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I am designing a pattern for a sand casting. It has only one critical dimension which is a 4" inner diameter cylinder. If I use a 4" core, will the casting shrink away from the core? In other words will the inner diameter of the cylinder end up being 4.1" or something like that? What kind of shrinkage can I expect for ductile iron?

The cylinder needs to be a sliding fit on a 4" diameter painted, mill finish steel column. The inner diameter of the cylinder should end up being no more than 4.05".

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  • $\begingroup$ What casting process will you be using? That affects the precision that you can get, and regardless of shrinkage, I'd be concerned about the ability of the casting process in general, even if shrinkage is accounted for, to meet the proper tolerances for a sliding fit. $\endgroup$ Aug 19 '15 at 16:53
  • $\begingroup$ Also, are you/your company doing the casting process yourselves, or are you designing the part and purchasing it from a foundry? $\endgroup$ Aug 19 '15 at 16:54
  • $\begingroup$ I have no control over the casting process. I am just making the pattern. I am not sure I trust the advice of anyone who calls a foundry a "casting house". $\endgroup$ Aug 19 '15 at 16:55
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    $\begingroup$ Is machining after casting not an option? That would seem to be the most straightforward option unless there is some other constraint. $\endgroup$
    – hazzey
    Aug 19 '15 at 18:01
  • $\begingroup$ @hazzey That would be overkill here. $\endgroup$ Aug 19 '15 at 18:06
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Ductile iron is actually likely to expand during the casting process, as shown in the table found on the Casting Wikipeda page, which is in part cited from this casting text by Stefanescu. However, as the tables show, the range of volume change is fairly large and not one-sided, which is to say the ductile iron may either expand or contract depending on other conditions of the casting process.

However, another concerning aspect is the range of precision that you're looking for in this casting. A tolerance of $^{+0.05"}_{-0}$ is pretty tight for a ductile iron casting. The ISO standard that lists general tolerances for castings (ISO 8062-3) says that for a machine-molded sand casting of ductile iron, the tolerance range on a linear dimension of $100-160mm$ is at least $\pm 0.035"$, and could be up to $\pm0.14"$. For a hand-molded casting, that goes from $\pm0.098"$ to $\pm0.24"$. These are general tolerances, so you could look for tighter tolerances on your critical feature, but I would suggest (if it's possible in your process) to plan on machining out that 4" diameter and set your core diameter up so that you are cleaning up the entire circumference of the bore. Otherwise, I think hitting the tolerance range you are looking for as-cast will be tricky.

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    $\begingroup$ Also, doesn't sand casting leave a fairly rough texture? Those irregularities may take up an appreciable amount of the tolerance? $\endgroup$
    – hazzey
    Aug 19 '15 at 18:12
  • $\begingroup$ The standard doesn't mention surface roughness, though you're correct that castings are not particularly smooth. I'm guessing that's covered in a different standard, but the surface roughness obviously can't be a wider range than the dimensional tolerance. That is another reason to clean up the diameter by machining though. $\endgroup$ Aug 19 '15 at 18:18

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