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## Paramaterized Sketch and dimensional instability bug.

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## Paramaterized Sketch and dimensional instability bug.

In this simple sketch, two parameterized angles maintain the correct relationship UNLESS they are 'drag and dropped' into a specific place.

So in the initial state we have two angles:

The (driven) internal_current_angle. This represents the position of the first tooth on a gear with internal teeth. It is driven so I can 'drag and drop' the gear into any position using its orange centreline.

Driven internal_current_angle for gear with internal teeth

Then we have a second (driving) external_current_angle representing the first tooth of a gear with external teeth. As you can see, it is proportional to the internal_current_angle

Driving external_current_angle for gear with external teeth is proportional to the internal angle

What I expect to see:

Here, I have moved the tooth (circle) of the gear with internal teeth to about 28 degrees by 'dragging and dropping' the orange centreline of the gear with the internal teeth and the tooth of the gear with external teeth has followed correctly.

[external_current_angle =internal_current_angle*internal_teeth/external_teeth]

[41.757                                = 27.838                             *           3            /          2]

First move is stable

What actually happens:

If  however, I move the tooth on the internal gear to about 122 degrees then

[external_current_angle =internal_current_angle*internal_teeth/external_teeth]

[183.321                               = 122.214                             *           3            /          2]

but, it has moved the dimension to the other side to show the complement of the external_current_angle

[ 176.679                              = 360 - 183.321]

So far so good, I cannot fault it at this point.

Second move when the external_current_angle exceeds 180 degrees

But then I let go of the 'drag and drop'

The angle of the tooth on the external gear flips BACK to 183.32 - OK

But this then drives the tooth on the internal gear backwards so instead of

[external_current_angle =internal_current_angle*internal_teeth/external_teeth]

[183.321                               = 122.214                             *           3            /          2]

it uses

[internal_current_angle =external_current_angle*external_teeth/internal_teeth]

[117.786                               = 176.679                             *           2            /          3]

And therefore, the tooth of the internal gear jumps back to a different position to where I left it !!

Completing the 'dr

It continues to maintain this relationship, smug with the knowledge that it has blatently flipped the angle to measure the wrong angle and all the gears grind to a halt !!!!

I include the Autodesk Fusion 360 Archive files for your amusement.

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