I spent probably 2-3 weeks drawing this in Sketchup. Finally got to where I was satisfied I understood how everything was supposed to work & ordered my prints from Shapeways. They came in this morning. There are painted gloss black to make them easier to mold.
This is a departure from my past approaches. Before, I tried to figure out how to make the shank rotate separately from the head. This time, I am attempting to model a prototypical draft gear & yoke.
First waxes:
Cool !
How are you making the wax castings? And when you print them, are you factoring in shrinkage? And how much?
I print my patterns at 103% to account for shrinkage.
After they’re sealed, I mount them in a mold frame. Vulcanized rubber molds require some special frames & clamps. But since I’m using RTV, I can use aluminum & Plexiglas. These can be bought ready-made, but they’re easy enough to make yourself with aluminum flat stock. Here’s what a single mold frame looks like after the RTV is poured:
And when I have several, and I’m lazy, here’s what I do:
After the RTV is cured, the mold has to be cut. This is a skill in itself, and I still feel a little green in this area. When cut correctly, the knife makes registration marks that keeps the halves aligned during wax injection:
This gets dusted with talc, and sandwiched between to blocks of 3/4" ply (hand pressure only) for injection:
This gets hot wax injected into it by this machine:
I should also add, the thing that mystified me when I was learning wax injection was the venting. I watched dozens of youtube videos. Everything I read said wax should be injected between 5 and 10 psi. I knew that wasn’t enough to crush air bubbles like you would when pressure casting resin at 60 psi. None of the patterns they were molding had venting attached - only a sprue for the wax to come in.
Then the light bulb came one. One video demonstrated vertical cuts for vents. When you have a horizontal cut, and you squeeze it, it gets tighter. If the cut is vertical, the pressure makes it open slightly, and thats how the air gets out. After the wax has filled the cavity, the pressure is released, and the mold goes back to its natural shape. This is why clamping is not desired, but hand-pressure only.
Looking great Burl!
What scale do these come out to be?
Compatible with other manufacturers’ couplers?
1:29, same as my other couplers. Should work with Kadees. I can’t guarantee that they’ll work with everything, as I don’t keep everything else on hand.
Perfect! Have lots of #1 Kadee’s all ready installed.
So do I.
Finally got all the waxes made up for this one & mailed them to the foundry yesterday:
I would have been finished a week ago, but this particular piece turned out to be more difficult than I had imagined:
I re-made the mold at least 5 times. I kept getting air bubbles in the spring pocket. My first attempts were to cast it without the openings. I planned to carved them out later. However, the RTV is so thick, I never could get it to flow completely into the cavity. Even after I opened up the pattern, I still had trouble getting it to go in. In my last attempt, I vacuumed the RTV (a second time) after I poured the mold. Still, I had a couple small air bubbles, but I could cut those out of the casting. I am currently pouring yet another mold, which has been vacuumed, and is now curing in my pressure pot at 60 PSI.
Looks amazing !
Looks great Burl! When will we see a video of the rotary coupler in action?
I hooked them up to a rotisserie and shot some video. First time I think I’ve tried something like this, so it may be a little rough.
Unreal
Awesome Burl!
Can’t wait to add these to my coal cars.
I had some time to work on the yoke drawing for the non-rotary end. This will eliminate a couple steps of assembly.
