Angle constraint when using command "Line: Tangent from curve"?


What is the way to constrain the angle of a line when the first point is tangent to an arc?

That is, the line must be at a defined angle to horizontal, and the start will be tangent to an arc.

When using command “Line: Tangent from curve”, the use of “<” in the command line seems not to work, nor using “Ortho” after setting “OrthoAngle”

There was a thread on this last November, but no answer.

I can’t even think of a work-around at the moment.


I seem to have answered my own question:
Draw a line at specified angle anywhere. Then “tangent” snap on, move the line proximal to expected tangent point, and let the object snap do the rest.

So, is that the best way?

(Margaret Becker) #3

It’s the only way I can think of, because a line of a certain angle can only be tangent to a curve at a restricted set of locations. Either the angle specification wins or the tangent specification wins.


OK, then that’s the technique I’ll remember. Midpoint snap point on the angled line moved proximally to tangent snap zone on the arc.

It’s a small observation, but in my case the result was not a perfect tangent snap, but one where the angled line microscopically intersected the arc, resulting in two intersection points. That became apparent when the line and arc were trimmed to each other and a join attempted of the remainders. When the join operation warned of a tolerance error, I needed to zoom in greatly and trim the chord that had remained. So, almost a tangent was achieved but not exactly so.


i have to do what you’re talking about fairly often… the way i used to do it, before learning about the tangent snaps, was by drawing a 3pt rectangle using the centerpoint of the arc, and the arc itself as the length… then i’d rotate the rectangle to the necessary angle and use it as a guide for my tangent line…

i’ve since started using the tangent snap instead and haven’t found a problem with it so far… it’s possible the error you experienced was because the tangent snap you used was actually the tangent of the ‘rubber band’ in between the move-from point and move-to point which will report ‘tangent’ such as these examples:

the snap you want to click to (as far as i can tell with your above description) is when the tooltip reports ‘Object Tan’

like this:

maybe play around with a similar drawing… you’ll see it pretty quick but fwiw, i do trust the ‘object tan’… i’ve checked it enough times against the rotated rectangle and it’s never been off.

[EDIT]- i’m on a mac so the tooltip may appear a little different for you? regardless, i’d imagine the same underlying theory is in place on both platforms.


Jeff, I just tried your method (thanks for taking the time to lay it out), and those steps of yours (i.e., look specifically for the Object Tan tooltip) also works in Windows.

Possibly you are right, that I might have snapped the ‘rubber band’ to Tan (tooltip), instead of snapping the object to Object Tan (tooltip). It’s quite hard to get the rubber band to make a Tan snap though.

In any case, I experimented with Object Tan snapping a few times and, after trimming, got a proper join every time. So I’m not sure what I got initially either.

As a fallback, that method of creating a three-point rectangle is very clever.


heh, yeah… that stems from having a sketchup background in which true arcs don’t exist… so i’d have to think of it this way then fill in the rest with a segmented arc.


I am trying to constrain a line tan from curve right now and recall simply entering <30 to constrain the line to e.g 30deg, a tip from Pascal, but cannot find the post.

…though its not working !

surely such would be good to have , a real simple entry like that.

Then “tangent” snap on, move the line proximal to expected tangent point, and let the object snap do the rest.

I have two snaps on Tan and Near, need near to be able to select the line with move command, but I can place it anywhere on my circle, I see no tan snap appear.

Once I have hold of it I turn off Near, so just Tan on, and it wont indicate a Tan even when I am the perfect place, Tan never appears.



Hi Ian
Try the below VBScript
Ciao Vittorio
Retta_Tangente_curva.txt (5.0 KB)


I shall give it a go.

Maybe this should be aded to V6 as lines and constraints should occur even in tan from curve etc.