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Help Modeling Pipe with Clocked Bends in Onshape

Hey folks,
I'm hoping someone can help me troubleshoot a problem I'm running into while trying to model a parametric pipe system in Onshape. Ultimately, I'd like to create a custom piping tool using FeatureScript, but I'm starting by manually building the geometry in a Part Studio—and I’m already hitting some snags that make me think this will be trickier than expected.

Here’s a video showing the issue (linked below), and I’ll do my best to describe it here: https://youtu.be/5n34GXwETfY

What I'm Trying to Do

The concept is simple: a pipe composed of straight segments and fixed-radius bends. All pipe diameters and bend radii are constant for now.

• The pipe begins with a straight segment L0.
• At the end of L0, there is a bend with angle B1, followed by another straight segment L1.
• At the end of L1, I begin the next segment. This involves:
  • A bend B2
  • A clocking angle C2 to rotate the bend around the previous straight segment (L1)
  • A new sketch plane based on that clocking angle

This process repeats: each segment consists of a bend (with a clocking angle) followed by a straight section. I'm driving all of this with a variable table.

The Problem

Everything works fine within certain ranges of angles. But when I change the bend angle or clocking angle, things start to break down. Specifically:

• The tangency on the bend segment often reverses unexpectedly
• I suspect the normal vector of the sketch plane flips, causing the arc I use for the bend to flip directions

This behavior makes the model unstable and unpredictable, which obviously wouldn’t work for a scripted solution down the line.

What I’m Looking For

• Is there a better way to handle this kind of construction in Onshape?
• Is there a known trick for stabilizing sketch plane orientation when chaining arcs and rotations like this?
• Would FeatureScript help avoid this kind of instability, or would I just run into the same reversal issues?

I’d love to hear any suggestions or strategies. I'm decent with Onshape modeling but still pretty green with FeatureScript—any guidance would be greatly appreciated!

Thanks!
—Dave

@GregBrown - OK I'm amazed that's the work around. Don't know why there isn't a planar face option. I don't think I ever would have found that. Thanks!

A good way to do this is with a proper NURBS curve, which you can make with the Control point curve custom feature.

This document has two examples. In each case the Weight of a particular control point is adjusted to be something other than 1 (thereby making it rational.) The effect it has on the curve is obvious, it "attracts" the curve towards it, and increases the curvature value. You can use it to approximate a circle, or tend towards a square, or something in-between.

In the first Part studio Periodic (closed)I have used a single degree 8 Control point spline (NURBS curve) and the corner Control points all have weighting of 2.

The curvature combs are pristine for this…

There is another part studio with one corner created from a Control point spline (degree 2, with the middle CP set to weighting of 2). This is then mirrored twice.

In both cases just play with the Weight parameter to get the desired effect.

The Circular Pattern tool would work well. It can be used in a sketch or outside the sketch by patterning a part or the features to create the part. - Scotty

You could also just use one of the gear feature scripts (such as Neil Cooke's) and make a full spur gear and cut away everything except for the sector you need.

https://cad.onshape.com/documents/5742c8cde4b06c68b362d748/v/1db29081376c095cf10e2a3d/e/c72760543a0d4412e72f6d38