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« on: January 14, 2015, 10:59:27 PM »
I am going to start a short series of posts on the pattern for the rear frame extension casting. This is the second largest pattern for the locomotive. Overall it is 56 1/2" wide by 15" tall and 20 3/4" long. Estimated weight of the finished steel casting is 521 lbs.
The function of this casting is to connect the forward frame to the rear frame just in front of the firebox. If you recall for #9, it needed a similar casting when it was rebuilt.
The first thing I did was take the CAD drawing and add machining allowance and draft. Then I scaled it by the expected shrinkage of 1/4" per foot.
The plan that Alan and I worked out for this casting is to break it into three parts, and to back it up with what is called a 'follow board'. This allows the pattern to serve as its own core box (to form a core), and to be able to extract the pattern from the sand mold. This will become more clear as the build progresses. Just follow along and see.
I started with the largest piece, a rectangular piece of plywood, laminated up to 1 1/64" thick, 2 degrees draft on all sides, and three 8" holes. The holes are reinforced by a 3/4" wide ring, which also gets 2 degrees draft on both sides.