Another Lesson I Learned in Casting.
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This project started with an evening phone call. A new member of Castinghobby news group (Yahoo! Groups : castinghobby), Rob Archer was in the area and was ready for his second visit. On his first I had promised we would do more than talk "next time." Now it was put-up or shut-up time. I have never liked the shut-up route. As Rob was winding his way into my neck of the woods, I looked around for something to cast. I had recently aquired an ornate cast iron cross I was itching to duplicate, so I dug it out.
When Rob arrived we stumbled our way out to the dark garage, turned on the lights and started assembling the gear needed. I put Rob to busting clods in my K-bond sand with a plastic scoop while I scrounged up a small flask and bottom board. I showed him how to ram the cope and then assembled the drag to the flask and had him finish ramming that side. The mold separated well, but I had trouble removing the cross from the sand and ruined the detail. We punched it out and started over.
I let Rob do this one while I put the burner in the furnace and hooked up the gas bottle. I lit a wadded up paper towel and dropped it into the furnace and fired it up. As it warmed up I dug through the pile to find some small pieces for the steel pipe crucible. No such luck. I had a fuel nozzle from a flight line that would fit into the crucible, but it was twice as tall. As it heated to the hot-short point we broke it with a steel rod and coaxed it down into the pot. Rob got the talk about hot-short temperature and the metal weakening to the point of breaking easily. He seemed impressed.
I opened the mold and stood the cope on it's side. I was a little luckier at pulling the cross from the drag this time. The cast iron was hard to get a hold of. I have some ideas for improving it next time, but Ill talk about that more in a minute. I showed Rob how to hold something flat against the sand of the cope to prevent pushing it out as I cut the sprue from the top. Then I cut a runner to feed one part of the pattern and let Rob cut another. After slicking and blowing loose particles out I put the cope back on.
So far so good, but success eluded us. Click the images for larger versions.
The detail that filled came out beautifully. By looking at the edge of the part that didn't fill I discovered the background part of the cross was only .115" thick. This is .010" less than 1/8th inch! I had no idea it was this thin. the cast iron cross weighs 1.1 pounds. The partially filled aluminum cross weighs .28, or a hair over a quarter pound.
As you can see, the back is inletted. I plan on filling this with Bondo to make it thicker and put some kind of finger grips in it to make drawing the pattern easier.
I also plan to put the sprue in the center of the cross next time to let the part be its own gating system. Hindsight is a wonderful thing!
But the important thing is Rob got to see it happen. "That is so cool!" I must have heard that at least ten times.<GRIN>
And I think it is, too.
Be sure to look at WOOD PATTERN MAKING.