# 'Perpendicular from curve' Seems Inconsistent

Hi, I was wondering if anyone could tell me why I am not able to get a perpendicular line to another line in the attached file using the ‘Perpendicular from curve’ command?

Sometimes it works, sometimes it doesn’t.

The line in this file was actually made using ‘perpendicular from curve’ on an actual curved line, but instead of giving a perpendicular line it would only give what appears to be a tangent line.

So I thought, ok I’ll just use that new line and from it’s end point use the same command, but doing so doesn’t give a perpendicular line - it comes close, but it looks more like about 85 deg.

So I’m thinking, ok, I’ll just use the draw a line option that let’s you specifiy an angle, so in my case I would go with 90 deg.

But I’m starting to think…if it’s not giving a true 90 deg perpendicular line, is the original line I made with that command really tanget? Does is just look that way? yet may also be off by a few degrees.

Suggestions on a better workflow/tool/command use that can give definite results is appreciated.

Thanks

Line Perpendicular to Curve.3dm (23.1 KB)

Hi BabaJ - I think you’ll find that the line is perpendicular, but it is one of an infinite number of possible perp lines that lie in the perpendicular plane at that location. When the line is skewed to the Cplane then there is no good way to know which perp line you’re after.

-Pascal

Hi, I’m not sure what your saying…except for the part that I do understand how a different plane could alter how it appears on another plane.

But in this case both my view and cplan are both co-planar…and so when I try to draw it perpendicular it doesn’t do so. If I use the angle measurement tool, it gives me something like 87 deg.

So if I am drawing on a construction plane that is also co-planar with my view, there is only two possibilities for perpendicularity, one at 90 deg. and the other at 270/-90.

Hi BabaJ - all of the blue lines are perp to the red one in the attached file.

Line Perpendicular to Curve_PG.3dm (61.1 KB)

-Pascal

Hi Pascal, yes I understand.

They are perpendicular, but on different planes.

I guess the question should be, is there not a command that can draw a line perpendicular to a point on a curve that is constrained to a specific cplane?

I stripped out alot of stuff in my original file I attached to keep things looking simple.

But the original curve and the resulting line were both created orthographicly on the same cplane.

But Rhino doesn’t seem to retain that information and is randomly choosing a plane of its’ own choice.

if there’s something else on the cplane (a point or curve for example), you can place your cursor on it during the line->perpendicular operation and rhino will draw the perpendicular line on the cplane… you can also use the tab key once you’ve registered to the cplane in order to lock it on the plane.

(not sure if this is the most understandable explanation… try it out… if you still can’t get it to work, reply back and i’ll try to say it more clearly)

Thanks again for your help Pascal…I will work with your idea and see.

Yes.
Turn off Grid snap, turn on Planar.
Use the Line-perpendicular from tool

you can also use the shift key to lock the perp line to a horizontal or vertical plane.

@BabaJ Turn on Planar. The use Line with the Perpendicular option to draw a line perpendicular to a curve in the view with the CPlane you want to be parallel to. With Planar on the end point of the line is constrained to be the same distance from the CPlane as the first end of the line.

Thanks guys for all your help…I think I found out why it’s not working.

The curve that I am attempting to draw perpendicular lines from isn’t exactly sitting on the cplane I’m working with.

The whole line seems to be about 0.005/6 mm off from it.( The colored red line).

This is what I believe is giving me unexpected results.

I tried drawing another line on the same Cplane and am not having trouble getting perpendicular lines. It’s working as expected with that one.( green colored line).

I suspect I know why my initial line ended up being off ever so slight from the plane, but that brings up another question that I think would be best for a new post, as it applies to other things as well.

Thanks again.

Perpendicular.3dm (28.2 KB)