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
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
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