Shrinkage allowance for patterns

[Ritz_physics]

While making a pattern for a casting do we increase or the bore diameter of the pattern, than actually required in the casting, taking into consideration the shrinkage allowance for metal?

 

[Vadar2012]

It depends on the material you're using to make the casting. There's a reason why Ferrari parts are made with molds made from sand.

 

[Ritz_physics]

I'll put the question in a different way. If we have a hot metal ingot with a bore, after the ingot cools down, whether the bore diameter will increase or decrease?

 

[pantaz]

Bore diameter will decrease.

 

[Ritz_physics]

Bore diameter will decrease.

So this is exactly what would happen during the cooling of a similar casting, after it has solidified?

 

[pantaz]

So this is exactly what would happen during the cooling of a similar casting, after it has solidified?

Generally speaking, yes, but...

... Some alloys will have overall linear shrinkage of up to 2.5%, whereas other alloys may actually experience no shrinkage or a slight "positive" shrinkage or increase in size in the casting process (notably type metal and certain cast irons). ...
http://en.wikipedia.org/wiki/Pattern_(casting)#Contraction_allowances_.2F_Shrinkage_allowance

 

[Ritz_physics]

Ok.. anyway for most metals (the ones that contract on cooling) the bore diameter will decrease during the shrinkage of casting, right?

 

[pantaz]

Ok.. anyway for most metals (the ones that contract on cooling) the bore diameter will decrease during the shrinkage of casting, right?

Again, yes. The amount of change will depend on many things. Getting the "as cast" dimensions to meet your required tolerances may require considerable trial & error.