How do you tell the dimension tool which side of a pair of points is the starting point?

AnOnshapeUser

How do you tell the dimension tool which side of a pair of points to make fixed?  I select the starting and ending points of my line, which I want to extend to the left 4 mm.  With the dimension tool I increase the length of the line by 4 mm.  But the dimension tool decides that I wanted to extend the right end of the line 4 mm, and extends it.  How do you identify to the dimension tool which end of the line is the end you want to extend when you increase the lingth of the line?  thx.

konstantin_shiriazdanov

usually you constrain one point to some fixed or full constrained geometry to avoid of the ambiguity of the solution

AnOnshapeUser

Konstantin: Does that trick always work?  And do you mean to say that no, Onshape will not let you choose which side to fix unless you do that?  If so, then it would have been more clear to hear that explicitly.

konstantin_shiriazdanov

I really never cared about in which side will be extended the geometry when dimensioned because the sketches should always be fully defined, so when you change one dimension the sketch will be solved in the way to satisfy your constraints. if the sketch is fully defined you shouldn't care about possible ambiguities. thought after some research I found that lines of sketch always extending in the positive derection of X axis, but I wouldn't reccomend you to rely on this behavior

Jake_Rosenfeld

@AnOnshapeUser ,

Fully constrained geometry (colored black) will not move in a sketch.  If you want to fix a point in space, you can use the "fix" constraint:
https://cad.onshape.com/help/Content/sketch-tools-fix.htm
You can always delete this constraint later by clicking the "Show constraints" box in the sketch dialog, then clicking the fix constraint and then "backspace" or "delete" (windows/mac respectively)

It may be better to put the point you care about in a specific location rather than just fixing it to an arbitrary point in space:

which is what @konstantin_shiriazdanov means when he says "constrained geometry": Geometry that can't move because its position is already fully defined.  If you were to apply a dimension to the line in the attached picture, the right point would have to move because the left point can't.