Monday, August 19, 2013

A way to reinforce investment for casting hollow parts.

In the past we use to invest the part and treat it very gingerly with the hope that the investment doesn't let us down. Then we started using scale to measure the water and the powder, thinking it could be controlled by precise measurement and temperature. Once you learn this stuff it stays with you, like muscle memory. I'll talk more about that in another post.....
So in our shop we still use solid walled flasks and over the years we found great uses for the stainless steel welding rods we use to provide vacuum vents is the investment. The one use I will be focusing on is using it as "reed bar" in the investment.

I was commissioned to make a sword handle for a very expensive hand made blade.
the original 300 year old handle
I was given this handle as a starting point to make the new handle for the new blade. So began the process...


I made an RTV mold of the original and an RTV mold of the tang of the blade ( the part where the handle is attached ) so that I did need to keep the blade ( $150K )...

the wax model

 So here is where the reed bar trick comes in. This handle had to be cast in one piece with an 11 inch by 1/4 by 2 inch hole. The tang of the blade had to fit inside perfectly because my teacher was going to mount it on the blade the same way the original 300 year old handle was mounted. Just like a katana with one pin through the center of the handle. In the picture of the original handle the red dot on it is wax covering the hole where the mounting pin went. It was covered because I didn't want RTV inside the handle, it was going to be put back on the Ken blade it came off of. Back to the investment story....
This handle was all most 14 inches long and had a diamond shaped tapered hole that was at least 11 inches deep. we had to wait to drill the hole side ways through the handle until it was finished and ready to mount. So the solution, I invested the wax in the picture with the open end facing away from the base so all the air could get out of the space. I also inserted three 13 inch long pieces of that stainless steel welding rod into the empty space inside the wax. The three rod stuck about a half inch out of the investment and the end of the handle was about 2 inches under the investment.

The finished handle was the proof that it worked. I ended up making two, the first was a test cast to figure out if there was anything i didn't think about or needed to watch out for.


My observation in the process is no noticeable cracking around the rods where they stuck out of the investment. Inside the investment there was no visible distortion or cracking around the rods. There was discoloration in the rods but they held the investment in place through the casting process. The stainless doesn't stick to bronze, brass or sterling. I don't ever file the end of the welding rod flat, I leave the sharp little pinched end the bolt cutters leave. I believe it helps prevent the steel from sticking to the casting. when ever we use this form of reinforcement in investment we always use 316L welding rod we also lower the water ratio of the investment to 38% ( our investment manufacturer recommends 40% water ratio ) to make its cured strength a little stronger.

So think about this the next time you need to do something the your investment might not want to do...









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