Large Scale Central

Brian Briggs' PS Mini-Hy Cube boxcar

Modeling Technical Modeling
#41

Okay, I see what part you are talking about now. If I remember correctly the trainline does not move with the centersill. The cushion drawbars were a known source of popping airhoses as slack ran in and out. So this would leave me to believe that the airline cut through the centersill. What does this information help out on the model, nope.

But, I think you highlighted part could be stronger if you attach it to the bolster instead of trying to cast it as a separate piece?

#42

If I make that part in brass, I can make it thinner, and make more room to tuck the trainline behind it. I came across a photo that finally made it click to me. Its low res, so it’s pretty grainy, but you can see that the trainline is attached to the center sill.

So I redrew mine to look like this:

#43

Yep. That’s how I remember it. When excessive slack pulled, it would pull the air hose apart because the air hose was a regular length hose, not a longer hose that most cushioned cars had. Autoracks were the worse at popping a airhoses.

#44

After stewing on it a couple days, I have found a design I like better. Funny how simpler designs seem like they are harder to come up with.

The outer center sill (green) will now be part of the main floor casting. It will have a channel that the bearings will ride in. The inner center sill (pink) will have slots for the bearing shafts. This should lock the whole thing in & keep it from sagging vertically. It also means I only have to reprint the center sill, and I can use the rest of the floor pattern with no further modifications.

This also allowed me to change the orientation of the pattern in the (eventual) RTV mold by 180 degrees. Now I can also model the spring for the cushioning unit as part of the casting.

#45

Very interesting stuff Burl…(http://largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-surprised.gif)

#46
While I’m waiting on my revised underframe parts to come in, I decided to work on the side patterns.  Started with the vertical panels.  Looks odd right now, because the glue shows through in places where the castings are very thin.  It should all even out when its painted though.
 
#47
My revised underframe prints came in last week.  When I started to glue them in, I was having trouble keeping them even.  After some double checking, I realized I had some variance in the I-beams.  So I stripped out the old glue, and made up this sanding stick out of aluminum C-channel & 220 grit sand paper:
 
 
 
With that, I was able to fine tune the fit & get a fairly straight line.  When I got ready to try gluing them again, I made up a spacer to represent the sliding part of the centersill (cut the same width), and added .015” styrene for clearance:
 
 
With that in place, I could tack it from the outside, remove the spacer & finish gluing from the inside.
 
 
 
Now everything looks like it fits well.  I don’t want to get in a hurry on this step, so I’ll look at it for a while.  If I can’t find anything wrong with it, I’ll pour another mold tomorrow (I have found sleeping on it helps me avoid mistakes due to haste):
 
 
#48

Is that brass bolster stamped out? I want more details on how you made that part.

#49

Lost wax casting

#50

Poured my molds for the revised lost wax parts yesterday, only to find the two I really needed had refused to completely cure. I was about to write them off as a total loss, but then I realized I could still get patterns out of them sufficient to finish the underframe casting.

I think the last thing I need on it is the divots for the new mounting holes:

(http://burlrice.com/_LS_MiniHyCube/underframe8b.jpg)

(http://burlrice.com/_LS_MiniHyCube/underframe8a.jpg)

I will remake the lost wax molds later. Still trying to figure out what inhibited the RTV, but they say platinum-based silicones are very touchy. When this has happened before, I cleaned the patterns, applied a fresh coat of paint, remolded, and they were fine the next time.

#51

Fine work there Burl

#52

Finished the centersill pattern & molded everything. Made up some castings to check the fit:

(http://burlrice.com/_LS_MiniHyCube/underframe9a.jpg)(http://burlrice.com/_LS_MiniHyCube/underframe9b.jpg)

#53

Nice craftsmanship Burl.

#54

Those brake hangers and brake beam looks really well done. Top notch!

#55

Sort of got derailed on this project. Shapeways took their sweet time on my last order (3-4 weeks). When it did come in, I got covered up at work.

Anyway, I got back around to working on the sides again. I’ve re-done some more work here. What I had before was OK, but I felt like I could get it closer. I had previously used my old trick of using Aluminum tape to imitate oil-canned sheet metal. Sometimes it worked well, sometimes it was hard to control the effect. And there was always un-evenness between the panels that made it hard to apply the weld bead decals later.

What I did different this time was to model the oil-canning in Fusion 360. I started with an image like this:

(http://burlrice.com/_LS_MiniHyCube/PS_oilcan.jpg)

I run this through a script that interpolates the z-coordinate based pixel lightness/darkness. Sort of imitates the result a 3d scan, without all the expensive hardware. I do some mesh reduction & smoothing on the raw image, and scale it to get the print to the thickness I want. Which looks something like this:

(http://burlrice.com/_LS_MiniHyCube/oilcan3.jpg)

To avoid obvious repetition, I had three prints made: one regular, one flipped horizontal, and one flipped vertical. The prints came in with obvious scan lines, which I expected. I sanded them with 220 grit sandpaper until they no longer looked like topographical maps. Then I made an RTV mold, and cast copies in resin. Using a combination of castings, some rotated 180 degrees, I built up the side pattern:

(http://burlrice.com/_LS_MiniHyCube/side3.jpg)

#56

Burl

I think that is Great!

Tom

#57

Thanks for this post Burl.