Stringer Spools

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    The following will be taken from “A Treatise on Wooden Trestle Bridges: According to the Present Practice on American Railroads” by Wolcott Cronk Foster pub 1897.

    Chapter V Floor Systems starts on page 42 talking about Corbels. The next page covers Stringers which is the subject of this article.


    A stringer should be placed immediately beneath each rail, and in order to guard against defective timber it ought to be “split” or composed of two or more pieces. These pieces should be separated from each other by either cast-iron washers or spools, or wooden packing-blocks, or both. A considerable difference exists in the present practice as to the amount of separation. It varies all the way from nothing to 13 in. From 1-1/2 in. to 2 in. is a very good distance. In Figs. 62 to 72 are shown a number of cast-iron separators ….

    It continues about using packing blocks for separating the stringers but we are focusing on the cast-iron spools. Below are Figs. 62 to 72 mentioned. Note the wide range of designs used by different railroads. Note that this book was published in 1897.  In the days before computers I think it was more of each engineering following his own ideas. This is in part determined by how each railroad used stringers (see page 44).

    For we modelers the best part of this illustration is the scale at the bottom – what more can we ask for but a good drawing with dimensions!

    O Scale

    Shawn Branstetter who requested these spools and I used the “Magic” of Facebook messenger to toss ideas back and forth, working toward a spool width (stringer separation) of 3″. There are a lot of factors that go into making a scale model and perhaps the most important is simply that it .. whatever “it” is .. needs to be large enough to be visible to the observer otherwise it only is there to salsify the OCD of the modeler. A 3″ separation of the stringers is 1/16″ in O scale – small yet wide enough for the spools to be visible. Looking back at the diagram, Fig. 66 seemed to me to be about the right width.

    I brought Fig. 66 into Sketchup as an image so I could draw on top to create a CAD drawing. Note that the overly exact dimensions are simply the CAD program dimensioning off of my drawing on top of blurry drawings and need to be taken with a very large grain of salt. Still this gave me a starting point.


    I went through several different designs finally settling on this design. There is a principal of design associated with late 19th and early 20th century architecture and industrial design – “Form Follows Function“. I can modify that a bit to “Function Dictates Design” as applied to creating 3D models in scale. Sometimes you need to make something larger simply because if it were exactly to scale it might not be easily visible to the naked eye and that is ultimately the reason for our modeling in the fist place to ..  well .. have some SEE the darn thing. The more important thing though for me in design is to make it physically printable .. we have to work within the limits of our printers and make it structurally strong enough for us clumsy humans to handle the thing.

    • Width: The 1.587 mm width is simply 3″ in 1:48 – O scale.
    • Bore: The 0.66 mm bore is 0,026″. A white head quilting pin (Walmart sewing) is ~ 0.022″ dia … we have to design for shrinkage of the resin during curing. The full size spools would have sat on large threaded rods that went through the stringers.
    • 0.4 mm wall: A wall thickness of 0.3 mm is about the thinnest you want to use for a 3D print. This is simply structural. Using ‘minimal’ seems to me to lead to failure .. bumping that to 0.4 mm is logical.
    • 0.412 mm rim: See above
    • Spool Dia: The 2.224 mm diameter is simply making the groove 0.4 mm deep – simply visually works. This would be 4.2″ diameter full-size.

    If you read through the bullets you can see that the pin dictated the bore, the minium wall thickness dictated the bottom of the groove. The groove could have been shallower – which visually would have been the wrong decision .. IMO .. or larger .. which would have not been proportional to the design of Fig. 66. The 0.4 mm groove depth was just .. “Good nuff”. This all gave the overall diameter .. and there is our spool.


    These things are SMALL. Seriously so. Here they are still attached to the supports. That is a dime to the left.


    This is a close-up with a spool mounted on one of the quilting ins. This was the last of a yellow resin. There is still a bit of the support on the spool that would be removed. I sent this to Shawn “as is” .. suggesting he find his Opti visor before working with these!! Rumors that he threaded the needles is I am sure false.

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