Constrain Sketch Line Length Relative to Other Line

[Anonymous]

I am trying to figure out how to make the length of one line equal some function of the length of another line without using any dimensions. For example, constraining one line to be half the length of another line (the same as the equal constraint works... but not equal, rather one length calculated based off of the value of a second length. An example of this could be an aspect ratio). 

Right now, the only way I have really found to do this is the following:

1. Draw a line and make it equal to the line in your sketch that you want half the length using the equal constraint.

2. Draw a construction line at the bottom of the line that is coincident and perpendicular to it.

3. Draw another construction line that is coincident at the midpoint of the other line and perpendicular to it.

4. Draw a line that is parallel to the first and whose endpoints are coincident to the two construction lines.

5. Apply the equal constraint to the last line and whatever line in your sketch that you would like to be equal to half the length of the line we started off with.

As you can see, this is ridiculous. Not only is it extremely monotonous, but it seems entirely unnecessary. Also I can't find some way around this limitation for more complex values. Let's say I have Line A and Line B for example, I would ideally like to say that the length of Line B should be constrained to always be equal to some f(x) where x is the length of Line A (i.e. Line B is constrained to always be 2/3 the length of Line A). I feel like this can't be that hard as it seems fairly straight forward but I have not been able to figure out how to do this. 

Though I know I can do something similar to this using a dimension referencing another dimension, I can't do this in my particular situation as I need the dimension of the particular thing I would like to be constrained to be derived from the rest of the sketch (where I do have another defined dimension but unfortunately do not know its relationship to other lines in the sketch). This is useful in things like solving geometry problems graphically, in situations where you do not want to have a fully defined sketch, or even defining something as simple as an aspect ratio between lines.

Is there any way to do this?

[matthewZYM62]

How about this:

• Using two construction lines, construct a right angle triangle with the source line as one of the perpendicular sides.
• Dimension the adjacent angle as atan(r) where r is the length ratio you want (e.g. 2/3)
• Now the length of the opposite side has the desired length. Constrain your target line to be equal to it.

I agree this is a bit less straightforward then the obvious solution of being able to use driven dimensions in subsequent calculated dimensions, but apparently that is not possible.

[Anonymous]

Would there be any way to achieve a line that is always 1 unit bigger than another. So Line A's length would always equal x and Line B would always equal x + 1?

[davebYYPCU]

You have the geometric result, 

you want to cut off your right arm, (without dimensions)

Put your arm back on,

go with dimension formulas 

Dimension A = 2

Dimension B = A/2. (Change value for A Will always update)

Dimension C = xxxx but can be referenced.

Models and their Sketches should be fully constrained, dimensions are part of this game.

Might help....

[Anonymous]

Yes this is the solution. Just to clarify, hover the mouse over one dimension constraint, it will show its instance name, something like d508. In the other dimension, type d508 / 2. Done.