Advice please method to create tangent line along angled pipe for sweep rail


To complete the surface between the two existing planar ones, ditto inside set, I need a curve on the pipe surface to be the rail (or profile…depending on what one decides to pick,or curve if using NetworkSrf, which I what I may use in fact).

That curve will be the TANGENT line of that pipe to the surface. The existing red lines are tangent on the pipe.

Where should the yet to be drawn red tangent line sit on the pipe and how is this created ?

If someone could actually do it, and explain method…please.

I did try as a starting ‘stab at it’ a line drawn between the two red lines and projected in top view onto curve, it didnt look bad BUT it does not leave the longer red line tangent to it.

Its a question of establishing the tangent points of that line required along that pipe !

INSIDE as well as OUTSIDE faces.
AdviceBestMethodProfileAroundCurvePipe.3dm (164.4 KB)

Why not just use BlendSrf between the planar surfaces?

I get a curve with handles, drag the sliders but its not easy to get the curve to lie on the surface of the pipe, and over the far side that area I have a 20mm radiused curve for it to use later on and its difficult to get that to be 20mm rad curve with handles.

The tangent line along the pipe will be used to slice the pipe into two, dispensing with rear half.

I had a go at using InterpCrv and start and end tangent, projecting the result in top view onto the pipe. Not sure if technically its correct.

It would seem to be as I then decided to dupEdge the surface BlendSrf created before any adjusting, and projected that in top view to pipe, I get a match on the curve I got with InterpCrv. assume both are applying same curve with tanstart and tan end :smile:

the blue curve.
AdviceBestMethodProfileAroundCurvePipe_BlendSrfDupEdgeProject.3dm (188.7 KB)


Get rid of the pipe surface between the planar surface. You’re putting the cart before the horse and making your life harder. While you’re at it, clean up those surfaces so the edges of your planar surfaces match the edges of your “pipe” surface. Neatness counts and what you have is sloppy. Use BlendSrf to create the surfaces between the planar sections:

Then use Sweep2 to create the “pipe” section in the blend region.

Hi, The pipe arc is an exact shape, the 20mm radius curve further aft projected to a planar surface there, it must not differ, so using BlendSrf and the variability of what it creates to dictate what pipe results is not befitting the hours spent in research establishing that radius and shape to start with, as depending on the handles etc it can create all sorts of bends there. As I see it.

The plan was to create the radiused 5mm edge, decided pipe command far easier than sweeps of an arc. , trim away its back half , use edges for rails and networkSrf the area between those edges and the rear of the item, (not in the file) . the rear shape having been projected to the angled planar surface to create curves for the pipe to follow, after they were inset 2.5mm.


You can do it my way and keep the 20mm pipe radius no problem, if you know what you’re doing. It’s about a two minute job. Really. Maybe you should take a break from banging your head against the wall work on the Level II material instead?

True, but if you are looking for a perfect exact ARC it must lie on a flat Plane.
Find that plane (project) and you’ll have your shape!
(I didn’t check the file) but probably, because the 2 lines doesn’t lie on the same plane, you, simply, can’t have an arc tangent to both. The curve in between need a Twist so isn’t an arc and it’s a more complex curve.

Just my opinion, hope can help you find your way.

Catch 22, have to finish this year and a half long job before allowed to get at Level2, deadline 1 week away. 5am stuff !!



pipe line used was planar, was projected to planar surface, so arc planar, 20mm circle became an elipse as surface was angled. The number of times I had to tweak and move that 20mm circle then project yet again so as to get this the correct size, as more data became available, seeing where the curve appeared, altering angle of planar surface, until it matched the target shape.


As I said the lines are not on the same plane, so a perfect arc could be possible only as a projection. (see the gap formed by the extension of the lines)
To get a perfect tangent on both side the only solution is to have at least degree 3 crv.


Given that it takes about a long weekend to go through the Level II work, and you’ve spent the last year and a half posting literally hundreds and hundreds of topics, how’s that “I’ll jump out of the plane first and then learn to skydive” approach working out?

The outer blend surface is within 0.01 mm of a 20 mm radius arc connecting the planar surfaces. The inner one is tighter, because of course it is.

NotDifficult.3dm (142.4 KB)


Hi Steve
What about running Silhouette on the pipe in a Right ( or Left ) view ?

Here is a method which seems to work Steve,

Silouette to side-on ortheo view of tubing,
Clean up overlapping and doubled up silhouette curves,
Tweencurve and extend centre line,
Extrude the centre line,
Intersect extrusion with tubing,
Delete extrusion,
Loft intersect curves,
Split created surf along it’s centre isocurve,
Offset the resulting half surfaces both sides,
DupEdge the offset surfaces,
EdgeSrf the dupedges with the silhouette edges.

You could also use these dupedges to create your trimming surface in the other thread.

Haven’t done the end bit but the principle is the same

Steve’s Tubing.3dm (324.2 KB) .

Here it is with the end bit Steve.

Steve’s Tubing 02.3dm (478.6 KB)

Solution attached (revised with fin used for finding centerline of pipe):AdviceBestMethodProfileAroundCurvePipeDC.3dm (156.7 KB)

Method somewhat similar to BrianM’s method above, but simplier:

Find centerline of bent pipe.
Orient CP perpendicular to both pairs of planar surfaces.
Project centerline onto “center” surface.
Create mid-surface by lofting between centerline and projected curve.
Offset mid-surface to create outer surfaces.
DupEdge to create the curves on the pipe which Steve asked for.

Hi David, what method do you use to find the tubing CL?

Radius of pipe cross section is 2.5 so I used Fin along the pipe seam to create a fin 2.5 high on the interior of the pipe. Edge of the fin is the pipe centerline. Another method which does not require knowing the pipe radius is to create two fins on the interior of the pipe which intersect, then find the intersection of the fins. That should work for pipes with variable radius as well as constant radius pipes.


OffsetNormal will help here as well.


BTW … IMO a Rhino command for that (centerline of pipes and fillets) would be fine … :smiley: