Circle tan tan rad fails in that order on simple L line

I have a simple line, see attached.
In perp view I choose Circle tan tan rad,

I follow the prompts, which follow the name tan tan rad.

First tangent curve…so I select the upright line (Z)
Second Tangent Curve or radius…so i select the second curve.
radius <8.00>: I type 10 and hit enter

and nothing is created.

Why ?

I also try for this…lets say I wish to drag and see a radius form then click when happy…
select first line, then second, then drag and a circle forms, when happy I click and it vanishes.

tan tan rad is not delivering as it says on the tin…tan tan rad !

I also try this in an ortho view. it still fails.
Circle tan tan rad fails on this simple L line.3dm (28.8 KB)


Hmm- it works if you simplify (SimplifyCrv) the more complex curve to be an actual two point line. I do not know why… I’ll add it to the pile.


Any guesses as to why the one line has so many points?
Where did it come from?
How was it made?


It appears that even with non measled lines there is still an issue.

I replaced the vert ones with new lines.

success second attempt on 10mm rad, though it failed first try if now first two trys.
the 12mm rad one you see wont receive a circle. if I trace it with line tool and move it aside a tad, then that will receive a circle.

reason for the measles…
It was an intersection of an extruded line and the surface fill (3 or 4 curve tool) .

I had network surfaced a tailplane tip (to be found in another post here), the last portion was sweep2rail. I followed Pascals divvying up .
Pascal saw this post.

Then had to slice its end off.

resulting edge when I did dupedge had many points

(layer ‘original curve’)…so I ran FitCrv and got the red line.

It was important not to alter the curve …rebuild I found did alter the curve.

I am not happy wiith the tip that was part of sweep but I couldnt improve on it, it used two rails and point option. Wingtips etc seem to see people using this method.

Then I surfaced it. with this curve and a straight line drawn with line tool at rear edge.

I then extruded a line as a surface and intersected that line with this planar surface. never looked to see how many points it had.

But if the original curve and also FitCrv lines are based on a straight line slice through this surface, thay are planar, so why has the intersect line not just got two points as its an intersect on a planar surface ?

source of line.3dm (276.1 KB)

Another example of circle tan tan rad failing.

cannot get any simpler than this, drawn two lines that intersect in a custom cplane ortho view.
circle will not form.

two lines intersecting, should they not be able to receive a circle ?

copy pasted in from the file they are acting up in, so they are not quite ortho in the ortho views,

even if I then create new cplane to the two lines, still no joy. that cplane saved in the 3dm.

was trying to create a 20mm rad circle, select first line, select 2nd, enter 20 and hit enter, no circle.

now if I do it as tan rad tan it works, but the command should do tan tan rad, its called that, and asks for that.

circle tan tan rad vanishes.3dm (25.7 KB)

It doesn’t work for me either but it works fine in V6.

Side note of sorts…

Is your intention to put an arc at the corner of these two lines? Like, to fillet them? If so, why not just use Fillet? (not FilletEdge, just plain old Fillet) It’s faster, it works, and your curves will already be split/trimmed. I’ve seen you use pipe in a lot of instances lately - that just seems like a super convoluted way to simple fillet a corner of either curves or surfaces.

As a workaround to a buggy tool, fillet would work, thanks

sounds like a bug then John.

pipe where I used it was the fact that I needed to create the edge, so that then I could create the surfaces. I couldnt get the twisting inner and outer edge of a half moon without first having the edge to work with. I had started with considering two rails and a sweep2 but couldnt create the rails in thin air, the pipe and projection down onto it of the angled surface gave me what I needed…but yes, I have also created radiused edges with fillet, works a treat.

…and maybe though if I had created that item as a solid, I could have done it that way. I think you have something there…I will add it to my notes on radiused edges…thanks.


I believe the bug was reported and the fix was determined to be too risky for V5, so it was fixed in V6 instead.

NOOOOOOOO!!! Fillet is not the workaround!!! FILLET IS THE “NORMAL” (NOT STEVE1) WAY OF DOING THINGS! Get it???

Sorry, ok, to create a radiused arc where two lines or curves meet, dont use tan tan rad, even if John hasnt proclaimed its got a bug, use fillet.

I for some reason see fillet as a thing for surfaces, its my upbringing, way before electronic calculators were out, before even slide rules were out !, not originally an engineer, chamfers and fillets are or were understood as being surfaces between surfaces, in a laymans way of putting it, in tech drawing we drew circles to get arcs in corners, 9h pencils for constructive etc, ring a bell with anyone ? I also used macromedia Freehand, and it had you use circles to create such arcs in corners, hope you understand. Still some school and old practices to forget.



Fillet is for curves. Then for surfaces, you can use FilletEdge. What you’ll notice is that there are tons of commands in Rhino that have a curve command, and then a surface command. A great example of this is Blend. BlendCrv works on curves, BlendSrf works on surfaces. Typically once you understand the curve version of a command, you’ll have a very good idea of how the surface version of the command works.

And Fillet is also about 10x faster to use. Seriously. Once you have your Fillet settings set the way you like (appropriate radius, and I usually set Join and Trim = Yes) then all you do is click the two lines. Boom, it’s created the arc, trimmed the excess from your original curves, and joined it all together. So typically that’s just starting the command and two clicks of the mouse. Your way requires that you:

Start the command.
Click on your first curve
Click on your second curve
Enter your radius
Now trim your original curves using your circle as a cutting object
Now trim your circle using your original curves as a cutting object
Now join them all together

See why everyone else just uses Fillet?