Stress releaving grey cast iron block

D

DLF

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Hello,

I have a question for the casting gurus out there.

I ordered at the local foundry a grey cast iron block with these dimensions: 23x13x11cm

I plan to machine a mill column spacer block for my benchtop mill.

The machined part will still be a 22x12x10 cm block with 4 through holes for the mounting bolts.

Considering the thickness of the finished part, do I need to stress releave the cast block before machining?

I mean, for complex parts with thin walls etc., stress releaving is mandatory. Buy what about this use case: thick parallelipedic block with just 4 mounting holes?

Thanks
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I don't think I would worry about it- over the course of many months or years it may undergo a tiny change in shape but you could always take
another light skim cut on the surface to square it again if necessary
-Mark
 
@DLF
You should probably ask the foundry what condition the piece is in and what to expect when you machine/scrape it.
I'm not an metallurgy or casting expert however, I would rough machine the block first (before annealing) because I'd expect that cutting the skin will have the greatest short term effect.
 
Asking the foundry would be the best bet. In many cases of machine frames, the rough casting is set outside for a year or three in the weather to "season" before machining. For a small piece, that may or may not matter. Ask though. . .

The big issue with cast iron is the surface where it comes out of the mold. A thick roughing cut is important for the " mill scale" where it comes out of the mold. Once the scale has been removed, finish cuts are on softer material. The scale is usually very hard and may contain sand and other materials extraneous to the iron. Use of old/cheap/worn tooling is a consideration there. New tooling will often be damaged by scale.

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With such a big block of uniform thickness, hopefully it will have cooled nice and slowly and won't be too hard. Were you not tempted to get it made hollow, though? It must weigh lot!!
 
+1 on asking the foundry. Though if I were doing that job, I'd bore out a cavity in the center to help relive internal stress. Somewhere in the dim recesses of my memory from engineering school, I remember something about one of the advantages of hollow castings, parts. Basically, it gives the material another place to go when its expanding or contracting from temperature changes. And reduces the amount of material needing to move in the first place.
 
The cast iron block is considered to be "green". It is freshly cast. The foundry let it cool for 7-8 days before taking it out of the mold.

Now it weighs a nice 28kg but machining will shave 10kg out of it.

Since this is a structural piece I would not hollow out the interior. I do not know how much wall thickness to leave.

I will ask the foundry about the stress releaving. Good hint on that

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Eddyde said:
+1 on asking the foundry. Though if I were doing that job, I'd bore out a cavity in the center to help relive internal stress. Somewhere in the dim recesses of my memory from engineering school, I remember something about one of the advantages of hollow castings, parts. Basically, it gives the material another place to go when its expanding or contracting from temperature changes. And reduces the amount of material needing to move in the first place.
Click to expand...
It makes sense. The outer skin would solidify first but the inner core would continue to shrink creating stress. If the casting had a hollow core, it would still cool from the outside to the center as the sand core is a relatively poor conductor of heat but the casting would pull away from the core as it shrunk, relieving some of the stress. However IMO, drilling the casting after it is removed from the mold won't do much to relieve stress unless the casting was heated to near molten state.
 
Not to shire but all the machine casting prints I see at work requires stress relieving and annealing very few don't if any. Most are Class 40 castings some are Class 30 but that is rare.
 
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