City Bridge

Whatisit?
I made a cut along the back of the layout so the On18 line would require a bridge. I have a running thread titled .. On18 City Bridge but the actual bridge designing dosen’t start until the fourth page – City Bridge – Pt IV.
Meshes etc.
GirderMain Girders: The plan is for the web to be made from .040″ styrene. This is of course way out of scale since this would be 2″ thick Full Size (FS)! In reality it would have been about 7/16″ thick. I can get away with this since the end of the web will be hidden by the abutments. The styrene will work better simply because of that thickness. One of the things that I have run into is I don’t have any .040″ styrene long enough (13-5/8″) so I will have to do what they did FS and splice it. I just find that amusing.

The main angles will be .080″ Evergreen angle. This is a little thick but I think thinning the edges slightly will pretty much hide that. The biggest problem will be the mass of rivets down the lengths of angle. I have some Micro-Mark water slide resin rivet decal sheets that SHOULD work for that.

Cross Frames: I originally planned to scratch the cross frames from Evergreen angle but I have since been working on creating them in Sketchup so they can be 3d printed.

snapshot00I need five each cross-frames for the bridge. Since the depth (front to back) of each assembly is less then 5mm I grouped two together with short sprues. I then duped this grouping twice in MeshLab and created one mesh.

The numbers – Vertices: 44,657 and Faces: 89,346 are for one grouping. The numbers in parentheses are 3x that. Shapeways has a maximum file-size of 65mb .. and 1 million polygons (faces).

Files: These are all in: .. MicroMimesis/On18 Plate Girder Bridge/Cross Frame

  • [CrossFrame_Pmm.skp] – single cross-frame, Pseudo mm
  • [CrossFrame_Pmm-r.stl] – .stl of above .skp file
  • [CrossFrame_Set_Pmm.skp] – two of the above grouped via sprues for meeting minimum print size
  • [CrossFrame_Set_Pmm-r.stl] – .stl of above .skp file
  • [CrossFrame_Set_6ea_Pmm.stl] – above file grouped three times for six cross-frames. This file is 4.3 mb which is well within Shapeways maximum. The faces at 269K are just past 1/4 the maximum for polygons.
LateralsV1Lateral Bracing: The plan was to scratch them up .. then print them .. and back to scratching them up. I am leaning toward scratching as these would be hard to print – requiring sprues to get the 5mm minimum print size – and then they would be prone to warping. They would be pretty easy to scratch up – some angle and flat strip – the most time would be putting rivets (resin).
Stiffeners: These angles are riveted to the web of the girders of deck girder bridges to help keep from buckling

Jogged_v2_60ea_Pmm_SWTo work the angles have to cross both the girder web and the thickness of the main angles. This means that if I used .060″ Evergreen angle I would have to pad out the angles where they cross the web. This was done in FS too. The better way is to use ‘Jogged’ angles .. where the ends of the stiffeners are jogged out so the stiffener/angle flows around the main angles.

Files: These are all in: .. MicroMimesis/On18 Plate Girder Bridge/Stiffeners/Jogged/V2

  • [single_jogged_v2_Pmm.skp] – single stiffener. This is the base design.
  • [single_jogged_v2_set_Pmm.skp] – 10 ea. grouped with sprue. Grouping is necessary to meet the minimum 5mm print size at Shapeways. This also allows me to replicate this mesh as many times as I need into a larger mesh using MeshLab – with the caveat that the resulting mesh is less then 64mb, 1m polygons and fits within the printer bounds.
  • [single_jogged_v2_set_Pmm-r.stl] – this is the above converted to .stl via MeshLab and then errors repaired with NetFabb Studio (the -r)
  • [Jogged_v2_60ea_Pmm.stl] – This should be the final mesh. I am planning to include this in a larger mesh of other parts to save $ on handling. (this is the rendering to the right)
Splice Plate: The heavily riveted plates that splice the web sections of a deck girder bridge could be scratched up – but I am guessing that a 3d printed version would look better.

If more than one web plate is used to build a longer bridge, splice plates are used to join the web plates. These are riveted flat panels.1

spliceplate_PmmI went into how I came up with the design for this splice plate in City Bridge – Pt VII.

I built a mesh with a pair of the plates spaced 5mm apart and linked with 1mm sprues. This is because the minimum size of objects at Shapeways is 5x5x5 mm. I have a small minimum thickness area where the sprues connect to the plates for removing the sprues. I have a minimum (.1mm) groove between the three plates to indicate separate pieces. The rivets are 3/4″ dia, spaced minimum 3-1/2″ apart and 1.75″ from any edge.

I will have to double this mesh for four plates and create a single mesh to upload but that will be after I create the mesh for the stiffeners. In the meantime, I uploaded the current mesh to Shapeways for quick test.

Current filename is Spliceplate Set Pmm-r

Files: These are all in: .. MicroMimesis/On18 Plate Girder Bridge/Web Splice

  • [spliceplate_Pmm.skp] – single splice-plate Pseudo mm
  • [spliceplage_Pmm-r.stl] – the above file converted to Pmm (Pseudo mm), to .stl and then repaired -r
  • [spliceplate_set_Pmm.skp] – paired splice-plates necessary for printing
  • [spliceplage_set_Pmm-r.stl] – paired splice-plates converted to .stl and repaired -r
WSF_printedBridge Shoes: Simple. Use the 3d printed ones I have on hand. These are printed in WSF (White Strong and Flexible). I normally don’t favor printing in this Nylon(ish) material because it has a noticeable grain – but this works fine for items that are supposed to be cast iron. In the photo the one on the left is as it came from Shapeways, still covered with white powder .. a few grains can be seen to the right of it (WSF is made by hitting powder with a laser and melting it into a shape). The center has been cleaned and on the right this has simply had an application of AI (Alcohol and India Ink) – WSF is somewhat porous and takes stains well.

Combined Mesh
snapshot00Here we are with the Cross-Frames, Stiffeners and Splice Plates combined into one mesh. This has 479,558 faces … and is 23,416 KB. I could double up and still be within the maximum polygon/file size that Shapeways has.

I went ahead and dropped an order. This is the part where you still don’t know if it will print. It passed all the ‘pre’ checks .. but they do another when it hits the printer (you would *think* they could do that BEFORE it goes to the printer .. but what do I know!)

  1. The Model Railroader’s Guide to Bridges, Trestles & Tunnels, Jeff Wilson, 1964 []

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