Putting a chimney on a model railway engine


The attached pic shows the smokebox and chimney of a small railway engine. I’ve drawn this full size, the smokebox diameter is about 44", the chimney is about 15" diameter across the rim (top).
Most loco chimneys have a flared base from which a segment is removed, to fit the profile of the cylindrical smokebox.
Inspection of the 3D model structure of the chimney shows that it is made of many small rectangles -presumably I need to remove that part of the chimney which is inside the smokebox, but I can’t simply delete a lot of the rectangles as this would create holes in the chimney above the smokebox surface. How should I tackle this (using SketchUp Make) ?
Note - the chimney is mainly hollow but I have left the bottom few inches as solid.

A supplementary question… Having drawn this full size, I want to scale it down to about 1:50th, but that makes the rim diameter only about 3/16" and the thickness looks much too small to print, so is making it hollow out of the question?!


First you need to correct the face orientation of the chimney.

Next, you need the smoke box and the chimney in the same context. Then use Intersect Faces to create the intersections needed so you can erase the edges and faces inside the cylinder. Best to do all that at full size. After that, you can scale the model down.


On a note unrelated to your question: that is going to be a tough print. At best there will have to be a LOT of supporting material. I would make it as two pieces (chimney and boiler) that you can join post print.


Yes, I realized that. I am also aware that the orientation of the item affects the smoothness of curved surfaces, and since the boiler and chimney are orthogonal, it seems sensible to do them separately so that each can be orientated optimally for printing.
I’ve got a lot to learn before I try printing this!!


I’ve been tinkering around creating cylindrical shapes using ‘follow me’. I am starting by drawing a 2D elevation, of the chimney but always produces reversed faces. Thinking about it I realized that the 2D drawing has of course got faces which end up inside the finished object, so what I really need to do is define the EDGE of the outline to be the outside face in the finished item. I’ve attached the skp file to show the outline and the circle that I’m using to direct ‘follow me’. Is there a better way to do this?

rev faces problem.skp (77.6 KB)


That happens sometimes with Follow Me. It’s no problem if you deal with it right away. Just select the result of Follow Me, right click on it and choose Reverse Faces.

If that profile is extruded around the circle as you have it set up, there’ll be no internal faces as a result of Follow Me. But if you want the hole down the middle, draw it in as part of the profile.


Ooh! So it does! Thanks.


As I said, deal with it when it happens. Don’t leave it until later. That’s sloppy work and it creates problems. For 3D printing, the face orientation determines which side the material is supposed to be on.

FWIW, I just ran FM on your profile and it shows the correct faces out. So it doesn’t always happen.


I didn’t understand what ‘context’ is but I seem to have got the result I was after using ‘intersect faces - model’.

here’s the skp file…

chimney 5 cut away base.skp (283.2 KB)

I have carefully worked around the base of the chimney flare to remove all the elements below the intersection line, and I have carefully done the same around the smoke box (the cylinder) to make the base of the chimney solid.
I found two small holes in the flare, above the line of the intersection (tiny triangles) which I have ‘filled’ by use of the pencil tool.

Two questions…
(1) have I done it right?
(2) do I need to increase the number of edges in the polygons to get a smooth edge to the chimney flare?
I’ve left the cylindrical smokebox in the file for reference to the profile of the chimney flare.


The chimney when grouped is not a ‘solid’ in Entity Info - it has a missing face near its bottom and a tiny ‘flap’ at its base:

When the flap is erased and the circle added it is a solid: I used a section cut to access the area…


I believe that the orientation of the original Follow Me face determines the orientation of the generated surfaces:


Also the direction of the path.


Thanks, I’ve fixed those bits now.


Tanks DaveR, it’s helpful to know what determines the behavior when using ‘follow me’.


Just saw this and figured I would inject an alternative workflow using Solid Tools and Fredo’s Round Corner extension:


That’s neat, but I guess you have to use SketchUp pro and us paupers don’t get Solid Tools! One day, perhaps!


Fredo’s tools don’t rely on Pro.
You could have a copy of the ‘chimney’ group’s geometry in the 'boiler’s context, select all of its geometry and intersect it, and get the cut-line, and then delete the unwanted parts below the ‘boiler’ [using a section cut to access the interior…]
Then apply the ‘fillet’ to the junction.
Of course, this assumes the two parts main are left ‘connected’…
If you decide to print them separately you need to group the two parts and ‘heal’ the open ends…



Here is a quick workflow for intersecting and deleting faces: