How to compensate for casting shrinkage when making models?

[Rolf]

I've bougth som butter Board and want to try and carve masters for casting rifle butt plates an trigger guards.
How much oversized should they be for wax casting?
How much oversized should they be for sand casting?
Hope to do test castings in brass and cast the ones I want in sterling silver. I inherited a collection of silver spoons (4 kilos). Had them checked at a auction house. They have only metal value. I've had them melted down an refined for casting.

Best regards
Rolf

[Kingsburyarms]

Rolf - I did the exact same thing, other than I used the silver set I inherited, and used existing waxes from Jim Kibler and others, someone on ALR with much more casting experience can chime in on the sizes. I really enjoyed re-purposing the set to live through more generations,

Jon

[JPK]

Brass has a shrink rate of 1.4 percent. Can’t help on sliver but it could be close to the same.

[smart dog]

Hi Rolf,
Plan on 3-5% shrinkage of the final cleaned up and polished object.  There is some shrinkage in the mold but it is usually very small particularly if you poured enough extra metal such that the hot mass of extra metal in the funnel leading to the sprue acts as a heat source keeping the metal flowing a little longer and completely filling the mold. If you are using sand, petrobond, and Delft clay casting, most loss occurs when you clean up the edges of the cast object. The object right out of the mold tends to have edges that are slightly rounded and If you need sharp clean edges you may end up with the final object a little smaller than planned.  I find, 3-5% extra size for the model solves all those issues.

dave 

[Jim Kibler]

I don't know about sand casting, but if I recall correctly, I typically get anywhere from about 1%-2.5% with investment castings.  It varies considerably due to geometry though. 

Jim