27/01/2020.

Assembling the bridge girders

Assembling a simple girder

Each girder consists of a U-shaped part, closed by a cover and by end parts. Bending the U-part is much easier using a bending tool.

After bending, let’s solder the inside spacers. We see here a small bit of solder, the length of which making it possible to measure the quantity brought. To prevent it from rolling, it’s a good idea to flatten it with pliers, which has not been done here! Flux is of course added, although it’s possible here to use electronic type solder.

Soldering of inside spacers

For assembly, the main difficulty here is to hold together very long and thin parts. For example, the bridge rail, made of 1 × 1 mm section nickel silver bar, must stay exactly at the edge of the girder. I have provided photo-etched templates which help with this, but which are insufficient, and which may be soldered at the same time as the parts.

To prevent the rail from sliding, I cut an aluminium wedge of just sufficient width, according to the cross-section diagram opposite.

I can thus use my technique of tying in conjunction with the templates.

Below is a view of an end engaged into a template.

Diagram of part positioning

End of a girder in a template

Tying with galvanized iron wire. I should have done the loops closer to each other. The 1.5 mm thick aluminium wedge is got from an unused electronics box front panel.

Tying with wire

Appearance after soldering with a torch. Aluminium foil protects the plywood backing. The end engaged in the template will be soldered afterwards.

Appearance after soldering with a torch

Assembling two simple girders

The single girders must be assembled in pairs to form the bridge girders. They are connected by connecting parts which are also U-shaped.

A first assembly attempt without particular precautions ended in a very bad result, with a significant warping of the whole. Again, it’s necessary here to provide a mounting template which will be the only way to ensure a correct geometry.

This template consists of a plywood support — whose flatness must be checked — and aluminium plates drilled and tapped to fix the parts to be assembled. These plates have a double function: they allow better drilling precision than wood, and they will protect the support from the torch flame. In the following photo, the parts are already mounted on the template. A central flange tightens the girders on the template, because these are not fixed at the ends. Soldering in progress. Note that using a soldering iron is possible here.

Soldering on template

Soldering finished. Because the parts are less joined, the solder has leaked out of them. The excess will be removed afterwards.

Soldering on template, result

The bulk of the assembly is now complete. Let’s check the correct assembly of the structure. Good ! There is just a 0.5 mm difference in distance between the top and bottom of the legs, which will be easily corrected with thin wedges judiciously placed between the legs and the girders.

Structure assembly

Assembling accessories

These are the end stops, located at the ends of the bridge rails, and cable festoon supports..

Festoon supports

No particular difficulty, except the smallness of the parts. I begin by soldering a support at each end, then one in the middle. Note: to avoid thickening the base of the supports, I started by using solder cream; this was not a good idea, the mechanical strength being very poor. Better to choose a tin-silver alloy, even if it means more cleaning to do.

Installation of the first festoon support

For the future version, a template will allow a much more comfortable assembly of these supports.

Installation of the “rail”

Here is the installation of the last support — of the first beam! We see the advantage of the presence of the “rail” which gives a second supporting point to the part being soldered.

Installation of the last support

End stops

The end stops, made up of two parts, being quite small (a few millimetres), it is convenient to tin them directly on the photo-etched plate.

Tinning of the end stops

Just bend the support, then assemble and solder the two parts.

Folding the stops

In the foreground, the very tiny solder bits used.

Soldering the end stops

We finish by mounting the end stops on the girders. The stop on the right is a bit askew. This will be corrected without difficulty.

Mounting the stops on the girders

Although the structural work is over, we still have a lot of work to do.