Which direction does surface grinding "curl" a cast-iron part?
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    Default Which direction does surface grinding "curl" a cast-iron part?

    If a cast-iron part (for example, a machine table) which is free of internal stress is surface ground on one side, does that side have a tendency to become concave or convex? I know that grinding creates internal stresses, but don't know if these tend to create tension (in which case the ground side would become concave) or compression (in which case the ground side would tend to become convex).

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    The part expands on the side towards the grinding wheel when heated by the grinding. When cooling down, there will be a resulting concavity. This is reduced when the cut is small and/or the cooling efficient.
    Ole

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    Hi Ole,

    Thanks for the quick reply!

    Quote Originally Posted by ole.steen View Post
    The part expands on the side towards the grinding wheel when heated by the grinding. When cooling down, there will be a resulting concavity.
    OK, so the ground side has a tendency to become concave. Is this purely because of heating/cooling effect that you describe? Or is also from the creation of an internal "tension" in the material?

    Cheers,
    Bruce

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    i believe grinding or milling warpage is from internal stress in the part. obviously part needs to be at same temperature. you cannot have temperature variation in part doing final machining. remember even if coolant is 5F cooler it can create warpage and size changes just using coolant especially if coolant only going on one end of a part
    .
    quite normal to rechuck at lighter and lighter holding force careful to not distort part in its being held or clamped. quite normal to rough machine both sides. rechuck semifinish both sides, rechuck finish machine both sides. if i need to hold a .0003" per 40" flatness tolerance its usually required to rechuck and take lighter and lighter cuts.
    .
    large parts if inspection says its off and they markup for recuts i take picture and record. if consistent error sometimes you can machine first time to compensate for normal expected errors. hard to explain. on horizontal cnc mill might machine at B270.001 degrees so inspections measures it as B270.000

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    Worth pointing out that a part such as a machine table should not be held with a magnetic chuck but rather fixtured so that the magnet does not pull the center down.

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    Quote Originally Posted by tdmidget View Post
    Worth pointing out that a part such as a machine table should not be held with a magnetic chuck but rather fixtured so that the magnet does not pull the center down.
    .
    normally use thickness gage stock .001, .0012, .0015" to support part so magnet dont distort part. if you use .0010" on one end and .0012"(.03mm) on other end thats a .0002" difference. if middle humped up .002" i usually stick .002 in so magnet dont pull it down. i have often seen .003" warpage recut, and still got .001" warpage and have to recut. sometimes have to sneak up on it. cant take warpage out all at once

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    Quote Originally Posted by DMF_TomB View Post
    i believe grinding or milling warpage is from internal stress in the part.
    This is very true. The OP's question is loaded to start with, is states "a casting that is free from internal stress", good luck finding that animal. Castings, by their nature are full of internal stress. The outside cools and solidifies first, leaving the inside to cool, and shrink(or at least want to) after the outside is solid. This causes tension within the casting, the inside is pulling in on the outer surfaces. As the cooling continues, the part reaches a point of equilibrium and maintains it's shape, until, material is removed from one side and the state of equilibrium is upset. The material in the center no longer has anything to pull against on that side, causing the casting to go convex on the machined side.
    Stress relieving will help remove a lot of the major stresses, but some will always remain. This is what causes a part to change shape.

    Various types of machining can induce stress into a part, especially the use of dull cutters and high pressure situations. This can have a burnishing effect and impart a compression stress into the very outer surface of the material. This effect is usually eliminated in taking light finish cuts with a sharp tool of free cutting grinding wheel.

    Best approach to machining a casting is to rough it out, at least cleaning all surfaces which require finishing, and leaving as little material as possible. This should be followed by stress relieving, then finishing.

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    Quote Originally Posted by Derek Smalls View Post
    This is very true. The OP's question is loaded to start with, is states "a casting that is free from internal stress", good luck finding that animal. Castings, by their nature are full of internal stress. The outside cools and solidifies first, leaving the inside to cool, and shrink(or at least want to) after the outside is solid. This causes tension within the casting, the inside is pulling in on the outer surfaces. As the cooling continues, the part reaches a point of equilibrium and maintains it's shape, until, material is removed from one side and the state of equilibrium is upset. The material in the center no longer has anything to pull against on that side, causing the casting to go convex on the machined side.
    Stress relieving will help remove a lot of the major stresses, but some will always remain. This is what causes a part to change shape.

    Various types of machining can induce stress into a part, especially the use of dull cutters and high pressure situations. This can have a burnishing effect and impart a compression stress into the very outer surface of the material. This effect is usually eliminated in taking light finish cuts with a sharp tool of free cutting grinding wheel.

    Best approach to machining a casting is to rough it out, at least cleaning all surfaces which require finishing, and leaving as little material as possible. This should be followed by stress relieving, then finishing.
    .
    yes i agree under right conditions just the act of machining can induce stress into surface of metal causing warpage. dull tool or running so fast its creating excess heat on part surface 2 common causes


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