My next challenge is trying to model these lower beams that are beneath the first two panels on each end of the bridge. The C channels won't be too difficult but all of the diagonal bracing will be tedious if built from individual pieces. The C channel according to my notes is 10" x 2.5". The diagonal straps are 2.25" x 0.25" stock. After checking the parts in the Central Valley kit I decided that there wasn't anything that was going to work. So I decided to build these from scratch. To make the lattice work a little more practical I decided to experiment with brass etching. K&S sells some etched lattice that is very similar but
unfortunately the dimensions aren't quite right.


The first step in the process is transferring the pattern on to the brass. I used my CAD software to draw up a lattice design. I then printed out a copy of this on plain paper using my ink jet printer. Since I don't have a laser printer I took this copy to work and used the copy machine to transfer the pattern on to a suitable medium. What you want to do is to print it on to a paper that has a glossy finish or maybe a transparency. The idea is to get the copier toner on to a medium that it can easily be extracted from. I made several copies on glossy photo paper and also some on transparency sheets made for copy machines. I then took one of the glossy photo paper copies and ironed the pattern on to the brass as seen above. The next step, trying to remove the paper from the toner, was rather tedious. I soaked the whole thing in warm water to try and break down the paper. This wasn't very easy and I spent a lot of time pulling off little scraps of paper. Eventually I got it all off without removing too much toner from the brass. I then touched up the imperfections in the toner with a "Sharpie" pen.

Now that the pattern is on the brass there is one more step to be done before it can be etched. The back side of the brass has no mask and would eat right away so it is necessary to coat the back side with etch resist. I painted the back side with some gray spray paint and then put clear packaging tape over it. This will protect the back side and the front is all set with the design to be etched.

To etch the pattern I decided to use some ammonium persulphate that I had around from an old circuit board project. This is in powdered form and was mixed with warm water in a plastic etching tray. Be sure to read all the warnings on the etchant bottle. This stuff is very corrosive and you should wear gloves and work in a well ventilated area.  After I had the etchant mixed up I placed the board into the etching tray, pattern side up, and gently rocked the tray to help speed the etching process. If you don't agitate the board the debris can settle down on the brass and it may never etch completely. I rocked and rocked the tray and after a long time to my dismay the pattern started coming loose from the brass before the brass etched all the way through.


Here is an example of my first failed attempt at etching brass. As you can see the toner has flaked off the brass before etching all the way through. You can see that the pattern was partially etched in the brass. In fact the brass is only about .002" thick outside the pattern. The original sheet was approximately .005 thick. Time to regroup and figure out how to proceed. I did some research on etching chemicals and read that sodium persulphate might be easier on the etch resist. So I headed down to the local Fry's Electronics and picked up some sodium persuphate crystals. This would also ensure that the problem wasn't due to the age of my etching chemicals.

This time I decided to try a better method of getting the pattern onto my brass sheet. Instead of using the glossy photo paper I decided to try a transparency. I ironed the pattern from the transparency on to the brass sheet. I carefully pulled the transparency away from the toner. I had more toner peel off than with the glossy paper but it wasn't too bad. I decided to overlay a second transparency and iron on a second layer of toner. This filled in a lot of the gaps and I touched up with a sharpie pen. As before I painted the back side with gray spray paint and then covered with clear packaging tape.

To etch this new piece I heated some water in the microwave. This was then mixed with the etching crystals in a plastic tray. The brass was then placed in the etchant and I began the agitation process. The etching took about an hour and twenty minutes using this process. With this new chemical I could see bubbles forming right way and the brass was etching quicker.


This is the how the sheet looked after the etching was complete. The gray paint shows between the masked areas to indicate that the brass has etched through. Note that the lower right corner was tapped with clear packaging tape since it was unused. You want to try and etch as little material as possible to keep the etchant from becoming saturated with metal. Also note that I added lots of little triangles between my pattern to reduce the surface area of the exposed brass. I also created some other panels that I might use in the lower left. I then soaked the finished sheet in paint thinner to loosen up the tape from the back side.


This image shows one of my completed beams and it's approximate location relative to the bridge frame. I took some K&S C channel stock and milled down the lips to about .040". The original size was a little deep at around .070". I then drilled three holes for the pins that will hold the vertical members.  00-90 bolts and nuts were used to provide proper spacing of the beams so that the lattice could be added. I attached two lattice pieces to the top with ACC. Two more were added on the bottom side but with the pattern reversed so that an X pattern is seen when viewed from above.


The next step will be building the pillars that the ends of the bridge rest upon. In this shot the beam that I just built is supporting my backpack. I'll be building the footing that it attaches to and then attach the bridge frame to the footing.


This is a side view of the south span. Here is another shot of the beam that I just made with the etched parts. Note that this beam only extends out for the first two panels of the bridge. The four center spans have a couple of heavy iron straps separating the panels. In this shot the very first one seems to have a nice kink in it. Maybe part of the twist that someone pointed out in my prior post.

Back to bridge frame construction



Home page