A little progress on the pattern for the underframe. All the black parts I had laser cut by Alan Friedland.
More patterns. This is the part of the center sill where the truck mounts (not sure what to call it). I tried initially casting it in a glove mold with no sprues, but the shape of the part did not lend well to resin flow from the top and the failure rate was high (hence, you can see where I filled in some of the bubbles with CA). I decided to make the production mold with a large fill sprue.
After I looked at the underframe for a while, I just wasn’t happy with the center sill. I had tried building it up out of styrene, but the profile just wasn’t lining up with the printed part of the center sill. I kept trying to think of what I could use to build the correct profile, then the light bulb finally came on… I poured three more of the printed sill, glued them together, and filled in the channel with styrene. Here’s the result:
Burl,
Are you sure that the center sill is filled in on the prototype? The plans that I have for the GN Snow Dozer show the center sill to be made up of 13" Z shapes, and the part that you filled in is open. Or are you filling it to make it easier to assemble/cast?
No, its not prototypical. For one, it is harder to cast under ambient conditions. Secondly, it gives the car more bottom weight.
I also want it thick right there for mounting a screw later for an idea I’m going to try.
That’s kind of what I figured, but I wanted to ask just in case. Is this going to be another rotational casting?
A little off topic, but when you draw parts up to be printed to you account for the resin shrinkage? I thought you posted once that resin parts shrink by 10%?
No, this will be a flat kit. I can keep the cost lower that way.
I account for 1% shrinkage in the resin under normal conditions.
Burl Rice said:
I account for 1% shrinkage in the resin under normal conditions.
So when you print out your parts from Shapeway you print them at 1:29 plus 1%?
Not to get further off track, but I’ve got some old casting Alumilite resin (4 years old), do you think it’s worth casting? I tried a test cast the other day (equal parts A& B) and the part never fully dried. It was sticky to the touch. I was just wonder if this resin was worth experimenting with (maybe more of A or B) to get a decent cast? I know that the resin goes bad 6-12 months so next time I order a bunch I’ll make sure that I have enough projects to cover the gallon. I think I would have actually used it all up, but we moved and the resin got packed away.
Not to change the subject. But I have found that the MTH 20-89010 is a great FRED for large scale. It is a yellow box mounted on a freight truck. I cut away the knuckle with my dremel and added a 9V battery plug. It’s red strobe is very bright, especially at night, and looks to scale. It has a small air hose on the bottom. I cut it off to about a quarter of an inch long, drilled a hole in the top of a Aristo Craft knuckle and slid it in. I mounted the one I purchased on the end of my double stacks and laid the 9V battery in the well. Looks very good.
Craig: that’s exactly what I do.
Resin at 4 years old is probably no good. Moisture is usually the biggest problem - it will foam when it starts to set. Not setting sounds like a mixing problem.
With Alumilite, the B side tends to form crystals (which you can strain out) and the casting will have dark spots. This is generally cosmetic, and I might use it for my own purposes.
Thanks for the info. I’ll try straining the B side because it has crystallized a bit. The gallon jug actually had formed a seal over the B side, so it might have preserved some life. Sounds like its worth experimenting with until it’s used up.
Burl,
i see diagonal parts below the center pivots.
were they necessary for 3D printing?
No, its necessary to release air from that part of the cavity during casting. From what I have been told, Shapeways uses a wax support material when you order something printed in acrylic. The wax is removed during the cleaning process, so you don’t have to design support structure for your models.
I poured the mold for the underframe a couple days ago:
Fantastic as always …Love your eye for detail and making it solid!
The brass patterns for the plug door came in today. I cheated a little bit & put some of them together on a sprue. Shapeways discourages that, but it went through.
The remainder of the parts I had ordered came in a couple days ago. I wanted to get a coat of paint on them before I make molds so I could see if any sanding would be required. You’ll have to look over the quality of the (iPhone) photos - there is a little bit of a fish-eye effect in them:
(http://burlrice.com/_LS_PS50.5boxcar/end.jpg)
Wow, real nice. If I was into making metal cars I would want me some of them.
I built up an under frame tonight. Not sure why Photoshop wanted to change the resin to gray, but I was having a hard time getting the color balance right. Tan resin on a white background is hard to shoot sometimes.
(http://burlrice.com/_LS_PS50.5boxcar/underframe5.jpg)
