‘Bonnet Tiles’ for Westhill Road

As you might have seen from other posts that I use a lot of 3D Printing in my layouts, so when my good friend, mentor, modeller and all round Southern Railway expert Graham Muz needed some specialist tiles creating for a building for his latest layout, then I was more than happy to help!

The tiles that were required are ‘Bonnet’ Tiles that run along the corner ridges of a roof, you can learn more about these tiles on Muz’s post blog here: Workbench Wittering#20 Out on the tiles and going potty with chimneys, Westhill Road’s station building continues to take shape – Southern Railway, Fisherton Sarum, Canute Road Quay & Westhill Road (southern-railway.com)

One of the key things required for production of a 3D printed model is good research / drawings on which to base the model. I’m afraid to say there are too many 3D Printing model companies that seem to not take as much time on this portion of the design process and produce models that are only a passing resemblance to the real thing they are meant to represent.

Thankfully in this case, Muz was good enough to provide me with this drawing of the tiles that provided most of the information:

As can be seen from the drawing, the tiles contain a number of complex compound curves that would require an interesting technique to produce the desired model. Muz and I had agreed that we really only need to portray the sense of the tile and its distinct ‘sawtooth’ profile, so it didn’t necessary require all the compound curves producing, but I wanted to see if I could do them justice, although I decided that producing just the top side of the tile would be enough.

For information, I use a free, personal use, version of the Autodesk Fusion 360 Software. I have previously used ‘Sketchup’ (formally a Google Produce, but now owned by Trimble), but was introduced to this software after a meeting of the Princes Risborough & District Model Railway Society, and have found it far superior in its capabilities. Obviously, the free version it is not as capable as the full CAD package such as AutoCAD or Solidworks, but it is more than enough for most modellers and is fairly easy to pick up.

I should point out that the following way of producing the tiles is what I did within my CAD software / knowledge rather than best or most efficient way, so don’t take this as the ultimate guide, although I am lucky in that I have been trained to use AutoCAD for 3D modelling and Bentley MicroStation for 2D modelling, so do have a lot of prior knowledge.

The first step in the CAD process was to create the initial outline of the tiles, albeit guessing some of the dimensions (using the power of the CAD). In Fusion 360, this is done using the sketch function:

To convert this to a 3D solid model, the ‘Extrude’ function was used to pull the profile of the files up, initial to the full height of the tiles, this created a solid ‘lump’ of virtual material that I could take a ‘digital saw’ to create the upper flowing surface of the tiles.

To do this, I created a cuboid the same width, length and depth as the shape I had just created, using the same functions. I then proceed to draw profile of the upper surface on the front the cuboid:

This profile then effectively divided the cuboid in two, and I could then use the ‘extrude’ tool and ‘extrude’ the bottom profile the opposite way to remove the material, this creates the negative impression of the top profile that I would use cut into the original tile shape:

Now, as the top curve is angled upward, I had to rotate this shape so that the top edge at the front was at the top edge of the tile shape and the back edge was at the to be wall thickness above the rear edge of the tile shape (I hope that makes sense):

So that when I cut the two shapes together the tile still has the ‘flange’ at the bottom of the tile, I had to create a plate so that the top shape can be cut using so that it has a flat bottom:

Next up, the two main shapes were aligned together:

I then used the software ‘split’ function, that allows me to cut the tile shape to the profile of the other shape. This produces several pieces which can then be deleted until I have the final shape I wanted:

Now that I have the shape of a single tile, I can then create a string of them as Muz wanted to that he could stick them straight onto the model as one long piece rather than do each tile. As Muz was using Wills Roof Tile sheets for the rest of the model, I copied the single tile and overlapped them by the width of the Wills Roof Tile (rather than the prototypical overlap):

Before I could hand the files over to Muz for Printing, I had to export them as a .stl (‘Standard Triangle Language’ or ‘Standard Tessellation Language’) file. Fusion 360 has an inbuilt exporter. I’ll be honest and I don’t understand all the settings, so I simply choose the defaults and export:

If you want to see the final painted model, head over to Muz’s excellent blog here: Workbench Witterings #23 Bonnet tiles, roof plumbing and painting putting the finishing touches to Westhill Road’s station building – Southern Railway, Fisherton Sarum, Canute Road Quay & Westhill Road (southern-railway.com)


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