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Behavior of dimensioned lines when moving an endpoint

todd_johnson276todd_johnson276 Member Posts: 1
edited July 2016 in Community Support
I'm new to CAD, but have been trying Fusion 360 for a week and OnShape for just a couple of days. I'm a bit puzzled by how dimensions interact with moving lines. In particular, if I draw a line, but do not dimension it, I see the following behavior in both Fusion 360 and OnShape:

Clicking and dragging the line moves the entire line without changing its length or angle.
Clicking an endpoint and moving it, changes both line and/or angle, because the other endpoint remains fixed in place (even though not constrained by a Fixed constraint)

This behavior is exactly what I expect based on non-CAD object-oriented drawing programs I've used.

However, as soon as I dimension a line's length, this behavior changes. Now the length and angle of the line is fixed regardless of whether I move the line or move an endpoint. I would expect the length to be fixed, but not the angle, such that moving an endpoint would allow me to rotate the line around the other endpoint. However, to do that I have to apply a Fixed constraint to the other endpoint.

My question is, why does simply dimensioning length lead to this behavior? It seems inconsistent with the undimensioned line's behavior and with my expectations. I also think this kind of behavior is causing me problems--mostly because I don't know what to expect when I move something in a sketch.

Is there an underlying conceptual model for what moves and doesn't when a sketch is only partially constrained?
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