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A Tale of Two Castings

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krv3000:
 right it brakes dawn any hard scale by rusting the outer surface

vtsteam:
Oh, okay, that makes sense!  :med:

vtsteam:
Fourth casting was a success!  :thumbup:

It's the twin of the last one, but with less finning. Surface finish is just a bit less shiny but free of sand sticking. This time I used a facing sawdust mix with spruce, instead of white pine (mainly because the pattern is spruce this time --the sawdust came from shaping it). The facing sand had a little more water in it and it had enough bond strength not to drop out at the edges, so the casting has less finning.

Photos tomorrow -- not enough light tonight.....

Tom, I don't think the sawdust absorbing water is likely to be a good thing. I think the faster it flashes to gas when the molten metal hits it, the more effective it is going to be in pushing the metal back from the sand surface. I mean it's probably only milliseconds difference between the burn time of very dry and somewhat moister sawdust, but the standard is coal dust, which has no water content and is more finely ground. I bet it flashes very fast. Well that's all guesswork on my part.

I'm thinking that maybe hardwood sawdust would be better than soft wood -- slower moisture absorption, denser, and higher energy content. I bet a higher percentage (by weight) in the sand won't decrease the bond strength -- because it is nearly twice as dense as softwood -- there is less volume of it for a given weight, and volume of wood is what will adversely affect bond strength.

I'll have to give it a try.

awemawson:
In the days before ‘just in time’ manufacturing one lathe maker piled his castings in the yard and apprentices were instructed to relieve themselves on them. They were left for up to a year before machining. I was told this was for the stresses from the casting process to relax. Presumably the rusting as Bob says would help get rid of any surface sand issues not that good cast iron rusts much short term.

PekkaNF:
Wet sandblasting and rusting were old ways to get rid of hard scale and aging did some stress relieving.

Fun anecdote: One mortar manufacturer had problems with tubes developing cracks after finishing or after few shots. Not ideal as you might guess. The tubes were made rotary hammer or something. There were some serious stress issues, heat treating would have been a logistical nightmare, but temporary solution was simple. They strapped full days production onto a pallet really tight and bolted that to a eccentric mass vibrator for overnight. That apparently did the trick!

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