FollowMe producing unexpected radius?

[john_mcclenahan]

With @quadhurst I’m trying to draw a tramway wiring insulator, which looks like this:

But we can’t get it quite right. This is close, but not perfect.

The overall dimensions are 4 3/8" H x 2 1/4" square.

I’ve used this path for FollowMe (highlighted in blue below) with the arc radii as shown:

All the straight edges are vertical.

Instead of generating a surface with a large radius of 1 1/8" (1.125"), it is 1.147"R. The generated edges don’t quite match the path and are not (quite) a circular arc.

The same is true at the ‘bottom’ of the FollowMe surface - it doesn’t match the half circle of 5/16" radius in the path.

Why?

I have also tried the path with half a circle rotated by half a segment, instead of a half-circle arc.

Like this:

It made no difference to the radius generated by FollowMe.

Any suggestions for how to draw this shape accurately? I have Curviloft if that might help.

Here’s the file in v2017.

Line insulator.skp (150.9 KB)

Am I wrong in my expectation of what FollowMe would do? Seems so.

LATER

Well, Curviloft may be the answer. Here’s a quarter of the insulator outer surface using Curviloft.

LATER STILL
Finished model with rounded edge - FollowMe half circle on a scaled up copy.

Line insulator 2 JWM v7.skp (303.6 KB)

But I’d still like to know how to draw this shape with native tools only.

[RTCool]

Hmm, beats me. What happens if you make a full circle first, and then cut it in half? How does that compare?

[DaveR]

If I were modeling that I would treat inches as meters and draw it large. I’d also use Follow Me for a 1/4 of it and make copies to make the a three quarters of it so that Follow Me doesn’t have so many direction changes to work through.

[john_mcclenahan]

Tried that on a simpler case. Same result. Comes out a fraction oversize. But I can see that’s because I set the mid-segment to mid-segment diameter at 5/8”, which makes the radius a bit bigger. On my phone and ready for bed (in UK at almost 2:30am.

More will have to wait until tomorrow evening.

[john_mcclenahan]

I will try that tomorrow, @DaveR. But I saw no overt signs of small edges issues.

[DaveR]

Maybe no overt signs but it doesn’t hurt to model large to avoid potential issues.

[john_mcclenahan]

Thanks for the reminder.

[Box]

I haven’t looked at your model, but remember that when follow me changes direction the profile does too and not necessarily how you want it to. So where you are expecting a midpoint follow you may be moving toward a vertex follow.

[mihai.s]

@john_mcclenahan

But I’d still like to know how to draw this shape with native tools only.

[john_mcclenahan]

Many thanks, Mihai.

If you have kept it, could you upload you model, saved at an early stage?

I ended up doing the outer curve with Curviloft/Skin contours to get a similar result.

Will try this myself now.

[mihai.s]

You’re welcome!

You will find the necessary geometry in the model (tags), and in the video you can see how I started to extract what was necessary for Follow Me (you can create the necessary edges only with the Line tool instead of the Zorro plugin)
3D model > li2-v7.skp (213 KB)

[john_mcclenahan]

Once I got the dimensions of the path lines correct, I was able to draw a quarter of the insulator.

The critical thing I noticed about your FollowMe path is the extra segment in the top arcs. That seems to force the outer edge to the desired radius.

Many thanks again, Mihai. It’s been driving me and @quadhurst mad that we couldn’t get the dimensions right.

LATER
Result

Still needs a little clean up, and mirroring/rotating to finish the full insulator.
Line insulator v8 Mihai’s method.skp (94.1 KB)

[mihai.s]

You will need your model to look like this for mirroring/rotating.