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Description of the Issue
In Fusion360, I've encountered a recurring issue where the software unpredictably solves new profile geometries, leading to errors that disrupt the reliability of parametric designs. This unpredictable behavior persists even when all parameters are within valid ranges, complicating the creation of dependable designs.
Demonstration
To demonstrate this problem, I've prepared a short video that walks through the construction of a basic object. This object is a single profile rotated around an axis. Throughout the video, I primarily accept the default constraints suggested by Fusion360, with a single exception where I opt for a colinear constraint over a parallel one to introduce additional restriction. The sketch is intentionally basic to highlight the issue at hand.
Design Objectives
The designed object, intended for 3D printing, features sidewalls with consistent thickness ("h_thickness") across its height. Both arcs in the design share the same radius and are offset by "h_thickness", ensuring the wall width remains constant in the XY plane—crucial for optimal 3D printing aesthetics. Other design specifics include:
- A fixed start angle of 30º for the arc.
- A rounded rim at the top with a 10mm width.
- A design constraint to not exceed a 40º print angle, leading the rim to terminate in a straight line for printing feasibility.
Core Issue
The problem arises when attempting to modify dimensions such as the diameter, which theoretically should not affect the profile's shape. Unexpectedly, changes to the diameter sometimes cause Fusion360 to solve constraints in an illogical manner, significantly altering the design.
Illustration of the Problem
The initial sketch, fully defined and indicated in black by Fusion360, incorporates multiple tangent constraints, pinpointed as the issue's source. Despite the sketch being marked as fully defined, altering the diameter unpredictably disrupts the design, deviating drastically from the intended geometry.
Potential Cause
The crux of the issue likely stems from the inherent ambiguity in tangent constraints between a line and an arc or circle, where two potential tangents exist. When aligning a circle with two lines via tangent constraints, the circle's position remains undefined, yet Fusion360 marks this as a fully defined state. This indicates that, beyond merely recording the objects bound by a tangent constraint, Fusion360 also attempts to determine the correct tangent, a process which appears to be flawed.
By sharing this report, my aim is to shed light on this critical issue affecting parametric design reliability in Fusion360, in hopes of prompt attention and resolution from the development team.
@TheCADWhisperer Thank you for your input! I appreciate you taking the time to review this design and for the hint that you think there is a missing vertical constraint. You're absolutely right about the importance of fully defining sketches for clarity and predictability in Fusion 360 designs.
Since the Fusion design is available for download, to simplify things, could you please demonstrate where and how to add the missing constraint to the sketch in this design? Your demonstration of how to do this would be highly appreciated.
Update: I found the missing constraint and realized I could have even deleted these lines since they are not important to the design. Now, the issue appears as follows:
Move the 140 dimension up.
You should now see an unconstrained Construction line to the right (in my image I dragged it out of vertical).
Add a Vertical Constraint to this line.
1. Note that it now turns black.
2. Note that there is now a Lock glyph on the sketch in the browser.
The missing Lock glyph was my clue that the sketch was not fully defined.
This line also showed unconstrained in your video and screenshot.
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