Unusual sketch behavior with negative coordinate system and pattern quantity expression

[JBerns]

Community,

I am experiencing difficulty with a rectangular sketch pattern given these conditions:
- the pattern uses a formula to calculate pattern quantity
- the formula uses a common parameter for the part length
- the pattern exists partially or wholly in a negative quadrant relative to the sketch coordinate system

Using the attached part, open the Parameters dialog.
Change the Length parameter to a shorter value. For example, 7 in.

The part length updates, the pattern quantity updates, but the pattern position is no longer aligned with the projected work plane.

If I "drag" the sketch after the parameter update, the sketch then updates its position correctly.

If the pattern quantity is changed from an expression to a fixed number, the pattern position tends to update correctly, regardless of coordinate system. 

If I move the sketch coordinate system to the lower left corner of the top face, the sketch position tends to be more reliable. Why does this combination of coordinate system and pattern quantity parameter expression cause this behavior?

I have tried various sketch coordinate systems, part and sheet metal templates, and pattern quantity expressions.

This impacts assemblies, which results in errors due to the sketch pattern in the wrong position. Also impacts sketch-driven patterns.

I generally recommend patterning at the feature level rather than sketch level, but regardless of preference, the option to pattern at the sketch level should result in accurate sketches. Would you agree?

Does anyone have suggestions to achieve a reliable sketch update after a parameter change and a pattern quantity that uses an expression AND does not require the sketch geometry to be in an all positive coordinate system?

If there is a deeper underlying condition in Inventor that causes this sketch pattern behavior, I hope the Inventor Development Team can solve it. I have tested this part in Inventor versions 2022 - 2024 and the problem persists.

Thank you for your time and attention. I look forward to your replies.

Regards,
Jerry

[johnsonshiue]

Hi Jerry,

I suspect this has something to do with flipping sketch geometry. Though Inventor 2D Sketch allows negative dimension value, the actual sketch compute still converts the value to an absolute value. A positive value or a negative value does not carry the sense of direction.

The flipping is an unpredictable behavior. It depends on the change of a particular value. Somehow the Sketch Solver thinks it is Ok to flip. For the sketch geometry prone to flip, the workaround is to create a workplane (perpendicular to the sketch plane) before the sketch. Project the workplane as the reference to the sketch.

Please feel free to share the file here or send it to me directly johnson.shiue@autodesk.com. I can help take a look to see if the above workaround applies to this case.

Many thanks!

[JBerns]

Johnson,

I am using a workplane to control the sketch point position.
I am aware of dimensions that can flip.
That is why I dont understand the behavior since I am using a workplane.
The file is attatched to the original post. I can send more examples if you
need them.

Regards,
Jerry

[johnsonshiue · Accepted solution]

Hi Jerry,

Many thanks for sharing the file and the steps! This is absolutely a bug. It seems that one additional update is required. Fortunately, Rebuild All seems to help. I created a simple iLogic rule.

ThisDoc.Document.Rebuild()

And set the trigger to user parameter change, model parameter change, and geometry change. Then the desirable behavior will happen.

When Rebuild All can correct things, it means there is a bug. But usually, this kind of bug can be tricky to resolve. I will work with the project team to understand it better and see what we can do.

Many thanks!

[JBerns]

Ackn. Thank you, Johnson.

I look forward to hearing of a resolution in a future version.

Regards,

Jerry

[johnsonshiue]

Hi Jerry,

Though there is a workaround, I have reported it as a defect INVGEN-76049. I think it should be fixed.

Many thanks!