What are the casting shrinkage ratios used in the ODP ?

turbo_1889

What are the casting shrinkage ratios used in the ODP ?

Postby turbo_1889 » Thu Mar 26, 2009 6:24 pm

Your Oline Design Program (ODP) lists the selected bullet diameter as a ''as cast'' diameter. Just wondering what the ratios for the various lead alloys you list is programmed in as. Industrial metal casting standard is to list this as either a ratio of a decimal fraction of an inch per inch or a percentage. Example: 0.0075'' or 0.75%

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Re: What are the casting shrinkage ratios used in the ODP ?

Postby mtngun » Thu Mar 26, 2009 7:06 pm

That's not part of the program.

It is a myth that as-cast diameter is predictable.

For one thing, the diameter of the mold cavity changes with mold temperature. So you don't really know the diameter of the cavity.

For another thing, before the bullet can "shrink," first it has to fill out the mold. "Fill out" is rarely perfect. If you "pressure cast" you will get a larger diameter because the cavity fills out better (I'm not recommending pressure casting, just making a point). The point is that casting technique affects fill which affects diameter.

Bullet design also affects diameter. Itty bitty bands don't fill out as well as long bands. Groove depth and shape affects fill, because the molten lead has to zig-zag around the grooves.

So there is no formula. There are many variables, and it is basically a guessing game.

turbo_1889

Re: What are the casting shrinkage ratios used in the ODP ?

Postby turbo_1889 » Thu Mar 26, 2009 10:51 pm

Thanks for the reply, I've just been looking over industrial engineering alloy specifications data sheets which list this kind of info for gravity and pressure casting for not only lead alloys but others as well. They, as you also state as well, point out that the specs in question pertain to the mold cavity size ''at casting temp'' and do very slightly with varying applications. I understand that casting technique plays a large part in this but apparently the shrinkage rate is somewhat perdictable at least on a rule of thumb basis from industrial experience data. Personally I think we little guys can learn a lot from the industrial 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 doing or own guess work from. I've also been comparing the actual room temperature cavity diameters of my molds with the diameter of the ''as cast'' bullets they drop with the particular alloy I'm working with. Nice to know it's not anything pre-programmed into the ODP, and that any adjustments to the dimensions for a different alloy need to be done on my end and the computer isn't going to be trying to do it's own adjustments and goober everything up; that will let me figure the math manually both for the increase in size of the cavity at casting temp and then the shrinkage of the actual cast from there. So far the WW alloy molds you have cut for me produce as cast diameters right on the money within the two thou. tolerance limit you specify. Just trying to recalculate to hit a given size dead on with a certain other alloy to avoid sizing. Going to aim for that perfect diameter or slightly less and then if necessary I can lap the mold out a thou. or two but don't want to try to lap it out any further than that. Take lapping beyond a two thou. expansion and you start loosing the relative dimensional precision and crispness of the original mold cut, the sharp corners and edges start turning into rounded ones.

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Re: What are the casting shrinkage ratios used in the ODP ?

Postby mtngun » Sat Nov 27, 2010 6:32 am

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.

In ANY 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.


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