turbo_1889 wrote:boys who do this casting thing on a massive scale and as a business. If they have learned from experince that with a particular alloy they need to cut their molds 0.75% bigger so that their parts are the right size when they come out of the mold, sounds like a good place to start
I wasn't aware that there was a massive industrial lead casting industry where diameters were controlled to tight tolerances ?
Industrial casting of mass produced parts made of things like zinc is done with pressure casting, not gravity pour.
casting process, dimensions are not tightly controlled, but instead, the casting is made oversize and the casting is finished by machining the critical dimensions. This is true whether you are talking about casting an engine block or casting a bullet.
My message didn't seem to register. I'll repeat it.Before the lead can "shrink," first it has to fill out the mold. In a gravity pour mold, with common bullet alloys, fill-out is never 100%. Fill-out depends on bullet design and casting technique.
If you cut the cavity with all the bands the same diameter, chances are that no two bands will cast the same diameter, because they will fill out differently. In order to make them cast the same the same diameter, it is often necessary to cut them to different diameters.
For example, I recently cut a 245 grain 0.432" bullet with 3 bands. The bottom band would drop at 0.433", the middle band at 0.430", and the top band at 0.431". In order to bring them into spec, it was necessary to cut each band differently -- because they were filling out differently.
As-cast diameter is different depending on whether the mold is iron, brass, or aluminum. Why ? 1) they have different coefficients of thermal expansion and 2) they fill out differently.
There are many variables. There is no reliable formula.