Module Progress--ICRR Bridge at Kankakee--NSF56K

[Zox]

Progress report for tonight:

First, I cut all the cores for the piers, and glued the sides to them.



You may note that the cores aren't centered on the pier sides, but rather flush with one side of them. I'll explain that shortly.

The next step was to fill the inside of the bridge with foam, to make it solid from side to side. Here's a picture showing all four steps of the process, although they're not quite in order.

• Fit a block of foam into the space between the sides and a pair of piers.
• With a hot-wire cutter, cut the foam to match the upper contour of the side profiles. The sides of the bridge act as guides for the wire, and since they're MDF the hot-wire cutter won't damage them.  Here is the reason I put the pier cores to one side: the foam fills in the missing area, and with the offset core you get one reasonably thick bit of foam on one side of the core instead of two thin, fragile bits on either side.
• "Skin" the top of the arch with glue and a sheet of paper, to insure a secure bond between the foam and the sides, and to hide the gap where they meet.
• Invert the bridge, and cut and "skin" the bottom of the arch.

Of course, it would be pretty inefficient to go through the whole process one arch at a time. So once I figured out what I was doing, I just went ahead and filled and top-cut for all the remaining arches. As it was after 9:00 at this point, I figured I could finish up in the morning.

I have to admit one mistake, and a bit of serendipity that rescued me. I designed the bridge for a 1/4" plywood deck, and the center of the arches actually comes a bit above the bottom of the deck. So I should have cut the foam so it would top out about 1/16" below the top of the arches, and I didn't. I could have gone back and tried to cut down the top, but it turns out the alleged 1/4" plywood I got from Home Depot is actually 3/16" thick--which gives me back the 1/16" I needed! I guess you could say I received a bit of St. Paddy's Day luck.

And just for the heck of it, here's a low-altitude shot of the work in progress. After the primary arches, the next job will be the secondaries, as seen on the bit of paper propped on the bridge side. Hopefully I can make a start on that tomorrow.

[SirTainly]

Rob, the bridge looks great but do wear a mask, the dust from MDF is not at all good for you...see here

http://www.design-technology.org/mdf.htm

Simon

[Zox]

Simon, thanks for the info on MDF. I don't plan on using any more of it in this project, but I'll make sure to wear a dust mask when I use it in the future. In fact, since I still have some work to do with Masonite/hardboard, I'll probably pick up some disposable masks tomorrow. Hardboard doesn't seem to carry the same health risks, but it's still not something that's pleasant to breathe.

I didn't make as much progress as I hoped today, both because of outside events and because the glue on the paper "skins" on the arches didn't dry as quickly as I expected. However, I did finish the skinning job. I also added 0.060" shims (close enough to 1/16") to the top of the piers, to compensate for the thinner-than-expected plywood deck. Basically, the shims bring the top of the deck back up to its intended height, level with the pier sides.



I'd hoped to start work on the sides above the arches, but it looks like that's a project for another time (probably tomorrow).

[Zox]

Well, this may or may not count as progress. Given that I'm coming up on the really fiddly little bits of the project, I accepted Lee's earlier advice and picked up one of these this evening:

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93012

It was on sale in the advertising flyer I got this weekend for $59.99, and the flyer also had a "15% off any item" coupon. I figured I'd take that as a sign.

[John]

Thats a real good price on the saw .. also, they have hot air guns for 9.99 .. perfect for shrink wrap

[Zox]

Tonight, I started working on the sub-arches above the main arches. For these, I'm using 1/8" hardboard (a.k.a. Masonite). Here's the template I'm working from.



The first step is to make hardboard rectangles of the right width, and slightly over-height. I did the width cut with my jigsaw, as laid out here.



It may be obvious, but if you need to follow a pencil line on a dark surface, it's much easier to see if you put a strip of masking tape down and draw the line on the tape.

Then: "Tonight on Futureweapons, the incredible 8.25-inch sliding compound miter saw. If you need to create a bunch of strips from a 6.5-inch wide piece of hardboard, this system will get the job done."



You may also notice something pink on the bridge. This is patching compound, which I'm using to smooth over a few last gaps between the sides and the foam. The compound goes on pink and dries white, although that doesn't really matter in this application.

Unfortunately, I'm stuck at this point until I can find my 9/16" brad-point drill bit, or pick up a new one. I used to be amazed that my tools knew exactly when to disappear for maximum inconvenience, but I've since learned that all tools are born with this ability.

[wm3798]

That's going to be awesome!

Lee

[Zox]

I'm hitting a bit of a snag tonight--it looks like the hardboard might not have enough structural integrity to hold together at the dimensions I'm working with (1/8" thick by 3/16" wide at some points). I'm still experimenting, but I may have to rethink my material choice here.

Any suggestions?

_{edit: stupid typo}

[John]

cut your hardboard as best as you can .. then lamintate cover it with styrene ..

[TiVoPrince]

Would Foamcore
be a better material?  Similar thicness but you will need to seal over the core but it may be more structurally sound for your purpose...

This is basically what stopped me twice on the Rio Hondo bridge.  Lacy thin structures and the available brittle/unstable materials that are the only ones actually workable for home tools made it truly frustrating...

[Zox]

Okay, I've achieved some success.

My process for creating the sides begins with drilling out the tops of the sub-arches. Then, I can either cut the large arch out first, or make the vertical cuts for the sub-piers first. Whichever order I did it, the oscillating action of the scroll saw was causing the sub-pier portions to tear off when I made the vertical cuts.

I therefore decided to try something from the Tim Taylor School of Extreme Overkill. After drilling, I stood the piece of hardboard against the fence on my miter saw, and used the vertical motion of the blade to slowly make the vertical cuts. This worked beautifully, and with a high degree of control.

I then used the scroll saw to make the large arch cut, which I'm still having trouble getting placed precisely--I've got a few things yet to try for that, though. Worst case, I may have to try the jigsaw again.

Here's two side pieces, set in place--the one on the with the vertical cuts done on the scroll saw, and the one on the right with the miter saw.



I do appreciate the suggestions, folks. John, if I was going to have to cut out styrene overlays, I'd probably just go ahead and do the whole side panel with styrene--and I may yet head that direction, depending on how things work out. TiVo, foamcore's a bit too thick--it's 3/16". I intend to use foamcore for the "insides" of the subpiers, though, because it is the right thickness for that.

[wm3798]

Use the scroll saw to rough cut the arch to within 1/16" to 1/8", then use a barrel sander on a Dremel to finish it.  You should be able to clamp 2-3 pieces together to speed the process.

Lee

[Ed Kapuscinski]

Would a drillbit of the correct width work to create the top arches?

[Sokramiketes]

Would a drillbit of the correct width work to create the top arches?

Looks like it worked quite well, considering that's what he used.   

I would go to styrene as the next best bet.  It's much less brittle than the hardboard.  Drill the arch tops the same way, use your scroll saw to run up the center of the archways to this hole.  Then you can scribe and snap the styrene from inside the arches.  (line cut with scroll saw is not critical then, just allows you to snap out each side individually.)

[TrainCat2]

My two cents...
I would have built a master out of styrene the proper thickness with all of the detail for one section and then made a mold for resin. Nasty work once, rewards the remainder of the time. Flat molds are real easy and a good start for anyone wanting to get in to it. The more resin work I do, the more applications for it's versatility I find.

Hind site is 20/20 most of the time from the sidelines. Zox, it is a great project. Thanks for sharing it with us.

Bob