How do I extrude an irregular component in an arc?

External Elevator Track and Lattice.skp (455.1 KB)
I am (still) trying to continue a track - actually 2, one in each inside corner - of a supporting structure that follows an arc 180 degrees. It is a half circle. But copying the track segments for my cog elevator/tram is both tedious and produces unsatisfactory results when I try to curve it all the way around.
I need 4 tracks, one in each corner, with two pairs facing each other as shown in the attached .skp file.
Follow me does not work for a 3D component…or does it?
Perhaps I need to step back and construct a single component and continue THAT all the way around the arc?

As I have suggested before, you need to do a Rotate/Copy about the centre of the arc.


  1. The teeth have become detached from the rail
  2. The rail isn’t at right angles to the radius at its centre
  3. The rail is slightly too long

I fixed 1.

Before fixing 2 and 3 I rotated the four sets of rails and teeth about the centre, and get this result:

External Elevator Track and Lattice.skp (551.8 KB)

Yes, I was trying to rotate the rail into place, but being on a curve that is so large it was all but impossible to “eyeball” it.

You shouldn’t need to ‘eyeball’ it. Just register the bottom corner, and rotate 0.5 degrees.

Sorry, having keyboard trouble at the moment - just trying another keyboard.

This is what the far end looks like after rotation by 1 degree, and copying 179 times. Your lattice module isn’t ending up at 180 degrees, and neither is your rail and teeth. Something in the module isn’t exactly one degree, or you didn’t rotate it exactly 1 degree.

Since the rotate/copy is dead easy to do, why not spend the time to get the first module just right, THEN copy all at once - lattice, rail, teeth all fitting properly?

And your cross pieces have a diagonal line across them - that face is not a plane, but should be.

Remember also that the inner teeth and rail is slightly shorter than the outer one. The teeth need to be a little smaller, and the rail shorter, to get the same number of teeth per degree.

Sorry to be so dense, but I can do rotate, but what is rotate/copy? Also, is it possible to lay the rail on the trackbed (the inner side of the arc and also the inside of the beams facing outward towards the outer rails)?

You aren’t being dense, just unfamiliar with the way SU works.When you select the Move or Rotate tools, you can tap the Ctrl key on Windows (or the Option/Alt key on Apple Mac) to toggle Copy on or off - you’ll see a little plus sign appear at the cursor when the Copy mode is ‘on’.

Before going much further with this, you need to get the basic architecture right. The big arc isn’t standing ‘on the ground’ - the bottom of the arc is some distance up the Z (blue) axis. You’ll find it easier to get things to line up properly if you start with the centres of the principal arc at the origin, and all the arc centres on a straight line along the green axis.

Let me try to draw a module and get the arcs properly aligned - give me a little time to try that.

Your main arc was (a) up in the air (centre and bottom weren’t on the red-green plane) and (b) had a small rotation about the Z axis - by the difference between the mid point and end point of the small segment on the inside of the arc where the face is coloured green.

I’ve just fixed that.

Now I find that the big arc segment isn’t 1 degree, it’s 1.048 degrees.

I really think you need to start over. Let me see if I can get you set up a bit better.

OK, thanks. But keep in mind I have to fit the arc into the underside of a much larger structure and I don’t have leeway on the size of that (which I am working on now).

Not changing the size.

What I’ve done is to open the arch structure for editing, and deleted everything EXCEPT the bottom face, which you have as a group.

Then I drew an arc with 180 sides, centred on the origin, outer radius as you had it (picked the endpoint).

Then exploded the end face, and used FollowMe tool to create the whole arch in one operation, with 180 sides subtending 1 degree each.

Now you can start to design the lattice module and it will fit the arch segment properly.

Arch alone.skp (354.3 KB)

And here’s a possible module - with outer but not inner teeth.

I redrew the teeth as a single component. And all the teeth are in a straight line, to match the arc segment. In reality, they’d probably need to be on a proper circular arc.

It is quite tricky to keep things aligned on the radius, and I’ve probably got minor inconsistencies in this. But you decide what thickness and appearance you want for your lattice.

Here’s the model so far. And that’s as far as I can take it tonight.
Arch lattice and teeth.skp (415.7 KB)

You could flip the outer teeth along their red axis, and shrink them slightly with the Scale tool to fit them on the inside arc, facing the outer teeth. And you could improve their profile by angling them slightly, as would be necessary in reality for a gear wheel to engage with them.

And as I said in an earlier thread about this subject, I really doubt that you should copy the teeth all the way round the arch - it will massively add to the edges and faces in the model, and it unlikely that anyone should need to be able to zoom in on any arbitrary part of the model - limit the teeth to a small detail section of a few arc segments where you need to show the passenger elevator car too.

Best wishes.

PS. Just found a minor error in the module - it was a fraction more than a degree, and didn’t quite fit when rotated and copied. Fixed in later upload. So if you downloaded the slightly earlier version from this post, delete it and take this one instead.

Just been tinkering when I should have been getting ready for bed. Here’s the result:

Arch lattice and teeth.skp (462.9 KB)

What is the desired outer arch radius? I’ve got it enormous now - about 660ft. Is that right? And it looks as if you may just have copied my 200m arch, since the radial length of the lattice module is still exactly 5 metres.

I remember somewhere a figure of 195ft and some inches and sixteenths as your original size.

But it really isn’t difficult to redraw. Just draw your arch cross section on the ground plane, draw a single arc of the right radius and axis with 180 sides (or perhaps even only 90 for a 2 degree module), then use FollowMe to redraw the arch at the right size.

And the module radial arms probably ought to be centred on whole degree radii, not offset from them as I have drawn them for simplicity. The very end modules at either end will then probably need to be very slightly different, to end properly on the ground plane.

External Elevator Track and Lattice 2.skp (558.5 KB)
I just went ahead and fixed up the track on the old lattice work. It took me hours, and it is just too hard to do over again.
I have to place the elevator inside of it still.
Hopefully, when I get to straighter lines, it’ll get easier.