I have not cast a crankshaft yet, but have been pondering the best way to do it.
I was chasing some additive (nickle-mag), and the sales guy said "What is your sulphur level?".
I said "I don't know.".
He said something like (and I paraphrase) "you are pissing in the wind without knowing the sulphur level".
And so he would not sell me any additive.
He did say there is a specially way to mix it with the iron, and said whatever you do, don't just dump the nickle-mag into a crucible of hot iron.
Here is a video where they mix and additive (some sort of magnesium alloy) in a ladle.
You can see that one has to be extremely careful if using a mag alloy. (see 1:20)
This video was taken in daylight, but the reaction was so bright that it looked like night time.
The pandemic hit after this video, and I have never had a chance to get with these folks and ask them the details of this pour, and what type of iron they made (I assume it was ductile iron, at least that is what the video title says).
I found a parts list for a Speedy Twin at the Soule Foundry, and the base cost for a Speedy Twin included a cast iron crankshaft, with a somewhat expensive option for a steel crankshaft (perhaps a $500.00 adder, and this was an old cost sheet).
I don't know if Soule cast ductile or malleable iron or not.
Your crankshaft iron looks to be pretty tough, judging from the break test.
For a model engine running without load, I suspect that a gray iron crankshaft may last a while.
For a gas model engine, a gray iron crankshaft may not hold up to the impact very long.
I visited Bob Pearson in Kansas a few years ago (Bob is the Cretors Popcorn Engine guy, and he has the original Cretors foundry patterns for the engines and other things).
Bob mentioned that the end of the No.01 cranshaft had a habit of breaking off, and so I suspect that a No.01 Cretors crankshaft was cast in gray iron.
A Cretors engine would not have much load on it.
Given the complexity of making ductile iron, I have decided to make crankshafts using a method used on many motorcycle engines, which is a pressed assembly using high strength steel parts.
For one-off crankshafts, I can't justify the expense of using ductile iron.
I had a brief email discussion with Kory Anderson, who is the guy who made his own 150 hp Case tractor, and he said he makes ductile iron using the cupola process. I was not able to get any details about exactly how he did it, but perhaps a stream added to the flow going into the ladle, or one of those ladles with a pocket for the additive.
As far as shrinage, I have noticed that any more than a slight amount of ferrosilicon can cause a lot of shrinkage, and I have heard this from multiple iron folks.
I think I have narrowed in on using between 0.04 and 0.06 oz of ferro per pound of iron.
For thick gray iron parts, I have gotten away with using no ferro at all, such as parts 1" thick or thicker.
According to good foundry practice, all the parts of a casting should be the same thickness, and I have noticed this looking at sections of old steam engines.
For some parts, such as a crankshaft, it may not be very practical to make everything the same thickness.
It looked like for your first crankshaft, the crank pin solidified first, and drew from the webs as it solidified.
For the second crankshaft, the webs were so large that it is possible that the crank pin solidified, and then perhaps the entire molten top of the riser shrank and solidified uniformly ? (just a guess)
After watching your video, my best guess at this point as to how I would cast a crankshaft would be to turn it 90 degrees so the webs were vertical, with the crank pin down, fill from the bottom on the side of either web, and have a generous round riser with neck at the top of each web.
Would require a larger flask and more sand, but that is how I would probably approach it.
The neck below the risers would simplify separating the riser from the casting.
I have followed 100model/luckygen1001/iroman's videos for many years, and he gets so many things right, as follows:
1. He uses a pouring cart with very good effect (no lifting and carrying a very heavy crucible full of iron).
And no, spilling molten iron on concrete is not the disaster everyone says it is.
For new and very hard concrete, I have seen it pop, but for my old concrete, it does nothing.
2. His pour technique is probably the best I have seen; ie: he uses a non-linear pour rate to fill the sprue very quickly, generally pouring from as low as possible, and then always keeps the sprue full during the mold fill, stopping the pour just as the mold fills.
This takes a lot of concentration and practice in order to not spill iron all over the place.
And he proves that you don't need a big pouring basin, as so many use.
A big pouring basin seems to be the best way to aspirate a lot of air down the spure, and I always pour directly down the sprue.
3. His castings always look good when he breaks them, with a nice clean gray break line.
4. He can pour very thin iron and get a complete fill, which can be rather a trick if you have ever tried it.
5. He is the most consistent iron pourer I have seen anywhere, and probably has the most experience among the hobby crowd with iron.
6. He runs a very low mass furnace, which is one of the secrets of getting very hot and fluid iron.
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