Friday, April 28, 2017
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Kitbashing a Micron Arts 1083 Pratt Truss Dual Mainline Bridge

Kitbashing a Micron Arts 1083 Pratt Truss Dual Mainline Bridge

It’s not that I’m so old. It’s just that there are traditions that are important. And one tradition, at least to me, is to fabricate as much as possible. I don’t like buying ready-to-run models. I prefer to build them.


Moreover, I love bridges. Being a devotee of John Allen’s Gorre and Daphetid, where he created over 100 bridges, any layout I create would have at least a few. And while I’m not opposed to building them using structural plastics if that’s what I have, I prefer metal. I can’t say why, but there is just a look and feel to metal (although I’m mightily impressed with some recent 3D printing being done by SouthernNScale, I must admit).


So when I started looking around the Z landscape, I found Micron Arts and their various bridge kits in etched brass. Perfect! True, I spent a lot more for the kits than I could have had I purchased a plastic assembly, but the fun is in the building.


Problem is my first Z layout has a dual mainline, whereas the Micron Arts kits are all single-track. But that’s not really a problem if one is willing to spend twice as much, take a risk and a leap of faith, and kitbash two single-track kits and transform them into a single, double-wide bridge. And that’s what I’ve done here.


In fairness I did assemble a kit as it’s supposed to be assembled, following the directions and producing the single-track version. I was more interested in seeing how it went together and if I would hit any roadblocks along the way. I found the kit easy to assemble, but I’ve been doing this awhile. For a beginner, take your time and make sure everything is fitting as it should! And since I lay my own track, I’m comfortable with soldering. I can’t imagine gluing a brass bridge together, but apparently you can according to the instructions. Here, I soldered, and I’m not entirely sure you could build this double-wide version using CA (cyano-acrylate, or “super glue”). Maybe epoxy, but soldering seemed the best option to me.


So first the big picture. My goal would be to use as much of the original kits as possible, keeping structural integrity and aesthetics in mind. Where I can’t use kit parts, I’ll prefer structural brass over other materials, in keeping with the spirit of the kit. I run my dual mainline track at 0.75” center-to-center, so all measurements will be based on that. The good news is half of 0.75” (3/4”) is 3/8”, so I don’t have odd measurements to deal with. I chose 3/4” because in HO scale, dual mainlines tend to run 2” apart. Doing some math, 2” in HO equates to 3/4" in Z.


Upon completion of the single-track version, I decided I would switch the order of assembly a bit. The order in which I assemble the chords (sides) isn’t relevant since they’re the same either way. But it’s very important to build the base correctly. The entire bridge depends on it. So I started with the base and essentially built the bridge from the ground up. That allowed me to get things aligned as well as create good solder joints along the track support girders. Once the base was entirely completed, then I created the chords and secured to the base. Finally I worked the top of the bridge.


From an instructional standpoint, I started with Step 5 of the instructions, completed Steps 6 and 7, then moved to Steps 1 through 4, finishing up with Steps 9 through 12. I skipped the final couple of steps. Those would have me secure the signage and end frames. I like the appearance of the bridge without those details, so I’ll leave them off.


So, let’s dig in. I first found the G1 gussets, which are the end gussets. G1 gussets have no tabs for securing the vertical support members. G2 and G3 have these tabs, with G3 gussets having longer tabs since there will be two diagonal supports instead of one. The longer tab is needed to go through the slots for all of the vertical and diagonal members. All gussets come as a flat piece which you’ll fold together to form a gusset twice as thick (Figure 1).



Using my ruler, I located the centerline on the back side of the gusset, the trimmed the end 3/8” away from center (Figure 2). Here you can see them butted against each other, forming the basis of the dual mainline configuration. To cut, I used Xuron sprue cutters (highly recommended).



It’s one thing to make the cut, but it’s something else entirely to weld the gusset pieces together. They absolutely must be properly aligned and square, but the cut isn’t guaranteed to be perpendicular to the track. If I just soldered them together as I cut them, the finished gusset could be crooked and angled. Can’t have that! So to prevent this, I created a soldering jig, which I used throughout the build. The kit comes in a thick cardboard envelope, so on the backside of this I glued a piece of scrap brass to be used as a straight edge (Figure 3). I’ll hold the gussets against the straight edge and solder the brass side not touching the guide. I don’t want to weld the gusset to the guide! The envelope will absorb heat, and I’m assured I won’t also weld my gussets to the cardboard.



I made my weld to both sections of the gusset, spinning it around in the jig. Then I folded the gusset and welded the two pieces together to form the unit that fits into the lower cross brace (Figure 4). This is the same process you find in Step 5 of the instructions if you’re following along.



The gussets fit into a slot, or well, in the lower cross brace. Normally the brace would extend side to side (as I’ll show later on with the top bracing), but in this case I elected to trim the ends from both pieces based on measurements I’d taken. (Figure 5).



With one side trimmed, I soldered the two lower cross braces together such that the wells are perfectly fitted to the gussets (Figure 6). Looking down from the top of the bridge, the brace angles are for a single track unit and therefore not correct for a bridge twice as wide. An expert engineer would notice the structural oddity, but it’s not a detail casual observers would notice. I was more interested in preserving the wells in the brace than cross brace appearance since it’ll not be that noticeable once assembled.



With the lower cross brace assembly complete, it was time to solder in the gussets (Figure 7). Not being an expert photographer, I decided to photograph things on white paper, hoping the detail would show up better than it does on my green cutting mat. That’s why you’ll see the different background starting with Figure 7. While I’m assembling all of this, I’m constantly monitoring the track center-to-center measurements and overall width. I want the bridge to square up in the end, after all. Also, one of the lower cross braces is lower than the other since I soldered them together one atop the other. The side that’s lower must have the end be adjusted upwards a tad because the pin of the brace and the tab of the gusset must mate with the vertical support members. If I didn’t adjust them, they wouldn’t line up and I couldn’t secure the side of the bridge.



Now is the perfect time to lay in the track support girders (Figure 8). Looking at the image, to my eye the bridge appears slightly curved. Old eyes? But in actual fact it’s perfectly straight.



Similarly, the tie plates are easy to add at this point (Figure 9).



With the tie plates secured, the base is complete. It’s time to assemble the chords! Here, I followed the instructions starting from Step 1. The only thing I deferred was the addition of the top plate (kind of the kit’s Step Zero). I find it easier to trim the pins we’ll be working with (Figure 10) without the plate installed. Once the pins were trimmed, I added the chord plates.



With both chords assembled, I carefully lined up the lower brace pins and gusset tabs, squared each side up, and soldered the sides according to the kit instructions. The completed unit so far is shown in Figures 11 and 12.




Everything to this point was pretty straight forward and relatively simple. And to be sure, we have the makings of our dual mainline bridge. The upper structures, however, will require some thought. I’d originally thought I’d cut the center cross braces from each upper sheer frame and then replace them with a slender, double-long piece I’d cut from brass plate. But looking at them I was a little concerned about damaging the outer frame itself, so I elected to try something different (Figure 13).



Here you can see that I trimmed the ends off of two sheer frames and soldered them together, similar to what I’d done for the base gussets. For the cross bracing, however, I trimmed one of the cut ends and used that as a vertical center support, soldering the cross braces to the center support. The angles aren’t quite as they should be, but I thought the overall effect was pretty cool. You can see the fine detail here, so I was careful with my cutting and splicing to as to not destroy the intricate design (see especially the upper banding).


So four main sheer braces and two end braces later, I had what you see in Figure 14.



Somewhat surprisingly, the center sheer frames were actually quite sturdy. I think my redesign helped in that the joint line wasn’t entirely vertical along a plane. That is, the sheer frames didn’t flex in the center since the joint included the center vertical brace. However, the end frames certainly did seem to want to bend along my solder joint, so to the upper rear of the end frames I soldered a 1/32” square brass bar as a horizontal stiffener. It’s hardly noticeable, but it gives great support to the frame. Figure 15 provides another view.



Now it was time for the upper ‘X’ cross brace. Here, I didn’t want to follow the approach I took with the lower brace. The lower brace had the gusset wells, but the upper brace is simply applied to the top of the chords. If I cut the ends off of the two existing kit pieces, it would look strange! But this is also easily solved with some 0.032” brass wire! It’s hard to see in Figure 16, but I’ve started weaving this wire along the top of the chords, soldering each joint as I go.



A slightly better image is Figure 17, where I’ve completed weaving the ‘X’ brace. Note I put a dab of solder where they meet to simulate a coupling plate. Figure 18 is a head-on view.




Now it’s all but completed. I added footers, as you can see from Figures 19 and 20. The structure squared up nicely and sits on a flat desktop with no rocking. At this point I’d put approximately 12 hours into the project, and taking my time, measuring frequently, and overall inspecting progress as I went has produced a nice piece, well capable of handling the traffic my little layout will throw at it!




Of course, it’s beautiful as a brass piece, but the intention is to actually use it. That being the case, I bathed it (liberally) in lacquer thinner to clean the solder flux from all surfaces. I then primed it using Tamiya’s (fine) surface primer and painted with Testor’s flat black, keeping good painting techniques in mind. It would have been a shame to paint a bunch of non-scale drips and runs at this point! In Figures 21 and 22 you can see a more final result. I’ll need to build some more track…the left track has no guard rail and the right track’s guard rail didn’t yield enough flange clearance (still learning!), but you can see the effect is tremendous. Once I do create good track and get that painted and glued into place, as well as some structural weathering, it’ll make for a nice addition to my burgeoning layout.




At long last (layout) progress!
Hand-Laid Turnouts the Easy Way


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