Quote:
Originally Posted by ItBites
David,
Not sure what temper you are using, but typically in a billet that thick, the center does not have the same temper as the outer couple of inches (can be gummy to machine too). When I have made big parts from a solid billet, I usually machine to near-net shape (the part moves after this due to removing internally stressed material), then re-heat-treat (it walks a little more during the re-heat treat). Then come back for the finish machining to final dimension, after all the stresses are stable again. This is normally followed and required practice for aerospace structures, but maybe not for this application?
Having seen you are an Engineer, I assume you have considered this. Was wondering what your plan was.
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You bring up really good, valid points. We haven't gone through all the discussion (yet) of why we did what we did. That will come a bit later. One of the things we did to mitigate the difference in hardness from inside to outside is to remove the entire bottom end of the motor (where the main caps are) and make that in a separate piece. (All the main caps are made from a single piece).
As for machining the part close to net and re-heat treating--yes, that certainly is a great option and, as you say, it is done on many aerospace parts. I don't think we will need to do that here--but we will see. In a car we have the luxury of pulling off to the side of the road and enduring the laughs of the Corvette guys as we pull over on the side of the road. It is a bit more difficult to pull over in an air plane when something goes wrong
We are very careful in our machining practices to remove most of the material around the block to let it stress relieve as we machine it. We also do not take heavy cuts as even heavy cuts can induce unwanted stresses into the part. After we get close we do a pre-final machine where we still leave material on the block. Then, finally, we do a small, overall, finish pass.
We will discuss more of our plan on this in the upcoming days. Right now we are buried with Darren's Flip Top project.
David
ps. I am not an engineer. I left BYU with one class left to go (as a Spanish Major with a Manufacturing Engineering Technology minor--on my way to med school to complicate matters further). My last semester I heard about this MiG factory in Poland that was looking for work...