with a line-to-line dimension, the line-to-line dimension is also deleted. However, if you delete the line-to-line dimension, the parallel constraint remains.

The Envelope Rule

The ASME Y14.5.1 Dimensioning and Tolerancing standard states that a size specification shall also control other variations (such as orientation) unless a more restrictive tolerance is specified for the other variations. For example, orientation is controlled by the size tolerance unless a more restrictive orientation tolerance is specified. This is known as the Envelope Rule.

Line-to-line dimensions (including the Cylindrical and Rectangular Feature of Size dimensions) have been designed to comply with the Envelope Rule. Line-to-line dimensions have a size component and an orientation component. The orientation component takes its tolerance from the size tolerance zone unless a more restrictive orientation tolerance is specified using a feature control frame.

Origin

In accordance with the ASME Y14.5M standard, Enventive Concept line-to-line dimensions use a small circle to denote the dimension's origin side, as illustrated below in Fig. 2-5 from the Y14.5.1 standard. The origin side is the second line to be selected when creating the dimension.

Although the origin is similar in nature to a datum, the standard explicitly states that the origin is not a datum. Therefore, if you need a more restrictive orientation tolerance, you must apply a datum to the origin side.

Cylindrical and Rectangular Feature of Size (FOS) dimensions are assumed to be symmetric, so they do not have a designated “origin” side.

To swap the origin side of a line-to-line dimension:

Select the dimension.
Click the origin side toggle in the Selected Parameter area of the Properties Explorer. This toggle is available only when the origin side is not specified as a datum.

Line-to-line dimensions involving centerlines

Important: The Envelope Rule does not apply to centerlines, because they are assumed to be infinite in length. As shown in the previous example, a long line segment cannot tilt as much as a short line segment, and therefore an infinitely long line cannot tilt at all.) For this reason, centerlines cannot be used on the non-origin (arrow) side of line-to-line dimensions.

Size and Orientation tolerances

When applying a size dimension that will also control orientation, line-to-line dimensions are generally preferable for analyzing tolerance on parallel lines, because they properly account for both size and orientation variations in accordance with the Y14.5 Envelope Rule. By default, the orientation tolerance associated with a line-to-line dimension is a zone equal to the width of the size tolerance.

Given the same size tolerance zone, a short line is allowed to tilt more than a long line, as illustrated by the angles of the two top surfaces below.

To specify a more restrictive orientation tolerance, convert the parallelism constraint associated with the line-to-line dimension to a feature control frame (FCF) and apply a tolerance value smaller than the size tolerance. The origin side must also be designated as a datum.

Tolerance analysis results for line-to-line dimensions

The orientation of line-to-line dimensions (and their derivatives, which includes FOS dimensions) is always reported as a statistical contributor in Enventive Concept's tolerance analysis report.

Orientation variation is not included in worst case results unless a leverage effect on an orientation contributor causes its worst case contribution to exceed that of the size component. In these cases, the orientation variation is used to compute the worst case results instead of (not in addition to) the size variation. Enventive Concept continues to report the size contribution and, when the orientation contribution exceeds the size, computes the additional impact of orientation on Worst Case results.

Example

For this example, we will run a tolerance analysis on the height of the following model:

The above assembly contains the following two instances. The tolerance type for the height of the red block is set to Basic so that we can focus on the interaction of size and orientation of the blue base component.

When we analyze the height in the assembly, the orientation component of the base height is listed as a statistical contributor, and its sensitivity is relatively low. (Click the plus/minus toggle in the column to the left of the contributor rows to expand the details for the complex contributors.)

Also note that the orientation has no contribution to the worst case results.

Now we will increase the length of the block instance from 80 to 200.

When we return to the assembly, we see that the 200-length block is now longer than the top of the base, which will have a leveraging effect.

After rerunning the analysis, we see the sensitivity for the line-to-line contributor's orientation is now higher than the sensitivity for the size:

When there is a leverage affect, the tolerance analysis report's Advanced Contributor Info results include an orientation contribution to the worst case results. The sigma values for size and orientation are interrelated rather than independent contributors.